Black holes are the most massive objects in the universe. Their gravitational pull is so strong that nothing can escape it — not even light. But according to a new NASA study, black holes may play a more complex role in galactic ‘ecosystems’. Specifically, a black hole was found to be contributing to the formation of a new star in its vicinity, offering tantalizing clues about how massive black holes develop in the first place.
A stellar nursery
Some ten years ago, Amy Reines, then a graduate student, discovered a black hole in a galaxy about 30 million light-years away from Earth, in the southern constellation Pyxis. She knew something was off right away, but it wasn’t until recently that new Hubble observations shed light on the situation.
“At only 30 million light-years away, Henize 2-10 is close enough that Hubble was able to capture both images and spectroscopic evidence of a black hole outflow very clearly. The additional surprise was that, rather than suppressing star formation, the outflow was triggering the birth of new stars,” said Zachary Schutte, Reines’ graduate student and lead author of the new study.
The galaxy, called Henize 2-10, is a so-called “starburst” galaxy — a galaxy where stars are being formed at a much higher rate than normal, around 1,000 times faster. The galaxy is also relatively small — a so-called dwarf galaxy — and has a black hole at its center, much like the Milky Way.
Researchers were already aware of an unusual cocoon of gas in the area, but Hubble managed to also image an outflow linked to the central black hole. Although the process is not fully understood, astronomers do believe that black holes (or at least some black holes) do have an outflow despite their massive gravity. In Henize 2-10, this outflow moves at about a million miles per hour, slamming into the gas cocoon — and as it turns out, newborn stars follow the path of the outflow.
In large galaxies, the opposite happens: material falling towards the black hole forms jets of plasma that don’t allow the formation of stars. But apparently, in the less-massive Henize 2-10, the outflow has just the right characteristics to precipitate new star formation. Previously, studies mostly focused on larger galaxies, where there is more observational evidence. Dwarf galaxies are still understudied, and it’s only thanks to Hubble that researchers were able to study this.
“Hubble’s amazing resolution clearly shows a corkscrew-like pattern in the velocities of the gas, which we can fit to the model of a precessing, or wobbling, outflow from a black hole. A supernova remnant would not have that pattern, and so it is effectively our smoking-gun proof that this is a black hole,” Reines said.
The role that black holes play in the universe is one of the biggest puzzles in astronomy, and the more data comes in, the more it’s starting to look like this is not a straightforward role, but rather a complex one. For instance, it was just recently demonstrated that researchers realized that most (if not all) galaxies have a black hole at their center. The more massive the galaxy, the more massive the central black hole — or possibly, the other way around, and the mass of the black hole is affecting the galaxy.
But we don’t really know how these central black holes (often called supermassive black holes) formed. Some researchers suspect they formed like “regular” black holes and somehow accumulated more and more mass; others believe they could only have formed in special conditions in the early stages of the universe; a further competing theory claims that the “seeds” of these black holes come from dense star clusters that collapse gravitationally. The black hole in Henize 2-10 could offer clues about these theories.
The black hole in the galaxy remained relatively small over cosmic time and did not accumulate a lot of material. This would suggest that it’s relatively unchanged since its formation, essentially offering a window into the early days of the universe.
“The era of the first black holes is not something that we have been able to see, so it really has become the big question: where did they come from? Dwarf galaxies may retain some memory of the black hole seeding scenario that has otherwise been lost to time and space,” Reines concludes.
Image of a Wolf Rayet star – potentially before collapsing into a black hole. ESO/L. Calçada, CC BY-SA
Astronomers are increasingly drawing back the curtains on black holes. In the past few years, we have finally captured actual photos of these fearsome creatures and measured the gravitational waves – ripples in spacetime – that they create when colliding. But there’s still a lot we don’t know about black holes. One of the biggest enigmas is exactly how they form in the first place.
My colleagues and I now believe we have observed this process, providing some of the best indications yet of exactly what happens when a black hole forms. Our results are published in two papers in Nature and the Astrophysical Journal.
Astronomers believe, on both observational and theoretical grounds, that most black holes form when the centre of a massive star collapses at the end of its life. The star’s core normally provides pressure, or support, using heat from intense nuclear reactions. But once such a star’s fuel is exhausted and nuclear reactions stop, the inner layers of the star collapse inward under gravity, crushing down to extraordinary densities.
Most of the time, this catastrophic collapse is halted when the star’s core condenses into a solid sphere of matter, rich in particles called neutrons. This leads to a powerful rebound explosion that destroys the star (a supernova), and leaves behind an exotic object known as a neutron star. But models of dying stars show that if the original star is massive enough (40-50 times the mass of the Sun), the collapse will simply continue unabated until the star is crushed down into a gravitational singularity – a black hole.
While stars collapsing to form neutron stars are now routinely observed throughout the universe (supernova surveys find dozens of new ones every night), astronomers are not yet entirely sure what happens during the collapse to a black hole. Some pessimistic models suggest the entire star would be swallowed up without much of a trace. Others propose that the collapse to a black hole would produce some other kind of explosion.
For example, if the star is rotating at the time of collapse, some of the infalling material may be focused into jets that escape the star at high velocity. While these jets wouldn’t contain much mass, they’d pack a big punch: if they slammed into something, the effects might be quite dramatic in terms of the energy released.
Up until now, the best candidate for an explosion from the birth of a black hole has been the strange phenomenon known as long-duration gamma-ray bursts. First discovered in the 1960s by military satellites, these events have been hypothesised to result from jets accelerated to mindboggling speeds by newly formed black holes in collapsing stars. However, a longstanding problem with this scenario is that gamma-ray bursts also expel abundant radioactive debris that continues to shine for months. This suggests most of the star exploded outward into space (as in an ordinary supernova), instead of collapsing inward to a black hole.
First image of a black hole. Event Horizon Telescope collaboration et al.
While this doesn’t mean a black hole can’t have been formed in such an explosion, some have concluded that other models provide a more natural explanation for gamma ray bursts than a black hole forming. For example, a super-magnetised neutron star could form in such an explosion and produce powerful jets of its own.
My colleagues and I, however, recently uncovered a new and (in our view) much better candidate event for creating a black hole. On two separate occasions in the past three years – once in 2019 and once in 2021 – we witnessed an exceptionally fast and fleeting type of explosion that, much like in gamma-ray bursts, originated from a small amount of very fast-moving material slamming into gas in its immediate environment.
By using spectroscopy – a technique that breaks down light into different wavelengths – we could infer the composition of the star that exploded for each of these events. We discovered that the spectrum was very similar to so-called “Wolf-Rayet stars” – a very massive and highly-evolved type of star, named after the two astronomers, Charles Wolf and Georges Rayet, that first detected them. Excitingly, we were even able to rule out a “normal” supernova explosion. As soon as the collision between the fast material and its environment ceased, the source practically vanished – rather than glowing for a long time.
This is exactly what you would expect if, during the collapse of its core, the star ejected only a small amount of material with the rest of the object collapsing downward into an enormous black hole.
The new study observed two events that may belong to third type of explosion, lasting only a short time. Credit: Bill Saxton, NRAO/AUI/NSF
While this is our favoured interpretation, it’s not the only possibility. The most prosaic one is that it was a normal supernova explosion, but that a vast shell of dust formed in the collision, concealing the radioactive debris from view. It’s also possible that the explosion is of a new and unfamiliar type, originating from a star we’re not familiar with.
To answer these questions, we will need to search for more such objects. Until now these kinds of explosions have been difficult to study because they are fleeting and hard to find. We had to use several observatories together in quick succession to characterise these explosions: the Zwicky Transient Facility to discover them, the Liverpool Telescope and the Nordic Optical Telescope to confirm their nature, and large high-resolution observatories (the Hubble Space Telescope, Gemini Observatory, and the Very Large Telescope) to analyse their composition.
While we didn’t initially know exactly what we were seeing when we first discovered these events, we now have a clear hypothesis: the birth of a black hole.
More data from similar events may soon be able to help us verify or falsify this hypothesis and establish the link to other types of unusual, fast explosions that our team and others have been finding. Either way, it seems this truly is the decade we crack the mysteries of black holes.
In 2019, astronomers with the Event Horizon Telescope (EHT) project made history after they imaged the shadow of a black hole for the very first time. The picture showed a dark circle surrounded by swirling, bright features resembling a ring of fire, just like Einstein’s general relativity predicted. Now, researchers affiliated with the EHT have done it again, this time for a much smaller black hole. The findings suggest that black holes behave similarly over a wide range of masses.
General relativity aces another test
The first historic direct image of a black hole revealed the event horizon of the supermassive black hole that sits at the heart of Messier 87, a distant galaxy located about 53.5 million light-years away. This is a huge black hole even by supermassive standards.
M87’s supermassive black hole packs the mass of several billion suns into a surprisingly tiny volume. It’s about 1,000 times more massive than the Milky Way’s black hole, Sagittarius A*.
