Redshift tells scientists how far away a certain gas cloud is from Earth and how far back in cosmic time the light from it was emitted. In this case, any shift in the dip in brightness expected at 21cm wavelength would give an indication of how the gas is moving and how far away it lies. The team measured a dip that covered a range of times in the cosmos – most dramatically back to when the universe itself was only 180m-years-old, compared to its grand age today of 13.9 billion years. This was the light from the very first stars.
Dark matter twist
The story doesn’t end there. The team was surprised to find that the amplitude of the signal was more than twice as large as predicted. This suggests that the hydrogen gas was much colder than expected of the background radiation.
These findings, published in another paper in Nature, have thrown a spanner in the work of theoretical physicists. This is because the physics suggests that at this time in the universe, it would have been easy to heat gas but difficult to cool it. In order to produce the extra cooling needed to explain the signal, the authors argue, the gas must have interacted with something even colder. And the only thing known in the early universe colder than this cosmic gas is dark matter. In fact, theorists must now decide whether they should extend the standard model of cosmology and particle physics to explain this effect.
We know that dark matter is five times more common than normal matter but we don’t yet know what it is made of. Several options for particles that could make up dark matter have been proposed, with the favourite candidate being the Weakly Interacting Massive Particle (WIMP).
The new research, however, suggests that the dark-matter particle would not be much heavier than a proton (which makes up the atomic nucleus along with the neutron). This is well below masses predicted for the WIMP. The analysis also suggests that the dark matter is colder than expected, and opens the exciting possibility of using “21cm cosmology” as a new probe of dark matter in the universe. Further discoveries with more sensitive receivers and less complications from terrestrial radio interference – which could be achieved by placing an interferometer on the dark side of the moon – could unveil more details about the nature of dark matter, maybe even probing the speed at which it is moving.
This comes at an opportune time for radio astronomers, who are developing the next generation of giant networks of radio telescopes or interferometers in Australia and South Africa called the Square Kilometre Array as well as other cutting-edge experiments dedicated to studying the cosmic dawn. It’s an exciting time to be a scientist.
- Carole Mundell is the head of physics at the University of Bath
-This article was first published at The Conversation