EXPERT REACTION: Signals from the first stars in the Universe light up the cosmic dawn

I have been working on the first stars and their violent deaths since some 20 years. While we know that some stars had to be the first, we were not able to directly observe their signatures until now. It is these first stars that are the origin of the first heavy elements in the universe, including the elements necessary to life. They are the seeds for the first galaxies, and they may be essential to understand the formation of the supermassive black holes that lurk in the centres of most galaxies.

These observations, if confirmed, will be a breakthrough, a milestone, in our quest to understand the transition of the ‘dark ages’ of the Universe - a Universe without stars - to the present state of the Universe, filled with stars and galaxies. The most successful method to learn about the first stars to date, has been to find them based on the chemical signatures they leave behind in other stars, in the form of ashes of their explosions that are then incorporated in the next generation of stars we can still find today.

This is a domain in which Australian observers are among the world leaders, for example, the most iron-poor star known has been discovered by the SkyMAPPER team. This new discovery will aid in allowing us to draw a more comprehensive picture of the cosmic dawn.

This is one of the most technically challenging radio astronomy experiments ever attempted. The lead authors include two of the best radio astronomy experimentalists in the world and they have gone to great lengths to design and calibrate their equipment so that they have convincing evidence that the signal is real. Dozens of other groups around the world have searched unsuccessfully for this signal and, with such a difficult experiment, we will have to wait for independent confirmation.

It’s true that they found a signal of roughly the expected form in the expected range of frequencies, but the fact that it is twice the most optimistic predicted amplitude and three to four times most predictions is not expected and was certainly not predicted.  Assuming the signal is real, this is an extremely important result and the statement that “this profile is largely consistent with expectations“ is an understatement. The accompanying theoretical paper proposes a specific dark matter explanation, but this is an explanation and not a prediction. More papers like this will follow, and this is makes this result even more important as we learn new things about our Universe.

This US team came to the MRO site in Australia because it has been protected as one of the most radio-quiet sites on Earth, and because it has sufficient infrastructure to conduct a difficult experiment. This is the site where the low-frequency SKA will be located and used for future experiments which will go on and measure the structure in the universe at this period in time when the first stars where forming.

The apparent detection of the signature of the first stars in the Universe will be a revolutionary discovery if it stands the tests of time.

While the detection appears robust, it is an incredibly challenging measurement, and needs to be confirmed. The fact that the detection is much stronger than expected and than can be easily explained, is particularly exciting.

Exciting because it might reflect new physics (as suggested in the paper and companion paper), but also exciting because it suggests that the upcoming generation of radio telescopes such as SKA-low will have lots to look at.

Amazingly Bowman et al. have detected a signature of the first stars that ever existed in the Universe! This is a signal from very, very early in the history of our Universe - 13.6 billion years ago - 9 billion years before our own star, the Sun, was even born. However the signal wasn't quite as expected - it was stronger. One explanation for this is that the first stars shone brighter than we have so far assumed.

Interestingly for me, an expert in the life cycles of ancient stars, the timing of the recorded event (just 180 to 270 million years after the Big Bang) means that smaller stars could have contributed to the signal.

It now appears smaller stars had enough time to be born and live out their lives, shining brightly to end the early, dark, starless phase of the Universe. This detection was made using a telescope in the remote desert of Western Australia, chosen because it is a perfect place to gaze into the far reaches of the Universe.

The signal detected by Bowman and collaborators is remarkable in its strength. So strong in fact that it doesn’t fit well with expectations of the current cosmological model.

If the signal is confirmed by other experiments (which may happen soon), the implications for our understanding of the evolution of the Universe and the nature of cosmic dark matter will be profound.

Moreover, detailed observations with future radio telescopes, such as the low-frequency portion of the Square Kilometre Array (due to be constructed in Western Australia) will yield images of the distant Universe at times much closer to the Big Bang than originally thought. A truly amazing result.

This Nature paper describes an extremely important and exciting result from Judd Bowman and colleagues. They have used a single antenna to watch the first stars turn on in the early Universe and to provide our first glimpse of the long-sought Cosmic Dawn. It is the first observational detection from a huge worldwide effort going back to the first predictions of such a signal almost 20 years ago.

The experiment is extremely difficult, requiring instrumental perfection of 1 part in 10,000 and they have achieved this. The success of the EDGES experiment hinges on the pristine radio-quiet environment at the Murchison Radio-astronomy Observatory (MRO) in Western Australia that is established and managed by CSIRO.