Black holes are the dark remnants of collapsed stars whose gravity is so intense that nothing, not even light, can escape. Because light cannot escape the gravitational pull of a black hole, it is virtually impossible to image or photograph one. However, matter and energy aggregate around the edge of a black hole, a ring-shaped feature known as the event horizon — and this can be imaged because some light manages to escape and reach us.
For M87, due to its behemoth mass, the jets of matter around the event horizon came to resemble a ring of light or an accretion disk.
Now, a team of astronomers led by Michael Janssen of the Max-Planck-Institut für Radioastronomie in Bonn, Germany, has made a new radio observation of the jet of plasma emitted from another supermassive black hole.
The researchers directed the Event Horizon Telescope — a combination of eight radio telescopes located all around the world that form a planetary-scale radio telescope — towards Centaurus A, the nearest active galaxy to Earth with a strong hot plasma jet that emits lots of radio emissions. It lies about 12 million light-years away in the southern constellation of Centaurus.
“It [Centaurus radio waves] is gigantic – 1.5 million light-years across. If you had radio eyes, you would see a structure that covers 200 times that of the full moon,” said Heino Falcke, professor of Astroparticle Physics and Radio Astronomy at Radboud University Nijmegen in the Netherlands and Janssen’s Ph.D. advisor.
“How is this possible? We think black holes are the source of all this. So, we wanted to see how this is happening,” Falcke told ZME Science.
Centaurus A had been imaged by radio observatories since the 1940s. Due to its powerful radio waves, it has always been an easy target for astronomers. However, this time Centaurus A’s black hole has been imagined with the powerful EHT at ten times higher resolution than previous observations.
“With the EHT we see the innermost region of jets and we can see their structure. That is very difficult and rare,” Falcke said.
“What we found was that Cen A looks remarkably like M87. Like a giant pair of scissors- telling us that these jets are hollow. Probably due to the action of magnetic fields ejecting the plasma.”
However, unlike M87, the researchers didn’t see a ring — and that’s good news!
As expected, the EHT observation could only see a faint wisp of plasma jet as it emerged from the event horizon. That’s because Centaurus A* is much smaller than M87*, about 100 times smaller. Due to its more modest size “we can’t see entirely down to the black hole yet, but we see how the plasma jet emerges and is collimated,” Falcke said.
These observations, which were performed with a resolution down to a scale of less than a light-day, are in agreement with general relativity. They also fill in the blanks and dispel many uncertainties scientists had about black hole behavior at different scales, suggesting that supermassive black holes are just like any other black hole, only at a grander scale.
It is very likely that Sagittarius A*, whose mass is between Centaurus A* and M87*, also behaves the same. Of course, this remains to be seen. The team of researchers is currently working on EHT observations of the Milky Way’s supermassive black hole and hopes to get something out by the end of the year.
Of course, that’s easier said than done. Black hole observations are a lot more demanding than meets the eye — and sometimes they can be outright dangerous. Janssen, the lead author of the new study, was working with a radio telescope in Mexico at 4,500 meters altitude when, one day in 2018, he was confronted by six heavily armed men in a black truck.
“We canceled the experiment at this point, but still got some data. All returned safely, but shocked,” said Falcke, who wrote more about such stories and his work imaging black holes in a best-selling book called Light in the Darkness.
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.”
Nestled between a myriad of stars inside the constellation Monoceros, researchers have found a tiny black hole hiding in plain sight. The cosmic object is just 1,500 light-years away, making it the closest black hole to home found thus far. Its mass is just three times that of the Sun, something that scientists didn’t even think possible until not too long ago. Due to this combination of unique factors, the black hole was aptly dubbed ‘The Unicorn’.
An invisible unicorn
Black holes are some of the most mysterious and elusive objects in the cosmos. Black holes usually form when a large star dies and collapses into itself, generating a singularity whose gravitational field is so powerful that not even light can escape its clutches.
Since black holes are essentially one-way doors into the universe, anyone visiting them is banished forever, never to return to tell the tale. That’s a frightening way of saying black holes are invisible to our direct observations, although since 2019 scientists have made a huge breakthrough by imaging a black hole’s event horizon for the first time — the boundary beyond which light can’t escape, but where matter congregates and emits radiation as it gets gobbled up by the voracious black hole.
Most of the black holes known to science have been discovered indirectly, typically due to their interaction with a companion star, which leads to the generation of a lot of X-rays that are actually visible to our instruments.
But the story of how The Unicorn was discovered is, of course, different. Astronomers at Ohio State University looked at data for observations of the black hole’s companion star documented by three telescope systems, including KELT, ASAS, and NASA’s TESS satellite that is on a mission to find exoplanets.
When Tharindu Jayasinghe, a doctoral student in astronomy at The Ohio State University, had a first look at this data, he noticed something odd. The red giant changed in intensity and appearance at various points around the orbit, suggesting it had a companion.
Just like the moon’s gravity is tugging at the Earth, producing high tides, so did this hitherto unidentified object distort the star into a football-like shape, with one axis longer than the other.
“When we looked at the data, this black hole—the Unicorn—just popped out,” said Jayasinghe, who is the lead author of the new study published in the journal Monthly Notices of the Royal Astronomical Society.
Based on the pulling effect incurred by the red giant, known as tidal distortion, as well as the velocity and period of orbit of the star, the astronomers calculated the black hole is about three solar masses.
Any black hole candidate less than five times the mass of the sun falls into a size window that astronomers call the “mass gap”. Although Einstein’s equations from his theory of relativity show that a leftover core from a dying star can turn into a black hole if its mass is at least three times as massive as the sun, scientists didn’t have confirmation that such a tiny black hole existed until very recently.
“When you look in a different way, which is what we’re doing, you find different things,” said Kris Stanek, study co-author, an astronomy professor at Ohio State. “Tharindu looked at this thing that so many other people had looked at and instead of dismissing the possibility that it could be a black hole, he said, ‘Well, what if it could be a black hole?'”
In the future, as more large-scale space experiments are set up, astronomers expect to encounter more such smaller black holes of this calibre. Perhaps The Unicorn isn’t that special after all.
“I think the field is pushing toward this, to really map out how many low-mass, how many intermediate-mass and how many high-mass black holes there are, because every time you find one it gives you a clue about which stars collapse, which explode and which are in between,” said Todd Thompson, co-author of the study, chair of Ohio State’s astronomy department.
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.
Physicists in Israel have devised a black hole analogue — only, rather than being a gravitational fortex that doesn’t allow light to escape, this particular object is a sonic black hole that doesn’t allow sound waves to escape. In the process, the researchers measured the long-theorized Hawking radiation, with important consequences for physics.
A black hole is a region of spacetime where gravity pulls so strongly that even light cannot get out once it crosses a point of no return, known as the event horizon — but you probably knew that already. What may be new to you is that almost 50 years ago, Stephen Hawking proposed a more nuanced view whereby black holes can also generate light.
According to Hawking, black holes can spontaneously emit photons at the event horizon thanks to transient quantum fluctuations known as virtual particles. Despite their name, virtual particles are indeed real particles — it’s just that they pop in and out of existence for a fleeting time.
Virtual particles appear in pairs and in the vast majority of cases they annihilate each other almost immediately. However, if they appear in the vicinity of a black hole, Hawking suggested that it is possible for one particle in a pair to get absorbed by the black hole, while the other escapes into space.
This stream of particles is known as stationary Hawking radiation, but since this phenomenon is so subtle, it’s virtually impossible for our instruments to detect it. But, with some thinking outside the box, it is possible to gain insight into this elusive cosmic phenomenon.
In order to study Hawking radiation, scientists from Technion-Israel Institute of Technology designed a scaled-down version of a black hole, or analogue, in the lab. An example of a black hole analogue can be found in your very own home: a bathtub vortex. The water swirling down the drain can be compared to black hole accretion of matter — but this isn’t what the physicists in Israel used.
Instead, the team cooled down 8,000 rubidium atoms to nearly absolute zero and trapped them in place with a laser beam. This nearly static gas was in an exotic state of matter known as Bose-Einstein Condensate (BEC), in which atoms become so densely packed they behave like one super atom, acting in unison.
A second laser beam created a stream of potential energy that caused the BEC gas to flow like water rushing down a waterfall. The boundary between the region where one half of the gas was flowing faster than the speed of sound while the other half flowed slowly was the event horizon of the sonic black hole.
Rather than pairs of photons spontaneously forming in the gas, the researchers were looking for pairs of phonons — quantum sound wave particles. Phonons in the faster half of the gas flow, beyond the event horizon, are trapped by the speed of the flowing gas. Just like in a black hole that has trapped light particles crossing the event horizon, the phonon cannot return to the other side of the sonic black hole’s event horizon.
