Tag Archives: Missing Baryon Problem

2020: A Year in Space

It’s difficult to mention the year 2020 without referencing COVID-19, but as more human beings than ever before were wishing they could take a break from the surface of the planet, space research continued to push our knowledge of the stars. Whilst much of the scientific community was consumed with combating a pandemic, physicists, astronomers, cosmologists, and other researchers were further pushing our understanding of space and the objects which dwell there.

These are some of my personal favourite space-related breakthroughs and research that have come about this year. The list is by no means exhaustive. 

Black Holes go silent

In terms of black hole science, 2019 was always going to be a difficult year to top being the year that brought us the first direct image of a supermassive black hole (SMBH). That doesn’t mean that 2020 has been a slow year for black hole developments, however.

One of the most striking and memorable examples of black hole research announced this year was the discovery of a ‘silent’ black hole in our cosmic ‘back yard.’ An international team led by researchers from European Southern Observatory (ESO) including found the black hole in the system HR 6819, located within the Milky Way and just 1,000 light-years from the Earth.

A silent and thus invisible black hole discovered lurking in our ‘solar backyard’ could be an indicator of a much larger population. (ESO/L. Calçada) Background: This wide-field view shows the region of the sky, in the constellation of Telescopium, where HR 6819 can be found. (ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin)

The observation marks the closest to Earth a black hole has ever been discovered and Dietrich Baade, Emeritus Astronomer at ESO in Garching believes that it is just ‘the tip of the Iceberg’. 

“It’s remarkable because not only is it the first of its kind found, but it’s also so nearby,” said Baade. “Discovering a first only an astronomical stone’s throw away is the biggest surprise one can probably imagine.”

The black hole was described as ‘silent’ by the team because it is not current accreting material — the destructive process that creates powerful x-ray emissions and makes these light-trapping objects observable. 

Close-up screen capture image of the LB-1 which, like HR 6819, could also host a silent black hole (Hubble/Public Domain)

“If there is one, there ‘must’ be more,” Baade remarked in May. “If the Earth is not in a privileged position in the Universe — and all available evidence suggests without a doubt it is not — this means that there must be many more silent black holes.”

Baade also remarked that as current cosmological models suggest that the number of stellar-mass black holes is between 100,000,000 to 1,000,000,000 and we have observed nowhere near this many objects, more quiet black holes are “badly needed” to confirm current models. “HR 6819 is the tip of an iceberg, we do not yet know how big the iceberg is.”

Silent black holes weren’t the only examples of this hind of science making noise in 2020, however. Long-missing Intermediate Mass Black Holes were discovered. And just like a proverbial bus, you wait decades for one and then two turn up at once.

Intermediate mass black holes found and found again

Missing black holes were the subject of another piece of exciting space science in September 2020, when researchers from the VIRGO/LIGO collaboration discovered the tell-tale signal of an intermediate-mass black hole (IMBH) in gravitational-wave signals. To add to the excitement, the signals originated from the largest black hole merger ever observed.

An artistic interpretation of the binary black hole merger responsible for GW190521. The space-time, figured by a fabric on which a view of the cosmos is printed, is distorted by the GW190521 signal. The turquoise and orange mini-grids represent the dragging effects due to the individually rotating black holes. The estimated spin axes, or self-rotations, of the black holes, are indicated with the corresponding coloured arrows. The background suggests a star cluster, one of the possible environments where GW190521 could have occurred. Credits: Raúl Rubio / Virgo Valencia Group / The Virgo Collaboration.)

The merger — identified as gravitational wave event GW190521 —was detected in gravitational waves and is the first example of 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,” Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS) said when announcing the team’s observation. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”

The black hole birthed in the detected merger appears to have a mass of between 100–1000 times that of the Sun — most likely 142 solar masses — putting it in the mass range of an IMBH — a ‘missing link’ between stellar-mass black holes and much larger SMBHs. 

Earlier in 2020, another team had used the Hubble Space Telescope X-ray data collected in 2018 to identify what they believed to be an IMBH with a mass 50,000 times that of the Sun named 3XMM J215022.4−055108 (or J2150−0551 for short). 

This Hubble Space Telescope image identified the location of an intermediate-mass black hole (IMBH), weighing over 50 000 times the mass of our Sun (NASA, ESA, and D. Lin (University of New Hampshire))

Whether GW190521 or J2150−0551 will go down in history as the first discovered IMBH is currently a little muddy, but what is less questionable is that 2020 will go down as the year in which these ‘missing link’ black holes were first discovered, bringing with them exciting implications for the future investigation of black holes of all sizes. 