The work spearheaded by CSIRO with the support of the Commonwealth and WA state governments and with industry to protect the MRO from radio frequency interference has been essential to allow the faint signal from the early Universe to be picked up by the sophisticated EDGES antenna system.

The signal detected by Bowman et al. covers a frequency range that would otherwise be swamped by terrestrial FM stations and other human-generated interference. It is the lack of such interference that makes the MRO such an important setting for radio-telescopes such as the Australian Square Kilometre Array Pathfinder (ASKAP), the Murchison Widefield Array (MWA) and the forthcoming Square Kilometre Array.

The MRO site will enable these telescopes to build on the remarkable new discovery by EDGES, as well as enable many other new science discoveries.

In this paper in Nature, Bowman et al. describe the detection of radio wave signals from the early Universe that trace the formation of the first stars. These observations give us a fundamental insight into how matter in the Universe was evolving with surprising rapidity, only 200 million years after the Big Bang.

The Big Bang was 13.8 billion years ago. If the age of the Universe is expressed as a human lifetime, Bowman et al. are looking at the Universe when it was the equivalent of a baby starting to walk (~1 year old). The evidence Bowman et al. have presented has been keenly sought by astrophysicists around the world for the last decade, making this a breakthrough result.

In Australia, as part of an international consortium, we are currently developing the $1B Square Kilometre Array (SKA) radio telescope, which has the detailed investigation of these signals as its highest priority science case. These results (and the predictions reported in the companion Nature paper by Barkana) are therefore incredibly welcome news for Australian and international radio astronomy.

Currently in Australia we are using a precursor to the SKA, the Murchison Widefield Array (MWA), to take the next steps beyond the results of Bowman et al., toward the SKA. The MWA and the instrument Bowman et al. use are located together in Western Australia, where the SKA will be built in the future.

We will be looking for confirmation of the Bowman et al. results from other instruments and this is sure to accelerate the international competition to obtain more detailed data.

The paper soon to appear in Nature, by Judd Bowman and colleagues, on the detection of an all-sky signal from the first stars, represents a landmark moment for astronomy. Over the past 50 years, our knowledge of the mechanics of the Universe (cosmology) has gone from being mostly educated guesses to a precision science. This progress was made possible by a small number of fundamental discoveries using extraordinary instruments on the ground and in space.

In 1964,  Arno Penzias and Robert Wilson, using an antenna intended for telecommunications studies, discovered the faint all sky radio echo of the Big Bang that occurred 13.8 billion years ago. Some 28 years later, the COBE satellite was able to map this all sky signal and show for the first time the tiny ripples in the matter content of the Universe that grew since the Big Bang into the galaxies and stars we see today. Both these discoveries were awarded Noble Prizes (in 1992 and 2006 respectively) underscoring their fundamental importance for cosmology and our understanding of the Universe. The all-sky signal detected by Bowman and colleagues is another such breakthrough. 

Their discovery now identifies that time in the Universe’s history when the first stars were formed. The location of that event, its duration and its subsequent end due to the heating of the Universe from these first stars, are new data that will further refine our knowledge of the Universe and the complex processes associated with star formation, galaxy formation and the nature of other major components of the Universe, like Dark Matter.

The next step is to try to map the signal detected by Bowman in a similar manner to COBE mapping the signal detected by Penzias and Wilson. These maps will identify the locations of the places where stars were born and thereby enable us to discover the processes of star formation and ultimately galaxy formation.

There are already telescopes around the world preparing to make these maps based on the breakthrough of Bowman and his team. That breakthrough would not have been possible with observations from the middle of a crowded city or even a small town.

The radio noise generated by people, and civilization in general, completely swamps the tiny signals in the low frequency part of the spectrum from the Universes’ first stars. The isolation of the Murchison Radio-astronomy Observatory (MRO) in Western Australia gives it a very clear view of the low frequency radio sky and it is the home base for Bowman’s instrument.

This new observatory, supported by the Australian Federal Government and the Government of Western Australia and operated by CSIRO, is also home to the Murchison Widefield Array (MWA) and will be the site for the low frequency component of the Square Kilometre Array (SKA). Both MWA and SKA are planning to construct maps of the sky to follow-up the major discover that Bowman and his collages have made. 