“Essentially, the event horizon is a black hole’s outer sphere, and inside it, there’s a small sphere called the inner horizon,” Prof. Jeff Steinhauer of Technion’s Physics Department said in a statement. “If you fall through the inner horizon, then you’re still stuck in the black hole, but at least you don’t feel the weird physics of being in a black hole. You’d be in a more ‘normal’ environment, as the pull of gravity would be lower, so you wouldn’t feel it anymore.”
It took 97,000 iterations of this experiment over 124 consecutive days for the physicists led by Steinhauer to confirm Hawking radiation. Luckily for them, their patience paid off.
“The experimental results of Prof. Steinhauer are of great importance and interest,” Prof. Amos Ori, a general relativity and black hole expert at the Technion Physics Department, said in a statement.
“Jeff measures stationary Hawking radiation emitted from a sonic black hole, in agreement with Hawking’s theoretical prediction. This gives very significant experimental support to Hawking’s analysis, which gets experimental approval for the first time in Jeff’s experiments.”
The experiments also revealed new insights that weren’t predicted during Hawking’s lifetime. After a certain time, the radiation emitted by the system began to intensify. This is likely due to the development of stimulated radiation following the formation of the inner horizon, the physicists explained in the journal Nature Physics.
“Our new long-term goal,” Steinhauer concluded, “is to see what happens when one goes beyond the approximations used by Hawking, in which the Hawking radiation is quantum, but space-time is classical. In other words, we would take into account that the analogue black hole is composed of point-like atoms.”
Astronomers have used the Very Long Baseline Array (VLBA) to discover that the first black hole ever detected is actually much larger than previously believed. So large, at 21 times the mass of the Sun, that it challenges existing theories about the evolution of stars and how they form black holes. These constraints should limit stellar black holes in binary systems to about 15 solar masses.
Cygnus X-1 is a Milky Way binary system that contains a black hole and a supergiant companion star feeding it gas and other material. First discovered in 1964, the binary system has gone on to become one of the most intensely studied objects in astronomy. Yet, our familiarity with Cygnus X-1 doesn’t mean it can’t still deliver a surprise or two.
In addition to finding the black hole is 50% more massive than prior estimates, near 21 solar masses as opposed to 15 solar masses, the team also discovered that the companion star also has a greater mass than previous measurements had revealed. The system as a whole is 20% further away than previously calculated–7,200 light-years from Earth as opposed to 6,100 light-years.
“We know that Cygnus X-1 hosts a black hole that is 21 times the mass of the Sun. We also learned that the supergiant companion star in Cygnus X-1 is also more massive than we had thought, with a mass of about 40 times the mass of the Sun,” Professor James Miller-Jones, the International Centre for Radio Astronomy Research (ICRAR), Curtin University, Australia, tells ZME Science. “The revised masses and distances also lead to an updated orbital separation between the star and the black hole — they orbit each other at a separation of one-quarter the distance from the Earth to the Sun.”
The finding, published in the latest edition of the journal Science, means that Cygnus X-1 contains the largest black hole created through the collapse of a star alone, that has ever been detected with traditional electromagnetic astronomy without the use of gravitational waves. Larger black holes do of course exist, but these are formed through other mechanisms such as mergers between smaller black holes after that initial stellar collapse.
Miller-Jones, the study’s lead researcher, goes on to explain that the team also learned that the black hole is spinning very rapidly , close to its maximum possible speed.
“With all this new information, we were able to propose a likely scenario for how this system formed, which can explain its observed properties.”
Professor James Miller-Jones, ICRAR, Curtin University
The finding doesn’t conform to current theories about black hole formation and stellar development in binary systems as its mass is greater than the limit imposed on such an object.
How Cygnus X-1 Challenges Theories of Stellar Evolution
The team chanced on their finding whilst conducting an ambitious project to observe Cygnus X-1 almost continuously over a full 5.6-day orbit with the network of radio telescopes that comprise VLBA and X-ray telescopes. The aim of the research was to better understand how gas being fed into a black hole from a binary partner via a spiraling accretion disc connects to powerful jets of material that launch out from near the central region at near light speed.
“We had not originally aimed to refine the distance and the mass of the black hole but realised that our data would allow us to do so, by accounting properly for the effects of the black hole orbit. But there is still a wealth of data from this rich observing campaign that we are looking to analyse more fully.”
Professor James Miller-Jones, ICRAR, Curtin University
“Black holes form from the deaths of the most massive stars when they run out of fuel and gravity takes over,” says Miller-Jones. “The mass of the resulting black hole is set by the initial mass of the star from which it formed — which we call the progenitor star — the amount of mass that star lost in winds over its lifetime, and any interactions with a nearby companion star.”
Miller-Jones continues, saying massive stars launch very powerful winds from their surfaces, which leads to significant mass loss over their few-million year lifetimes. Some of the later phases of star’s evolution have particularly strong winds — determined by the abundance of elements heavier than helium in the gas from which the star was formed. More heavy elements mean stronger winds, and ultimately, a less massive star immediately before gravitational collapse.
While some stars can also lose further mass in supernova explosions as they collapse to form a black hole, the evidence suggests that in Cygnus X-1, there was no explosion, and the star collapsed directly into a black hole,” says Miller-Jones. “The stronger the stellar winds during the late evolutionary phases of the star, the less massive we would have expected the black hole to be.”
At first, the team wasn’t totally aware of just how significant their discovery of mass disparities in the Cygnus X-1 binary system was. “I think that our biggest surprise was when we appreciated the full implications of our measurements,” Miller-Jones says. “As observational astronomers, my team and I had already found that we could revise the source distance and the black hole mass. However, it was not until I visited a colleague, Professor Ilya Mandel of Monash University, who is a theoretical astronomer, that we realised how important this actually was.”
Mandel–co-author on the resulting paper– realised that a 21-solar mass black hole was too massive to form in the Milky Way with the constraints in place due to the current prevailing estimates of the amount of mass lost by massive stars in stellar winds.
“The existence of such a massive black hole in our own Milky Way galaxy has shown us that the most massive stars blow less mass off their surface in winds than we had previously estimated. This improves our knowledge of how black holes form from the most massive stars.”
Professor James Miller-Jones, ICRAR, Curtin University
Cygnus X-1: No Stranger to Contraversy
The team’s findings have allowed them to put forward a scenario that would allow the formation of a 21 solar mass black hole in a binary system. “We suggest that the star that eventually collapsed into a black hole began its life a few million years ago with a mass of 55-75 times the mass of the Sun,” Miller-Jones tells ZME. “Over its lifetime, it was close enough to its companion–the current supergiant–that gas from its surface was transferred onto its companion. This removed the outer layers of the black hole progenitor and caused it to rotate more rapidly because the two stars were always keeping the same face towards one another.
“Eventually, possibly as recently as a few tens of thousands of years ago the progenitor star collapsed directly into a black hole–of close to its current mass of 21 times the mass of the Sun–without a supernova explosion.”
Professor James Miller-Jones, ICRAR, Curtin University
Additionally, as well as gaining an insight into the black hole’s birth, Miller-Jones believes the team’s results could also indicate how the system could end its life. “Finally, we considered the eventual fate of this system,” the paper’s lead author says. “While the current companion star may eventually form a black hole, the separation of the two stars is such that the two black holes are unlikely to merge on a timescale comparable to the age of the Universe.”
A companion paper appearing at the same time in the Astrophysical Journal will delve deeper into these elements of the research.
This isn’t the first time that Cygnus X-1, and more specifically its black hole, has sparked discussion in the fields of astronomy and cosmology. As speculation grew during that the intense X-ray source in the region was the result of a black hole, renowned physicist Stephen Hawking bet fellow scientist Kip Thorne–well known for his black hole work–in 1974 that Cygnus X-1 did not contain a black hole.
“This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would win me four years of the magazine Private Eye. If black holes do exist, Kip will get one year of Penthouse.”
Stephen Hawking, A Brief History of Time
Hawking lost the bet, conceding by breaking into Thorne’s office whilst he was on a trip to Russia and signing the framed bet.
The team now intend to apply the technique that led them to this finding to investigate further black holes. This should enable them to better understand how massive stars lose mass through stellar winds. With that said, as Cygnus X-1 is relatively unique in the Milky Way–as one of the few black holes so far detected in orbit with a massive companion star– Miller-Jones believes that they are unlikely to find any more binary systems in which the masses of the constituent star and black hole diverge so drastically from current estimates.
“Most excitingly for me, the advent of cutting-edge new telescopes such as the Square Kilometre Array radio telescope (SKA) will allow us to detect many more black holes, and study their properties, including how matter flows into and away from them, in more detail than ever before,” concludes Professor Miller-Jones. “It’s an exciting time to be in this field!”
