Astronomers have discovered a one-kilometre wide asteroid orbiting the Sun at a distance of just 20 million km (12 million miles). Not only does this make the asteroid–currently designated 2021 PH27–the Sun’s closest neighbour, but it also means that as it completes an orbit in just 113 days, it is also the solar system’s fastest-orbiting asteroid. 2021 PH27 skirts so close to the Sun that its discoverers say its surface temperature is around 500 degrees C–hot enough to melt lead.
Scott S. Sheppard of the Carnegie Institution of Science first spotted asteroid 2021 PH27 in data collected by the Dark Energy Camera (DECam) mounted at the prime focus of the Victor M. Blanco 4m Telescope at Cerro Tololo Inter-American Observatory (CTIO), Chile. Brown University astronomers Ian Dell’antonio and Shenming Fu took images of the asteroid on 13th August 2021 at twilight–the optimum time for hunting asteroids that lurk close to the Sun. Just like the inner planets–Mercury and Venus–asteroids that exist within the Earth’s orbit become most visible at either sunrise or sunset.
The discovery was followed by measurements of the asteroid’s position conducted by David Tholen of the University of Hawai‘i. These measurements allowed astronomers to predict asteroid 2021 PH27’s future position, leading to follow-up observations on the 14th of August by DECam and the Magellan Telescopes at the Las Campanas Observatory in Chile.
These observations were then subsequently followed on August 15th by imaging made with the LasCumbres Observatory network of 1- to 2-meter telescopes located in Chile and South America by European Space Agency (ESA) researcher Marco Micheli.
The findings were so significant that many astronomers cancelled their scheduled projects to use telescope time with a variety of sophisticated instruments to further observe the asteroid. “Though telescope time for astronomers is very precious, the international nature and love of the unknown make astronomers very willing to override their own science and observations to follow up new, interesting discoveries like this,” explains Sheppard.
What makes the discovery of asteroid 2021 PH27 so special, and of great interest to astronomers, is the fact that it belongs to a population of solar system bodies that have been, thus far, notoriously difficult to spot.
Hunting For Inner Solar System Asteroids
Interior asteroids that exist close to the Sun tend to be difficult for astronomers to spot because of the glare from our central star. This difficulty is amplified by the fact that as they get close to the Sun these objects experience intense gravitational, tidal, and thermal forces that breaks them up into smaller–thus tougher to spot–fragments.
That means tracking an intact interior asteroid could have benefits for our understanding of these objects and the conditions they experience. In particular, if there are few asteroids experiencing a similar orbit to asteroid 2021 PH27 it may indicate to astronomers many of these objects were loose ‘rubble piles.’ This may, in turn, give us a good idea of the composition of asteroids on a collision course with Earth, and crucially, how we could go about deflecting them.
“The fraction of asteroids interior to Earth and Venus compared to the exterior will give us insights into the strength and make-up of these objects,” Sheppard continues. “Understanding the population of asteroids interior to Earth’s orbit is important to complete the census of asteroids near Earth, including some of the most likely Earth impactors that may approach Earth during daylight and that cannot easily be discovered in most surveys that are observing at night, away from the Sun.”
In addition to this, asteroid 2021 PH27’s orbit is so close to the Sun that our stars exerts considerable gravitational effects upon it, something that could make it a prime target for the study of Einstein’s geometric theory of gravity–better known as general relativity.
This close proximity to the Sun may actually be a recent development for asteroid 2021 PH27.
Asterod 2021 PH27 is on the Move
Planets and asteroids don’t move around their stars in perfectly circular orbits, but in ellipses–flattened out circles. The ‘flatter’ the circle the greater we say its eccentricity is. The widest point of the ellipse is the semi-major axis and for an orbit, this represents the point at which a body is farthest from its parent star.
Asteroid 2021 PH27 has a semi-major axis of 70 million kilometres (43 million miles or 0.46 au) which gives it a 113-day orbit crossing the orbits of both Venus and Mercury. But it may not have always existed so close to the Sun.
Astronomers believe that the asteroid may have started life in the main asteroid belt between Mars and Jupiter, with the gravitational influence of the inner planets drawing it closer to the Sun. This would make it similar to the Near-Earth Object (NEO) Apophis, which has only recently been ruled out as a potential Earth impactor, which was also dragged closer to the Sun by gravitational interactions.
There is also some evidence arising from 2021 PH27’s high orbital inclination of 32 degrees that the asteroid may have a slightly more exotic origin, however. This could imply that the asteroid is actually an extinct comet that comes from the outer edge of the solar system pulled into a close orbit as it passed an inner-terrestrial–rocky–planet. Astronomers will be looking to future observations to determine which of these origins is correct, but unfortunately, this will have to wait. 2021 PH27 is about to enter solar conjunction which means that from our vantage point on Earth it is about to move behind the Sun. That means the asteroid will only become available for further observations in 2022.
These follow-up observations will allow astronomers to better determine its orbit. And with this better determination will come a new official name that is hopefully a bit less of a mouthful than 2021 PH27. But what is certain is that this asteroid is not set to become any less interesting.
In a major breakthrough for the investigations of Earth-like planets outside the solar system, astronomers have used the Very Large Telescope (VLT) to spot previously ‘hidden’ exoplanets orbiting the star L 98-59–located around 35 light-years from Earth.
Amongst the exoplanets, which resemble the inner worlds of our planetary system, is the smallest exoplanet yet to be discovered by astronomers using a detection method known as the radial velocity technique. The innermost exoplanet–which has been designated L 98-59b–has half the mass of Venus.
This isn’t the only important find in the planetary system, however. The team has also found signs of a world dominated by oceans and even the possibility of a terrestrial planet like Earth within the star’s habitable zone.
“We have scrutinized the L 98-59 planetary system. Combining the best instruments currently in operation for the study of exoplanets, achieving the smallest mass measurement using the radial velocity methods,” Olivier Demangeon, a researcher at the Instituto de Astrofísica e Ciências do Espaço, University of Porto, Portugal, tells ZME Science. “We have discovered a new planet in this system and a planetary candidate in the habitable zone. We have also measured the mass and radius of the three inner planets with exquisite precision and inferred their internal structure and composition.”
The research, documented in the journal Astronomy & Astrophysics, represents a major breakthrough for the radial velocity method, already an import tool for exoplanet hunters.
“The radial velocity method measures the movement of the star caused by the presence of the planets,” María Rosa Zapatero Osorio, an astronomer at the Centre for Astrobiology in Madrid, Spain, and Chair of the “Atmospheric Characterisation” working group of the ESPRESSO science team at Centro de Astrobiología (CSIC-INTA), tells ZME Science. “In planetary systems, all bodies–the star and the planets–move around the center of mass, the gravitational central point of the system. Displacement in the velocity of the star is determined via the radial velocity method.”
What Makes the L 98-59 Planetary System Such an Important Find?
As Demangeon explains, there are a number of factors that make the L 98-59 planetary system so special. “At just 35 light-years away it is relatively close to Earth, its central star is bright, it hosts at least four planets, and it might host a planet in the habitable zone,” the researcher says. “Other systems also have a couple of these properties, but it is the combination of all these properties that makes this system unique.”
Astronomers had already investigated the system with the Transiting Exoplanet Survey Satellite (TESS) and using dips in light from the stars caused by exoplanets passing across the face of the stars, finding three planets. The fact that the system contains at least four planets came as something as a surprise to the astronomers, who initially set about investigating the system to measure the mass of L 98-59b.
“This fourth planet does not transit its parent star, we can only characterize it thanks to the gravitational pull that it produces on its parent star. If this planet was alone, we would only have a lower estimate of its mass,” says Demangeon. “However as it has siblings which transit the parent star, we can make reasonable assumptions and reach an accurate knowledge of its mass and orbit. What remains unknown is its radius.”
Not only was the fourth exoplanet around L 98-59b previously undiscovered, but the team strongly suspects there is also a fifth planet in the system.
It isn’t just the number of planets in the system that the team has been able to ascertain. Even though they are as of yet unable to determine the radius of the fourth world, they have also made impressive headway in determining the size and composition of some of the previously discovered worlds.
“We were able to measure precisely the mass and radius of the three inner planets, and we know that these three planets are terrestrial, meaning that they are mostly composed of rocky material–iron and silicates–but their water contents are noticeably different,” explains Demangeon.
The researchers are clear, this new knowledge about the L 98-59 planetary system would not have been available with the impressive observing power of the ESPRESSO instrument and the VLT.
An Important Breakthrough for Exoplanet Hunters
The observation of such a small planet as L 98-59b with the radial method is an impressive feat that was only possible by employing the VLT and the ESPRESSO instrument, an achievement that should solidify its position as one of astronomy’s most impressive tools.
“ESPRESSO is, as of today, one of the best instruments to hunt planets,” says Zapatero Osorio. “It was built with this objective in mind and it is stabilized mechanically and thermally. This means that It delivers radial velocities with unprecedented quality for bright stars like L 98-59.”
Zapatero Osorio adds that the reason that ESPRESSO is so well adapted for radial velocity measurements is that it can see stellar activity in such detail that it no longer obscures the signal presented by small worlds. Hence why it was able to spot an exoplanet with half the mass of Venus, and spotting such a small world could massively improve our understanding of the formation of planetary systems.
“One of the major open questions in physics today is how stars and planets are formed from the protoplanetary nebula, the planetary dynamics within the systems, and their evolution,” says Zapatero Osorio. “Determining the masses of the smallest planets with better signal-to-noise ratio is necessary for understanding how the planetary system was formed and evolved and to find the right place for each planet in the planetary mass-radius diagram.”
The team is unfortunately currently unable to make an in-depth examination of the exoplanets’ atmospheres even with the powerful VLT and ESPRESSO combination. There is hope that this could change in the future will the launch of the James Webb Space Telescope (JWST).
“The next step, the one that I am most excited about, is to attempt the detection and the study of the atmosphere of the three transiting planets. In particular planet d, with its high water content is an ideal candidate,” concludes Demangeon. “Several movies and books have depicted what the atmosphere of terrestrial extrasolar planets could look like.
“With L98–59, we might not have to imagine for very long.”
We are closer than ever before to understanding the composition of Mars thanks to the first observations of seismic activity on the planet made by the InSight lander. The NASA-led project, which landed on the surface of the Red Planet in November 2018 with the goal of probing beneath the Martian surface, observed several so-called ‘marsquakes’ which reveal details about its crust, mantle, and core.
InSight’s primary findings which are detailed in three papers published today in the journal Science, represent the first time scientists have been able to produce a detailed picture of the interior of a planet other than Earth.
“We are seeking to understand the processes that govern planetary evolution and formation, to discover the factors that have led to Earth’s unique evolution,” says Amir Khan, ETH Zurich and the University of Zurich, whose team used direct and surface reflected seismic waves to reveal the structure of Mars’ mantle. “In this respect, the InSight mission fills a gap in the scientific exploration of the solar system by performing an in-situ investigation of a planet other than our own.”
The results from the ongoing NASA mission–with the full title ‘Interior Exploration using Seismic Investigations, Geodesy and Heat Transport’— could reveal key insights into the Red Planet‘s formation and evolution, as well as helping us understand the key differences between our planet and Mars.
“One big question we would like to understand is why Earth is the only planet with liquid oceans, plate tectonics, and abundant life?” adds Khan. “Mars is presently on the edge of the solar system’s habitable zone and may have been more hospitable in its early history. Whilst we don’t yet know the answers to these questions, we know they to be found are on Mars, most likely within its interior.”
InSight first detected the presence of marsquakes from its position in Elysium Planitia near the Red Planet’s equator in 2019 and has since picked up more than 300 events–more than 2 a day–tracing many of them back to their source.
What is really impressive is what researchers can do with these quakes, using them as a diagnostic tool to ‘see’ deep into the planet’s interior.
“Studying the signals of marsquakes, we measured the thickness of the crust and the structure of the mantle, as well as the size of the Martian core,” Simon Stähler, a research seismologist at ETH Zurich, tells ZME Science. “This replicates what was done on Earth between 1900 and 1940 using the signals of earthquakes.”
From the Crust of Mars…
The observations made by InSight have allowed researchers to assess the structure of Mars’ crust, allowing them to determine its thickness and other properties in absolute numbers for the first time. The only values we previously had for the Martian crust were relative values that showed differences in thickness from area to area.
“As part of the bigger picture on the interior structure of Mars, we have determined the thickness and structure of the Martian crust,” Brigitte Knapmeyer-Endrun, a geophysicist at the University of Cologne’s Institute of Geology, tells ZME Science. “Previous estimates could only rely on orbital data–gravity and topography–that can accurately describe relative variations in crustal thickness, but no absolute values. These estimates also showed a wide variability.”
With data collected regarding the crustal thickness at InSight’s landing area, new seismic measurements, and data collected by previous missions, the team could map the thickness across the entire Martian crust finding an average thickness of between 24 and 72 km.
Knapmeyer-Endrun explains that the data she and her team collected with InSight’s Seismic Experiment for Interior Structure (SEIS), particularly the very broad-band (VBB) seismometer–an instrument so sensitive it can record motion on an atomic scale–and information from the Marsquake Service (MQS) at ETH Zurich, suggest that the Red Planet’s crust is thinner than models have thus far predicted.
“We end up with two possible crustal thicknesses at the landing site–between 39 and 20 km– but both mean that the crust is thinner than some previous estimates and also less dense than what was postulated based on orbital measurements of the surface.”
Knapmeyer-Endrun continues by explaining that the InSight data also reveals the structure of the Martian crust as multi-layered with at least two interfaces that mark a change in composition. In addition to this, the team can’t rule out the presence of a third crustal layer before the mantle.
“The crust shows distinct layering, with a surficial layer of about 10 km thickness that has rather low velocities, implying that it probably consists of rather porous–fractured–rocks, which is not unexpected due to the repeated meteorite impacts,” says the geophysicist adding that we see something similar on the Moon, but the effect is more extreme due to that smaller body’s much thinner atmosphere.
Knapmeyer-Endrun is pleasantly surprised regarding just how much information InSight has been able to gather with just one seismometer.”It’s surprising we were really able to pull all of this information about the interior of Mars from the recordings of quakes with magnitudes of less than 4.0 from a single seismometer,” she explains. “On Earth, we would not be able to even detect those quakes at a comparable distance. We typically use 10s or even 100s of seismometers for similar studies.”
And the marsquake data collected by InSight has not just proven instrumental in assessing the thickness and composition of the planet’s crust, it has also allowed scientists to probe deeper, to the very core of Mars itself.
…To the Martian Mantle and Core
Using direct and surface reflected seismic waves from eight low-frequency marsquakes Khan and his team probed deeper beneath the surface of Mars to investigate the planet’s mantle. They found the possible presence of a thick lithosphere 500km beneath the Martian surface with an underlying low-velocity layer, similar to that found within Earth. Khan and his co-author’s study reveals that the crustal layer of Mars is likely to be enriched with radioactive elements. These elements heat this region with this warming reducing heat in lower layers.
It was these lower regions that Stähler and his colleagues investigated with the use of faint seismic signals reflected by the boundary between the Martian mantle and the planet’s core. What the team discovered is that the Red Planet’s core is actually larger than previously calculated, with a radius of around 1840 km rather than previous estimates of 1600km. This means the core begins roughly halfway between the planet’s surface and its centre.
From the new information, we can also determine the core’s density and extrapolate its composition.
“We now know for sure the size of the core and it’s significantly larger than it had been thought to be for a long time,” says Stähler. “Because we found that the core is quite large, we, therefore, know it is not very dense. This means that Mars must have accumulated a substantial quantity of light, volatile elements such as sulfur, carbon, oxygen, and hydrogen.”
This ratio of lighter elements is greater than that found within Earth’s denser core, and it could give us important hints about the differences in the formation of these neighbouring worlds.
“Somehow these light elements needed to get into the core. It may mean that the formation of Mars happened faster than Earth’s,” Stähler says. “These observations have fueled speculation that Mars might represent a stranded planetary embryo that depicts the chemical characteristics of the solar nebula located within the orbit of Mars.”
As just Knapmeyer-Endrun did, Stähler expresses some surprise regarding just how successful InSight has been in gathering seismological data, emphasising the role good fortune has played in the mission thus far.
“We were able to observe reflections of seismic waves from the core–like an echo–from relatively small quakes. And the quakes were just in the right distance from the lander. Had we landed in another location, it would not have worked out,” the seismologist says. “And the landing site was only selected because it was flat and had no rocks, so it was really pure luck.”
