“We found it!” – Fast Radio Bursts shed light on missing matter

We've used FRBs to weigh all the matter in the Universe, and we found the half that was missing!" said Associate Professor Jean-Pierre Macquart from the Curtin University node of the International Centre for Radio Astronomy Research.

Our Universe is a rather dark place. Unlike our experience here on Earth, where we have a brilliant source of light only 8-light minutes away, space is exceedingly dark.

Sure, there are likely trillions upon trillions of stars out there which give off light, sprinkled across the night, and collecting in beautiful island cities known as galaxies. Yes, there are enormous nebulae that stretch for hundreds of light-years—who emit and reflect light from nearby stellar companions and light up the sky resembling familiar objects. And yes, there’s even our Moon, which has for millennia captivated many beings, reflecting solar photons and illuminating corners of the Earth where the shadows fell.

But for all these observations that we witness and document, there is one common denominator in all of our findings: the Universe is dark. So dark, that for years it has been very difficult to try and figure out how much ‘stuff’ it is made of.

Historically, the amount of matter in the Universe has intrigued astronomers and physicists—giving rise to fundamental philosophical questions like “where did we come from?” and “what are we made of?”—so an understanding of where and what the ‘stuff’ is, has always been important for our scientific objectives.

The search for the amount of matter that can be seen out there in space has driven scientists to consider new ideas and abstract concepts, even theorising about observed phenomena that accounts for the unseen, for example why spiral galaxies rotate at the speeds that they do (dark matter), or how some of the first elements were formed (early nucleosynthesis).

This search has continuously driven us to utilise new tools that help us measure the Universe—astronomical laboratories where scientists devise methods to test and model theory against obtained and peer-reviewed data. Tools such as observing a supernova’s light curve increase and decrease in brightness over time as a method of calculating distance. Or, that of the bending and lensing of a bright light source around a massive object to determine the mass of the object as it transits across our line of sight.

Now, a new tool—one yet to be fully understood—has been used to weigh the Universe.

Astronomers have used a series of Fast Radio Bursts (FRBs) to probe the intergalactic medium (IGM) and measure the amount of matter that resides between the galaxies – a decades-old mystery that now can define the ‘missing baryonic matter’ problem in astrophysics.

This is not to be confused with Dark Matter – which to date remains unobserved in the electromagnetic spectrum, presenting itself only through its gravitational influence on objects. The missing matter problem relates to normal matter – the stuff that we see all around us in space – like galaxies, stars, nebulae, planets, dust, and gas.

The research, which today appears in the journal Nature, outlines the detection of several FRBs originating in different galaxies through observations made using an array of radio telescopes followed up with optical telescopes. This has enabled astronomers to calculate the density of the material that lies between us and the origin galaxies, resolving the mystery in astronomy that has been ongoing for 20 years.

The results are made more intriguing in that FRBs themselves are still enigmatic to astronomers – they appear as random bursts of energy, originating from all directions in the sky and lasting for only milliseconds. It is unknown what can cause such a huge outpouring of energy in such a small amount of time, the equivalent to the amount released by the Sun in 80 years – and still remain undetectable in other electromagnetic frequencies (that is, there is no optical counterpart that has yet been observed).

To date, all FRBs (excluding one that has only been detected last month) originate in external galaxies – outside our own Milky Way. Whilst this adds further intrigue to what the cause of these progenitor events could be, scientists have determined that they can use these powerful beams of energy as probes to study the IGM – the stuff between our Galaxy and others.

Excitingly, this new study is the first of its kind to apply FRBs as cosmic probes to learn about the Universe with the science they produce. It’s expected as FRBs are further understood more science could be derived to better analyse other remaining problems in understanding the IGM, such as how the matter is distributed between galaxies or what temperature range it occupies.

The findings were made by a global collaboration of scientists, led by Associate Professor Jean-Pierre Macquart from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), and also include other Australian institutions, such as CSIRO, Swinburne University of Technology, and Macquarie University.

The Australian teams collaborated with researchers from the University of California Santa Cruz, The Kavali Institute for Physics and Mathematics in Japan, the University of Washington in Seattle, and the Instituto de Física in Chile.

Observations of the FRBs were made using CSIRO’s ASKAP radio telescope, located in the Murchison Radio-astronomy Observatory in central Western Australia.

“We know from measurements of the Big Bang how much matter there was in the beginning of the Universe,” said Associate Professor Macquart.

“But when we looked out into the present Universe, we couldn’t find half of what should be there. The matter in intergalactic space is very sparse,” he said.  “The missing matter was equivalent to only one or two atoms in a room the size of an average office, so it was very hard to detect this matter using traditional techniques and telescopes.”