The core of a galaxy is typically brightly lit due to highly luminous objects known as quasars — the brightest in the universe actually — that are powered by gas spiraling at high velocity from an extremely large black hole. One such bright source was recently discovered by astronomers over 13 billion light-years away. The quasar in question is thus associated with the farthest and, at the same time, the oldest supermassive black hole encountered so far.
The quasar dubbed J0313-1806 is of great significance for scientists since it formed just 670 million years after the universe as we know it came into existence. As such, the mysterious cosmic object may help shed light on how the ancient universe functioned, setting the stage for its evolution in its current configuration. Previously, the oldest black hole found by scientists was 690 million years old.
But how can black holes be so luminous — aren’t they supposed to suck up everything in their vicinity, even light? That’s of course still true, but when a black hole reaches gargantuan proportions millions to billions of times more massive than the sun, its gravity can create a whirlwind of gas and dust. As matter is funneled into the black hole, it forms a trail of debris shaped like a disk around the black hole. This debris revolves at dizzying speeds and, hence, high energy, which is expelled into the universe. Here on Earth, we can see these eruptions of electromagnetic radiation in the visible spectrum.
And this radiation is extremely intense. The authors of the new study — who studied the quasar using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the world’s largest radio telescope, and two observatories on Mauna Kea in Hawaii — found that J0313-1806 is about 1,000 times brighter than all of the Milky Way galaxy.
But as it often has it when speaking of black holes, for every new thing we learn about them, there are also new questions that scientists can’t answer yet.
Calculations performed by the team of researchers, which included lead author Feige Wang, a Hubble Fellow at the University of Arizona’s Steward Observatory, suggest that the quasar’s black hole has a mass of around 1.6 billion times that of the sun — far too great for such a short period of growth time. Where did all that mass come from considering the quasar is so young?
Some of the available ‘food’ for black holes in the early universe would have been first-generation massive stars largely made of hydrogen and lacking in heavy elements that make up later stars. But even if the black hole formed as early as 100 million years after the Big Bang and grew as quickly as possible, it still would have had to have at least 10,000 solar masses from the very beginning. Things don’t add up, which tells us that astronomers are still missing some pieces of the puzzle. But the authors of the new study have some ideas.
“This tells you that no matter what you do, the seed of this black hole must have formed by a different mechanism,” said co-author Xiaohui Fan, Regents Professor and associate head of the University of Arizona Department of Astronomy. “In this case, one that involves vast quantities of primordial, cold hydrogen gas directly collapsing into a seed black hole.”
The monster black hole at J0313-1806 is still growing at a rapid pace, gobbling up the equivalent of 25 suns every day. How exactly such a supermassive black hole came into existence could be answered with data provided by the upcoming and much-anticipated James Webb Space Telescope, slated to launch on October 31.
Astronomers surveying the heart of the giant galaxy cluster Abell 2261 were expecting to find a hefty supermassive black hole, with an appetite for matter and energy that matched the enormous scale of its cosmic home. But although the supermassive black hole should have had a mass between 3 to 100 billion times that of the sun — very conspicuous to most observations — there was no object to be found. NASA scientists don’t know what to make of the missing black hole and its elusive nature may be yet another reminder that we still know very little about these mysterious objects.
Just like stars cluster around the core of a galaxy, so do galaxies group together, bound by the gravity of an extremely dense core. Most known galaxies, including the Milky Way, have a supermassive black hole at their core that keeps their stars together. Similarly, clusters of hundreds or thousands of galaxies have a core, with an even larger supermassive black hole bounding all those galaxies in place. The galaxy nearest this core is known as the brightest cluster galaxy (BCG) — this is where the cluster’s most important supermassive black hole should be found.
Abell 2261’s BCG is a staggering one million light-years across, making it about 10 times larger than the Milky Way. What’s more, it has a huge core 10,000 light-years across, the largest galactic core ever found.
But, defying all expectations, a team of astronomers led by Kayhan Gultekin from the University of Michigan couldn’t find any supermassive black hole in the galaxy in the middle of Abell 2261, located about 2.7 billion light-years from Earth.
The astronomers employed the most sophisticated techniques and searched the sky using NASA’s Chandra X-Ray Observatory and the Hubble Space Telescope, however they were quite surprised when they came out empty-handed. After all, an object that could weigh as much as 100 billion times the mass of the sun shouldn’t be able to hide easily. For comparison, the Milky Way’s supermassive black hole has 4 million solar masses.
As a black hole devours matter, some of the material becomes superheated at it falls towards the black hole, producing X-rays in the process. However, the scientists did not detect any such source. This is the second time that researchers have failed to find the supermassive black hole at the center of Abell 2261’s large central galaxy, after previous attempts that employed Chandra data obtained in 1999 and 2004. So perhaps there really isn’t any black hole — at least not where scientists have been expecting.
It’s possible the black hole was ejected from the host galaxy’s center as a result of two galaxies merging to form the observed one. When two black holes merge, they produce gravitational waves that may be stronger in one direction than another, so the newly formed supermassive black hole might have been ejected from the center of the galaxy in the opposite direction. Scientists call this a recoiling black hole.
But that’s all in theory. We don’t have any evidence that supermassive black holes can merge, let alone evidence of a black hole recoiling. To date, researchers have only confirmed the mergers of much smaller black holes.
Alternatively, the black hole may not be active enough to produce noticeable amounts of X-rays to show up in Chandra observations, although that sounds a bit implausible given the expected scale. Finally, the black hole may simply not exist.
These hypotheses may be verified once NASA’s upcoming James Webb Telescope comes into operation.
The findings are due to be published in the journal of the American Astronomical Society. In the meantime, it is available online on the pre-print server arXiv.
In 2019, a massive consortium of international scientists made the announcement of the century: they had the first picture of a black hole. The historic picture showed a ring of light swirling around the supermassive black hole at the center of the galaxy M87, located some 55 million light-years away. Now, astronomers have revisited data from earlier observations of the black hole, which they used to reconstruct the images for each year starting from 2009 all the way to 2019.
When you stitch together these images, you end up with a timelapse movie where each frame represents a snapshot of M87* for one year. It’s not the most fluid or eye-catching film you’ve ever seen, but its implications for science are worthwhile. What’s more, in the future, as more data is gathered, we might actually be treated to a more detailed recording of a black hole.
The original 2019 snapshot of M87* showed a dark circle, which is the black hole itself, surrounded by a swirling, bright ring. This bright ring is actually superheated matter that is spiraling into the void at high velocity. The interface between the void and the bright ring is the black hole’s event horizon — the point of no return from which nothing can escape, not even light.
To produce this image, more than 200 scientists affiliated with the Event Horizon Telescope (EHT) project combined data from eight radio telescopes located all around the world. By employing a technique called Very Long Baseline Interferometry, or VLBI, the instruments of all these observatories located thousands of miles from one another were linked to form a “virtual telescope” the size of planet Earth.
Ultimately, this allowed the scientists to discern the shape of the M87* event horizon, a feat comparable to resolving the shape of a doughnut on the surface of the moon from Earth.
Maciek Wielgus, a radio astronomer at Harvard University in Cambridge, Massachusetts, wanted to see what M87* looked like in previous years. Although the 2019 snapshot of the black hole was modeled from copious amounts of data that weren’t available in previous years, Wielgus and colleagues gained access to EHT observations since 2009.
Albeit of lower quality, these observations were enhanced by combining the limited data with mathematical models. The resulting images were much better than the team of researchers expected, revealing the same accretion disk around the event horizon.
The 2020 snapshot of M87* has been postponed due to the COVID-19 pandemic, but the astronomers hope to resume observations in 2021. They even plan on including additional observatories, one in Greenland, the other in France.
Gravitational waves have been detected from what appears to be the largest black hole merger ever observed. The powerful and previously unobserved hierarchical merger resulted in an intermediate-mass black hole, an object never before detected.
A massive burst of gravitational waves equivalent to the energy output of eight Suns has been detected by the LIGO laser interferometer. Researchers at LIGO and its sister project VIRGO believe that the waves originate from a merger between two black holes. But, this isn’t your average black hole merger (if there is such a thing). The merger — identified as gravitational wave event GW190521 — is not only the largest ever detected in gravitational waves — but it is also the first recorded example of what astrophysicists term a ‘hierarchical merger’ occurring between two black holes of different sizes, one of which was born from a previous merger.
“This doesn’t look much like a chirp, which is what we typically detect,” says Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), comparing the signal to LIGO’s first detection of gravitational waves in 2015. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”
Even more excitingly, it seems that black hole birthed in the event has a mass of between 100–1000 times that of the Sun, putting it in the mass range of an intermediate-mass black hole (IMBH). Something that researchers have theorised about for decades, but up until now, have failed to detect.
The gravitational wave signal–spotted by LIGO on 21st May 2019–appears to the untrained eye as little more than four short squiggles that lasted little more than one-tenth of a second, but for Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration, whose group focuses on the development of highly-accurate waveform models, it holds a wealth of information. “It’s amazing, but from about four gravitational-wave cycles, we could extract unique information about the astrophysical source,” she tells ZME Science.