“Studying the origin and evolution of the intermediate-mass black holes will finally give an answer as to how the supermassive black holes that we find in the centres of massive galaxies came to exist,” said Natalie Webb of the Université de Toulouse in France, part of the team that found J2150−0551. And IMBHs weren’t the only missing element of the Universe that turned up in 2020.

Discovering the Universe’s missing mass

In May astronomers, including Professor J. Xavier Prochaska of UC Santa Cruz, announced that they had found the missing half of missing baryonic matter demanded by cosmological models. 

“The matter in this study is ‘ordinary’ matter — the material that makes up our bodies, the Earth, and the entirety of the periodic table. We refer to this matter as ‘baryonic’–matter made up of baryons like electron and protons,” Prochaska said when he spoke exclusively to ZME Science earlier this year. “Of particular interest to astronomers is to ascertain the fraction of the material that is tightly bound to galaxies versus the fraction that is out in the open Universe — what we refer to as the intergalactic medium or cosmic web.”

The matter the team discovered isn’t ‘dark matter’ — which accounts for roughly 85–90% of the Universe’s matter content — but rather ‘ordinary’ matter that has been predicted to exist by our models of universal evolution but has remained hidden.

The team made the discovery using mysterious Fast Radio Bursts (FRBs) and the measurement of the redshift of the galaxy from which they originate as a detection method. FRBs can be used as a probe for baryonic matter because as they travel across the Universe, every atom they encounter slows them down by a tiny amount.

This means that they carry with them a trace of these encounters along with them in the spectral splitting as seen above. This allowed the team to infer the presence of clouds of ionised gas that are invisible to ‘ordinary’ astronomy because of how diffuse they are. 

Asteroid Samples Returned by Hayabusa2

Japan’s Hayabusa2 probe and its continued investigation of the asteroid Ryugu has been the gift that has just kept giving in 2020. Just this month the probe returned to Earth samples collected from an asteroid — which has an orbit that brings it between Earth and Mars — for the first time.

Though probes have landed on asteroids and collected samples before, these samples have been examined in situ. Thus this is the first time researchers have been able to get ‘up close and personal’ with matter from an asteroid.

Artist’s impression of the Hayabusa2 probe achieving touchdown on Ryugu. Image credit: JAXA

Hayabusa2 arrived at Ryugu in late June 2018, making its touch-down on the surface of the asteroid in February of the following year after months careful manoeuvring conducted by the Japan Aerospace Exploration Agency (JAXA) and the selection of an optimal region from which to collect samples. 

Ahead of the return of samples on December 5th, the probe sent back some stunning images of the asteroid’s surface. These images were more than purely aesthetic, however. Examination of dust grains on the surface of Ryugu gave the team, including Tomokatsu Morota, Nagoya University, Japan, indications of a period of rapid heating by the Sun. 

The surface of near-Earth carbonaceous asteroid 162173 Ryugu, as observed by the Hayabusa2 spacecraft just before its landing. This image was produced from images obtained by ONC-W1 at the bottom and ONC-W2 on the side of the spacecraft. The spacecraft’s solar ray paddle casts a shadow on Ryugu’s surface. Image credit: JAXA/U. Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Inst. Tech./Meiji U./U. Aizu/AIST

“Our results suggest that Ryugu underwent an orbital excursion near the
Sun,” said Morota in May. “This constrains the orbital transition processes of asteroids from the main belt to near-Earth orbit.”

Impressive though this achievement is, its the collection of samples from the asteroid and their subsequent safe return to earth that is the ‘main course’ of the Hayabusa2 mission. “The most important objective of the touchdown is sample collection from Ryugu’s surface,” Morota explained. 

Animation created from CAM-H and ONC-W1 data obtained during the 1st touchdown operation (Feb. 21, 2019). Image credit: JAXA/U. Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Inst. Tech./Meiji U./U. Aizu/AIST

It is hoped that access to these samples will help answer lingering questions about asteroid composition as well as assisting researchers to confirm Ryugu’s suspected age of 100 million years old — which actually makes it quite young in terms of other asteroids. 

Asteroids like Ryugu can act as a ‘snapshot’ of the system’s in which they form at the time of that formation. This is because whereas planets undergo a lot of interaction with other bodies, asteroids remain pretty much untouched. 

Whilst researchers will no doubt be elated by the return of the Ryugu samples and the continuing success of the Hayabusa2 mission, 2020 wasn’t all good news for fans of asteroid research. 