Whilst this density is extremely diffuse (for comparison the air we are breathing has approximately 1028 electrons per cubic meter and weighs about 1.2 kg/m3 at sea level) – this material is what makes up the majority of the baryonic matter in the Universe. As such, understanding its properties (like density, distribution, temperature) is vital to further refine cosmological and astrophysical models.

Associate Professor Macquart said the team detected the missing matter by using fast radio bursts as “cosmic weigh stations”.

“The radiation from fast radio bursts gets spread out by the missing matter in the same way that you see the colours of sunlight being separated in a prism,” he said.

“We’ve now been able to measure the distances to enough fast radio bursts to determine the density of the Universe,” he said. “We only needed six to find this missing matter.”

Complementing the detection of FRBs using ASKAP, were optical observations using large, ground-based optical telescopes such as the Keck Telescopes to obtain the spectrum of the host galaxies from which the FRBs originated. In doing so, an analysis of this spectrum allowed the team to determine the redshift, and thus the distance to each host galaxy.

Co-author Professor J. Xavier Prochaska, from UC Santa Cruz, said astronomers have unsuccessfully searched for this missing matter with our largest telescopes for more than 20 years. 

“The discovery of fast radio bursts and their localisation to distant galaxies were the key breakthroughs needed to solve this mystery," he said. 

“When the burst arrives at the telescope, it records a live-action replay within a fraction of a second,” said Dr Keith Bannister from Australia’s national science agency, CSIRO, who designed the pulse capture system used in this research.

“This enables the precision to determine the location of the fast radio burst to the width of a human hair held 200m away,” he said.

For years, astrophysicists and researchers have looked up into the night sky and studied the rotational velocities of galaxies and found that the majority of matter is actually something called Dark Matter – a non-interacting, invisible substance that exerts a gravitational force, though we are yet to still fully understand what it could be. They’ve also observed a number of supernovae at a variety of distances – and established a speed of expansion of the Universe – and much to everyone’s surprise, this expansion is accelerating and being propelled by another invisible force, known as Dark Energy. We also have no idea what causes this.

If we leave aside the concepts of Dark Matter and Dark Energy, what can we say of the ordinary matter in the Universe? The non-exotic stuff that makes up what we can see and observe? The technical name is baryonic matter and though it seems like we’re surrounded by it (stars, galaxies, planets, moons, nebulae, humans, oceans, trees – the stuff made of atoms) it only makes up a very small portion of the all of the total baryonic matter in the Universe.

The estimated density values of baryonic matter comes from two theoretical postulates, derived from independent cosmological methods. The first is the theoretical limit of protons and neutrons that came together to form heavier elementary species like helium-4 and deuterium during a very early period of the Universe’s history, known as Big Bang Nucleosynthesis.

The prediction from the Big Bang theory states that in the very early stages of the Universe’s history, temperatures cooled enough for certain species of elements (the lightest ones) to form. The abundance of these values can be predicted based on known nuclear reactions, and this value is found to be approximately 5%.

To observe the abundance of these primordial elements as close to their original composition (the Universe has since chemically evolved with the generation of heavier elements being mixed as it has aged), astrophysicists have devised a few methods to observe this value.

This includes searching for primordial helium-4 amongst dwarf galaxies, estimating the values of deuterium amongst high-redshift quasars, attempting to determine the abundance of helium-3 within HII regions found across the Milky Way, and finally looking for the composite value of lithium-7 in some of the oldest known stars.

When applying all of the above observations, the values more or less match what the theory of Big Bang Nucleosynthesis states should occur.

A relic from the Big Bang itself, the Cosmic Microwave Background (CMB) also provides a secondary prediction about the baryonic matter content for the Universe. In particular, the interaction of matter with photons should leave an impression on the CMB – and in 2015, observations made by the orbiting Plank observatory (and published by the Plank Collaboration) found this value to be in agreement, measuring in at 4.9%.

Observations, as indicated above, in addition to the inference of spectra from diffuse gas and dust indicate that of this 5%, there was still a portion of this value (50% of the 5%) that was considered ‘missing’.

Taking all this into account, the composition of baryonic matter across the Universe can be summarised as follows (values are not absolutes, and contain uncertainties):

• Star and galaxies – 7%
• Cold gas in galaxies – 1%
• Hot gas in galaxies – 5%
• Hot gas in galactic clusters – 4%
• Cool, intergalactic gas – 28% (high uncertainty)
• Warm, intergalactic gas – 15%
• Hot, intergalactic gas – 40%

Since the first FRB was discovered in 2007, amongst archival data collected in 2001 by CSIRO’s Parkes radio telescope, scientists have been working towards how these extragalactic signals can advance science.