Stähler says that he and his team will now attempt to use seismic waves that have crossed the core of Mars to determine if the planet’s core possesses a solid-iron inner-core like Earth, or if it is entirely liquid. Just one of the lingering questions that Knapmeyer-Endrun says InSight will use marsquakes to tackle over the coming years.
“There are still multiple open questions that we’d like to tackle with seismology. For example, which geologic/tectonic features are the observed marsquakes linked to? At which depth do olivine phase transitions occur in the mantle? And Is there a solid inner core, like on Earth, or is the whole core of Mars liquid?” says the geophysicist.
And if we are to go by track record, the smart money is on InSight answering these questions and more. “Within just 2 years of recording data on Mars, this single seismometer has been able to tell us things about the crust, mantle and core of Mars that we’ve been speculating about for decades.”
The surfaces of neutron stars may feature mountains, albeit ones that are no more than millimetres tall, new research has revealed. The minuscule scale of neutron star mountains is a result of the intense gravity produced by these stellar remnants that are the second densest objects in the Universe after black holes.
Because neutron stars have the mass equivalent to a star like the Sun compressed into a diameter that is about the size of a city on Earth–about 10km– they have a gravitational pull at their surface that is as much as 40,000 billion times stronger than Earth’s.
This presses features on that surface flat, making for almost perfect spheres. Yet the new research, presented at the National Astronomy Meeting 2021 shows that these stellar remnants do feature some tiny topological deformations, analogous to mountains on a planet’s surface.
The finding was a result of complex computer modelling by a team of researchers led by the University of Southhampton’s Fabian Gittins. The Ph.D. student’s team simulated a realistic neutron star and then calculated the forces acting upon it. What the research really shows is how well neutron stars can support deviations from a perfect sphere without its crust being strained beyond breaking point.
This revealed how mountains could be created on such dense stellar remnants and demonstrated that such formations would be no taller than a fraction of a millimetre.
“For the past two decades, there has been much interest in understanding how large these mountains can be before the crust of the neutron star breaks, and the mountain can no longer be supported,” says Gittins. These results show how neutron stars truly are remarkably spherical objects. “Additionally, they suggest that observing gravitational waves from rotating neutron stars maybe even more challenging than previously thought”.”
Mountain formation has been formulated for neutron stars before, but these new findings suggest such features would be hundreds of times smaller than the mountains of a few centimetres previously predicted. This is because those older models took the crusts of neutron stars to the edge of breaking point at every single point; something the up-to-date research suggests is less than realistic.
Neutron stars form when massive stars run out of fuel to power nuclear fusion. This means that the toward force balancing against gravity’s inward pull is cancelled and leads to the gravitational collapse of the star. During the course of this collapse, the massive star ejects its outer material in supernova explosions and leaving behind a core of ultradense material. This stellar remnant is only protected from further collapse–and in turn, becoming a black hole–by the quantum mechanical properties of the neutron-rich material that composes it.
The finding may have implications that go beyond the modelling of neutron stars. Tiny deformations on the surface of rapidly spinning neutron stars called pulsars could launch gravitational waves–the tiny ripples in spacetime predicted by general relativity and detected here on Earth by the LIGO/Virgo collaboration.
Unfortunately, as precise and sensitive as the LIGO laser interferometer is, it is still not powerful enough to detect gravitational waves launched by these ant-hill like mountains. It is possible that future upgrades to these Earth-based detectors and advancements such as the space-based gravitational wave detector LISA could make observing the effect of these tiny bumps possible.
During the 20th century, the idea that the Universe existed in a steady state was seriously challenged and eventually dismissed by the discovery that the Universe is not only expanding but is also doing so at an accelerating rate. The reason for this accelerating expansion is thus far unknown, but scientists have given this force a name–albeit one that is a placeholder–dark energy.
Explaining this accelerating rate of expansion has become one of the most challenging problems in cosmology. The fact that the value for this acceleration varies wildly between theory and practical observations has created a raft of problems in itself. This means that the net result arising from any attempt to explain dark energy creates more questions than it answers.
Dark Energy: The Basics
Dark energy is whatever is causing the expansion of the Universe to accelerate. One of the most striking things about this mysterious energy is just how much ‘stuff’ in the Universe it accounts for. If you consider the contents of the cosmos to be matter and energy –more formally known as the Universe’s mass-energy density–then dark energy accounts for between 68% to 72% of all cosmic ‘stuff.’
Dark matter is the second-largest contributor, with just a tiny proportion of the Universe’s ‘stuff’ made up of the baryonic matter consisting of atoms that we see around us on a day-to-day basis.
This is even more staggering when you consider that all the stars, planets, dust, gas and cosmic bodies that make up the visible Universe are contained in this tiny fraction of cosmic stuff that doesn’t even amount to 5% of the Universe’s contents.
Dark energy could very roughly be described as a force acting in opposition to gravity. Whereas the more familiar everyday force of gravity holds objects like planets and stars together in orbits, dark energy acts as a repulsive force, driving galaxies themselves apart. But, whereas gravity acts upon objects themselves, dark energy acts on the very fabric of spacetime between objects. Because dark matter is the largest contributor to gravity, this means this mysterious substance is locked in what has been termed a ‘cosmic tug of war.’ And it’s clear during our current epoch, dark energy is winning!
A popular analogy for this is the description of the Universe as the surface of a balloon. Galaxies are represented by two marker ink dots on this surface. As the balloon is inflated the points move apart with the very space between them expanding.
Repeat the experiment with three unevenly placed dots and it is clear that the dots that are initially further apart recede away from each other more rapidly. This extremely rough analogy carries over to galaxies. The further apart the galaxies are, the more quickly they recede.
An Expanding Surprise
The discovery that the Universe is expanding came as a considerable shock to the scientific community when it was confirmed by Edwin Hubble in 1929. Hubble had built upon theory provided by George Lemaitre and Alexander Friedman who had used the equations of general relativity to predict that the Universe was non-static, something that very much contradicted the scientific consensus of the time.
Albert Einstein who devised general relativity had also found that his geometric theory of gravity predicted a non-static Universe, something he wasn’t exactly comfortable with. Despite having already killed many of the sacred cows of physics, Einstein was unwilling to do away with the concept of a static Universe. To recover a static Universe that was neither expanding nor contracting, the world’s most famous scientist introduced to his equations a ‘fudge factor’ called the cosmological constant–commonly represented with the Greek letter lambda.
The cosmological constant was in danger from the start. Once Hubble managed to persuade Einstein that the Universe was indeed expanding, the physicist abandoned the cosmological constant, allegedly describing it as his ‘greatest blunder’ in his later years. The cosmological constant wouldn’t stay in the cosmological dustbin for very long, however.
If physicists had been surprised by the discovery at the beginning of the 20th Century that the Universe is expanding, they would be blown away when at the end of that same century when the observations of distant supernovae made by two separate teams of astronomers revealed that not only is the Universe expanding, it is doing so at an accelerating rate.
To understand why this is shocking and how it leads to the conclusion that some repulsive force is driving this expansion, it is necessary to journey to the very beginning of time… Let’s take some balloons too…
Escaping the Big Crunch
When thinking about the initial expansion of the Universe it makes sense to conclude that the introduction of an attractive force within that Universe would slow and eventually halt this expansion. That is exactly what gravity should do, and it seems did do during the early stages of expansion that had nothing to do with dark energy (we think).
Some cosmologists are willing to go a step further. If there is no outward pressure but an inward attractive force, shouldn’t the Universe actually start to contract?
This leads to the theory that the Universe will end in what physicists term the ‘Big Crunch’–an idea that dark energy could make obsolete. Think about how counter-intuitive this was to scientists when the idea was first suggested and evidenced. Let’s return to the balloon analogy; imagine you stop blowing into the balloon, and instead start sucking the air out of it.
How shocked would you be to find that the balloon isn’t contracting, it’s continuing to expand? And not just that, it’s actually expanding faster than it was when you were blowing into it!
With that in mind, consider the initial moments of the Universe. Beginning in an indescribably dense and hot state, squeezed into a quantum speck, the Universe undergoes a period of rapid expansion. This period of expansion wasn’t driven by dark energy. As it expands, the Universe cools allowing electrons to form atoms with protons and neutrons, which in turn frees photons to travel the cosmos.
Soon there is enough matter in the Universe to allow the attractive force of gravity to slow its expansion. And this does seem to be what happened in the early cosmos. The rapid inflation of the infant Universe is believed to have halted at around 10 -32 seconds after the Big Bang, with the Universe still expanding, albeit at a much slower rate.
This period of expansion continued to slow as a result of the growing dominance of matter during what cosmologists call the ‘matter-dominated epoch.’ But, at around 9.8 billion years into the Universe’s 13.8 billion year history, something strange begins to happen. The Universe begins to expand again, this time at an accelerating rate.
This is the dawn of the dark energy dominated epoch.
The Cosmological Constant is Back and Still Causing Trouble
That explains why the accelerating expansion of the Universe is so troubling and the need to introduce the placeholder concept of dark energy to explain it. Yet this accelerating expansion would still need a mathematical representation in equations used to describe the Universe. To do this cosmologists would return to the cosmological constant and its symbol, lambda.
This new iteration of the cosmological constant would be used in a different way to Einstein’s version. Whereas the earlier cosmological constant was used to balance gravity and hold the Universe steady and static, this new version would be used to overwhelm gravity and account for the acceleration of its expansion. But, this revised use of the cosmological constant does not mean it is any less troublesome than Einstein had found its predecessor.
In fact, the difference between the cosmological constant’s measured value, found by measuring the redshift of distant Type Ia supernovae, diverges from the value predicted by quantum field theory and particle physics by a value as large as 10121 (that’s 1 followed by 121 zeroes). Thus, it should come as no surprise that this value has been described as the worst prediction in the history of physics.
And as it represents the action of dark energy, that makes dark energy itself cosmology’s biggest conundrum.
OK… But What is Dark Energy?
So by now, you might well be thinking all of this is all fine and good, but this article specifically asks ‘what is dark energy?’ Isn’t it time to get to answering this question? It should come as no surprise that the answer is no one knows. But, that doesn’t mean that cosmologists don’t have some very good ideas.
One of the explanations for dark energy says that it could be vacuum energy, an underlying background energy that permeates that Universe and is represented by the cosmological constant. The most commonly cited evidence for vacuum energy–the energy of ’empty’ space which manifests as the Casimir effect.
Without delving too deeply into this, as relativity states that energy and mass are equivalent and mass has gravitational effects, then it stands to reason if empty space has vacuum energy, this too should contribute to the effect of gravity across the cosmos. That contribution has been factored in as a negative repulsive influence acting against the attractive influence of gravity.
The big problem with this is that quantum field theory suggests that this negative pressure contribution from vacuum energy should arise from all particles and thus, should give lambda a value that is tremendously larger than that obtain when our astronomers measure the redshift of Type Ia supernovae in distant galaxies.
This problem could be solved by dark energy’s effects being the result of something other than vacuum energy, of course. The Universe’s accelerating expansion could be due to some, as of yet undiscovered fundamental force of nature. Alternatively, it could indicate that our best current theory of gravity–general relativity– is incorrect.
A new generation of cosmologists is currently actively tackling the dark energy puzzle with new and revolutionary ideas. These include the idea that dark energy could have started work in the early Universe, an idea proposed by Early Dark Energy (EDE) models of the Universe. Another alternative is that dark energy does not influence the curvature of the Universe, or perhaps does so weakly–a theory referred to as the ‘well-behaved cosmological constant.’
As unsatisfying an answer as it is, the only honest way of addressing the question ‘what is dark energy?’ right now is by saying; we just don’t know. But, science wouldn’t be anywhere near as fascinating without mysteries to solve, and revolutionary ideas to be uncovered.
Astrophysicists have finally observed the spiralling merger between a neutron star and a black hole. The cataclysmic event was witnessed in a gravitational wave signal by the LIGO/Virgo/KAGRA collaboration and is the first time that one of these elusive but titanic ‘mixed’ merger events has been spotted and had its nature confirmed. And just like buses, you wait for an age for one to come and then two arrive at once.
The researchers also detected a gravitational wave signal from another event of the same nature just ten days after the first, with the signals picked up by LIGO/Virgo on 5th January 2020 and the 15th January 2020 respectively.
The finding is significant because of the three types of mergers between stellar remnant binaries–neutron star/neutron star mergers, black hole/ black hole mergers, and neutron star/ black hole or mixed mergers–this latter category is the only one we hadn’t detected until now and has proved fairly elusive.
“With this new discovery of neutron star- black hole mergers outside our galaxy, we have found the missing type of binary,” says Astrid Lamberts, a CNRS researcher at Observatoire de la Côte d’Azur, in Nice, France. “We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way.”
These detections of signals from separate mixed merger events come just six years after the LIGO/Virgo collaboration first detected the gravitational waves confirming predictions regarding ripples in the fabric of spacetime by Einstein’s theory of general relativity a century previous.
Though further observations are needed, the results produced by the team could help astronomers and astrophysicists refine their knowledge of systems in which these elusive mergers occur determining both how these mixed binary pairings form and how frequently their components spiral together and merge.
“Gravitational waves have allowed us to detect collisions of pairs of black holes and pairs of neutron stars, but the mixed collision of a black hole with a neutron star has been the elusive missing piece of the family picture of compact object mergers,” says Chase Kimball, a Northwestern University graduate student. “Completing this picture is crucial to constraining the host of astrophysical models of compact object formation and binary evolution. Inherent to these models are their predictions of the rates that black holes and neutron stars merge amongst themselves.
“With these detections, we finally have measurements of the merger rates across all three categories of compact binary mergers.”
Chase Kimball, Northwestern University
Kimball is the co-author of a study published in the Astrophysical Journal Letters and part of a team that includes researchers from the LIGO Scientific Collaboration (LSC), the Virgo Collaboration and the Kamioka Gravitational Wave Detector (KAGRA) project.
A Gravitational-Wave Signal Signal One Billion Years in the Making
One of the most astounding things about the detection of gravitational waves is just how precise a piece of equipment has to be to detect these tiny ripples in the fabric of spacetime. Since that first key detection in 2015, the National Science Foundation’s (NSF) operators at the LIGO laser interferometer and their counterparts at the Virgo detector in Italy have detected over 50 gravitational wave signals from mergers between black hole pairs and neutron star binaries.
The first mixed neutron star/black hole merger spotted by the collaboration on January 5th is believed to be the result of a merger of a black hole six times the mass of the Sun and a neutron star with a mass 1.5 times that of our star. The event which has been designated GW200105 occurred 900 million light-years away from Earth and was picked up as a strong signal at the LIGO detector located in Livingstone, Louisiana.
LIGO Livingstone’s partner detector located in Hanford, Washington, missed the signal as it was offline at the time. Virgo on the other hand caught the signal but it was somewhat obscured by noise. “Even though we see a strong signal in only one detector, we conclude that it is real and not just detector noise,” says Harald Pfeiffer, group leader in the Astrophysical and Cosmological Relativity department at Max Planck Institute for Gravitational Physics (AEI) in Potsdam, Germany. “It passes all our stringent quality checks and sticks out from all noise events we see in the third observing run.”
The fact that GW200105 was only strongly picked up by one detector makes it difficult to pinpoint in the sky with the international team only able to ascertain that it came from a region about 34 thousand times the size of the Moon.
“While the gravitational waves alone don’t reveal the structure of the lighter object, we can infer its maximum mass,” says Bhooshan Gadre, a postdoctoral researcher at the AEI. “By combining this information with theoretical predictions of expected neutron star masses in such a binary system, we conclude that a neutron star is the most likely explanation.”
Despite the fact that the second mixed merger occurred farther away–1 billion light-years distant from Earth– its signal was spotted by both LIGO detectors and the Virgo detector. This means that the team have been able to localise the merger–named GW200115– more precisely, to a region of the sky that is around three thousand times the size of Earth’s moon. This second merger is believed to have occurred between a black hole nine times the mass of our Sun and a neutron star almost twice the size of the Sun.
These Black Holes Weren’t Messy Eaters
Because of the extraordinary distances involved, astronomers have yet to confirm either merger in the electromagnetic spectrum upon which traditional astronomy is based. Despite being informed of the event almost immediately astronomers could not find telltale flashes of light indicating the mergers.
This is unsurprising as any light from such distant events would be incredibly dim after one billion years of journeying to Earth no matter what wavelength it is observed in, or how powerful the telescope is that is used to attempt the follow-up observation.
There also remains another possibility why no light could be seen from these events. The lack of a signal in electromagnetic radiation could be because the neutron star elements of these mergers were swallowed whole by their black hole partners.