Goodbye to Arecibo

The iconic radio telescope at the Arecibo Observatory in Puerto Rico collapsed at the beginning of December, ahead of its planned demolition. The telescope which will be familiar to moviegoers as the setting of the climactic battle in Pierce Brosnan’s first outing as James Bond, 1995’s Goldeneye, had been in operation up until November, playing a role in the detection of near-Earth asteroids and monitoring if they present a threat to the planet.

An image of the radio telescope before its December 1st collapse (NSF)

The collapse of the radio telescope’s 900-tonne platform which was suspended above the telescope’s 305-metre-wide dish, on December 1st, followed the snapping of one of its main cables in November.

The US National Science Foundation (NSF), which operates the observatory had announced that same month that the telescope would be permanently closed citing ‘safety concerns’ after warnings from engineers that it could collapse at any point.

Following the collapse, the NSF release heart-wrenching footage of the radio telescope collapsing recorded by drones. The footage shows cables snapping at the top of one of the three towers from which the instrument platform was suspended. The platform then plummets downward impacting the side of the dish. 

Video shows the radio telescope’s instrument platform fall and collision into the side of the dish (NSF)

The observatory had played a role in several major space-science breakthroughs since its construction in 1963. Most notably, observations made by the instrument formed the basis of Russell A. Hulse and Joseph H. Talyor’s discovery of a new type of pulsar in 1974. The breakthrough would earn the duo the 1993 Nobel Prize in Physics. 

Some good could ultimately come out of the collapse of Arecibo. Questions had been asked about the maintenance of the radio telescope for some time and the fact that the cable which snapped in November dated back to the instrument’s construction 57 years ago has not escaped notice and comment.

As a result, various space agencies are being encouraged to make efforts to better maintain large-scale equipment and facilities so that losses like this can be avoided in the future.

This aerial view shows the damage at the Arecibo Observatory after one of the main cables holding the receiver broke in Arecibo, Puerto Rico, on December 1, 2020. – The radio telescope in Puerto Rico, which once starred in a James Bond film, collapsed Tuesday when its 900-ton receiver platform fell 450 feet (140 meters) and smashed onto the radio dish below. (Photo by Ricardo ARDUENGO / AFP) (Photo by RICARDO ARDUENGO/AFP via Getty Images)

For most of us, 2020 is going to be a year that we would rather forget. Whilst very few of us come honestly comment that we have had anything approaching a ‘good year’ space science has plowed ahead, albeit mildly hindered by the global pandemic.

Our knowledge and understanding of space science are better off at the end of 2020 than it was twelve months earlier, and that is at least something positive that has emerged from this painful year.

CSIRO’s ASKAP telescope continues to detect new FRBs, adding to the catalogue of these mysterious objects. Credit: ICRAR and CSIRO/Alex Cherney

The Universe’s Missing Matter Problem is Solved

Our theories of the Universe have a missing matter problem: half of its matter is missing. But now this ‘missing baryon problem’ one of the most lingering puzzles in cosmology has been solved.

Analysis of the Cosmic Microwave Background (CMB), the radiation left over from an event that occurred shortly after the ‘Big Bang’ which evenly fills the entire Universe, tells physicists that ‘normal’ or baryonic matter  —  the stuff that forms planets, stars, and our bodies  — should account for roughly 5% of the total matter and energy in the Universe.

The problem has been that until now, roughly half of this baryonic matter (essentially the building blocks of everything we see around us )  has been missing.

Australian astronomers have used fast radio bursts, mysterious blasts of radio-wave radiation that occur in random directions through deep space, whose origins are as of yet unknown, to trace this missing matter for the first time. In the process, the discovery helps confirm that both theories of nucleosynthesis in the early Universe, and our concept of how the cosmos developed immediately following the Big Bang are correct. 

The team’s findings are published in the journal Nature. 

CSIRO’s ASKAP telescope continues to detect new FRBs, adding to the catalogue of these mysterious objects. Credit: ICRAR and CSIRO/Alex Cherney
CSIRO’s ASKAP telescope continues to detect new FRBs, adding to the catalogue of these mysterious objects. Credit: ICRAR and CSIRO/Alex Cherney

“The matter in this study is ‘ordinary’ matter — the material that makes up our bodies, the Earth, and the entirety of the periodic table. We refer to this matter as ‘baryonic’–matter made up of baryons like electron and protons,” says the paper’s co-author Professor J. Xavier Prochaska, UC Santa Cruz, says. The researcher adds that it’s worth noting that this matter isn’t ‘dark matter’ which accounts for roughly 85–90% of the Universe’s matter content.