“The waves are fingerprints of the source that has produced them.”
Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration.
As well as containing vital information about black holes and a staggering merger event, as the signal originated 17 billion light-years from Earth and at a time when the Universe was half its current age, it is also one of the most distant gravitational wave sources ever observed. The incredible distance the signal has travelled may initially seem at odds with the fact that the Universe is only 14.8 billion years old, but the disparity arises from the fact our universe is not static but is expanding.
Details of the international team’s important findings are featured in a series of papers publishing in journals such as Physical Review Letters, and The Astrophysical Journal Letters, today.
Missing Intermediete-Mass Black Holes
Thus far, the black holes discovered by astronomers have either been those with a mass inline with that of larger stars–so-called stellar-mass black holes, or supermassive black holes, with masses far exceeding this. Black holes that exist between these masses have remained, frustratingly hidden. Until now.
“The LIGO and Virgo collaborations detected a gravitational wave corresponding to a very interesting black hole merger. This was named GW190521 and corresponds to two large black holes during the final orbit and merger,” Pedro Marronetti, program director for gravitational physics at the National Science Foundation (NSF) responsible for the oversight of LIGO, tells ZME Science.
“What makes GW190521 extraordinary in comparison to other gravitational wave events is the mass of the black holes involved, the product of the merger is a 142 solar mass black hole and the first object of its kind with mass above 100 solar masses but below a million solar masses to be discovered.”
Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)
Thus, the resultant black hole of 142 solar masses exists in that crucial, thus far undetected, mass range indicating an intermediate-mass black hole (IMBH).
“These black holes, heavier than 100 solar masses but much lighter than the supermassive black holes at the centre of galaxies — which can be millions and billions of solar masses — have eluded detection until now,” Marronetti says. “Additionally, the heavier of the original black holes with 85 solar masses also presents an enigma.”
Pair Instability and Hierarchical Black Hole Mergers
The enigma that Marronetti refers to is the fact that heavier of the two black holes that entered the merger, is of a size that suggests it too must have been created by a merger event between two, even smaller, black holes. “The most common channel of formation of black holes involves heavy stars that end their lives in supernova explosions,” the NSF program director points out. “However, this formation channel prevents the creation of black holes heavier than 65 solar masses but lighter than 130 solar masses due to a phenomenon called ‘pair-instability’.”
As nuclear fusion ceases, there is no longer enough outward radiation pressure to prevent gravitational collapse. “The star suddenly starts producing photons that are energetic enough to create electron-positron pairs,” Marronetti explains. “These photons, in turn, create an outward pressure that is not strong enough to stop the star from collapsing violently due to its self-gravitational pull.”
This results in a difference in gravitational pressure between the star’s core and its outer layers. As a massive shock travels through these ‘puffed out’ outer layers they are blown away in a massive explosion. With smaller stars, this leaves behind an exposed core that becomes a stellar remnant such as a white dwarf, neutron star or black hole. But if the star is a range above 130 solar masses, but below 200 solar masses, the result is more disastrous.
“The resulting supernova explosion completely obliterates the star, leaving nothing behind– no black hole or neutron star is produced,” Marronetti says. “It will take stars heavier that 200 solar masses to collapse into black hole fast enough to avoid this complete disintegration.”
As Marronetti points out; this means that the 85-solar mass back hole could only be formed by the merger of two smaller black holes, as at these masses, collapsing stars can’t form black holes. “This is a quite unusual event that can only occur in regions of dense black hole population such as globular clusters,” the researcher adds. “GW190521 is the first detection that is likely to be due to this ‘hierarchical merger’ of black holes.”
Marronetti continues by explaining that a hierarchical merger consists of one or more black holes that were produced by a previous black hole merger. This hierarchy of mergers allows for the formation of progressively heavier and heavier black holes from an original population of small ones.
“We don’t really know how common these hierarchical mergers are since this is the first time we have direct evidence of one. We can only say that they are not very common.”
Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)
LIGO Delivering Discoveries and Surprises
The team uncovered the unusual nature of this particular merger by assessing the gravitational wave signal with a powerful state of the art computational models. Not only did this reveal that GW190251 originates from the most massive black hole merger ever observed and that this was no ordinary merger but a hierarchical merger, but also crucial information about the black holes involved in the event.
“The signal carries information about the masses and spins of the original back holes as well as their final product,” Marronetti adds, alluding to the fact that the LIGO -VIRGO team were able to measure that spin and determine that as the black holes circled together, they were also spinning around their own axes. The angles of these axes appeared to have been out of alignment with the axes of their orbit. This misaligned spin caused the black holes to ‘wobble’ as they moved together.
“Our waveform models were used to detect GW190521 and also to interpret its nature, extracting the properties of the source, such as masses, spins, sky location, and distance from Earth. For the first time, the waveform models included new physical effects, notably the precession of the spins of the black holes and higher harmonics,” Buonanno says. “What we mean when we say higher harmonics is like the difference in sound between a musical duet with musicians playing the same instrument versus different instruments.
“The more substructure and complexity the binary has — for example, black holes with different masses or spins—the richer is the spectrum of the radiation emitted.”
Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration.
Unanswered Questions and Future Investigations
Even with the staggering amount of information the team has been able to collect about the merger that gave rise to the signal GW190251, there are still some unanswered questions and details that must be confirmed.
The LIGO-VIRGO detectors use two very distinct methods to search the Universe for gravitational waves, an algorithm to pick out a specific wave pattern most commonly produced by compact binary mergers, and more general ‘burst’ searches. The latter searches for any signal ‘out of the ordinary’ and it’s the mechanism via which the researchers found GW190215.
Morronetti expresses some surprise that the methods used by the team were able to unlock these secrets, believing that this result demonstrates the versatility of LIGO. “My main surprise was that this event was detected using a search algorithm that was not specifically created to find merger signals,” says the NSF director. “This is the first detection of its kind and shows the capability of LIGO to detect phenomena beyond compact mergers.”
“This is of tremendous importance since it showcases the instrument’s ability to detect signals from completely unforeseen astrophysical events. LIGO shows that it can also observe the unexpected.”
Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)
This leaves open the small chance that the signal was created by something other than a hierarchical merger. Perhaps something entirely new. The authors hint at the tantalising prospect of some new phenomena, hitherto unknown, in their paper, but Marronetti is cautious: “By far, the most likely cause is the merger of two black holes, as explained above. However, this is not as certain as with past LIGO/Virgo detections.
“There is still the small chance that the signal was caused by a different phenomenon such as a supernova explosion or an event during the Big Bang. These scenarios are possible but highly unlikely.”
Confirming the nature of the event that gave rise to the GW190251 signal is something that the LIGO team will be focusing on in the future as the interferometer also searches for similar events via the gravitational waves they emit. “
With GW190521, we have seen the tip of the iceberg of a new population of black holes,” Buonanno says, adding that LIGO’s next operating run (O4) will explore a volume of space 3 times larger than the current run (O3). “Having access to a larger number of events, which were too weak to be observed during O3, will allow us to shed light on the formation scenario of binary black holes like GW150921.”
The universal speed limit, which we commonly call the speed of light, is fundamental to the way the universe works. But human imagination knows no real limits. In science fiction, you often hear about ‘wormholes’, which are objects that enable faster-than-light travel by instantaneously transferring passengers from one point in spacetime to another.
Although the General Theory of Relativity and the Standard Model of Physics can theoretically support the existence of wormholes within their frameworks, they forbid traversable wormholes.
But physicists at Princeton flexed some serious mathematical muscles and found a loophole. By exploiting quirks of quantum mechanics within a five-dimensional universe, the researchers claim that it may be possible to create a wormhole large enough for humans and their spacecraft to travel through it and instantly emerge somewhere else at the other side of the universe. Alas, such a thing likely cannot exist in nature and an artificial traversable wormhole would be impossible to create with today’s technology.
Plausible? Perhaps. Practical? Definitely not
A wormhole or a Lorentzian wormhole is a sort of theoretical ‘tunnel’ through space-time, often used as the preferred mode of interstellar travel in movies like Star Trek. The opening is a shortcut through intervening space to another location in the Universe. That seems to be in stark contrast to a black hole which is less of a tunnel and more of a meat grinder. However, some physicists claim that there are many characteristics which both black holes and wormholes share.
The existence of wormholes was first proposed by Karl Schwarzchild, whose solutions to Einstein’s field equations form the basis for the inference of black holes. Sometimes, blackholes or blackhole binary systems may form connections between different points in spacetime.
The problem is that these wormholes collapse almost immediately, which would block matter from crossing from one end to the other. All hope isn’t lost yet, though.