“These were not events where the black holes munched on the neutron stars like the cookie monster and flung bits and pieces about,” explains Patrick Brady, a professor at the University of Wisconsin-Milwaukee and Spokesperson of the LIGO Scientific Collaboration, colourfully. “That ‘flinging about’ is what would produce light, and we don’t think that happened in these cases.”
Whilst these are the first two confirmed examples of such mixed mergers, there have been suspects spotted by their gravitational-wave signals in the past. In August 2019 a signal designated GW190814 was detected which researchers say involved a collision of a 23-solar-mass black hole with an object of about 2.6 solar masses. This second object could have been eitherthe heaviest known neutron star or the lightest known black hole ever found. That ambiguity left this signal unconfirmed as the product of a mixed merger event and other similar finds have been plagued with similar ambiguities.
Now that two confirmed detections of mixed mergers have been made, astrophysicists can set about discovering if current estimates that say such collisions should occur at a frequency of around one per month within a distance of 1 billion light-years of Earth are correct.
They can also set about discovering the origins of such binaries, possibly eliminating one or two of the proposed locations in which such events are believed to occur: stellar binary systems, dense stellar environments including young star clusters, and the centers of galaxies.
Key to these investigations will be the fourth observation run of the laser interferometers that act as our gravitational wave detectors, set to begin in summer 2022.
“The detector groups at LIGO, Virgo, and KAGRA are improving their detectors in preparation for the next observing run scheduled to begin in summer 2022,” concludes Brady. “With the improved sensitivity, we hope to detect merger waves up to once per day and to better measure the properties of black holes and super-dense matter that makes up neutron stars.”
One of the major problems which has hindered our understanding of planet formation has been the lack of direct measurements of the mass of planet-forming protoplanetary discs. Now, by successfully measuring the mass of a unique protoplanetary disc for the first time, astronomers have confirmed that gravitational instabilities play a key role in the formation of planets.
The team of astronomers, led by Teresa Paneque-Carreño, a PhD student at the University of Leiden and the European Southern Observatory (ESO), used gas velocity data collected using the Atacama Large Millimeter/submillimeter Array (ALMA) to make observations of the young star Elias 2-27 which is surrounded by a disc of gas and dust with some extraordinary features.
The star which is located just under 400 light-years from Earth in the constellation Ophiuchus has been a popular target for investigation by astronomers for at least five decades which paid off in 2016 with the discovery that the young star is surrounded by a disc of gas and dust. This marks the first time, however, that such a mass measurement has been made and gravitational instabilities have been confirmed.
“How exactly planets form is one of the main questions in our field. However, there are some key mechanisms that we believe can accelerate the process of planet formation,” explains Paneque-Carreño. “We found direct evidence for gravitational instabilities in Elias 2-27, which is very exciting because this is the first time that we can show kinematic and multi-wavelength proof of a system being gravitationally unstable.
“Elias 2-27 is the first system that checks all of the boxes.”
Teresa Paneque-Carreño, University of Leiden
Paneque-Carreño is the first author of one of two papers detailing the team’s findings–which give astronomers the key to unlocking the mystery of planet formation– published in the latest edition of The Astrophysical Journal.
What makes Elias 2-27 the Ideal System for Cracking the Planet Formation Mystery?
Researchers have known for some time that protoplanetary discs of gas and dust surrounding young stars are locations of planet formation and we have certainly no shortage of studies of such structures. But, despite having this knowledge and a wealth of observational data, the exact process that leads to the birth of a planet has remained a puzzle.
Fortunately, telltale evidence of gravitational instabilities around Elias 2-27 made it the ideal star for astronomers in order to conduct a thorough investigation of planet formation.
“We discovered in 2016 that the Elias 2-27 disk had a different structure from other already studied systems, something not observed in a protoplanetary disk before: two large-scale spiral arms,” remarks principal investigator Laura Pérez, Assistant Professor at the Universidad de Chile. “Gravitational instabilities were a strong possibility, but the origin of these structures remained a mystery and we needed further observations.”
It was Pérez who suggested that ALMA–a series of 66 radio telescopes located in the Atacama Desert of northern Chile–should be trained on the spiral of gas and dust surrounding this young star.
It was this further study that revealed, not only does Elias 2-27 possess a protoplanetary disc with signs of gravitational instabilities within it, it also has something unique for such a structure: spiral arms.
Elias 2-27: A Unique and Chaotic Young Star System
The presence of spiral arms in the protoplanetary disc is believed to be the result of perturbations caused by density waves throughout the gas and dust that comprise it.
It is the first star-forming disc discovered with such features. But, to Paneque-Carreño it signals the presence of something else within the disc, chaos. This chaotic nature also gives rise to another characteristic never seen in a disc such as this.
“There may still be new material from the surrounding molecular cloud falling onto the disc, which makes everything more chaotic,” says the graduate of the Universidad de Chile. “The Elias 2-27 star system is highly asymmetric in the gas structure. This was completely unexpected, and it is the first time we’ve observed such vertical asymmetry in a protoplanetary disc.”
It is the double-punch of this vertical asymmetry and large-scale perturbations giving rise to a spiral structure that Cassandra Hall, Assistant Professor of Computational Astrophysics, University of Georgia, believes has major implications for our theories of planet formation.
“This could be a ‘smoking gun’ of gravitational instability, which may accelerate some of the earliest stages of planet formation,” says Hall, a co-author of one of the papers detailing these findings. “We first predicted this signature in 2020, and from a computational astrophysics point of view, it’s exciting to be right.”
This research has cracked the problem of measuring the mass of a protoplanetary disc, thus removing a significant barrier in our understanding of planet formation. This was possible in large part due to the high sensitivity of ALMA’s observing bands, particularly band 6 which covers light with a wavelength of 1.1 to 1.4 nanometres in combination with bands 3 and 7–which cover 2.6 – 3.6 nm and 0.8 -1.1 nm, respectively.
“Previous measurements of protoplanetary disc mass were indirect and based only on dust or rare isotopologues. With this new study, we are now sensitive to the entire mass of the disc,” says the second paper’s lead author Benedetta Veronesi, a postdoctoral researcher at École normale supérieure de Lyon. “This finding lays the foundation for the development of a method to measure disc mass that will allow us to break down one of the biggest and most pressing barriers in the field of planet formation. “
“Knowing the amount of mass present in planet-forming discs allows us to determine the amount of material available for the formation of planetary systems, and to better understand the process by which they form.”
Benedetta Veronesi, École normale supérieure de Lyon
More Planet Formation Mysteries to Solve
Even though this research has answered some of the questions surrounding the process of planet formation, like the best scientific discoveries, it has also given rise to new questions.
“While gravitational instabilities can now be confirmed to explain the spiral structures in the dust continuum surrounding the star, there is also an inner gap, or missing material in the disk, for which we do not have a clear explanation,” explains Paneque-Carreño.
Many of these questions are difficult to answer because of the vast difference between the timescales on which we live and those taken by the processes that birth planets.
“Studying how planets form is difficult because it takes millions of years to form planets. This is a very short time-scale for stars, which live thousands of millions of years, but a very long process for us,” said Paneque-Carreño. “What we can do is observe young stars, with disks of gas and dust around them, and try to explain why these disks of material look the way they do. It’s like looking at a crime scene and trying to guess what happened. “
Fortunately, researchers like Paneque-Carreño, Cassandra Hall, and Benedetta Veronesi are prepared to tackle this monumental challenge and solve planet formation’s remaining mysteries.
“Our observational analysis paired with future in-depth analysis of Elias 2-27 will allow us to characterize exactly how gravitational instabilities act in planet-forming discs and gain more insight into how planets are formed,” concludes Paneque-Carreño.
In late 2019 and early 2020 Betelgeuse, a red supergiant in the constellation of Orion, made headlines when it underwent a period of extreme dimming. This dip in brightness for the star, which is usually around the tenth brightest in the night sky over Earth, was so extreme it could even be seen with the naked eye.
Some scientists even speculated that the orange-hued supergiant may be about to go supernova, an event which would have been visible in daylight over Earth for months thanks to its power and relative proximity–700 light-years from Earth. Yet, that supernova didn’t happen and Betelgeuse returned to its normal brightness.
This left the ‘great dimming’ of Betelgeuse–something never seen in 150 years of studying the star–an open mystery for astronomers to investigate.
Now, a team of astronomers led by Miguel Montargès, Observatoire de Paris, France, and KU Leuven, Belgium, and including Emily Cannon, KU Leuven, have found the cause of this dimming, thus finally solving this cosmic mystery. The researchers have discovered that the darkening of Betelgeuse was caused by a cloud of dust partially concealing the red supergiant.
“Our observations show that the Southern part of the star was hidden and that the whole disk of the star was fainter. The modelling is compatible with both a cool spot of the photosphere and a dusty clump in front of the star,” Montargès tells ZME Science. “Since both signatures have been detected by other observers, we conclude that the Great Dimming was caused by a cool patch of material that, due to its lower temperature, caused dust to form in gas cloud ejected by the star months to years before.”
The ‘great dimming’ of this massive star lasted a few months presented a unique opportunity for researchers to study the dimming of stars in real-time.
“The dimming of Betelgeuse was interesting to professional and amateur astronomers because not only was the appearance of the star changing in real time we could also see this change with the naked eye. Being able to resolve the surface of a star during an event like this is unprecedented.”
Emily Cannon, KU Leuven
The team’s research is published in the latest edition of the journal Nature.
A Unique Opportunity to Capture a Dimming Star
Montargès and his team first trained the Very Large Telescope (VLT)–an ESO operated telescope based in the Atacama Desert, Chile–on Betelgeuse when it began to dim in late 2019. The astronomers took advantage of the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument at the VLT as well as data from the telescope’s GRAVITY instrument. This allowed them to create stunning images tracking the great dimming event allowing them to distinguish it from regular dips in brightness demonstrated by the supergiant stars.
Betelgeuse has been seen to decrease in brightness before as a result of its convection cycle, which causes material to rise and fall throughout the star’s layers based on its temperature. This convection cycle results in a semi-regular dimming cycle that lasts around 400 days.
When the ‘great dimming’ was first observed in October 2019 astronomers had assumed this was due to its natural dimming cycle. That assumption was dismissed by December that same year when the star became the darkest that it had been in a century. The star had returned to its normal brightness by April 2020.
“No other red supergiant star has been seen dimming that way, particularly to the naked eye. Even Betelgeuse that has been closely monitored for 150 years has not shown such behaviour.”
Miguel Montargès, Observatoire de Paris, France
Not only does this finding solve the mystery of this star’s dimming, but it also provides evidence of the cooling of a star causing the creation of stardust which goes on to obscure the star.
Even though Betelgeuse is much younger than the Sun–10 million years old compared to our star’s age of 4.6 billion years–it is much closer to the supernova explosion that will signal the end of its lifecycle. Astronomers had first assumed that dimming was a sign that the red supergiant was exhibiting its death throes ahead of schedule.
Thanks to the work of Montargès and his team, we now know this isn’t the case. The dimming is the result of a veil of stardust obscuring the star’s southern region.
“We have observed dust around red supergiant stars in the past,” Cannon explains. “However, this is the first time we have witnessed the formation of dust in real-time in the line of sight of a red supergiant star,”
This stardust will go on to form the building blocks of the next generation of stars and planets, and the observations made by Montargès, Cannon and the team represent the first time we have seen an ancient supergiant star ‘burping’ this precious material into the cosmos.
The Giant that Burped Stardust
The surface of Betelgeuse–which with its diameter of around 100 times that of the Sun would consume the orbits of the inner planets including Earth were it to sit in our solar system–is subject to regular changes as bubbles of gas move around it, change in size, and swell beneath it. Montargès, Cannon and their colleagues believe that sometime before the great dimming began the red supergiant ‘burped’ out a large bubble of gas.
This bubble moved away from the star leaving a cool patch on its surface. It was within this cool patch that material was able to solidify, creating a cloud of solid stardust. The team’s observations show for the first time that stardust can rapidly form on the surface of a star.
“We have directly witnessed the formation of so-called stardust,” says Montargès. “The dust expelled from cool evolved stars, such as the ejection we’ve just witnessed, could go on to become the building blocks of terrestrial planets and life.”
With regards to the future, the researchers point to the Extremely Large Telescope (ELT), currently under construction in the Atacama Desert as the ideal instrument to conduct further observations of Betelgeuse. “With the ability to reach unparalleled spatial resolutions, the ELT will enable us to directly image Betelgeuse in remarkable detail,” says Cannon. “It will also significantly expand the sample of red supergiants for which we can resolve the surface through direct imaging, further helping us to unravel the mysteries behind the winds of these massive stars.”
For Montargès solving this mystery and observing a phenomenon for the first time, solidifies a lifetime of fascination with Betelgeuse and points towards a deeper understanding of the stardust that is the building blocks of stars, planets, and us. “We have seen the production of star dust, materials we are ourselves made of. We have even seen a star temporarily change its behavior on a human time scale.”
Astronomers have spotted a rare giant ‘blinking’ star towards the centre of the Milky Way. The team believes the serendipitous discovery, which came after 17 years of observation, represents another example of a rare class of ‘blinking giant’ stars that represents an eclipsing binary system.
The giant star with a mass around 100 times that of the Sun–designated VW-WIT-08–was spotted by the international team of researchers as it decreased in brightness by a factor of 30. A dimming extreme enough to result in the star almost disappearing entirely from the sky.
Changes in brightness such as this are usually associated with stars that pulsate or stars that exist in a binary system and are eclipsed by their companion star.
This giant star, which is located around 25,000 light-years away from Earth, dimmed for a period of several months in 2013 and then lightened again. A characteristic not commonly associated with the dimming mechanisms listed above.
The team of astronomers that have been investigating VW-WIT-08 believe that the dimming it demonstrated eight years ago and has not repeated since is the result of an as-of-yet unseen object orbital companion eclipsing the giant star.
They add that this eclipsing object could be another star or a planet, but one thing that is fairly certain is that it is surrounded by some form of an opaque disc which is responsible for causing the star’s extreme dimming.
“It’s amazing that we just observed a dark, large and elongated object pass between us and the distant star, and we can only speculate what its origin is,” says Sergey Koposov from the University of Edinburgh.
Alongside Leigh Smith from the Institute of Astronomy, the University of Cambridge, and Philip Lucas from the University of Hertfordshire, Koposov is one of the authors of a paper detailing the discovery published in the journal Monthly Notices of the Royal Astronomical Society.
VW-WIT-08 isn’t the only example of a star dimming in this unusual fashion, but arguably it is the most extreme example discovered thus far.
What’s Going On with Giant Blinking Stars?
Another example of this form of an eclipsing binary system is Epsilon Aurigae, first discovered in 1821 by German astronomer Johann Heinrich Fritsch. The visible component of this binary system is the supergiant star Almaaz–an Arabic name meaning the he-goat–which dims by around 50% every 27 years.
Though this dimming is less pronounced than that of VW-WIT-08, it lasts for a prolonged period of time; between 640 and 730 days–around two years. This means the dimming component of this binary system must be something truely immense, probably another star surrounded by a thick ring of obscuring dust, angled edge-on from our perspective.
Whilst this two-year eclipse which last occurred between 2009 and 2011 may seem extreme, it’s topped by the eclipse seen in another similar system discovered more recently–TYC 2505-672-1 found around 10,000 light-years from Earth.
This system currently holds the record for the longest known eclipse. Every 69 years the massive star component of this system is dimmed by a magnitude of 4.5 for a period of around 3 and a half years.
Thanks to the team that found VW-WIT-08 the catalogue of these eclipsing binary systems looks set to expand as the astronomers have currently found two more giant blinking stars ripe for further investigation.
“Occasionally we find variable stars that don’t fit into any established category, which we call ‘what-is-this?’, or ‘WIT’ objects,” remarks Lucas. “We really don’t know how these blinking giants came to be.”
What Does the Future Hold for Giant Blinking Stars?
The team made the discovery of VVV-WIT-08 using data collected by VISTA Variables , part of the Via Lactea (VVV Survey) which ran from 2010 to 2016. The survey’s main mission was the observation of the Milky Way’s central bulge and southern disc in near-infrared. The project utilised the capabilities of the VISTA telescope located at the Parnal Observatory, Chile.
Lucas adds: “It’s exciting to see such discoveries from VVV after so many years planning and gathering the data.”
The dimming of VVV-WIT-08 was also captured by the Gravitational Lensing Experiment (OGLE) operated by researchers at the University of Warsaw. Our galaxy’s central bulge was also a primary target for OGLE which makes its observations in light closer to the visible range of the electromagnetic spectrum.