The two-decade-long hunt for the missing baryonic matter

Thus far astronomers have spent at least two decades searching for this matter using a very precise estimate of the total mass of baryons in our Universe derived from an analysis of data from the early Universe. 

“The hunt for missing matter engaged astronomers across the globe over the past 20 years,” Prochaska explains. “An unofficial accounting estimates 1000+ orbits with the Hubble Space Telescope, thousands of hours with the Chandra X-ray Observatory, and 100+ nights on the largest optical telescopes; these total over $50M USD in operations alone.” 

Yet, despite this monumental effort, a successful census of all the matter in the Universe has not been carried out — until now. Current best measurements account for only around half the baryonic matter, leaving the other half effectively missing.

“When we looked out into the present Universe, we couldn’t find half of the matter that should be there,” says lead author, Associate Professor Jean-Pierre Macquart, Curtin University, International Centre for Radio Astronomy Research (ICRAR). “It was a bit of an embarrassment.”

The issue with finding this missing matter is primarily because it is extremely diffuse and space is extremely sparse. Macquart compares that searching for this matter to searching for just a few atoms in a room the size of the average office. The problem with this matter existing in such a diffuse and tenous gas is that it doesn’t emit light itself, and when background light passes through it, none is absorbed. 

Thus this missing matter doesn’t leave any ‘fingerprint’ and it is, for all practical purposes, invisible. This means that traditional telescopes and the techniques associated with them simply aren’t effective enough to spot such diffuse matter. 

The missing matter that the team of astronomers has located fits within the yellow slice of a pie chart showing the Universe's total energy andmatter content. It isn't dark matter, or dark energy, both of which remain 'missing.'

“The missing matter has a density and temperature that make it effectively invisible to any other technique used to observe it,” explains Prochaska. “Indeed, we have not imaged it either.”

“In a way, you can think of this as ‘grey matter’ in that, it isn’t dark matter,” Macquart explains. “It’s ordinary matter that we could see if it weren’t so diffuse.”

The team of astronomers, therefore, had to find another way to detect that matter. That means finding some other impact or effect that this matter could have, that could be spotted. This is where the phenomenon of Fast Radio Bursts (FRBs) comes in to play.  

Fast Radio Bursts as a detection method

The first Fast Radio Burst was detected in 2007 and since then these, seemingly random blasts of radiation lasting just a few milliseconds, have been recorded many times. This includes the observation in April this year, of the first FRB within our own galaxy, and the first regularly repeating FRB —named FRB 181112 —  in 2018.

Yet, the actual origins of these emissions and their causes are still unknown. 

The FRB leaves its host galaxy as a bright burst of radio waves. (ICRAR)
The FRB leaves its host galaxy as a bright burst of radio waves. (ICRAR)

Despite the mystery around their emission, researchers have still found a way to both localize their source to a host galaxy and to utilize FRBs to measure other phenomena in the Universe. Last year, Prochaska himself was part of research to investigate diffuse gas haloes using FRBs as a probe. The researcher says that from their very discovery by Duncan Lorimer and his student David Narkevic thirteen years ago, he believed that FRBs could be employed in the search for missing baryonic matter. 

“In 2007, I recall discussing Duncan Lorimer’s paper on the first fast radio burst — sometimes referred to as the ‘Lorimer Burst’ — the morning it published,” Prochaska says. “It hit me right then that this would be the best way to find the missing matter.”

To make the detection the team utilized two distinct methods, the dispersion measure and the redshift of the FRB’s origin galaxy. The dispersion measure allows researchers to count the number of encountered electrons as these slow the propagation of the FRB  — with different frequencies that make up the burst be affected to varying degrees.

Whilst, analysis of the spectrum yields the redshift which is effectively a measure of the distance to the FRB’s origin galaxy. This latter part is important because the researchers need to know just how much ‘space’ the FRB has traveled through. 

“Thus, combining redshift and dispersion measure, we can assess the total mass in baryons in the universe,” Prochaska explains. 

It’s all about timing…

FRBs can be used as a probe for baryonic matter because as they travel across the Universe, every atom they encounter slows them down by a tiny amount. This means that they carry with them a trace of every atom they encounter along their line of travel. Even those astronomers can’t see.

The tenous gas cloud that the FRB passes through consists of atoms that have been ionised, this means that the protons and electrons have been separated and float freely in the gas. 

The density of the missing matter is calculated using the distance of the FRB from Earth and the delay between the wavelengths of the FRB, (Credit: ICRAR)
The density of the missing matter is calculated using the distance of the FRB from
Earth and the delay between the wavelengths of the FRB, (Credit: ICRAR)

When radio waves pass through this ionised gas, the different frequencies that make up the FRB propagate at slightly different speeds. This means that the different frequencies which start off perfectly aligned become slightly spread out bt the time the FRB reaches the other side of the gas cloud. 

