The Big Bang may have been bright and dramatic, but immediately after that, the universe went dark, and for a very long time. Scientists believe that the first stars appeared in a muddy broth of matter 200 million years after the hot start. Since modern telescopes are not sensitive enough to observe the light of these stars directly, astronomers are looking for indirect evidence of their existence.
And so a team of scientists managed to pick up a faint signal from these stars using a tabletop-sized radio antenna called EDGES. Spectacular measurements that open up a new window into the early universe show that these stars appeared 180 million years after the Big Bang. The work published in Nature also suggests that scientists may rethink what "dark matter" - a mysterious type of invisible matter - is made of.
Models showed that the first stars to illuminate the universe were blue and short-lived. They immersed the universe in a bath of ultraviolet light. The very first observable signal of this cosmic dawn has long been thought of as an "absorption signal" - a drop in brightness at a particular wavelength - caused by the passage of light and affecting the physical properties of clouds of hydrogen gas, the most abundant element in the universe.
We know that this drop should be detected in the radio wave part of the electromagnetic spectrum at a wavelength of 21 cm.
Complex measurement
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In the beginning there was a theory that predicted all this. But in practice, finding such a signal is extremely difficult. This is because it intertwines with many other signals in this region of the spectrum that are much stronger - for example, the common frequencies of radio broadcasts and radio waves from other events in our galaxy. The reason the scientists succeeded was partly because the experiment was equipped with a sensitive receiver and a small antenna, allowing it to cover a large area of the sky relatively easily.
To make sure that any drop in brightness they found was due to the starlight of the early universe, the scientists looked at the Doppler shift. You are familiar with this effect by the lowering of the pitch when a car with a blinker and a siren passes by you. Likewise, as galaxies move away from us due to the expansion of the universe, light shifts towards red wavelengths. Astronomers call this effect "redshift".
The redshift tells scientists how far a cloud of gas is from Earth and how long ago, by cosmic standards, light was emitted from it. In this case, any shift in brightness expected at 21cm wavelength will indicate gas movement and distance. Scientists measured the drop in brightness that occurred in different cosmic periods of time, until the moment when the universe was only 180 million years old, and compared it with its current state. It was the light of the very first stars.
Hello dark matter
The story doesn't end there. Scientists were surprised to find that the signal amplitude was twice as large as predicted. This suggests that the hydrogen gas was much colder than expected from the microwave background.
These results were published in another article in Nature and threw a bait hook for theoretical physicists. This is because it becomes clear from physics that at this time of the existence of the universe, the gas was easy to heat up, but difficult to cool. To explain the additional cooling associated with the signal, the gas had to interact with something even colder. And the only thing colder than cosmic gas in the early universe was dark matter. Theorists must now decide if they can extend the standard model of cosmology and particle physics to explain this phenomenon.
We know that there is five times more dark matter than ordinary matter, but we do not know what it is made of. Several variants of particles have been proposed that could make up dark matter, and the favorite among them is the weakly interacting massive particle (WIMP).
The new study, however, suggests that the dark matter particle should not be much heavier than the proton (which enters the atomic nucleus along with the neutron). This is well below the masses predicted for the WIMP. The analysis also suggests that dark matter is colder than expected, and opens up a fascinating opportunity to use "21cm cosmology" as a probe for dark matter in the universe. Further discoveries with more sensitive receivers and less interference from terrestrial radio could reveal more details about the nature of dark matter and perhaps even indicate the speed at which it travels.
Ilya Khel