The main advantage of OGLE is the fact that it makes frequent observations, something that was vital for building a model of VVV-WIT-08. This combination of observations also showed the astronomers that the giant star dims in both the visible spectrum and the infrared spectrum.
The team’s findings show that there are undoubtedly more eclipsing binary systems in the Milky Way left to be discovered. But this may not be the most difficult part of the process of investigating these systems.
“There are certainly more to be found, but the challenge now is in figuring out what the hidden companions are, and how they came to be surrounded by discs, despite orbiting so far from the giant star,” Smith concludes. “In doing so, we might learn something new about how these kinds of systems evolve.”
Astronomers have completed the first in-depth census of molecular clouds in the nearby Universe. The study has revealed that these star-forming regions not only look different but also behave differently. This finding runs in opposition to previous scientific consensus, which considered these clouds of dust and gas to be fairly uniform.
The project–Physics at High Angular Resolution in Nearby GalaxieS (PHANGS)–consisted of a systematic survey of 100,000 molecular clouds in 90 galaxies in the local Universe. The primary aim of the PHANGS was to get an idea of how these star-forming regions are influenced by their parent galaxies.
The census was conducted with the use of the Atacama Large Millimeter/ submillimeter Array (ALMA) located on the Chajnantor plateau, in the Atacama Desert of northern Chile. Whilst not marking the first time stellar nurseries have been studied with ALMA, this is the first census of its kind to observe globular clusters across more than either one galaxy or a small region of a single galaxy.
“We have carried out the first real ‘census’ of these stellar nurseries, and it provided us with details about their masses, locations, and other properties,” Adam Leroy, Associate Professor of Astronomy at Ohio State University (OSU) tells ZME Science. “Some people thought that all stellar nurseries across every galaxy look more or less the same, and it took having a really big, sensitive, and high-resolution survey of many galaxies with a telescope such as ALMA to see that this is not the case. This survey allows us to see how the stellar nurseries change across different galaxies. “
As a result, this is the first time that astronomers have been granted a look at the ‘big picture’ when it comes to these star-forming regions. Erik Rosolowsky, Associate Professor of Physics at the University of Alberta, and a co-author of the research points out that what ALMA has allowed the team of astronomers to create is essentially a new form of ‘cosmic cartography’ consisting of 90 maps of unparalleled detail detailing the regions of space where the next generation of stars will be born.
“By doing this we will combine what we are learning from ALMA about the clouds that form stars with pictures of newly formed stars from these other telescopes. This promises to give us the best view ever of the full life cycle of these stellar nurseries, and our most complete picture ever of the full cycle of star birth and death.”
“Our survey is the first one to capture the demographics of these stellar nurseries across a large number of the galaxies near the Milky Way,” adds Leroy, the lead author of a paper presenting the PHANGS ALMA survey. “We used these measurements to measure the characteristics of these nurseries, their lifetimes, and the ability of these objects to form new stars.”
How Galactic Neighborhoods Influence Star-Forming Clouds
The variety displayed by the molecular clouds surveyed in the PHANGS project was visible due to ALMA’s ability to take millimeter-wave images with the same sharpness and quality as images taken in the visible spectrum.
“While optical pictures show us light from stars, these ground-breaking new images show us the molecular clouds that form those stars,” says Leroy. “That helped us to see that stellar nurseries actually change from place to place.”
The team compared the changes displayed by molecular clouds from galaxy to galaxy to changes in houses, neighbourhoods and cities from region to region here on Earth.
“How stellar nurseries relate to their parent galaxies has been a big question for a long time. We’re able to answer this because our survey expands the amount of data on stellar nurseries by a factor of almost 100,” says Leroy. “Before this, it was very common to study a few hundred nurseries in one galaxy. So it was kind of like trying to learn about houses in general by looking only at neighbourhoods in Columbus, Ohio.
“You will learn some things about houses, but you miss the big picture and a lot of the variation, complexity, and commonality With this survey we looked at houses in many cities across many countries.”
Adam Leroy, Ohio State University
Leroy continues by explaining that stellar nurseries ‘know’ about their neighbourhood, meaning that molecular clouds are different depending on what galaxy they live in or where in that galaxy they are located. “So the stellar nurseries that we see in the Milky Way won’t be the same as those in a different galaxy, and the stellar nurseries in the outer part of a galaxy–where we live–aren’t the same as those near the galaxy centre.”
The team found clouds in the dense central regions of galaxies tend to be more massive, denser, and more turbulent than those located on the outskirts of a galaxy. In addition to this, the census revealed the lifecycle of clouds also depends on their environment. Annie Hughes, an astronomer at L’Institut de Recherche en Astrophysique et Planétologie (IRAP) explains that this means that both the rate at which a cloud forms stars and the processes that ultimately destroy clouds both seem to depend on where the cloud lives.
How Differences in Globular Clusters Influence the Birth of Stars
Because all stars are formed in molecular clouds, understanding the differences in these clouds of gas and dust and how they are caused by the conditions in which they exist is key to better understanding the processes that are driving the birth of stars like our own Sun.
These molecular clouds are so vast that they can birth anywhere from thousands to hundreds of thousands of stars before being exhausted of raw materials. These new observations have shown astronomers that each cosmic neighbourhood can have an effect on where stars are born and how many stars are spawned.
“Every star in the sky, in fact, every star in every galaxy, including our Sun, was born in one of these stellar nurseries. These are really the engines that build galaxies and make planets, and they’re just an essential part of the story of how we got here.”
Adam Leroy, Ohio State University
The next step for the astronomers will be to combine the data provided by ALMA with surveys conducted by other telescopes including the Hubble space telescope, and the Very Large Telescope (VLT) also located in the Atacama desert, Chile. Leroy hopes that this along with observations made with the James Webb Space Telescope (JWST), will help astronomers answer the question of how the diversity of molecular structures affects the stars which form within them. He explains: “By doing this we will combine what we are learning from ALMA about the clouds that form stars with pictures of newly formed stars from these other telescopes.
This promises to give us the best view ever of the full life cycle of these stellar nurseries, and our most complete picture ever of the full cycle of star birth and death.”
Adam Leroy, Ohio State University
Leroy concludes by pointing out why the study of these star-forming regions is so important. “This is the first time we have gotten a clear view of the population of these stellar nurseries across the whole nearby universe,” the researcher says. “It’s a big step towards understanding where we come from.”
Whilst it may not have the snappiest name, the event GW150914 is pretty significant in terms of our understanding of the Universe. This event, with a name that includes ‘GW’ as a prefix which is an abbreviation of ‘Gravitational Wave’ and the date of observation–15/09/14– marked humanity’s first direct detection of gravitational waves.
This was groundbreaking on two fronts; firstly it successfully confirmed a prediction made by Albert Einstein’s theory of general relativity almost a century before. A prediction that stated events occurring in the Universe do not just warp spacetime, but in certain cases, can actually send ripples through this cosmic fabric.
The second significant aspect of this observation was the fact that it represented an entirely new way to ‘see’ the Universe, its events and objects. This new method of investigating the cosmos has given rise to an entirely new form of astronomy; multimessenger astronomy. This combines ‘traditional’ observations of the Universe in the electromagnetic spectrum with the detection of gravitational waves, thus allowing us to observe objects that were previously invisible to us.
Thus, the discovery of gravitational waves truly opened up an entirely new window on the cosmos, but what are gravitational waves, what do they reveal about the objects that create them, and how do we detect such tiny tremblings in reality itself?
Gravitational Waves: The Basics
Gravitational waves are ripples in the fabric of spacetime.
These ripples travel from their source at the speed of light.
The passage of gravitational waves squash and stretch space itself.
Gravitational waves can be detected by measuring these infinitesimally small changes in the distance between objects.
They are created when an object or an event that curves spacetime causes that curvature to change shape.
Amongst the causes of gravitational waves are colliding black holes and neutron stars, supernovae, and stars that are undergoing gravitational collapse.
Imagine sitting at the side of a lake, quietly observing the tranquil surface of the water undisturbed by nature, the wind, or even by the slightest breeze. Suddenly a small child runs past hurling a pebble into the lake. The tranquillity is momentarily shattered. But, even as peace returns, you watch ripples spread from the centre of the lake diminishing as they reach the banks, often splitting or reflecting back when they encounter an obstacle.
The surface of the lake is a loose 2D analogy for the fabric of spacetime, the pebble represents an event like the collision of two black holes, and our position on Earth is equivalent to a blade of grass on the bank barely feeling the ripple which has diminished tremendously in its journey to us.
Gravitational waves were first predicted by Henri Poincare in 1905 as disturbances in the fabric of spacetime that propagate at the speed of light, but it would take another ten years for the concept to really be seized upon by physicists. This happened when Albert Einstein predicted the same phenomenon as part of his revolutionary 1916 geometric theory of gravity, better known as general relativity.
Whilst this theory is most well-known for suggesting that objects with mass would cause warping of spacetime, it also went a step further positing that an accelerating object should change this curvature and cause a ripple to echo through spacetime. Such disturbances in spacetime would not have been permissible in the Newtonian view of gravity which saw the fabric of space and time as separate entities upon which the events of the Universe simply play out.
But upon Einstein’s dynamic and changing stage of united spacetime, such ripples were permissible.
Gravitational waves arose from the possibility of finding a wave-like solution to the tensor equations at the heart of general relativity. Einstein believed that gravitational waves should be generated en masse by the interaction of massive bodies such as binary systems of super-dense neutron stars and merging black holes.
The truth is that such ripples in spacetime should be generated by any accelerating objects but Earth-bound accelerating objects cause perturbations that are far too small to detect. Hence why our investigations must turn to areas of space where nature provides us with objects that are far more massive.
As these ripples radiate outwards from their source in all directions and at the speed of light, they carry information about the event or object that created them. Not only this, but gravitational waves can tell us a great deal about the nature of spacetime itself.
Where do Gravitational Waves Come From?
There are a number of events that can launch gravitational waves powerful enough for us to detect with incredibly precise equipment here on Earth. These events are some of the most powerful and violent occurrences that the Universe has to offer. For instance, the strongest undulations in spacetime are probably caused by the collision of black holes.
Other collision events are associated with the production of strong gravitational waves; for example the merger between a black hole and a neutron star, or two neutron stars colliding with each other.
But, a cosmic body doesn’t always need a partner to make waves. Stellar collapse through supernova explosion–the process that leaves behind stellar remnants like black holes and neutron stars– also causes the production of gravitational waves.
To understand how gravitational waves are produced, it is useful to look to pulsars–binary systems of two neutron stars that emit regular pulses of electromagnetic radiation in the radio region of the spectrum.
Einstein’s theory suggests that a system such as this should be losing energy by the emission of gravitational waves. This would mean that the system’s orbital period should be decreasing in a very predictable way.
The stars draw together as there is less energy in the system to resist their mutual gravitational attraction, and as a result, their orbit increases in speed, and thus the pulses of radio waves are emitted at shorter intervals. This would mean that the time it takes for the radio wave to be directly facing our line of sight would be reduced; something we can measure.
This is exactly what was observed in the Hulse-Taylor system (PSR B1913±16), discovered in 1974, which is comprised of two rapidly rotating neutron stars. This observation earned Russell A. Hulse and Joseph H. Taylor, Jr, both of Princeton University, the 1993 Nobel Prize in Physics. The reason given by the Nobel Committee was: “for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation.”
Though inarguably an impressive and important scientific achievement, this was still only indirect evidence of gravitational waves. Whilst the effect Einstein predicted of shortening of the pulsar’s spin was definitely present, this wasn’t an actual direct detection.
In fact, though not alive to witness this momentous achievement, Einstein had predicted that this would be the only way we could ever garner any hint of gravitational waves. The great physicist believed those spacetime ripples would be so faint that they would remain impossible to detect by any technological means imaginable at that time.
Fortunately, Einstein was wrong.
How do we Detect Gravitational Waves?
It should come as no surprise that actually detecting a gravitational wave requires a piece of equipment of tremendous sensitivity. Whilst the effect of gravitational waves–the squashing and stretching space itself–sounds like something that should pre-eminently visible, the degree by which this disturbance occurs is so tiny it is totally imperceptible.
Fortunately, there is a branch of physics that is pretty adept at deal with the tiny. To spot gravitational waves, researchers would use an effect called interference, something demonstrated in the most famous quantum physics experiment of all time; the double-slit experiment.
Physicists realised that a laser interferometer could be used to measure the tiny squashing and stretching of space as it would cause the arms of the equipment to shrink by a minute amount. This means when splitting a laser and sending it through the arms of an interferometer the squeezing of space caused by the passage of a gravitational wave would cause one laser to arrive slightly ahead of the other–meaning they are out of phase and causing destructive interference. Thus, this difference in arrival times causes interference that gives an indication that gravitational waves have rippled across one of the arms.
But, not just any laser interferometer would do. Physicists would need an interferometer so large that it constituents a legitimate feat in engineering. Enter the Laser Interferometer Gravitational-wave Observatory (LIGO).
The LIGO detector uses two laser emitters based at the Hanford and Livingstone observatories, separated by thousands of kilometres apart to form an incredibly sensitive interferometer. From these emitters, lasers are sent down the ‘arms’ of the interferometer which are actually 4km long vacuum chambers.
This results in a system that is so sensitive it can measure a deviation in spacetime that is as small as 1/10,000 the size of an atomic nucleus. To put this into an astronomical context; it is equivalent to spotting a star at a distance of 4.2 light-years and pinpointing its location to within the width of a human hair! This constitutes the smallest measurement ever practically attempted in any science experiment.
And in 2015, this painstaking operation paid off.
On 14th September 2015, the LIGO and Virgo collaboration spotted a gravitational wave signal emanating from the spiralling in and eventual merger of two black holes, one 29 times the mass of the Sun, the other 36 times our star’s mass. From changes in the signal received the scientists were also able to observe the resultant single black hole.
The signal, named GW150914, represented not just the first observation of gravitational waves, but also the first time humanity had ‘seen’ a binary stellar-mass black hole system, proving that such mergers could exist in the Universe’s current epoch.
Different Kinds of Gravitational Waves
Since the initial detection of gravitational waves, researchers have made a series of important and revelatory detections. These have allowed scientists to classify different types of gravitational waves and the objects that may produce them.
Continuous Gravitational Waves
A single spinning massive object like a neutron star is believed to cause a continuous gravitational wave signal as a result of imperfections in the spherical shape of this star. if the rate of spin remains constant, so too are the gravitational waves it emits–it is continuously the same frequency and amplitude much like a singer holding a single note. Researchers have created simulations of what an arriving continuous gravitational wave would sound like if the signal LIGO detected was converted into a sound.
The sound of a continuous gravitational wave of the kind produced by a neutron star can be heard below.
Compact Binary Inspiral Gravitational Waves
All of the signals detected by LIGO thus far fit into this category as gravitational waves created by pairs of massive orbiting objects like black holes or neutron stars.
The sources fit into three distinct sub-categories:
Binary Black Hole (BBH)
Binary Neutron Star (BNS)
Neutron Star-Black Hole Binary (NSBH)
Each of these types of binary pairing creates its own unique pattern of gravitational waves but shares the same overall mechanism of wave-generation–inspiral generation. This process occurs over millions of years with gravitational waves carrying away energy from the system and causing the objects to spiral closer and closer until they meet. This also results in the objects moving more quickly and thus creating gravitational waves of increasing strength.
The ‘chirp’ of an eventual merger between neutron stars has been translated to sound waves and can be heard below.
Stochastic Gravitational Waves
Small gravitational waves that even LIGO is unable to precisely pinpoint could be passing over Earth from all directions at all times. These are known as stochastic gravitational waves due to their random nature. At least part of this stochastic signal is likely to have originated in the Big Bang.
Should we eventually be able to detect this signal it would allow us to ‘see’ further back into the history of the Universe than any electromagnetic signal could, back to the epoch before photons could freely travel through space.
The simulated sound of this stochastic signal can be heard below.
It is extremely likely given the variety of objects and events in the Universe that other types of gravitational wave signals exist. This means that the quest to detect such signals is really an exploration of the unknown. Fortunately, our capacity to explore the cosmos has been boosted tremendously by our ability to detect gravitational waves.
A New Age of Astronomy
GW150914 conformed precisely to the predictions of general relativity, confirming Einstein’s most revolutionary theory almost exactly six decades after his death in 1955. That doesn’t mean that gravitational waves are done teaching us about the Universe. In fact, these ripples in spacetime have given us a whole new way to view the cosmos.
Before the discovery of gravitational waves, astronomers were restricted to a view of the Universe painted in electromagnetic radiation and therefore our observations have been confined to that particular spectrum.