The most radio quiet place on Earth has just set the astronomical airwaves buzzing with a cosmic discovery.

The discovery of the Big Bang and the 50 year campaign to measure its properties was a tour de force by radio astronomers to find cosmic signals at the bottom of an ocean of background noise.

Now an Arizona group may have done it again with an experiment called EDGES at the Murchison Radio Observatory in Western Australia, which has found the signature of the neutral hydrogen that preceded the formation of the first stars and galaxies.

The time and duration of this signal, starting 150 million years after the Big Bang and ending 100 million years later, was not a surprise.

The temperature of the hydrogen gas at this time, however, is a big surprise. Its low temperature seems to demand a role for dark matter, normally considered to be an inert bystander in the history of the Universe so early on.

An accompanying Nature paper by a physicist at Tel-Aviv University, Rennan Barkana, says, “An excess 21-cm absorption signal is a clear sign of scattering of baryons and dark-matter particles.”
 
Dark matter is famous for its role in holding galaxies together by gravity and for making up a quarter of the energy density of the Universe, some 5 or 6 times more than the chemical elements we know.
 
Could this be the biggest clue yet about the nature of dark matter? This would mean dark matter is not ‘sterile’; it does interact with ordinary matter.
 
This is what we are looking for in numerous dark matter experiments around the world, including the SABRE-South experiment, which our consortium is placing in the first underground physics laboratory in the Southern Hemisphere at Stawell, western Victoria. So far, the dark matter particle has escaped detection. But if the particle mass is as low as this interaction requires, there is no contradiction with current laboratory work.
 
There are other experiments at Murchison Radio Observatory that will map the neutral hydrogen, culminating in the Square Kilometre Array. This will open up a new window on the 100 million year old Universe. And it could be the biggest clue yet about the nature of dark matter.

The Nature press release describes the signal as ‘mostly as expected’. This is something of an understatement! The work by Judd Bowman and collaborators describes the long-sought detection of the famous '21 cm' radio signal caused by the first stars to appear in the universe 13.6 billion years ago. Astronomers have been searching for this signal for the last decade and it was expected that it would take quite a few more years.

Amazingly, this detection in 2015-2016 was done by a small aerial, only a few meters in size, coupled to a very clever radio receiver and signal processing system. This is the most important astronomical discovery since the detection of gravitational waves in 2015.
 
No one quite knew how strong the signal would be or at what frequency to search. A number of experiments worldwide are vying to find it. The detection by the EDGES experiment at 78 Mhz (near what we call the FM band) in Western Australia tells us that the first stars appeared only 180 million years after the Big Bang.

Because of their radiation exciting the intergalactic gas, we are able to see the cold shadow of hydrogen in the early Universe absorbing the background radio emission. An even bigger surprise is this absorption signal is about twice as strong as expected, telling us that this early Universe gas was probably much colder than we thought. It is quite difficult to explain this as most new things we can think of (exotic stars, black holes) would make the Universe hotter.

One plausible explanation that theorist Rennan Barkana (detailed in an accompanying paper) has come up with is that the atoms on the early Universe could have been cooled by collisions with ‘cold dark matter’, the mysterious particles whose gravity dominates the Universe but which we have virtually no information on what they are. If true, this would be a stupendous advance and could lead us to new physical laws. This would be the first time that dark matter has demonstrated any physical interaction apart from gravity.
 
A note of caution is warranted. This is a very difficult signal to detect, it is thousands of times fainter than the background radio noise, even for the remote location in Western Australia. The authors have spent over a year doing a multitude of tests and checks to make sure they have not made a mistake.

The detection is an impressive technical achievement but astronomers worldwide will be holding their breath until the result is confirmed by an independent experiment. If it is, then this will open the door to a new window on the early Universe and potentially a new understanding of the nature of dark matter.

It will be exciting news for Australia in particular, Western Australia is the most radio-quiet zone in the world and will be the prime location for future searches. New FM band telescopes with more power, such as the Murchison Widefield Array and the future Square Kilometre Array, could make a picture of the 21cm signal on the sky and this could reveal the nature of the first stars, and even tell us what dark matter is made of.

Radiation from the intense light of the first stars, in particular Lyman alpha, altered giant clouds of gas 180m years after the Big Bang. These clouds then blocked the light of the afterglow of that Big Bang, the Cosmic Microwave Background. We see these clouds, silhouetted against the fireball all around us. The continuing growth of stars, and the first galaxies, eventually heated that gas until it itself began to glow, within just 100m years.