Juan Maldacena, the Carl P. Feinberg Professor of theoretical physics from the Institute of Advanced Study and Alexey Milekhin, a graduate student of astrophysics at Princeton University, wrote a new paper in which they discuss the conditions that may allow the existence of traversable wormholes.
In their paper, the two physicists outline some very exotic circumstances that may allow wormholes stable enough for humans to cross through. This includes the existence of negative energy, for instance.
In the theory of general relativity, we usually assume that the energy is greater than zero, at all times and everywhere in the universe. This has a very important consequence for gravity: Energy is linked to mass via the formula E=mc². So negative energy would consequently also imply negative mass. Positive masses attract each other, but with a negative mass, gravity could suddenly become a repulsive force.
Quantum theory, however, allows negative energy. According to quantum physics, it is possible to borrow energy from a vacuum at a certain location, like money from a bank. In their paper, the authors mention the Casimir Effect, in which quantum fields may produce negative energy while propagating along a closed circle.
“However, this effect is typically small because it is quantum. In our previous paper, we realized that this effect can become considerable for black holes with large magnetic charge. The new idea was to use special properties of charged massless fermions (particles like the electron but with zero mass). For a magnetically charged black hole these travel along the magnetic field lines (In a way similar to how the charged particles of the solar wind create the auroras near the polar regions of the Earth),” Maldacena and Milekhin explained to Universe Today.
These sort of wormholes would be allowed by the Standard Model of particle physics. However, they would be so tiny, not even a strand of human hair would have room to pass through. What’s more, they would only exist over equally tiny distances.
In order to support wormholes large and stable enough for humans to use, the physicists had to think outside the Standard Model box. The pair of researchers turned to the Randall-Sundrum II model, also known as the 5-dimensional warped geometry theory, which describes the universe in terms of five-dimensions.
This five-dimensional model of spacetime can enable scientists to describe physics that would normally be undetectable, allowing negative energy to exist.
The produced wormholes would look like medium-sized, charged black holes. They would be large enough for a spacecraft to travel through, however, the pilot would have to navigate very powerful tidal forces.
If the pilot could somehow navigate through this chaos, entering the wormhole would instantly propel the crew to another point in spacetime — but the instant factor is only true from the perspective of the traveler. From the perspective of an outside observer, the travel would take as much time as light would take to travel from point A to point B, which is consistent with the Theory of General Relativity.
However, it would be next to impossible for this to work. Such wormholes do not exist in nature and artificially creating them would involve engineering negative mass.
In other words, it’s unlikely that wormholes will ever become a practical means of traveling through space. Nevertheless, it’s always fascinating to hear about concepts once solely thought to be of the realm of fiction that may actually be plausible.
The paper was published in the pre-print serverarXiv.
Astronomers have come across a monstrously large black hole with a gargantuan appetite. Each passing day, the insatiable void known as J2157 consumes gas and dust equivalent in mass to the sun, making it the fastest-growing black hole in the universe.
The sheer scale of J2157 is almost unfathomable, but we can try pinning some numbers on it nevertheless.
According to Christopher Onken, an astronomer at the Australian National University who was part of the team that originally discovered the object in 2019, J2167 is 8,000 times more massive than the supermassive black hole found at the heart of the Milky Way. That’s equivalent to 34 billion times the mass of the Sun.
In order for Sagittarius A*, the Milky Way’s supermassive black hole, to reach a similar size, it would have had to gobble two-thirds of all the stars in the galaxy.
For their new study, astronomers turned to ESO’s Very Large Telescope in Chile to get a more accurate assessment of the black hole‘s mass. The researchers already knew they were dealing with a black hole of epic proportions, but the final results surprised everyone.
“We knew we were onto a very massive black hole when we realized its fast growth rate,” said team member Dr. Fuyan Bian, a staff astronomer at ESO.
“How much black holes can swallow depends on how much mass they already have. So, for this one to be devouring matter at such a high rate, we thought it could become a new record holder. And now we know.”
Although black holes can’t be imaged directly because they don’t let light escape, J2157 is actually classed as a quasar, or “quasi-stellar radio source” — extremely bright objects powered by black holes at least a billion times as massive as our sun.
The bright signal of the quasar is formed by particles of dust and gas accreting around the edge of the supermassive black hole that are accelerated away at almost the speed of light. Practically, the black hole acts like an extremely powerful natural particle accelerator.
Luckily for us, the black hole is located many billions of light-years away. But this also means that astronomers are measuring J2157’s gravitational influence as it appeared in the distant past when the universe was still very young.
“We’re seeing it at a time when the universe was only 1.2 billion years old, less than 10 percent of its current age,” Dr Onken said.
“It’s the biggest black hole that’s been weighed in this early period of the Universe.”
Since then, J2157 likely grew even bigger, perhaps merging with several other black holes across the eons.
“With such an enormous black hole, we’re also excited to see what we can learn about the galaxy in which it’s growing,” Dr Onken said.
“Is this galaxy one of the behemoths of the early Universe, or did the black hole just swallow up an extraordinary amount of its surroundings? We’ll have to keep digging to figure that out.”
Researchers affiliated with European Southern Observatory (ESO) have caught a massive star in the midst of a disappearing act. Typically, stars in its class end their life cycle with a bang, in a supernova — the most powerful explosion in the universe. However, the star disappeared without a trace, perhaps directly collapsing into a black hole, with important consequences for astronomy.
Astronomers have been tracking the luminous blue variable star located in the Kinman Dwarf galaxy since 2001. The extremely massive star was of particular interest because scientists still don’t know much about how such objects behave towards the end of their lifetimes, especially in metal-poor environments such as the Kinman Dwarf galaxy.
Andrew Allan of Trinity College Dublin, along with colleagues from Chile and the US, pointed ESO’s Very Large Telescope (VLT) in Chile’s Atacama Desert towards the distant galaxy in 2019 for a new survey. Much to their surprise, the star’s signal vanished.
How could such a luminous star, which was about 2.5 million times brighter than the sun, simply disappear? That’s still a mystery, but Allan’s team has some ideas.
“One of the most memorable moments was when we noted the absence of the massive star signature in our first observation which was obtained with the ESPRESSO instrument of ESO’s Very Large Telescope. As the conditions were not perfect on the day this observation was made, we wanted to make sure the signature was absent. For this, we needed to request a follow-up observation. This is usually a long process, however as Ireland recently agreed to join ESO, we were able to apply for a fast-track observation reserved for important unusual events. This time we used the X-Shooter instrument of the Very Large Telescope and were happy to find that this also pointed towards the star disappearing!” Allan told ZME Science in an e-mail.
Although it is extremely unusual for such a massive star to disappear without producing a bright supernova explosion, old data supplied by the ESO Science Archive Facility suggests that the massive object could have been undergoing a strong outburst period that likely ended sometime after 2011.
“This indicated the extreme nature of the massive star and was achieved by developing computer simulations,” Allan said.
Based on their observations, the researchers think that there are only two plausible explanations for the star’s sudden disappearance and lack of a supernova.
In one scenario, the outburst may have ‘downgraded’ the luminous blue variable star into a less bright star, whose signature may be partly obstructed by dust.
The second, more exciting, explanation is that the star could have simply collapsed into a black hole.
Massive stars usually end their life cycle by exploding into a supernova. What’s left of the star either turns into a neutron star or a black hole. However, the absence of a supernova in such cases is almost unprecedented.
“If true, this would have major implications for astronomy. Such an event has been observed only once, in the galaxy of NGC 6946 where a smaller massive star seemed to disappear without a bright supernova explosion. The larger mass of the star we study as well as it being from a low metallicity galaxy makes the finding unique and could hold important clues as to how stars could collapse to a black hole without producing a bright supernova,” Allen said.
The astronomers will have the chance to find out more about the star’s fate once ESO’s Extremely Large Telescope (ELT) comes into operation in 2025. ELT can image at high resolution very distant stars such as those in Kinman Dwarf, which is located more than 75 million light-years away. In the meantime, Allen and colleagues plan on performing additional observations with the Hubble Space Telescope.
“As Hubble already imaged the galaxy prior to the star’s disappearance, we are hoping this will enable us to confirm the star’s disappearance and determine the true cause of its disappearance,” Allan said.
Researchers affiliated with the European Southern Observatory (ESO) have detected an invisible object orbiting a double-star system. Upon closer inspection, the object was revealed to be a black hole. Lying just 1,000 light-years away from Earth, this makes it the closest black hole found thus far. In fact, the stellar system that the black hole calls home can be seen with the naked eye.
The first stellar system with a black hole that can be seen with the unaided eye
Originally, the astronomers were observing HR 6819 as part of a study of double-star systems. However, after many months of observations, the researchers noticed that one of the two visible stars orbits an invisible object every 40 days, while the second star is at a relatively large distance from this object. This invisible object turned out to be a black hole, making HR 6819 a triple system.