Using the electromagnetic spectrum alone, astronomers have been able to discover astronomical bodies and even thecosmic microwave background (CMB) radiation, a ‘relic’ of one of the very first events in the early universe, the recombination epoch when electrons joined with protons thus allowing photons to begin travelling rather than endlessly scattering. Therefore, the CMB is a marker of the point the universe began to be transparent to light.
Yet despite the strides traditional astronomy has allowed us to make in our understanding of the cosmos, the use of electromagnetic radiation is severely limited. It does not allow us to directly ‘see’ black holes, from which light cannot escape. Nor does it allow us to see non-baryonic, non-luminous dark matter, the predominant form of matter in galaxies–accounting for around 85% of the universe’s total mass. As the term ‘non-luminous’ suggests dark matter does not interact with the electromagnetic spectrum, it neither absorbs nor emits light. This means that observations in the electromagnetic spectrum alone will never allow us to see the majority of the matter in the universe.
Clearly, this is a problem. But one that can be avoided by using the gravitational wave spectrum as both black holes and dark matter do have considerable gravitational effects.
Gravitational waves also have another significant advantage over electromagnetic radiation.
This new form of astronomy measures the amplitude of the travelling wave, whilst electromagnetic wave astronomy measures the energy of the wave, which is proportional to the amplitude of the wave squared.
Therefore the brightness of an object in traditional astronomy is given by 1/distance² whilst ‘gravitational brightness’ falls off by just 1/distance. This means that the visibility of stars persists in gravitational waves for a much greater distance than the same factor persists in the electromagnetic spectrum.
Of course, none of this is to suggest that gravitational wave astronomy will ‘replace’ traditional electromagnetic spectrum astronomy. In fact the two are most powerful when they are unified in an exciting new discipline–multimessenger astronomy
Sources and Further Reading
Maggiore. M., Gravitational Waves: Theory and Experiments, Oxford University Press, 
Maggiore. M., Gravitational Waves: Astrophysics and Cosmology, Oxford University Press, 
Collins. H., Gravity’s Kiss: The Detection of Gravitational Waves, MIT Press, 
The Dark Energy Survey (DES) is an ambitious cosmological project that aims to map hundreds of millions of galaxies. In the process, the project will detail hundreds of millions of galaxies, observe thousands of supernovae, map the cosmic web that links galaxies, all with the aim of investigating the mysterious force that is causing the Universe to expand at an accelerating rate.
Using the 570-megapixel Dark Energy Camera on the National Science Foundation’s Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), Chile, the DES has observed a map of galaxy distribution and morphology that stretches 7 billion light-years and captures 1/8 of the sky over Earth.
Now new results from the DES which collects the work of an international team of over 400 scientists from over 25 institutions from countries including the US, UK, France, Spain, Brazil, and Australia, are in. The findings are detailed in a ground-breaking series of 29 papers and comprises of data collected during the DES’ first three years of operation providing the most detailed description of the Universe’s composition and expansion to date.
The survey was conducted between 2013 to 2019 cataloging hundreds of millions of objects, with the three years of data covered in these papers alone containing observations of at least 226 million galaxies observed over 345 nights.
The fact that some of these galaxies are close to the Milky Way and others are much more distant–up to 7 billion light-years away– gives researchers an excellent picture of the evolution of the Universe over around half of its lifetime.
The results seem to confirm the standard model of cosmology, currently the best-evidenced theory of the Universe’s composition and evolution which suggests the Universe was created in a ‘Big Bang’ event and has a composition of 5% ordinary or baryonic matter, 27% dark matter, and 68% dark energy.
The snapshot of the Universe provided by the DES does seem to show that the Universe is less ‘clumpy’ than current cosmological models suggest, however.
Illuminating the Dark Universe
The fact that the ‘Dark Universe’ consists of 95% of the matter and energy in the known cosmos means that there are huge gaps in our understanding of the evolution of the Universe, its past, present, and its future.
These gaps include the nature of dark matter, whose gravitational influence holds galaxies together, and dark energy, the force that is expanding space between the galaxies driving them apart at an accelerating rate.
These effects seem to be in opposition, with one holding matter together and the other working upon space itself to drive matter apart. And it is this cosmic struggle that shapes the Universe which the DES aimed to investigate.
There are two key phenomena which the survey used to do this. Studying ‘the cosmic web’ that links galaxies together in clusters and loose associations gives hints at the distribution and influence of dark matter.
The second phenomenon used by the DES is the bending of light as it travels past curvatures in spacetime created by objects of tremendous mass like galaxies. This effect predicted by Einstein’s theory of gravity–general relativity–is known as ‘gravitational lensing.’
The DES relied on a form of this effect called ‘weak gravitational lensing’ to assess how dark matter is distributed across the Universe, thus inferring its ‘clumpiness.’
The data collected by the DES was cross-referenced against measurements carried out by the European Space Agency (ESA) operated mission, the Planck observatory. The orbiting observatory, which operated between 2009 and 2013 and studied the cosmic background radiation (CMB)–an imprint leftover from an event shortly after the Big Bang in which electrons and protons connected thus allowing photons to travel freely for the first time.
Observing the CMB reveals conditions that were ‘frozen in’ to it at the time of this event known as the last scattering and thus gives a detailed picture of the Universe when it was just 400 thousand years old for the DES team to draw from.
Setting the Scene for Future Surveys
The DES intensely studied ten regions labeled as ‘deep fields’ which were repeatedly imaged during the course of the survey. These images were stacked which allowed astronomers to observe distant galaxies.
In addition to allowing researchers to see further into the Universe and thus further back in time, information regarding redshift– an increase in wavelength caused by objects receding which can arise as a result of the Universe’s expansion–taken from these deep fields was used to calibrate the rest of the survey. This constituted a major step forward for cosmic surveys providing the researchers with a picture of the Universe painted with stunning precision.
Whilst the DES was concluded in 2019, the sheer wealth of data collected by the survey requires a huge amount of computing power and time to assess. This is why we are only seeing the first three years of observations reported and likely means that the DES still has much more to deliver.
This will ultimately set the scene for the Legacy Survey of Space and Time (LSST) which will be conducted at the Vera C Rubin observatory–currently under construction on the El Penon peak of Cerro Pachon in northern Chile.
Whereas the DES surveyed an inarguably impressive 1/8 of the sky over the earth, the wide-field camera that will conduct the LSST will capture the entire sky over the Southern hemisphere, meaning it will view half of the entire sky over our planet.
A major part of the LSST’s mission will be the investigation of dark matter and dark energy, meaning that when the data from the DES is finally exhausted and its secrets are revealed, a worthy successor will be waiting in the wings to assume its mission of discovery.
Cell membranes are one of the most important elements of life on Earth holding together the genetic material and metabolic machinery of every living cell. Whilst we know those cell membranes are made of phospholipids, the origin of these molecules with their hydrophobic heads and hydrophobic tails is currently poorly understood.
Currently, scientists can’t explain where the elements that make up these molecules come from.
Extraordinary new research reveals that the source of a key ingredient of phospholipids–the simplest of the molecules that make up that hydrophilic head–could have originated in space, incorporated into smaller solar system bodies that were drawn into our planet as it formed. From there these molecules could eventually kick start the evolution of the first cellular membranes.
The discovery comes courtesy of a team of astronomers, including Víctor M. Rivilla, The Spanish Astrobiology Center, Madrid, Spain, who found prebiotic ethanolamine in a molecular cloud located close to the centre of the Milky Way.
“We have discovered in an interstellar molecular cloud the molecule ethanolamine, which is part of the head of phospholipids, the biomolecules that build up the cellular membranes,” says Rivilla. “I can add that these cells were absolutely crucial for the origin of life because they keep together the genetic material and the metabolic machinery.
“Without these cells, the origin of life is not possible.”
Rivilla also tells ZME Science that the discovery made by he and his colleagues could constitute a major step forward in our understanding of life here on Earth and elsewhere in our galaxy.
Delivered to Earth or Here From the Start?
This isn’t the first time that prebiotic ethanolamine has been discovered in space, researchers have previously found the molecules in meteorites. One striking example of this is the discovery of ethanolamine in the Almahate Sitta meteorite which entered Earth’s atmosphere on October 7, 2008, and exploded 37 kilometres above the Nubian Desert in Sudan.
These findings led to speculation that this vital ingredient in cell membranes was seeded on Earth via meteorite strike after forming within the meteorites from simpler amino acids. Lab impact experiments have shown that a significant amount of prebiotic molecules in comets and meteorites could survive the passage into our atmosphere and the impact with the surface of the planet lending credence to this theory.
Yet, this delivery method fails to explain the formation of cell membranes and more complex molecules as there are serious doubts as to whether it would result in enough ethanolamine available on an early Earth to allow these processes to begin.
Fortunately, there is another mechanism by which this molecule could have been present in Earth’s early epochs. It could have been present all along–incorporated during planetary formation. This would mean that ethanolamine would have to be present in the planetesimals and small bodies that initially came together to form our planet–seeded there by the interstellar medium.
“Panspermia was originally invoked as a way of transporting life from another place to Earth. This is different,” says Rivilla. “We are talking here about the chemical feedstock–the basic molecular precursors of biomolecules such as lipids–delivered at the dawn of our planet. This is sometimes called molecular panspermia.”
In order to support this idea, which would mean ethanolamine forms in space from smaller precursors, researchers have been searching the interstellar medium–the gas and dust that exist between planetary systems–for this molecule. Until now these searches had proved mostly unsuccessful.
Thus, this research provides the first evidence of ethanolamine in the interstellar medium.
A Small Part of Life’s Big Picture
Rivilla and his colleagues found the clear signal of prebiotic ethanolamine in the region of the molecular cloud G+0.693 using the 30-meter IRAM telescope located on Pico Veleta mountain in the Spanish Sierra Nevada in conjunction with the Yebes radiotelescopes.
The region has been at the centre of the search for organic molecules in space previously. In 2020 researchers–also including Rivilla–spotted an organic molecule that is key in the formation of amino acids in G+0.693-0.027. The molecule–propargylimine–is unstable here on Earth but thrives in low-density, low-temperature environments such as those found in the interstellar medium.
This latest discovery supports the idea that ethanolamine forms in interstellar space and then becomes part of planetesimals which are incorporated into forming planets. The researchers conclude that the availability of ethanolamine on early Earth, together with amphiphilic fatty acids or alcohols, may have contributed to the assembly and early evolution of primitive cell membranes. This theory is something that is also well supported by experiments that simulate the conditions on early Earth.
That isn’t the end for this line of investigation, however. The team will now set about finding some of the other key elements for living cells in the interstellar medium.
“We have detected a piece of the head of a phospholipid in space,” says Rivilla. “But there are other important pieces, such as the phosphate group, glycerol, or the tails, formed by fatty acids/alcohols. We will try to also detect them, to see if a whole phospholipid itself can be synthesised in space.”
Using data collected by the Very Large Telescope (VLT) a team of astronomers has discovered iron and nickel in the atmosphere of around 20 different solar system comets–including some located far away from the Sun.
These findings will come as a surprise to astronomers because even though such heavy metals have been known to exist in solid form within comet interiors before, the vapour of such elements has only previously been associated with cometary atmospheres in hot environments.
This is the first time such vapour has been seen in the cooler atmospheres of comets that exist far from a star and could indicate some previously unknown mechanism or material on the surface of comets.
“It was a big surprise to detect iron and nickel atoms in the atmosphere of all the comets we have observed in the last two decades, about 20 of them, and even in ones far from the Sun in the cold space environment,” says Jean Manfroid, of the University of Liège, Belgium.
This wasn’t the only surprise the team found, however. The Belgian astronomers–who have been studying comets with the VLT for 20 years–observed nickel and iron in the atmosphere of the comet in equal amounts.
Generally, iron is about ten times more abundant in the solar system than nickel, and comets are believed to be material left over from the formation of planetary bodies within the solar system. That means it’s something of a mystery why the comets the team observed should have such a relatively large abundance of nickel.
“Comets formed around 4.6 billion years ago, in the very young Solar System, and haven’t changed since that time. In that sense, they’re like fossils for astronomers,” Emmanuel Jehin, also from the University of Liège. “This discovery went under the radar for many years.”
Manfroid and Jehin are two of the authors of a paper published in the latest edition of the journal Nature documenting the team’s findings. And that isn’t the only research revealing metal in the atmosphere of such a body published in Nature this month.
The discovery is accompanied by the revelation that a separate team of researchers, this time located in Poland, has also found traces of nickel vapour in the atmosphere around the interstellar visitor 2l/Borisov.
This comet may sound familiar as it made headlines in 2019 when it became only the second object found within the solar system which originated from outside our planetary system.
A paper detailing this second finding is also published in this month’s Nature.
Heavy Metal Rocks
Astronomers have known for some time that a variety of metals exist within the icy and rocky interiors of comets. There have even been suggestions that spent comets could be mined for precious or useful metals like gold, silver, platinum and iron.
These solid metals within comets were not expected to be found as gases in the body’s atmosphere, though, unless that body is passing within close vicinity to a star.
It is the heat from these close brushes with stars like the Sun that causes solid metals within comets to ‘sublimate’–the process by which solid material changes directly into a gaseous state.
That means that distant comets in the cold environment of space away from the heat of the Sun shouldn’t have heavy metal atmospheres.
Yet, despite this, researchers have now found nickel and iron vapour in the atmospheres of comets up to 480 million kilometres from the Sun. A distance that is three astronomical units, or three times the distance between the Sun and the Earth.
In order to make this discovery, the team employed the technique of spectroscopy which reveals the signatures of specific chemical elements and the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on the VLT to assess the chemical composition of comets’ atmospheres.
The spectral lines of nickel and iron found by the team in comets’ atmospheres were extremely faint, which leads them to believe that the reason such elements have been missed in past is due to their tiny abundance. The team says that for every 100kg of water in the atmosphere of the comets they studied there is just one gram of iron and nickel respectively.
The Belgian astronomers believe that the equal amounts of iron and nickel together with the sublimation at low temperatures means there is something undiscovered at the surface of the comets they studied.
“Usually there is 10 times more iron than nickel, and in those comet atmospheres we found about the same quantity for both elements,” explains Damien Hutsemékers, also a member of the Belgian team from the University of Liège.”We came to the conclusion they might come from a special kind of material on the surface of the comet nucleus, sublimating at a rather low temperature and releasing iron and nickel in about the same proportions.”
The team intends to attempt to use new telescope technology such as the Mid-infrared ELT Imager and Spectrograph (METIS) on ESO’s upcoming Extremely Large Telescope (ELT)–currently under construction in the Atacama Desert region of Northern Chile– to discover what this material is.
The findings of this team are accompanied by the revelation that nickel vapour has also been discovered in the atmosphere of 2I/Borisov.
2I/Borisov: The Interstellar Intruder that keeps giving
The discovery that metal is also present in the atmosphere of the interstellar visitor 2I/Borisov was made by a team of astronomers in Poland. The team also used the VLT to catch a glimpse of the interstellar comet as it passed through the solar system.
The data collected with the VLT’s X-Shooter spectrograph revaled nickel vapour in the cold envlope surround 2I/Borisov.
ESO/L. Calçada/O. Hainaut, P. Guzik and M. Drahus
The discovery marks another surprise for astronomers, as again it details the discovery of sublimated heavy metals in a cold atmosphere.
“At first we had a hard time believing that atomic nickel could really be present in 2I/Borisov that far from the Sun,” says Piotr Guzik, the Jagiellonian University, Poland, a co-author on this second study. “It took numerous tests and checks before we could finally convince ourselves.”
This latter study shows that nickel was not uniquely present during the formation of our solar system, but as it can be seen in a comet from another planetary grouping, it may well be common in many such conglomerations.
“All of a sudden we understood that gaseous nickel is present in cometary atmospheres in other corners of the Galaxy,” Michał Drahus, also from the Jagiellonian University and another of the paper’s co-authors, says.
In unison, both these studies indicate that the comets of this solar system and the interstellar visitor 2I/Borisov share many similarities. Dahus adds: “Now imagine that our Solar System’s comets have their true analogues in other planetary systems — how cool is that?”
Jehin, meanwhile, believes these studies could inspire future research into cometary bodies and their atmospheres, and a re-examination of data already collected.
“Now people will search for those lines in their archival data from other telescopes,” the University of Liège researcher concludes. “We think this will also trigger new work on the subject.”
By the mid-20th Century, physicists had begun to understand the fundamental structure of matter to such a degree that a theory was needed to encapsulate the Universe’s particles, the interactions between them and the forces that govern those interactions. That theory was the Standard Model of Particle Physics or just the Standard Model for short.