To date, we are still unaware of what is causing these short, powerful events – though a lot of researchers are now progressing towards the origins occurring in highly magnetised stellar remnants – magnetars, a type of pulsar with a powerful magnetic field.

A major piece of science that FRBs will shed light on would be determining with confidence what is the progenitor's cause and objects. We know it is energetically powerful, so an explosion of some sort would make sense – but no explosion to date has been detected.

Complicating matters even further, several FRBs are thought to repeat. One has even been found to repeat on a regular basis, every 16 days or so. So where does this leave the explosion model? Do objects continually explode on repeat?

Radio astronomers have categorised FRBs into two sub-groups: repeaters and non-repeaters, and then subdivided these into three potential populations: young stellar population, general stellar population or non-stellar models. When considering this categorisation most of the models presented are related to supernovae and remnants of massive stars – neutron stars, pulsars, magnetars, white dwarfs, and black holes. In some cases, merger concepts are used to explain these outcomes.

By using FRBs, a more complete scientific stellar evolution model can be established for these remnants, post entering their compact object phase after a supernova. Or maybe, these models are all wrong and it’s something more exotic like superconducting cosmic strings or a neutron star colliding with an active galactic nucleus.

Towards the end of April 2020, astrophysicists – using a variety of telescopes and observatories around the world – reported a potential FRB from within the Milky Way. An analysis is currently underway about this curious object, which will once again change our understanding of what FRBs are, or can do.

Whilst it is important to understand what is causing FRBs, it is equally important to understand how we can use these FRBs as probes to better quantify the Universe around us – very much as this new study has, by measuring the density of the IGM.

Indeed, this study has now provided an independent analysis and quantification of the missing baryonic matter value. It also raises more questions that further FRB probes can help answer with regards to the IGM – in particular, how is the IGM density distributed with regards to galaxies, dark matter, and even galactic clusters? Is the temperature uniform across this distribution? And what do both these results tell us about cosmological evolution over deep time?

Like supernovae off in distant galaxies, which help scientists measure distance and the rate of expansion across the Universe, could FRBs also be used to analyse some of the Universe’s fundamental aspects and cosmology?

Associate Professor Macquart thinks so, saying the research team had also pinned down the relationship between how far away a fast radio burst is and how the burst spreads out as it travels through the Universe.

"We've discovered the equivalent of the Hubble-Lemaitre Law for galaxies, only for FRBs," he said.

"The Hubble-Lemaitre Law, which says the more distant a galaxy from us, the faster it is moving away from us, underpins all measurements of galaxies at cosmological distances.”

Scientists are now modelling how FRBs might indicate something similar, or a new way to check this.

To make these observations, Associate Professor Macquart and the collaboration used one of Australia’s most advanced astronomical instruments ever developed – the Australian Square Kilometre Array Pathfinder (ASKAP).

Located in a government-designated radio quiet zone in central Western Australia, the interferometer features 36 dish antennas that work together to gather radio waves from some of the furthest regions of our Universe. 

Each of the 12m ASKAP dishes is fitted with a unique, specialised receiver known as a ‘phased array feed’ – custom technology developed by CSIRO, which allows each antenna to scan the sky using 36 separate and simultaneous beams. When combining this data from all dishes, a massive 30 square degrees of the sky (about 60 times the size of the full Moon) can be surveyed quickly, accurately, and with unprecedented resolution.

Just the type of instrument scientists would need to catch FRBs – which are to date, unpredictable (we don’t know where or when they will appear).

Associate Professor Ryan Shannon, another co-author from Swinburne University of Technology, said the key was the telescope used, CSIRO’s ASKAP.

“ASKAP both has a wide field of view, about 60 times the size of the full Moon, and can image in high resolution,” he said. “This means that we can catch the bursts with relative ease and then pinpoint locations to their host galaxies with incredible precision.”

Using ASKAP, Macquart and his colleagues were able to determine the arrival time of the radio waves as a function of their frequency to determine the dispersion of the signal as it arrived at each antenna. This critical information outlines how much ‘stuff’ the signal has passed through to get from the source and into the ASKAP dish.

Another enormous advantage that ASKAP provided to this study, and indeed above other global radio telescopes around the world who also find FRBs is the ability to localise, or pinpoint, where in space – and in particular which galaxy – the FRB originated in. Once this localisation was accurately determined, the giant optical telescopes stepped in to measure the electromagnetic spectrum of the galaxies to determine its redshift.

By then combining the findings of dispersion from ASKAP with the redshift from the optical telescopes, the team was able to accurately determine how much baryonic matter there was in the Universe.

