The Big Bang may have been bright and dramatic, but immediately after it occurred, the universe became extremely dark for a long time. Until now we believed that it took up to 200 million years for the first stars to begin to emerge from a dark soup of matter.
However, a team of astronomers from the University of Arizona and the Massachusetts Institute of Technology (USA) has managed to detect a signal that emitted hydrogen gas from the primordial universe, verifying that they were generated just 180 million years after that. Big explosion. It is the first evidence of hydrogen found in the universe.
The weak signal from these stars has been picked up with a table-size radio antenna called EDGES in a remote and arid area of Australia, free of interference.
This new brand opens a new window to the early universe, by showing that these first stars emerged approximately 180 million years after the Big Bang. But this finding is accompanied by a more unexpected one: before the stars were born, something cooled the surrounding gas, perhaps the mysterious dark matter.
Models have shown that the first stars to illuminate the universe would have been blue and short-lived, producing a bath of ultraviolet light.
The observable signal from this cosmic sunrise was theorized as an “absorption signal” – a drop in brightness at a particular wavelength – caused by this light passing through and affecting the physical properties of hydrogen gas clouds, which is the most abundant element in the cosmos. In practice, this signal is extremely difficult to find. This is because it overlaps with many other signals in this region of the spectrum that are much stronger, such as common frequencies on the FM radio dial and radio waves from other events in our galaxy.
The reason scientists have been successful was in part due to the experiment’s sensitive receiver and small antenna, which allows it to cover a large area of the sky more easily. The scientists were surprised to find that the amplitude of the signal was more than twice as predicted, suggesting that the hydrogen gas was much colder than expected.
Considering that at this point in the universe it would have been easy to heat the gas but difficult to cool it, something must have interacted to produce that additional cooling. The only thing known in the early universe colder than this cosmic gas is dark matter.
In fact, theorists must now decide whether to extend the standard model of cosmology and particle physics to explain this effect.
We know that dark matter is five times more common than ordinary matter, but we still don’t know what it is made of.
The new study suggests that the dark matter particle would not be much heavier than a proton (which makes up the atomic nucleus along with the neutron) and would be cooler than we thought.
However, the authors acknowledge that there could be another cause of the excessive cooling of the primordial gas: “What we see is absorption, by gas, of radio waves. The other possible explanation is that there were more and more intense radio waves in the early universe than we expect, produced by some process when stars began to form. This would also be a great surprise ”, clarifies Rennan Barkana, leader of the work published by the journal Nature.
These discoveries come at a key moment 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 Kilometer Array , as well as other cutting-edge experiments dedicated to studying the cosmic dawn.
Referencia. Judd D. Bowman et al. An absorption profile centred at 78 megahertz in the sky-averaged spectrum, Nature (2018). DOI: 10.1038/nature25792
Rennan Barkana. Possible interaction between baryons and dark-matter particles revealed by the first stars, Nature (2018). DOI: 10.1038/nature25791