First devised in the 1970s, the Standard Model would be used to predict a wide variety of phenomena, meet various experimental challenges, before being confirmed by the discovery of the Higgs Boson in 2012. Yet, as successful and fruitful a theory as the Standard Model is, it can’t explain everything. Gravity still evades confinement within the Standard Model, and physicists have caught tantalizing glimpses of physics beyond the theory’s limits.
Before these glimpses can be confirmed and a new chapter in physics is opened, let’s take a trip through the particle zoo and discover the wonders of the Standard Model.
The Matter Particles
The everyday matter that surrounds us is comprised of building blocks called elementary particles. Of these building blocks, there are two main families; fermions and bosons.
Of the fermions, the two mains classes are leptons and quarks. Within each of these groups are six particles that group into three pairings that physicists call generations.
The first generation of leptons and quarks are made up of the lightest and most stable particles. These are the particles responsible for forming the elements of the Universe we are most familiar with–the stars, planets, moons, and us. The second and third generations are made up of increasingly more massive and less stable particles. The greater in mass these particles are, the quicker they decay into their lighter cousins.
Starting with the quarks is an easy way to introduce some of the qualities and values associated with the particles in the Standard Model. One thing you will notice is the interesting naming convention for these qualities. They reflect things we commonly encounter in the everyday macroscopic world such as flavour, colour and spin, but really shouldn’t be confused with those things.
So, let’s make quarks the first stop in our walk through the particle zoo.
The six quarks that make this family of particles are known as up, down quarks, which make up the first generation of quarks, these second comprises of the more massive charm and strange quarks. And the third generation contains the most massive particles, known as the top and bottom quarks.
These are generally known as the ‘flavours’ of quarks, each of which has its own antiquark. Of course, I’ve primed you to realise that this name has nothing to do with how these quarks taste!
Of the four fundamental forces, quarks ‘feel’ electromagnetism, the strong and weak nuclear forces, and gravity, but the latter is too weak to have an effect on quarks’ tiny mass.
The strong nuclear force binds together quarks in nucleons, whilst the weak nuclear force can actually cause quarks to switch flavours something that we’ll look at further when we get to the force-carrying particles.
But these elementary particles don’t just come in flavours–they also come in ‘colours.’
It is this quality–again nothing to do with wavelengths of light, quarks are large enough to reflect light in such a way to have a conventional colour– that determines how quarks come together to form other, more massive, particles.
Quarks join up to make particles called baryons, the most common of which are the protons and neutrons that come together to form the elements and the matter we interact with on an everyday scale.
Protons are made up of one down quark and two up quarks, whilst neutrons are comprised of two down quarks and an up quark.
Considering these arrangements and the fact that each flavour of quark has its own charge it’s easy to see why the proton has a positive charge whilst the neutron is neutral. It should also be apparent that when the weak nuclear force causes an up quark to switch to a down quark it also charges the nucleon it is part of from a proton to a neutron.
There are a multitude of other exotic arrangements of quarks like mesons which consist of a quark and its antiquark, and tetra and pentaquarks made up of three and five quarks respectively.
Considering how quarks come together to form particles is important because despite being fundamental particles, quarks are found wandering the particle zoo on their own. They are always found in conglomerations.
There is another important quality of fundamental particles that need to be considered–and yes, just as with ‘flavour’ and ‘colour’ it has a slightly misleading name.–these particles also have ‘spin.’
This shouldn’t be considered as representing a particle constantly revolving. It’s more a description of how a particle reacts when it interacts with a magnetic field.
Quark, like all fermions, are 1/2 spin particles and are described as having ‘up’ or ‘down’ spin. Unlike the spin of a macroscopic object, say a football after it is kicked, the spin of fundamental particles doesn’t change.
Like quarks, leptons are particles with 1/2 spin. They also come in six flavours and across three generations. But, unlike quarks, leptons are freely found wandering the particle zoo alone. The most famous lepton is possibly also the most famous fundamental particle. The electron–a generation I particle possessing a charge of –e.
Leptons can also be sub-divided into two groups; charged, which includes electrons and electron like muons, and chargeless leptons like the neutrinos. The charged leptons also possess a more considerable mass than their uncharged cousins. The reason that uncharged leptons have such smaller masses is not explained by the Standard Model of Particle Physics and doing so requires an extension to the model.
The lack of charge and practical lack of mass of neutrinos has led them to be labelled ‘ghost particles’ and means that 100s of thousands of them can stream through every square inch of your body every second without the slightest interaction with the matter that composes you.
Just like with quarks, each particle has its own anti-particle, including perhaps the most famous example of such symmetry–the antiparticle of the electron, the positron. One possible quirk to this symmetry is the possibility is that neutrinos are their own antiparticles.
Like quarks, leptons interact with gravity and the electromagnetic force, but unlike quarks leptons don’t feel the strong nuclear force.
Leptons obey the Pauli exclusion principle. This means that no two particles can share the same quantum numbers. This is key to the range of chemical elements that exist within the Universe as it forces electrons to occupy increasingly energetic shells around an atomic nucleus. The number of valance electrons in an element’s outer shell determines the chemical properties that the element will have.
The Pauli exclusion principle can be overwhelmed. A neutron star is protected from becoming black holes by this phenomenon, but when it exceeds a certain mass it can no longer rely on this to protect against complete gravitational collapse.
The Force Carriers
There are four fundamental forces in the Universe that we are currently aware of–the strong force, the weak force, the electromagnetic force, and the gravitational force. All of these forces work over different distances with different strengths. For instance, gravity is the weakest of the forces–even though there is actually a force explicitly called the ‘weak force’– but works over a potentially infinite distance. Meanwhile, the electromagnetic force also works over a long-range but is much more powerful than the force of gravity.
The strong and weak nuclear forces work over much shorter ranges; dominating the forces for sub-atomic particles. As the name implies, the strong force is the strongest of all the four forces, whilst the weak force is the weakest barring gravity.
We are certain that three of the four fundamental forces–electromagnetism, the strong and weak forces–are communicated by carrier particles called bosons. Particles exchange these bosons to communicate these forces.
Unlike leptons and quarks–collectively known as Fermions–Bosons have full integer spin. This means that they are not forced to obey the Pauli exclusion principle.
The electromagnetic force is carried by the most familiar of these particles–the photon. The strong force which ‘glues’ quarks together in protons and neutrons is communicated by gluons, and the weak force that influences particles to switch flavours is transmitted by W and Z bosons.
So what about gravity?
Put simply, the force that we are most familiar with and experience every moment of every day isn’t part of the Standard Model. Physicists think that this outsider force is also transmitted by a boson which they have given the provisional name the graviton. As of yet, however, there is no experimental sign of this hypothetical boson. Thus it can’t yet be seen in our particle zoo.
The exclusion of gravity isn’t a massive problem for particle physics, because the model deals with particles that are so small and the fact that gravity is so weak, the force doesn’t really have an effect on this sub-atomic world.
But what this omission does tell us, is that despite its importance and the fact that it has been experimentally verified to an impressive standard, and can now predict that outcome of a wide array of experiments, the Standard Model is by no means a complete description of the physical world.
That means we need extensions to this model to obtain that more accurate description. The problem is, no one can quite agree on what those extensions should look like.
Beyond the Standard Model
The force of gravity isn’t the only element of the Universe that physicists can’t squeeze into the Standard Model at the moment. Despite being a great description of sub-atomic particles, the theory can’t account for dark matter. As this mysterious form of matter that isn’t made up of baryons like protons and neutrons, accounts for around 85% of the mass in the known Universe, this isn’t an insignificant shortfall.
Likewise, the model can’t explain why matter dominates the Universe rather than antimatter. Processes that birth particles produce matter and antimatter in equal amounts. If the Universe had started with these balanced they would have likely met and annihilated each other before large scale structure had the oppotunity to form. That means there must be some reason beyond the Standard Model why the Universe initial favoured matter and allowed an imbalance.
Another potential issue with the Standard Model could result from the particle that was heralded as marking its completion: the Higgs Boson.
This particle is believed to emerge from the Higgs field and endow mass to most particles. But, the Standard Model isn’t the only theory that posits the existence of the Higgs Boson. The Higgs particle suggested by this theory is the simplest version. The particle that was measured by the CMS detector at the Large Hadron Collider (LHC) certainly conforms to the description given by the Standard Model, but it’s not a perfect fit.
That means that even as we create more Higgs Bosons at the LHC and continue learning more about the particle, the possibility of discovering that it conforms better to another theory remains.
One of the most well-supported extensions to the Standard Model is Supersymmetry (SUSY). This hypothesises a connection between fermions and bosons and suggests that all particles have a superpartner –or sparticle–with the same mass, and quantum numbers but a spin that differs by 1/2.
That means that each 1/2 lepton is a partner ‘slepton’ with a full integer spin–or more simply a boson. So, for the electron, SUSY posits the slepton with the same mass, charge, but with a spin of 1 rather than 1/2 called the selectron. For quarks, there are squarks, and so on.
SUSY could provide a dark matter candidate as the lightest particle suggested by the extension to the Standard Model would, if it existed, be a dead-ringer for dark matter.
Unfortunately, despite some tantalising hints at physics beyond the Standard Model of Particle Physics, experiments have thus far failed to turn up anything substantial. For SUSY specifically, sparticles that should be created in collisions at the LHC have thus far not been detected.
At least until the completion of high-luminosity upgrades at the LHC provide more collisions and thus a greater chance of spotting exotic phenomena, the Standard Model will remain our best, albeit incomplete, description of the sub-atomic world.
Sources and Further Reading
Manton. N., Mee. N., The Physical World, Oxford University Press, .
Martin. B. R., Shaw. G., Particle Physics, Wiley, .
Astronomers have observed two pairs of quasars in the distant Universe, closer together than any previously observed examples of similar pairings. The team followed the discovery–made with the Hubble Space Telescope and Gaia spacecraft–with spectroscopic observations made by the Gemini North Telescope.
The discovery is significant as it points towards the possible existence of supermassive black hole (SMBH) pairs. As the quasar pairs exist in merging galaxies, the finding also grants researchers an insight into how such events could have proceeded in the early Universe.
It is the relatively close proximity between the quasars in the two pairings of just 10-thousand light-years that suggests to the astronomers that they belong to merging galaxies.
“We estimate that in the distant Universe, for every one thousand quasars, there is one double quasar,” says Yue Shen, an astronomer at the University of Illinois. “So finding these double quasars is like finding a needle in a haystack.”
Shen is the lead author of a paper published in the latest edition of the journal Nature Astronomy.
Quasars sit at the centre of galaxies in an area known as the active galactic nuclei (AGN) blasting out powerful jets of radiation. They are powered by SMBHs devouring material like gas and dust that surrounds them.
Quasars are so powerful that they profoundly affect the evolution of galaxies around them. This means that studying them is also a great way of learning how galaxies come together.
These particular quasars are 10-billion light-years from Earth meaning they existed just four billion years after the Big Bang. Double quasars are, in of themselves, rare, especially at such great distances. But, what makes these pairs particularly interesting is the fact they point to even rarer, hitherto undiscovered, SMBH binaries.
“This truly is the first sample of dual quasars at the peak epoch of galaxy formation that we can use to probe ideas about how supermassive black holes come together to eventually form a binary,” says Nadia Zakamska, Johns Hopkins University, part of the team that made the discovery.
The team’s discovery will excite scientists currently involved in the search for SMBH binaries. Current theories suggest that as monstrous as they are these black holes, which are believed to lurk at the centre of most galaxies, do not always exist in isolation.
Alessandra De Rosa is a research astrophysicist at the National Institute of Astrophysics, Italy, and the author of a recent review paper which summarizes what we know thus far about SMBH pairs.
“Searching for high z dual Active Galactic Nuclei at such small separations is a fundamental piece of information to understand how SMBHs could form and grow and to probe what we know about galaxy formation and evolution,” DeRosa, who was not involved in the team’s study, tells ZME Science. “Moreover, these systems are the most direct precursors of binary SMBHs which are amongst the loudest emitters of gravitational waves in the low-frequency ranges.”
DeRosa continues by explaining that the search for these objects at such great distances is extremely challenging due to instrument limitations that prevent them from being individually distinguished.
Until now it has been believed that these pairings would find the black holes in such close proximity that they could only be distinguished by the gravitational waves launched by their eventual merger.
This new research could offer another way to at least study how such SMBH pairings come together and form binaries.
Tracking Down Quasar Pairs
As DeRosa points out, tracking down these quasar pairs at a distance of around 10-billion years was no easy task. In order to do this, the astronomers employed a novel new method that unites data from several space-based and ground-based telescopes.
It takes an extremely powerful telescope to view objects at such distances limiting the team’s choice to the Gemini North telescope in Hawai’i, and the Hubble Space Telescope. Because observing time on these telescopes is extremely limited, sweeping the entire sky for quasar pairs was out of the question.
To work around this, the team selected 15 quasars from the 3D map created from data collected by the Sloan Digital Sky Survey (SDSS). Observations from the Gaia spacecraft were then used to narrow these 15 quasars to candidates that could actually be pairs.
The last step of the process was using Hubble to get a better look at these suspects. In this way, the team was able to confirm that two of the objects they selected were indeed quasar pairs.
Further investigation with Gemini North and its Gemini Multi-Object Spectrograph (GMOS) instrument allowed the astronomers to resolve the quasars’ individual spectra. Locked within this light signature is information regarding the distance from Earth and the quasars’ compositions.
“The Gemini observations were critically important to our success because they provided spatially resolved spectra to yield redshifts and spectroscopic confirmations simultaneously for both quasars in a double,” says Yu-Ching Chen, part of the team and a graduate student at the University of Illinois. “This method unambiguously rejected interlopers due to chance superpositions such as from unassociated star-quasar systems.”
The Next Steps for Studying Quasar Pairs
Whilst the team is extremely confident that they have discovered quasar pairs in merging galaxies, there does remain the slight chance that they have actually captured a double image of a single quasar.
This kind of doppelganger illusion can be caused by strong gravitational lensing, the bending of light from a distant source when an object of great mass passes between it and our line of sight.
In extreme cases, this lensing can cause objects to appear at multiple points in the sky due to light being forced to take different paths across the Universe. Striking examples are so-called Einstein crosses and rings when single light sources appear at numerous points in a geometrical pattern.
The researchers believe that this can be discounted in the case of their research as the light from the distant quasars did not pass an intersecting foreground galaxy.
The next step for the researchers is the research for more quasar pairs, hopefully leading to the development of a census of such duos in the early universe.
“This proof of concept really demonstrates that our targeted search for dual quasars is very efficient,” Hsiang-Chih Hwang, the principal investigator of the Hubble observations and a graduate at John Hopkins University, concludes. “It opens a new direction where we can accumulate a lot more interesting systems to follow up, which astronomers weren’t able to do with previous techniques or datasets.”
Black holes are cosmic bodies that pack an immense amount of mass into a surprisingly small space. Due to their extremely intense gravity, nothing can escape their grasp — not even light which defines the universe’s speed limit.
April 10th, 2019 marked a milestone in science history when the team at the Event Horizon Telescope revealed the first image of a supermassive black hole. As a result, these areas of space created when stars reach the end of their nuclear fuel burning and collapse creating massive gravitational wells, completed their transition from theory to reality.
This transition has been further solidified since with the revelation of a second, much clearer image of the supermassive black hole (SMBH) at the centre of the galaxy Messier 87 (M87). This second image revealing details such as the orientation of the magnetic fields that surround it and drive its powerful jets that extend for light-years.
The study of black holes could teach us much more than about these spacetime events and the environments that home them, however. Because cosmologists believe that most galaxies have an SMBH sat at their centre, greedily consuming material like a fat spider lurking at the centre of a cosmic web, learning more about these spacetime events can also teach us how galaxies themselves evolve.
The origin of black holes is one that runs in reverse to that of most astronomical objects. We didn’t discover some mysterious object in the distant cosmos and then began to theorise about it whilst making further observations.
Rather, black holes entered the scientific lexicon in a way that is more reminiscent of newly theorised particles in particle physics; emerging first from the solutions to complex mathematics. In the case of black holes, the solutions to the field equations employed by Einstein in his most important and revolutionary theory.
Just as a physical black hole forms from the collapse of a star, the theory of black holes emerged from the metaphorical collapse of the field equations that govern the geometrical theory of gravity; better known as general relativity.
One of the most common misconceptions about black holes arises from their intrinsic uniqueness and the fact that there really isn’t anything else like them in the Universe.