The observation was performed using the FEROS spectrograph on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile.
“The dullest object one can imagine”
“Because it is not being “fed” by its companion star, it does nothing, except for orbiting its companion star. It is the dullest object one can imagine, just the dead remains of a once-powerful star. However, that is exactly what makes this boring individual exciting because there should be many more awaiting their discovery,” Dietrich Baade, Emeritus Astronomer at ESO in Garching and co-author of the study, told ZME Science.
But this is just the tip of the iceberg. The astronomers believe that silent black holes are actually extremely abundant in the universe. Already, the researchers believe that their findings apply to another system, called LB-1, which may also be a triple system.
“Perhaps, the biggest challenge was to keep going with the observations and their analysis until our friend Stan Štefl could determine the orbital period of 40 days. We are still sad that he died in a car accident in 2014 and cannot receive his share in the recognition of our work. Already in 2010, he laid the foundation to the ‘Aha!’ we experienced at the end of 2019 when we realized that the then newly published observations of the system LB-1 were hard to distinguish from ours of HR 6819. This prompted us to resume our work on HR 6819 which confirmed the conclusion reached back in 2010 that this system consists of 2 luminous stars and one invisible object. LB-1 has the same architecture (contrary to the discovery announcement, which portrayed LB-1 as a binary star). Our more in-depth analysis now demonstrates clearly the black-hole nature of the unseen object in HR 6819,” Baade said.
Unlike most black holes identified thus far, the one lurking in the HR 6819 system does not interact too much with its environment. Since it doesn’t engulf a lot of matter and energy, it doesn’t draw attention, appearing truly ‘black, which added to the challenge of discovery.
“Not only did we find one of the first 2 or 3 black holes (BHs; perhaps even the very first one) that do not shine brightly in X-rays (because they are not fed with gas from a luminous companion star) but it is also the most nearby of all types of BHs. The latter implies that such BHs must be very common because the solar system is not at a place with special properties,” Baade told ZME Science.
Baade and colleagues believe that there may be many more black holes that are just as inconspicuous as the one lurking in the HR 6819 system.
Previous work that modeled stellar populations suggests that there should be 100 million to one billion stellar-sized black holes in the Milky Way. However, no more than two dozen black holes have been found thus far in our galaxy.
The astronomers believe that this low black hole count is due to the fact that most of them likely do not emit bright X-ray light. There may be hundreds of millions of X-ray-quiet black holes silently populating the Milky Way, according to Baade.
“Since isolated BHs are next to impossible to detect in significant numbers, multiple stars in which at least one of the luminous stars is forced into an orbit about the BH, are the best way forward towards filling in the giant gap between observations and models,” he added.
In addition to offering clues about the behavior of X-ray-quiet black holes, the findings might also explain the occurrence of certain cosmic mergers that trigger gravitational waves powerful enough to be detected from Earth. Distant outer objects, such as the distant star in the HR 6819 triple system, can gravitationally influence the merge of the inner pair (innermost star and black hole), astronomers suggest.
“We shall try to obtain more observations to find and characterize more systems so that their commonalities can identify the patterns of their formation and evolution. In addition, we shall investigate published reports about other systems for possible alternative interpretations (cf. LB-1). In this way, our work will contribute to further complementing our understanding of the evolution of single stars, multiple stars, and the Milky Way. This also includes the history and the future of our Sun and the solar system,” Baade concluded.
An artist’s conception of two black holes entwined in a gravitational tango. NASA/JPL-Caltech/SwRI/MSSS/Christopher Go.
Do supermassive black holes have friends? The nature of galaxy formation suggests that the answer is yes, and in fact, pairs of supermassive black holes should be common in the universe.
I am an astrophysicist and am interested in a wide range of theoretical problems in astrophysics, from the formation of the very first galaxies to the gravitational interactions of black holes, stars and even planets. Black holes are intriguing systems, and supermassive black holes and the dense stellar environments that surround them represent one of the most extreme places in our universe.
The supermassive black hole that lurks at the center of our galaxy, called Sgr A*, has a mass of about 4 million times that of our Sun. A black hole is a place in space where gravity is so strong that neither particles or light can escape from it. Surrounding Sgr A* is a dense cluster of stars. Precise measurements of the orbits of these stars allowed astronomers to confirm the existence of this supermassive black hole and to measure its mass. For more than 20 years, scientists have been monitoring the orbits of these stars around the supermassive black hole. Based on what we’ve seen, my colleagues and I show that if there is a friend there, it might be a second black hole nearby that is at least 100,000 times the mass of the Sun.
At the center of our galaxy is a supermassive black hole in the region known as Sagittarius A. It has a mass of about 4 million times that of our Sun. ESA–C. Carreau.
Supermassive black holes and their friends
Almost every galaxy, including our Milky Way, has a supermassive black hole at its heart, with masses of millions to billions of times the mass of the Sun. Astronomers are still studying why the heart of galaxies often hosts a supermassive black hole. One popular idea connects to the possibility that supermassive holes have friends.
To understand this idea, we need to go back to when the universe was about 100 million years old, to the era of the very first galaxies. They were much smaller than today’s galaxies, about 10,000 or more times less massive than the Milky Way. Within these early galaxies the very first stars that died created black holes, of about tens to thousand the mass of the Sun. These black holes sank to the center of gravity, the heart of their host galaxy. Since galaxies evolve by merging and colliding with one another, collisions between galaxies will result in supermassive black hole pairs – the key part of this story. The black holes then collide and grow in size as well. A black hole that is more than a million times the mass of our son is considered supermassive.
If indeed the supermassive black hole has a friend revolving around it in close orbit, the center of the galaxy is locked in a complex dance. The partners’ gravitational tugs will also exert its own pull on the nearby stars disturbing their orbits. The two supermassive black holes are orbiting each other, and at the same time, each is exerting its own pull on the stars around it.
The gravitational forces from the black holes pull on these stars and make them change their orbit; in other words, after one revolution around the supermassive black hole pair, a star will not go exactly back to the point at which it began.
Using our understanding of the gravitational interaction between the possible supermassive black hole pair and the surrounding stars, astronomers can predict what will happen to stars. Astrophysicists like my colleagues and me can compare our predictions to observations, and then can determine the possible orbits of stars and figure out whether the supermassive black hole has a companion that is exerting gravitational influence.
Using a well-studied star, called S0-2, which orbits the supermassive black hole that lies at the center of the galaxy every 16 years, we can already rule out the idea that there is a second supermassive black hole with mass above 100,000 times the mass of the Sun and farther than about 200 times the distance between the Sun and the Earth. If there was such a companion, then I and my colleagues would have detected its effects on the orbit of SO-2.
But that doesn’t mean that a smaller companion black hole cannot still hide there. Such an object may not alter the orbit of SO-2 in a way we can easily measure.
The physics of supermassive black holes
Supermassive black holes have gotten a lot of attention lately. In particular, the recent image of such a giant at the center of the galaxy M87 opened a new window to understanding the physics behind black holes.
The first image of a black hole. This is the supermassive black hole at the center of the galaxy M87. Event Horizon Telescope Collaboration, CC BY-SA.
The proximity of the Milky Way’s galactic center – a mere 24,000 light-years away – provides a unique laboratory for addressing issues in the fundamental physics of supermassive black holes. For example, astrophysicists like myself would like to understand their impact on the central regions of galaxies and their role in galaxy formation and evolution. The detection of a pair of supermassive black holes in the galactic center would indicate that the Milky Way merged with another, possibly small, galaxy at some time in the past.
That’s not all that monitoring the surrounding stars can tell us. Measurements of the star S0-2 allowed scientists to carry out a unique test of Einstein’s general theory of relativity. In May 2018, S0-2 zoomed past the supermassive black hole at a distance of only about 130 times the Earth’s distance from the Sun. According to Einstein’s theory, the wavelength of light emitted by the star should stretch as it climbs from the deep gravitational well of the supermassive black hole.
The stretching wavelength that Einstein predicted – which makes the star appear redder – was detected and proves that the theory of general relativity accurately describes thephysics in this extreme gravitational zone. I am eagerly awaiting the second closest approach of S0-2, which will occur in about 16 years, because astrophysicists like myself will be able to test more of Einstein’s predictions about general relativity, including the change of the orientation of the stars’ elongated orbit. But if the supermassive black hole has a partner, this could alter the expected result.
Finally, if there are two massive black holes orbiting each other at the galactic center, as my team suggests is possible, they will emit gravitational waves. Since 2015, the LIGO-Virgo observatories have been detecting gravitational wave radiation from merging stellar-mass black holes and neutron stars. These groundbreaking detections have opened a new way for scientists to sense the universe.
Any waves emitted by our hypothetical black hole pair will be at low frequencies, too low for the LIGO-Virgo detectors to sense. But a planned space-based detector known as LISA may be able to detect these waves which will help astrophysicists figure out whether our galactic center black hole is alone or has a partner.