This detected silhouette would be a first of its kind, one of the earliest detections of forming stars/galaxies and acts as a trailblazing observation in low frequency radio astronomy for years to come. Yet that’s not even the most exciting part - it could be the very first confirmation of the dark matter particle. A hidden component of our universe, outweighing everything we can see five times over. 

In the billions of years since the silhouette blocked this light from the Big Bang afterglow, the Universe has expanded, and the fireball has long since cooled to just 2.7K above absolute zero. It's now visible in microwaves by radio telescopes such as EDGES (the Experiment to Detect the Global Epoch of Reionizaton Signature).

It searches at low frequencies of radio, from 50 - 100 MHz, similar to radio stations such as ABC RN or TripleJ. Just like your car radio that you tune into different stations at different frequencies, these radio telescopes tune into different regions of space. The lower the frequency, the further away from us, and further back in time we look into the Universe.

At 78 MHz a silhouette is seen where something appears to be blocking the CMB radio station. To see this silhouette required hundreds of hours of patient observing of the Southern sky by EDGES. Like your favourite song playing at a whisper over the radio while standing in the noisiest, most chaotic city traffic imaginable, the team first had to filter out the other signals. Ten thousand times louder than the desired signal was the radio emission of all of the fast-moving electrons spiralling along magnetic fields of our Galaxy itself. Once filtered away, there was left the silhouette. 

This is a giant cloud of gas, unexpectedly cold and efficiently blocking that signal. The early Universe was a simple time, as quite literally there had not been much time for things to form that would complicate the picture. Most objects, from stars to black holes, that might exist would tend to heat the gas. Not many things can cool the gas. Indeed the lowest bound on the signal of the silhouette is 50% colder than anything we might expect. The only thing possibly colder than this gas? Dark matter. 

The dark matter is a ghost, able to travel through solids, much less a gas, without ever colliding. Yet if the dark matter somehow collided with gas in the early universe then it would leach the energy from it and cool it just as seen by EDGES. This is potentially one of the greatest clues as to the nature of dark matter. All other experiments that have indicated the presence of dark matter have used its gravitational force to trace it. This would be the first glimpse into the dark matter interacting with atoms with some different kind of force. 

The very coldness of the gas and hence relative stillness in reference to the ultra-cold dark matter allows any mutual-interaction to be highlighted. Indeed, non-standard Coulomb-like scattering suppress interaction of the two by the fourth power of their velocities. Increase the relative velocities from 1km/s to 10km/s and the interaction between dark matter and gas becomes ten thousand times less. Today, near the Earth, the relative velocities are hundreds of km/s, meaning the dark matter truly is a ghost, 100 million times less interacting. 

Experiments are underway in Australia, such as SABRE, to directly detect such interactions in the laboratory. This signal from space would be an incredibly exciting guide of what mass of dark matter they should try to be most sensitive too. Early results, if confirmed, suggest it may well be lighter than is commonly searched for, potentially less than 4 times the mass of a proton rather the hundreds of times more massive in WIMP models.

If this is confirmed, and several other competitors are close, then the future for dark matter investigation is low frequency radio telescopes. The largest telescope ever conceived, the Square Kilometre Array, starts construction in Australia and South Africa imminently. All eyes in astronomy were already following this telescope, now all eyes in physics will be as well.

The drive to understand the nature of dark matter may even require telescopes to be built in evermore radio quiet environments. The best location? The far side of the Moon, where the bulk of the Moon blocks the radio stations of Earth and is free from the blurring effects of our planet’s ionosphere on radio signals from space.

Understanding that the effect can be explained in various ways, it's always exciting to see another tantalising hint of dark matter particles from the early Universe.

Unfortunately, so far, these hints indicated something other than dark matter, but there's a first time for every discovery.

Considering the chances of this one being "it": there's nothing surprising about a gigaelectronvolt mass dark-matter particle.

Such a particle can easily hide from other probing experiments, provided its interaction strength with ordinary matter is low enough. An assumed interaction cross-section greater than about 10^−21 cm^2 between dark and standard matter particles, however, should lead to other detectable signals.

So, there's good reason for optimism, because this signal should soon be verified by other experiments.