That’s Warped: Black Holes and Their Effect on Spacetime
General relativity introduced the idea that mass has an effect on spacetime, a concept fundamental to the idea that space and time are not passive stages upon which the events of the universe play out. Instead, those events shape that stage. As John Wheeler brilliantly and simply told us; when it comes to general relativity:
“Matter tells space how to curve. Space tells matter how to move.”
The most common analogy is for this warping of space is that of placing objects on a stretched rubber sheet. The larger the object the deeper the ‘dent’ and the more extreme the curvature it creates. In our analogy, a planet is a marble, a star an apple, and a black hole a cannonball.
Thus, considering this a black hole isn’t really ‘an object’ at all but, is actually better described as a spacetime event. When we say ‘black hole’ what we really mean is an area of space that is so ‘warped’ by a huge amount of mass condensed into a finite point that even light itself doesn’t have the necessary velocity to escape it.
This point at which light can no longer escape marks the first of two singularities that define black holes–points at which solutions of the equations of general relativity go to infinity.
The Event Horizon and the Central Singularity
The event horizon of a black hole is the point at which its escape velocity exceeds the speed of light in vacuum (c). This occurs at a radius called the Schwarzchild radius–named for astrophysicist Karl Schwarzschild, who developed a solution for Einstien’s field equations whilst serving on the Eastern Front in the First World War.
His solution to Einstein’s field equations–which would unsurprisingly become known as the Schwarzschild solution– described the spacetime geometry of an empty region of space. It had two interesting features — two singularities — one a coordinate singularity the other, a gravitational singularity. Both take on significance in the study of black holes.
Dealing with the coordinate singularity, or the Schwarzchild radius first.
The Schwarzchild radius (Rs) also takes on special meaning in cases where the radius of a body shrinks within this Schwarzschild radius (ie. Rs >r). When a body’s radius shrinks within this limit, it becomes a black hole.
All bodies have a Schwarzschild radius, but as you can see from the calculation below for a body like Earth, Rs falls well-within its radius.
That’s part of what makes black holes unique; their Schwartzchild radius is outside their physical radius because their mass is compressed into such a tiny space.
Because the outer edge of the event horizon is the last point at which light can escape it also marks the last point at which events can be seen by distant observers. Anything past this point can never be observed.
The reason the Schwarzschild radius is called a ‘coordinate singularity’ is that it can be removed with a clever choice of coordinate system. The second singularity can’t be dealt with in this way. This makes it the ‘true’ physical singularity of the black hole itself.
This is known as the gravitational singularity and is found at the centre of the black hole (r=0). This is the end-point for every particle that falls into a black hole. It’s also the point the Einstein field equations break down… maybe even all the laws of physics themselves.
The fact that the escape velocity of the event horizon exceeds the speed of light means that no physical signal could ever carry information from the central singularity to distant observers. We are forever sealed off from this aspect of black holes, which will therefore forever remain in the domain of theory.
How to Make a Black Hole
We’ve already seen that for a body with the mass of Earth to become a black hole, its diameter would have to shrink to less than 2cm. This is obviously something that just isn’t possible. In fact, not even our Sun has enough mass to end its life as a black hole. Only stars with around three times the mass of the Sun are massive enough to end their lives in this way.
But why is that the case?
It won’t surprise you to learn that for an astronomical body to become a black hole it must meet and exceed a series of limits. These limits are created by outward forces that are resisting against the inward force that leads to gravitational collapse.
For planets and other bodies with relatively small masses, the electromagnetic repulsion between atoms is strong enough to grant them stability against total gravitational collapse. For large stars the situation is different.
During the main life cycle of stars–the period of the fusion of hydrogen atoms to helium atoms–the primary protection against gravitational collapse is the outward thermal and radiation pressures that are generated by these nuclear processes. That means that the first wave of gravitational collapse occurs when a star’s hydrogen fuel is exhausted and inward pressure can no longer be resisted.
Should a star have enough mass, this collapse forces together atoms in the nucleus enough to reignite nuclear fusion— with helium atoms now fusing to create heavier elements. When this helium is exhausted, the process happens again, with the collapse again stalling if there is enough pressure to trigger the fusion of heavier elements still.
Stars like the Sun will eventually reach the point where their mass is no longer sufficient to kick start the nuclear burning of increasingly heavier elements. But if it isn’t nuclear fusion that is generating the outward forces that prevent complete collapse, what is preventing these lower-mass stars from becoming black holes?
Placing Limits on Gravitational Collapse
Lower-mass stars like the Sun will end their lives as white dwarf stars with a black hole form out of reach. The mechanism protecting these white dwarfs against complete collapse is a quantum mechanical phenomenon calleddegeneracy.
This ‘degeneracy pressure’ is a factor of the Pauli exclusion principle, which states that certain particles– known as fermions, which include electrons, protons, and neutrons– are forbidden from occupying the same ‘quantum states.’ This means that they resist being tightly crammed together.
This theory and the limitation it introduced led Indian-American astrophysicist Subrahmanyan Chandrasekhar to question if there was an upper cap at which this protection against gravitational collapse would fail.
Chandrasekhar –awarded the 1983 Nobel Prize in physics for his work concerning stellar evolution– proposed in 1931 that above 1.4 solar masses, a white dwarf would no longer be protected from gravitational collapse by degeneracy pressure. Past this limit — termed the Chandrasekhar limit — gravity overwhelms the Pauli exclusion principle and gravitational collapse can continue.
But there is another limit that prevents stars of even this greater mass from creating black holes.
Thanks to the 1932 discovery of neutrons— the neutral partner of protons in atomic nuclei — Russian theoretical physicist Lev Landau began to ponder the possible existence of neutron stars. The outer part of these stars would contain neutron-rich nuclei, whilst the inner sections would be formed from a ‘quantum fluid’ comprised of mostly neutrons
These neutron stars would also be protected against gravitational collapse by degeneracy pressure — this time provided by this neutron fluid. In addition to this, the greater mass of the neutron in comparison to the electron would allow neutron stars to reach a greater density before undergoing collapse.
By 1939, Robert Oppenheimer had calculated that the mass-limit for neutron stars would be roughly 3 times the mass of the Sun.
To put this into perspective, a white dwarf with the mass of the Sun would be expected to have a millionth of our star’s volume — giving it a radius of 5000km, roughly that of the Earth. A neutron star of a similar mass though would have a radius of about 20km — roughly the size of a city.
Above the Oppenheimer-Volkoff limit, gravitational collapse begins again. This time no limits exist between this collapse and the creation of the densest possible state in which matter can exist. The state found at the central singularity of a black hole.
We’ve covered the creation of black holes and the hurdles that stand in the way of the formation of such areas of spacetime, but theory isn’t quite ready to hand black holes over to practical observations just yet. The field equations of general relativity can also be useful in the categorisation of black holes.
The four types of black holes
Categorising black holes is actually fairly straight-forward thanks to the fact that they possess very few independent qualities. John Wheeler had a colourful way of describing this lack of characteristics. The physicist once commented that black holes ‘have no hair,’ meaning that outside a few characteristics they are essentially indistinguishable. This comment became immortalised as the no-hair theorem of black holes.
Black holes have only three independent measurable properties — mass, angular momentum and electric charge. All black holes must have mass, so this means there are only four different types of a black hole based on these qualities. Each is defined by the metric or the function used to describe it.
This means that black holes can be quite easily catagorised by the properties they possess as seen below.
This isn’t the most common or most suitable method of categorising black holes, however. As mass is the only property that is common to all black holes, the most straight-forward and natural way of listing them is by their mass. These mass categories are imperfectly defined and so far black holes in some of the categories–most notably intermediate black holes– remain undetected.
Cosmologists believe that the majority of black holes are rotating and non-charged Kerr black holes. And the study of these spacetime events reveals a phenomenon that perfectly exemplifies their power and influence on spacetime.
The Anatomy of a Kerr Black Hole
The mathematics of the Kerr metric used to describe non-charged rotating black holes reveals that as they rotate, the very fabric of spacetime that surrounds them is dragged along in the direction of the rotation.
The powerful phenomenon is known as ‘frame-dragging’ or the Lense-Thirring effect and leads to the violent churning environments that surround Kerr black holes. Recent research has revealed that this frame-dragging could be responsible for the breaking and reconnecting of magnetic field lines that in-turn, launch powerful astrophysical jets into the cosmos.
The static limit of a Kerr black hole also has an interesting physical significance. This is the point at which light–or any particle for that matter– is no-longer free to travel in any direction. Though not a light-trapping surface like the event horizon, the static limit pulls light in the direction of rotation of the black hole. Thus, light can still escape the static limit but only in a specific direction.
British theoretical physicist and 2020 Nobel Laureate Sir Roger Penrose also suggested that the static limit could be responsible for a process that could cause black holes to ‘leak’ energy into the surrounding Universe. Should a particle decay into a particle and its corresponding anti-particle at the edge of the static limit it would be possible for the latter to fall into the black hole, whilst its counterpart is launched into the surrounding Universe.
This has the net effect of reducing the black hole’s mass whilst increasing the mass content of the wider Universe.
We’ve seen what happens to light at the edge of a black hole and explored the fate of particles that fall within a Kerr black hole’s static limit, but what would happen to an astronaut that strayed too close to the edge of such a spacetime event?
Death by Spaghettification
Of course, any astronaut falling into a black hole would be completely crushed upon reaching its central gravitational singularity, but the journey may spell doom even before this point has been reached. This is thanks to the tidal forces generated by the black hole’s immense gravitational influence.
As the astronaut’s centre of mass falls towards the black hole, the object’s effect on spacetime around it causes their head and feet to arrive at significantly different times. The difference in the gravitational force at the astronaut’s head and feet gives rise to such a huge tidal force that means their body would be simultaneously compressed at the sides and stretched out.
Physicists refer to this process as spaghettification. A witty name for a pretty horrible way to die. Fortunately, we haven’t yet lost any astronauts to this bizarre demise, but astronomers have been able to watch stars meet the same fate.
For a stellar-mass black hole, spaghettification would occur not just before our astronaut reaches the central singularity, but also well before they even hit the event horizon. For a black hole 40 times the mass of our Sun — spaghettification would occur at about 1,000 km out from the event horizon, which is, itself, 120 km from the central gravitational singularity.
As well as developing the Oppenheimer-Volkoff limit, Oppenheimer also used general relativity to describe how a total gravitational collapse should appear to a distant observer. They would consider the collapse to take an infinitely long time, the process appearing to slow and freeze as the star’s surface shrinks towards the Schwarzschild radius.
An astronaut falling into a black hole would be immortalized in a similar way to a distant observer, though they themselves–could they have survived spaghettification– they would notice nothing. The passing of Rs would just seem a natural part of the fall to them despite it marking the point of no return.
Much More to Learn…
After emerging from the mathematics of general relativity at the earlier stages of the 20th Century, black holes have developed from a theoretical curiosity to the status of scientific reality. In the process, they have indelibly worked their way into our culture and lexicon.
Perhaps the most exciting thing about black holes is that there is so much we don’t yet know about them. As a striking example of that, almost all the information listed above resulted just from theory and the interrogation of the maths of Einstein’s field equations.
Unlocking the secrets held by black holes could, in turn, reveal how galaxies evolve and how the Universe itself has changed since its early epochs.
Sources and Further Reading
Relativity, Gravitation and Cosmology, Robert J. Lambourne, Cambridge Press, .
Relativity, Gravitation and Cosmology: A basic introduction, Ta-Pei Cheng, Oxford University Press, .
As an interstellar visitor–an object from outside the solar system–the rogue comet 2I/Borisov is already a source of great interest for astronomers. But researchers have now also discovered that this interstellar comet is composed of pristine material similar to that which exists when star systems first form.
Not only does this make 2I/Borisov even more exciting than previously believed, it means that studying the material that composes it and its coma –an envelope of gas and dust that surround comets– could unlock secrets of planetary system formation.
“2I/Borisov could represent the first truly pristine comet ever observed,” says Stefano Bagnulo of the Armagh Observatory and Planetarium, Northern Ireland, UK. The astronomer tells ZME Science: “We presume this is because it has travelled in the interstellar medium without interacting with any other stars before reaching the Sun.”
Bagnulo is the lead author of one of two papers published in the Nature family of journals detailing new in-depth analysis of 2I/Borisov.
Reflecting on 2I/Borisov
The team was able to make its detailed study of 2I/Borisov–the second interstellar comet found trespassing in our solar system after the cigar-shaped Oumuamua–using the Very Large Telescope (VLT) located in the Acatma Desert, Northern Chile.
In particular, they employed the FOcal Reducer and low dispersion Spectrograph (FORS2) instrument–a device capable of taking mages of relatively large areas of the sky with very high sensitivity–and a technique called polarimetry to unlock the comet’s secrets.
“Sunlight scattered by material, for instance, reflected by a surface, is partially polarised,” explains Bagnulo comparing this to polaroid sunglasses which absorb the polarised component of the light and thus dampen reflected light suppressing glare. “In astronomy, we are interested in that polarised radiation because it carries information about the structure and composition of the reflecting surface or scattering material.”
Bagnulo continues by explaining that because light reflected by a darker object is polarised more than the light reflected by a brighter object, polarimetry may be used to estimate the albedo of an asteroid. This makes it a tool regularly used to study comets and allowed the team to compare 2I/Borisov to comets that begin life in our solar system.
“We found that the polarimetric behaviour of 2I/Borisov is different than that of all other comets of our solar system, except for one, Comet Hale-Bopp,” Bagnulo says. “We suggest that this is because Hale-Bopp is a pristine comet.”
It also implies that 2I/Borisov and Halle-Bopp formed in similar environments, thus giving us a good picture of conditions in other planetary systems.
Whilst, Bagnulo and his team were conducting this research with data collected by the VLT, another team was using a different method to examine the material that comprises this interstellar comet.
The Secrets in the Dust of 2I/Borisov
Bin Yang, is an astronomer at ESO in Chile, who also took advantage of 2I/Borisov’s intrusion into the solar system to study this mysterious comet, but using the Atacama Large Millimeter/submillimeter Array (ALMA).
“I had the idea of observing the thermal emission from the dust particles in the coma of 2I/Borisov using ALMA. My co-author Aigen Li constructed theoretical models to fit the ALMA observation and set constraints on the dust properties,” Yang, the lead author of the second paper detailing the 2I/Borisov investigation, tells ZME Science. “The composition of 2I/Borisov is similar to solar system comets, consists of dust and various ices. The major ices are water ice, carbon monoxide ice and the minor species include hydrogen cyanide and ammonia.”
Yang goes on to explain that the team was not able to precisely determine the composition of 2I/Borisov’s dust component. The astronomer adds that it could be composed of silicates or carbonaceous materials or a mixture of both.
The team also found that the comet’s coma contains compact pebbles and grains of around 1mm and above.
Additionally, as 2I/Borisov neared the Sun the relative amounts of water and carbon they detected from it changed quite drastically.
“We found that the dust coma of Borisov consists of compact, millimeter-sized and larger pebble-like grains, which formed in the inner region near the central star,” Yang says. “We also found the cometary nucleus consists of components formed at different locations in its home system.”
“Our observations suggest that Borisov’s system exchanged materials between the inner regions and the outer regions that are far from the central star, perhaps due to gravitational stirring by giant planets much like in our own solar system.”
Bin Yang, ESO.
These characteristics indicate that 2I/Borisov formed by collecting materials from different locations in its own planetary system. It also imnplies that the system from which it originated likelty featured the exchange of materials between its inner and outer regions. Something that Yang says is also common in our solar system.
“So, it is possible that chaotic material exchanging processes are common phenomena for young planetary systems,” says Yang. “We want to know if other planetary systems form like our own. But we cannot study these systems to the level of their individual comets.”
“Interstellar objects represent the building blocks of planets around other stars. Comet Borisov provides a rare and valuable link between our own solar system and other planetary systems.”
The Journey of 2I/Borisov
2I/Borisov was first discovered by Gennedy Borisov, an amateur astronomer and telescope maker, in August 2019. It was only the second visitor from outside the solar system to be found within our planetary system. That means that as it passed the Sun it presented a unique opportunity to compare conditions in our small corner of the galaxy to those found in other planetary systems.
“2I/Borisov is quite a small comet and it didn’t get very close to the Earth and the Sun, so the emission from this comet is quite weak. We were happily surprised that we actually detected the thermal emission from this alien comet. Because of this detection, we are able to set constraints on the dust properties of this comet,” says Yang. “Comets in other planetary systems are simply too far away and too small to be seen by our telescopes.
“We are extremely lucky to find a comet that is from a planetary system far far away from us. Even more luckily, we managed to take many pictures and spectra of this alien comet during its short visit.”