In a galaxy far, far away… astronomers have discovered the most massive black hole in the local observable universe. According to recent observations, the black hole at the center of the Holm 15A galaxy has a staggering 40 billion solar masses. For comparison, the Milky Way’s supermassive black hole measures only 4 million solar masses.
The discovery came to light while a team of astronomers, from the Max Planck Institute for Extraterrestrial Physics and the University Observatory Munich, was surveying Abell 85. It is a galaxy cluster located about 740 million light-years from Earth, which consists of more than 500 individual galaxies.
Holm 15A came to the researchers’ attention when they noticed a huge dark patch at its center. For a galaxy whose stars are equivalent to 2 trillion solar masses — in other words, a very bright galaxy — this was highly surprising to see.
Observations suggested that the murky and diffuse center of Holm 15A is almost as large as the Large Magellanic Cloud, a big clue hinting towards the presence of a black hole with a very high mass.
“There are only a few dozen direct mass measurements of supermassive black holes, and never before has it been attempted at such a distance,” said Jens Thomas, a Max Planck researcher and the lead author of the study. “But we already had some idea of the size of the Black Hole in this particular galaxy, so we tried it.”
With the help of the observatory at the University Observatory Munich and the MUSE instrument at the Very Large Telescope in Chile, the astronomers were able to estimate the black hole’s mass by measuring the motion of stars around the galaxy’s core. The results suggest that at the heart of Holm 15A lies a behemoth with 40 billion solar masses — the most massive black hole yet known to scientists in the local universe.
“This is several times larger than expected from indirect measurements, such as the stellar mass or the velocity dispersion of the galaxy,” says Roberto Saglia, senior scientist at the Max Planck Institute for Extraterrestrial Physics.
The diffuse galactic core also suggests that Holm 15A formed after two smaller galaxies merged. Both had supermassive black holes at their center, so when the galaxies merged, so did the black holes. As the black hole became more massive, so did the rate at which stars were expelled from the center due to the gravitational interactions between the merging elements, a physical process known as core-scouring. And, because there is no gas left in the galactic core to form new stars, it will look depleted, dim, and diffuse.
“The newest generation of computer simulations of galaxy mergers gave us predictions that do indeed match the observed properties rather well,” Thomas said. “These simulations include interactions between stars and a black hole binary, but the crucial ingredient is two elliptical galaxies that already have depleted cores. This means that the shape of the light profile and the trajectories of the stars contain valuable archaeological information about the specific circumstances of core formation in this galaxy—as well as other very massive galaxies.”
The study’s key insight lies in the newly established relationship between black hole mass and a galaxy’s surface brightness. In the future, astronomers could use this insight to estimate a black hole’s mass in more distant galaxies where our instruments are unable to measure stellar motions in the vicinity of a galactic core.
In the move Interstellar, former NASA pilot Joseph Cooper (Matthew McConaughey) is sent hurtling through time and space via a wormhole in order to save humanity. Essentially, humans have screwed up this planet to the extent that in order to survive, we all have to move to a less-screwed-up planet. The quickest way to this new future home is through a wormhole near Saturn. This opening is the entrance to a distant galaxy located near a black hole named Gargantua. And, of course, if anyone can do it, it’s the bad-ass McConaughey.
Wormholes have always been the fascinating stuff of sci-fi, because come on, intertwining dimensions by bending space and time is pretty cool. Unfortunately, they’ve never been proven to actually exist. Luckily, however, that hasn’t put off scientists from trying. Now a new study out of the University of Buffalo (UB) has attempted to take a look at what they would look like if they were real.
Obviously, the first place you would look for a wormhole would be a black hole or a binary black hole system, which involves two black holes circling one another. Theoretically, the insane amount of gravity would pull them together and create a tunnel.
For their study, the researchers focused on Sagittarius A*, an object that’s thought to be a supermassive black hole at the heart of the Milky Way galaxy. While there’s no evidence of a wormhole there, it’s a good place to look for one because wormholes are expected to require extreme gravitational conditions, such as those present at supermassive black holes.
Black holes are massive pits of gravity that will bend space-time due to their incredibly dense centers, or singularities. When a massive star dies, it collapses inward, and as it does so, the star explodes into a supernova — a catastrophic expulsion of its outer material. This dying star will continue to collapse until it becomes either a neutron star or a singularity — something consisting of zero volume and infinite density. This seemingly impossible contradiction is what causes a black hole to form.
The UB scientists believed that if a wormhole does exist at Sagittarius A, nearby stars would be influenced by the gravity of stars at the other end of the passage. As a result, it would be possible to detect the presence of a wormhole by searching for small deviations in the expected orbit of stars near Sagittarius A.
“If you have two stars, one on each side of the wormhole, the star on our side should feel the gravitational influence of the star that’s on the other side. The gravitational flux will go through the wormhole,” says Dejan Stojkovic, PhD, cosmologist and professor of physics in UB’s College of Arts and Sciences. “So if you map the expected orbit of a star around Sagittarius A*, you should see deviations from that orbit if there is a wormhole there with a star on the other side.”
The research, which was published in Physical Review D, focuses on how scientists could hunt for a wormhole by looking for perturbations in the path of S2, a star that astronomers have observed orbiting Sagittarius A*.
While current surveying techniques are not yet precise enough to reveal the presence of a wormhole, Stojkovic says that collecting data on S2 over a longer period of time or developing techniques to track its movement more precisely would make such a determination possible. These advancements aren’t too far off, he says, and could happen within one or two decades.
The good doctor cautions, however, that although this new method might be used to detect a wormhole if one is there, it will not strictly prove that a wormhole is present.
“When we reach the precision needed in our observations, we may be able to say that a wormhole is the most likely explanation if we detect perturbations in the orbit of S2,” he says. “But we cannot say that, ‘Yes, this is definitely a wormhole.’ There could be some other explanation, something else on our side perturbing the motion of this star.”
Wormholes were originally conceived in 1916 by Ludwig Flamm. The Austrian physicist was reviewing another physicist’s solution to the equations in Albert Einstein’s theory of general relativity when he believed another solution might, in fact, be possible. His “white hole” was a theoretical time reversal of a black hole. Entrances to both black and white holes could be connected by a space-time conduit.
Though, if we ever do find one, it probably won’t be the one that science fiction has shown us.
“Even if a wormhole is traversable, people and spaceships most likely aren’t going to be passing through,” says Stojkovic . “Realistically, you would need a source of negative energy to keep the wormhole open, and we don’t know how to do that. To create a huge wormhole that’s stable, you need some magic.”
Black holes are the most massive objects in the universe. They’re so massive that light itself can’t escape them — hence the name. All (or almost all) galaxies have a supermassive black hole at their center, millions or billions of times more massive than the sun. But many other black holes are not as huge.
Intermediate black holes (IBH) are hundreds or thousands of time more massive than the sun, and they come in a bizarre place. They’re a sort of “missing link” between the types of black holes we can readily detect: small, stellar black holes (2-100 times more massive than the sun) and the gargantuan supermassive black holes.
Since the first observations of black holes in the previous century, a few tens of stellar-mass black holes have been detected via X-rays and more recently, gravitational waves. However, black holes are only a very tiny fraction of the black holes that exist in our
Galaxy. All the stellar black holes astronomers have discovered were part of a binary system with a star, which makes them much easier to detect. However, it is believed that there exists a large population of isolated black holes without a companion star.
The key element to this, the researchers argue, is the black holes’ “greed”. With their strong gravitational field, they sip matter from the interstellar medium around them (dust and stuff floating between the stars). However, they’re not very efficient at it: instead of absorbing everything, much of this matter is ejected at great speed. This ejecta, the paper argues, produces radio waves that can be detected — if we can filter it from the surrounding noise.
“A naive way to observe IBHs is through their X-ray emission via accretion of gas in the interstellar medium,” the study reads
“These outflows can possibly make the IBHs detectable in other wavelengths. The outflows can interact with the surrounding matter and create strong collisionless shocks at the interface. These shocks can amplify magnetic fields and accelerate electrons, and these electrons emit synchrotron radiation in the radio wavelength,” Daichi Tsuna of the University of Tokyo and Norita Kawanaka of Kyoto University continue.
While the proposed method is intriguing, it’s not an easy task.
There are currently only a handful of objects that astronomers suspect as isolated black holes — but it’s unclear if this is actually the case. Even if it is, recent research has suggested that there are millions of these objects scattered throughout the galaxy. It’s also not clear if our telescopes have the necessary sensitivity for this task. This is where upcoming arrays, such as the Square Kilometer Array (SKA), whose expected sensitivity surpasses existing radio surveys by orders of magnitude, can come in handy.
However, at the very least, it’s an interesting hypothesis which is probably worth pursuing.