Macquart describes this phenomenon as exactly what we see when light passes through a prism.

This dispersion in timing is tiny but builds up over the vast distances travelled between galaxies, and the amount of dispersion is exactly proportional to the amount of ionised matter the radio pulse has passed by. 

When travelling through completely empty space, all wavelengths of
the FRB travel at the same speed, but when travelling through the missing matter, some wavelengths are slowed down. (Credit: ICRAR)
When travelling through completely empty space, all wavelengths of
the FRB travel at the same speed, but when travelling through the missing matter, some wavelengths are slowed down. (Credit: ICRAR)

Thus, when the team measure these FRBs they can calculate how smeared out in time the frequencies are, revealing just how much-ionised matter they had passed through on their journey to us. Once the distance of the origin galaxy from which the bursts originated is known, the astronomers could also then measure the average density of the ionised matter, and check this against theoretical predictions.

Leading the way with ASKAP

This combination of methods also required a combination of telescopes and techniques. To conduct their study the team of astronomers turned to CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) — a radio telescope consisting of 36 12-inch antennas located roughly 500 miles north of Perth in Western Australia — and some of the world’s most powerful optical telescopes. Prochaska is clear here though: “ASKAP led the way.”

The team used ASKAP to measure the positions of the FRB and thus determine the galaxy from which they originated, unsurprising, as the instrument has become the foremost piece of technology in terms of localising FRBs. 

CSIRO’s ASKAP
measures the delay between the wavelengths of the FRB, allowing
astronomers to calculate the density of the missing matter (Credit: ICRAR and CSIRO/Alex Cherney)
CSIRO’s ASKAP measures the delay between the wavelengths of the FRB, allowing
astronomers to calculate the density of the missing matter (Credit: ICRAR and CSIRO/Alex Cherney)

ASKAP found the smeared-out bursts of radio emission, which gave the researchers two key pieces of information — how much ionised matter was between the source of the burst and us on Earth, and where to go looking for the galaxy that the burst had originated in. This is why ASKAP was key to the team, whilst other radio telescopes are able to localise FRBS, almost none of them can pinpoint the location of the burst precisely enough to determine which galaxy they came from.

“To ID an FRB we had to scour through weeks of data to find a single interesting measurement,” Dr Keith Bannister, an Astronomer at CSIRO explains. “Like a needle in a haystack the size of 100 football stadiums.” Fortunately, ASKAP has what Bannister calls a ‘live-action replay’ saving the last 3 billion measurements–just 3 seconds worth of data–from each antenna.

The ASKAP could only solve 2/3 of the missing baryon puzzle, however. Other telescopes were used to measure the redshift of these host galaxies, most incredibly distant and faint, with this measurement giving the team an estimate of the distance and the speed at which the expansion of the Universe is carrying it away. Once the FRB travel distance was known the team could calculate the density of all the atoms along the line of sight.

We know the missing matter is there, now we need to know where ‘there’ is

The idea of ‘finding’ this missing matter may be a touch misleading. The team are quick to point out, that even though they now are able to show that this matter is there, they are yet to determine exactly where it is and how it is distributed.

The next question they will aim to answer is whether this gas is evenly spread smoothly between galaxies, or if it is gathered in haloes and filaments?

Core antennas of CSIRO’s ASKAP radio telescope in Western Australia pointing at the Milky Way. (Credit: CSIRO/ Alex Cherney)
Core antennas of CSIRO’s ASKAP radio telescope in Western Australia pointing at the Milky Way. (Credit: CSIRO/ Alex Cherney)

“Of particular interest to astronomers is to ascertain the fraction of the material that is tightly bound to galaxies versus the fraction that is out in the open Universe — what we refer to as the intergalactic medium or cosmic web,” Prochaska says. But, for him in particular, this finding marks the end of a twenty-year journey. 

“I began this pursuit in 2000 as a postdoc at Carnegie Observatories,” Prochaska concludes. “Ten years after the discovery of Fast Radio Bursts as new radio experiments were tooling up to join the search, I formed a team called ‘Fast and Fortunate for FRB Follow-up’ ) to perform the follow-up with optical telescopes. 

“I never dreamed it would go this smoothly…”

References: Macquart. JP, Prochaska. J. X, McQuinn. M, et al, ‘A census of baryons in the Universe from localized fast radio bursts,’ Nature, [2020].