Bin Yang, ESO.
As Yang points out, 2I/Borisov is only in our solar system for a short time before it must continue its interstellar journey, so the time available to astronomers to study it is limited. But, with interstellar visitors to the solar system believed to be fairly common, but difficult to spot, improving telescope technology could offer future opportunities to study other objects with similar interstellar origins.
Bagnulo points to both the upcoming Vera C Rubin telescope and ESA’s comet interceptor, set to launch in 2029, as future technology that could help us spot and investigate interstellar comets.
“We expect to detect at least one interstellar object per year,” Yang concludes. “So, we will have more opportunities to study alien materials.”
In a series of papers beginning in 1905 Einstein’s theory of special relativity revolutionized the concepts of space and time, uniting them into a single entity–spacetime. But, the most famous element of special relativity–as famous as the man himself–was absent from the first paper.
Mass-energy equivalence, represented by E=mc2, would be introduced in a later paper published in November 1905. And just as Einstein had already unified space and time–this paper would unite energy and mass.
So what does the mass-energy equivalence tell us and what is the equation E=mc2 saying about the Universe?
If you wanted to walk away from this article with one piece of information about the equation E=mc2 (and I hope you won’t) what would that be?
Essentially the simplified version of the equation of special relativity tells us that mass and energy are different forms of the same thing– mass is a form of energy. Probably the second most important piece of information to take away is the fact that these two aspects of the Universe are interchangeable, and the mitigating factor is the speed of light squared.
Still with us? Good!
Perhaps the most surprising thing about the equation E=mc2 is how deceptively simple it is for something so profound. Especially when considering that as the equation that describes how stars release energy and thus make all life possible. Mathematical formulae don’t get much more foundational.
Gathering Momentum: Where Does the Mass-Energy Equivalence Come From?
There are actually a few ways of considering the origin of E=mc2. One way is by considering how the relationship it describes can emerge when comparing the relativistic equation for momentum and its Newtonian counterpart. The major difference between the two, as you’ll see below, is multiplication by the Lorentz factor — you might remember in the last part of this guide to special relativity —concerning space and time— I told you it gets everywhere in special relativity!
Whilst you could argue that the only difference between the two is that velocity (v) has been replaced with a more complex counterpart that approaches v when speeds are far less than light–everyday speeds that we see everyday objects around us move at–but some physicists find this more significant than a mere substitution.
These scientists would argue that this new factor ‘belongs’ to the mass of the system in question. This view means that mass increases as velocity increases, and this means there is a discernable difference between an object’s ‘moving mass’ and its ‘rest mass.’
So, let’s look at that equation for momentum again with the idea of rest mass included.
So, if mass is increasing as velocity increases, what is responsible for this rise?
Let’s conduct an experiment to find out. Our lab bench is the 2-mile long linear particle accelerator at SLAC National Laboratory, California. Using powerful electromagnetic forces, we take electrons and accelerate them to near the speed of light. When the electrons emerge at the other end of the accelerator we find that their relativistic mass has increased by a staggering factor of 40,000.
As the electrons slow, they lose this mass again. Thus, we can see it’s the addition of kinetic energy to the object that is increasing its mass. That gives us a good hint that energy and mass are interconnected.
But, this conclusion leads to an interesting question; if the energy of motion is associated with an object’s mass when it is moving, is there energy associated with the object’s mass when it is at rest, and what kind of energy could this be?
An object at rest without kinetic energy can, with the transformation of an infinitesimally small amount of mass, provide energy enough to power the stars.
As the equation E=mc2 and the fact that the speed of light squared is an extremely large number implies, in terms of energy just a little mass goes a very long way. To demonstrate this, let’s see how much energy would be released if you could completely transform the rest mass of a single grain of sugar.
That’s a lot of energy!
In fact, it is roughly equivalent to the amount of energy released by ‘little boy’– the nuclear fission bomb that devastated Hiroshima on the 6th August 1945.
That means that even when an object is at standstill it has energy associated with it. A lot of energy.
As you might have guessed by this point, as energy and mass are closely associated and there are many forms of energy there are also many ways to give an object increased mass. Heating a metal rod, for example, increases the rod’s mass, but by such a small amount that it goes unnoticed. Just as liberating a tiny bit of mass releases a tremendous amount of energy, adding a relatively small amount of heat energy results in an insignificant mass increase.
We’ve already seeen that we can accelerate a particle and increase its relativistic mass, but is there anything we can do to increase a system’s rest mass?
E=mc2: Breaking the Law (and Billiard Balls)!
Until the advent of special relativity two laws, in particular, had governed the field of physics when it comes to collisions, explosions, and all that cool violent stuff: the conservation of mass and the conservation of energy. Special relativity challenged this, suggesting instead that it is not mass or energy that is conserved, but the total relativistic energy of the system.
Let’s do another experiment to test these ideas… The first location we’ll travel to in order to do this… a billiard table at the Dog & Duck pub, London.
At the billiard table, we strike a billiard ball 0.17 kg toward a stationary billiard ball of the same mass at around 2 metres per second. We hit the ball perfectly straight on so that all of the kinetic energy of the first ball is transferred to the second ball.
If we could measure the kinetic energy of the initial ball, then measure the kinetic energy of both balls after the collision, we would find that–accounting for the small losses of energy to heat and sound–the total energy of the system after the collision is the same as the energy before the collision.
That’s the conservation of energy.
Let’s rerun that experiment again, but this time we launch the billard ball so hard that instead of knocking the target ball across the table, it shatters it. Collecting together the fragments of the shattered ball and remeasuring the mass of the system, we would find the final mass is exactly the same as the initial mass.
And that’s the conservation of mass.
We’re starting to get funny looks from the Dog & Duck regulars now, and the landlord looks angry about the destruction of one of his billiard balls. Luckily, the third part of our test requires we relocate to CERN, Geneva. So we down our drinks, grab our coats and hurry out the door.
Trying the experiment a third time, we are going to replace the billiard table with the Large Hadron Collider (LHC)–that’s some upgrade– and the billiard balls with electrons and their equal rest mass anti-particles– positrons.
Using powerful magnets to feed these fundamental particles with kinetic energy we accelerate them to near light speed, directing them towards each other and colliding them. The result is a shower of particles that previously weren’t present. But, unlike in our billiard ball example, when we measure the rest mass of the system it has not remained the same.
Just one of the particles we observe after the collision event is a neutral pion–a particle with a rest mass 264 times the rest mass of an electron and thus 132 times the initial rest mass we began with.
Clearly, the creation of this pion has taken some of the kinetic energy we poured into the electrons and converted it to rest mass. We watch as the pion decays into a muon with a rest mass 204 times that of an electron, and this decays into particles that are lighter still. Each time the decay releases energy in the form of pulses of light.
Relativistic Energy .vs Rest Energy
By now it is probably clear that in special relativity rest mass and relativistic mass are very different concepts, which means that it shouldn’t come as too much of a surprise that rest energy and relativistic energy are also separate things.
Let’s alter that initial infographic to reflect the fact that the equation E=mc2 actually describes rest energy.
This raises the questions (if I’m doing this right that is) what is the equation for relativistic energy?
It’s time for another non-surprise. The equation for relativistic energy is just the equation for rest energy with that Lorentz factor playing a role.
Ultimately, it is this relativistic energy that is conserved, thus whilst we’ve sacrificed earlier ideas of the conservation of mass and the conservation of energy, we’ve recovered a relativistic version of those laws.
Of course, the presence of that Lorentz factor tells us that when speeds are nowhere near that of light — everyday speeds like that of the billiard balls in the Dog & Duck–the laws of conservation of mass and energy are sufficeint to describe these low-energy systems.
The Consequences of E=mc2
It’s hard to talk about the energy-mass equivalence or E=mc2 without touching upon the nuclear weapons that devasted Hiroshima and Nagasaki at the close of the Second World War.
It’s an unfortunate and cruel irony that Einstein–a man who was a staunch pacifist during his lifetime–has his name eternally connected to the ultimate embodiment of the most destructive elements of human nature.
Nuclear radiation had been discovered at least a decade before Einstein unveiled special relativity, but scientists had struggled to explain exactly where that energy was coming from.
That is because as rearranging E=mc2 implies, a small release of energy would be the result of the loss of an almost infinitesimally small amount of rest mass –certainly immeasurable at the time of discovery.
Of course, as we mention above, we now understand that small conversion of rest mass into energy to be the phenomena that power the stars. Every second, our own star–the Sun– takes roughly 600 tonnes of hydrogen and converts it to 596 tonnes of helium, releasing the difference in rest mass between the two as around 4 x 1026 Joules of energy.
We’ve also harnessed the mass-energy equivalence to power our homes via nuclear power plants, as well as using it to unleash a terrifying embodiment of death and destruction into our collective imaginations.
We could probably ruminate more about special relativity and its elements, as its importance to modern physics simply cannot be overstated. But, Einstein wasn’t done.
Thinking about spacetime, energy and mass had open a door and started Einstein on an intellectual journey that would take a decade to complete.
The great physicist saw special relativity as a great theory to explain physics in an empty region of space, but what if that region is occupied by a planet or a star? In those ‘general’ circumstances, a new theory would be needed. And in 1915, this need would lead Einstein to his greatest and most inspirational theory–the geometric theory of gravity, better known as general relativity.
Sources and Further Reading
Stannard. R., ‘Relativity: A Short Introduction,’ Oxford University Press, .
Lambourne. R. J., ‘Relativity, Gravitation and Cosmology,’ Cambridge University Press, .
Cheng. T-P., ‘Relativity, Gravitation and Cosmology,’ Oxford University Press, .
Fischer. K., ‘Relativity for Everyone,’ Springer, .
Takeuchi. T., ‘An Illustrated Guide to Relativity,’ Cambridge University Press, .
Using the Event Horizon Telescope (EHT) to observe the supermassive black hole at the centre of the galaxy Messier 87 (M87), astronomers have once again produced another first in the field of astronomy and cosmology.
Following up on the image of M87’s black hole published two years ago–the first time a black hole was imaged directly–astronomers at the EHT collaboration have captured a stunning image of the same black hole, this time in polarized light.
The achievement marks more than just an impressively sharp and clear second image of this black hole however–it also represents that first-time researchers have been able to capture the polarization of light around such an object.
Not only does this reveal details of the magnetic field that surrounds the supermassive black hole, but it also could give cosmologists the key to explaining how energetic jets launch from the core of this distant galaxy.
“M87 is a truly special object! It is tied for the largest black hole in the sky with the black hole in our galaxy–Sagittarius A*, ” Geoffrey C. Bower, EHT Project Scientist and assistant research astronomer at the Academia Sinica Institute of Astronomy and Astrophysics, tells ZME Science. “It’s about one thousand times further away but also one thousand times more massive.
“The M87 black hole’s home is in the centre of the Virgo Cluster, the nearest massive cluster of galaxies, each with its own black hole. This makes it a great laboratory for studying the growth of galaxies and black holes.”
Geoffrey C. Bower, Academia Sinica Institute of Astronomy and Astrophysics.
Along with Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University, Netherlands, Bower is one of the authors on two papers detailing the breakthrough published in the latest edition of The Astrophysical Journal Letters.
“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy,” says Mościbrodzka.
“We have never see magnetic fields directly so close to the event horizon,” the astronomer tells ZME Science.
“We now, for the first time, have information on how magnetic field lines are oriented close to the event horizon and how strong these magnetic fields are. All this information is new.”
Deeper Into the Heart of M87
The release of the first image of a black hole on the 10th of April 2019 marked a milestone event in science, and ever since then, the team behind that image has worked hard to delve deeper into M87’s black hole. This second image is the culmination of this quest. The observation of the polarized light allows us to better understand the information in that prior image and the physics of black holes.
“Light is an electromagnetic wave which has amplitude and direction of oscillation or polarization,” explains Mościbrodzka. “With the EHT we observed that light in the M87’s surrounding ring is polarized meaning that waves oscillation have a preferred direction.”
This polarization is a property of synchrotron radiation that is produced in the vicinity of this black hole. Polarization occurs when light passes through a filter–think of polarized sunglasses blocking out light and thus giving you a clearer view–thus the polarization of light in this picture accounts for this clearer view of M87’s black hole, which reveals a great deal of information about the black hole itself.
“The polarization of the synchrotron light tells us about the orientation of magnetic fields. So by measuring light polarization we can map out the magnetic fields around the black hole.”
Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor, Radboud University
Capturing such an image of polarized light at a distance of 55 million light-years is no mean feat, and is only possible with the eight linked telescopes across the globe that comprise the EHT. Together these telescopes–including the 66 antennas of the Atacama Large Millimeter/submillimeter Array (ALMA)–form a virtual telescope that is as large as the Earth itself with a resolution equivalent to reading a business card on the Moon.
“As a virtual telescope that is effectively as large as our planet the EHT has a resolution power than no other telescope has,” says Mościbrodzka. “The EHT is observing the edge of what is known to humans, the edge of space and time. And for the second time, it has allowed us to bring to the public the images of this black hole.”
This image–as the above comparison shows– has had its clarity enhanced immensely by calibration with data provided by the Atacama Pathfinder EXperiment (APEX).
Of course, these magnetic fields are responsible for much more than just giving us a crystal clear image of the black hole they surround. They also govern many of the physical processes that make black holes such powerful and fascinating events–including one of M87’s most mysterious features.
How Magnetic Fields Help Black Holes ‘Feed’
The M87 galaxy–55 million light years from Earth– is notable for its powerful astrophysical jets that blast out of its core and extend for 5000 light-years. Researchers believe that these jets are caused when some of the matter at the edge of the black hole escapes consumption.
Whilst other matter falls to the surface of the central black hole and disappears to the central singularity, this escaping matter is launched into space as these remarkable jets.
Even though this is a more than plausible explanation, many questions still remain about the process, namely, how an area that is no bigger than our solar system creates jets that are greater in length than the entire galaxy that surrounds it.
This image of the polarized light around M87’s black hole which offers a glimpse into this inner region finally gives scientists a chance to answer these mysteries.
“Our planet’s magnetosphere prevents ionized particles emitted by the Sun from reaching the Earth’s surface. In the same way, strong black hole magnetic fields can prevent or slow down the accretion of matter onto the black hole,” Bowers says. “Those strong magnetic fields are also powerful for generating the jets of particles that flow at near the speed of light away from the black hole.”
By mitigating the feeding process of their central black holes, however, these magnetic fields may have an influence that like the jets they create may extend even further than M87 itself. They could be affecting the entire galactic cluster.
“Magnetic fields can play a very important role in how black holes ‘eat.’ If the fields are strong enough, they can prevent inflowing material from reaching the black hole. They are also important in funnelling matter out into the relativistic jets that burst from the black hole region. These jets are so powerful that they influence gas dynamics amongst the entire cluster of galaxies surrounding M87.”
Geoffrey C. Bower, Academia Sinica Institute of Astronomy and Astrophysics.
This means a better understanding of the magnetic fields around M87’s black hole also gives researchers an improved understanding of how the matter behaves at the edge of that black hole and perhaps of how such things affect neighbouring galaxies and their evolution.
And from the image of M87’s black hole the EHT team have developed, it looks clear that of the various models that cosmologists have developed to describe the interaction of matter at the edge of black holes, only those featuring strongly magnetized gases can account for its observed features.
“We have now a better idea about the physical process in the ring visible in the image,” Mościbrodzka says. “We now know more precisely how strong magnetic fields can be near a black hole. We also know more accurately at what rate the black hole is swallowing matter. And we have a better idea of what the black hole might look like in the future.”
In terms of what is next for black hole imaging, both Mościbrodzka and Bowers are clear; they have their sights set on a black hole that is closer to home than M87–the one that sits at the centre of the Milky Way, which despite being closer to home, could be a tougher nut to crack in terms of imaging.
“We’re hard at work on a problem that we know everyone wants to see; an image of the black hole at the centre of our galaxy,” says Bowers. “This is really tricky because the gas around the black hole moves so fast that the image may be changing on same the time scale that it takes to snap our picture. We think we know how to handle this problem but it requires a lot of technical innovation.”
Given the advancements already made by the EHT collaboration team, it would be unwise to bet against them achieving this lofty goal at some point in the not too distant future.
“We’ve gone from imagining what happens around black holes to actually imaging it!” Bowers concludes. “In the near future, we’ll be able to show a movie of material orbiting the black hole and getting ejected into a jet. I never thought I would see anything like this.
“Black holes are the simplest but most enigmatic objects in the Universe. These observations are just the beginning of the road to understanding them.”
Geoffrey C. Bower, Academia Sinica Institute of Astronomy and Astrophysics.