Australasian Science Magazine

By Alan Duffy

CSIRO’s Murchison Radio-astronomy Observatory has detected a faint silhouette of the first stars after the Big Bang. Its extreme coldness indicates the existence of the dark matter particle.

Radiation from the intense light of the first stars, in particular Lyman alpha, altered giant clouds of gas 180 million years after the Big Bang. These clouds then blocked the light of the afterglow of that Big Bang, the Cosmic Microwave Background (CMB). 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 100 million years.

The silhouette detected (see Nature, https://goo.gl/nJ5Ykv) would be a first of its kind, one of the earliest detections of stars and galaxies forming, 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). This searches at low frequencies of radio, from 50–100 MHz, similar to radio stations such as ABC RN or Triple J. 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 it is away from us and the 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 the magnetic fields of our galaxy itself. Once these were 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 but 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, let alone 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 of 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 suppresses interaction of the two by the fourth power of their velocities. Increase the relative velocities from 1 km/s to 10 km/s and the interaction between dark matter and gas becomes 10,000 times less. Today, near the Earth, the relative velocities are hundreds of kilometres per second, meaning the dark matter truly is a ghost interacting 100 million times less.

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 to. Early results, if confirmed, suggest it may well be lighter than is commonly searched for, potentially less than four times the mass of a proton rather than 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.