Dr. Keith Bannister from the CSIRO designed the system where ASKAP would capture the pulse amongst the enormous volume of data that is produced by the telescope’s 36 antennas. 

“The telescope produces about one billion measurements per second for us to look for FRBs. Searching through all this data is like trying to find a needle in a haystack the size of a football stadium - but after about a week, we’ll find an FRB,” he said.

The pulse capture system is designed to ‘catch out’ FRBs within half a second of them occurring. The search machine then downloads a slow-motion, live-action replay of the last 3 seconds of data from the telescope - providing a deeper level of information than originally searched. 

This recorded, 3-second snapshot is then saved and analysed later - and it is through this analysis that the FRB’s position can be worked out, down to a level of accuracy that compares to finding the width of a human hair in the haystack stadium.

The findings were rolled out as part of Commensal Real-time ASKAP Fast Transients (CRAFT) survey, a global collaboration of scientists and institutions that utilise ASKAP’s capabilities to localise events to within a few arcseconds, its large instantaneous field of view, and its longer baselines which can rule out local terrestrial sources of noise vs. real astronomical signals.

Using ASKAP, the CRAFT project surveys the Universe between 700 MHz and 1500 MHz, with 1 millisecond time resolution and at 1 MHZ spectral resolution across 337 MHz. The array can be operated in two different modes, which includes ‘fly-eye’ mode where each antenna looks at a different portion of the sky, or in collimated incoherent mode – where all antennas are focused on a common target.

The CRAFT survey is an ideal project to keep ASKAP’s eye on the sky for FRBs – as these fast, transient bursts of radio waves are over within milliseconds. Through a collaborative approach of all CRAFT project members, the team is able to trigger rapid response times for global optical telescopes to turn towards the location of the FRB, soon after one is registered by ASKAP.

The ability to do so could change some fundamental questions radio astronomers are trying to resolve about FRBs – could an optical counterpart signal be confirmed one day and help finalise what is causing these events? Or are FRBs coming from a particular type of galaxy, like the spiral structure the Milky Way has, in which we reside?

Learnings from both ASKAP’s development and radio astronomy capabilities, combined with discoveries made through surveys like CRAFT – in particular demonstrating technology like the phased array feed system, or testing how hardware and real-time detection algorithms are able to deal with large volumes of data – are all heading towards a larger project known as the Square Kilometre Array.

This science mega-project – which includes an impressive list of 20 participating nations, tens of thousands of antennas and a radio telescope that spreads across two continents will become one of the most advanced scientific machines ever built by humans.

Construction of the SKA is soon to start (scheduled to break ground in 2021) and is scheduled to finish towards the end of the decade. Between now and then, projects like CRAFT and instruments like ASKAP are continually finding the path forward through innovation, development, testing, and research of technology and science that is being pushed to its limits.

Once the SKA opens its great eye, it will delve into some of science’s most fundamental questions like what Dark Energy is, the evolution of the first galaxies, the nature of time and space in the first few instances after the Big Bang, and if we are alone in the Universe.

FRBs are expected to be better understood by then as well, so the SKA will likely add a high number of additional samples to be studied by future astrophysicists who commence working in the field.

So where does the FRB story go from here?

Findings like those reported in this paper are only the first step in learning about these mysterious occurrences that have traveled across the vastness of space and time to be detected and deciphered here on Earth.

FRB signals tell us that our complete understanding of the Universe is not complete, but that we can use these signals to learn more about it, how it formed and evolved and what it is made from.

They’ll help us to answer some of our most fundamental questions like where did all the matter in the Universe come from, what is it made from, and why did it evolve the way it did, in the locations we currently observe it.

As this paper has demonstrated, these mysterious bursts – like bolts from the blue – are excellent scientific probes that can be used to support or rule out astrophysical hypotheses and modelling of the Universe. There’s certainly more science to come from FRBs, including a better understanding of what causes them, and excitingly if we can pinpoint more cases of them occurring in our own Galaxy.

As our scientists try to keep up with the rapidly advancing field of study surrounding FRBs, our technology capabilities continue to grow to meet the demands of seeing more, seeing better, seeing further into the Universe. Advancements that will be applied to large scale infrastructure projects, like the SKA in the near future.

When we look out into the night sky, it's humbling to remember that most of what we see is only a small portion of everything else. For all the brightness of galaxies, stars, planets, and moons – our insignificant minute portion of the baryonic matter gives us perspective to think about the grandness of the cosmos, and the mysteries that lay within it.

Yes, our Universe is exceedingly dark. But we now have a new light to shine upon it.