The light of the first stars can change our notion of dark matter

The Big Bang may have been bright and dramatic, but right after that the universe faded, and for a very long time. Scientists believe that the first stars appeared in a cloudy 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 now a group of scientists managed to catch a weak signal of these stars with the help of a radio antenna the size of a table top called EDGES. Impressive dimensions that open a new window to the early universe show that these stars appeared 180 million years after the Big Bang. The work published in Nature also suggests that scientists can rethink what the “dark matter” consists of-the mysterious type of invisible matter.

Models showed that the first stars that illuminated the universe were blue and short-lived. They immersed the universe in a bath of ultraviolet light. The very first observed signal of this cosmic dawn has long been considered an “absorption signal” – a decrease in brightness at a certain wavelength – caused by the passage of light and influencing the physical properties of clouds of gaseous hydrogen, the most common 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

In the beginning there was a theory that predicted all this. But in practice it is extremely difficult to find such a signal. All because it is intertwined with a lot of other signals in this area of ​​the spectrum, which are much stronger – for example, the common frequencies of broadcasting 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, which makes it possible to cover a large area of ​​the sky relatively easily.

To be sure that any drop in brightness that they found was due to the starlight of the early universe, scientists looked at the Doppler shift. You this effect is known for lowering the pitch, when a car passing by with a flasher and a siren passes past you. Similarly, since the galaxies are moving away from us because of the expansion of the universe, the light is shifted toward red wavelengths. Astronomers call this effect a “red shift”.

The red shift tells scientists how far the cloud of gas is from the Earth and how long the cosmic standards emitted light from it. In this case, any shift in brightness expected at 21 cm wavelength will indicate the movement of gas and the remoteness of its location. 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 with its current state. It was the light of the very first stars.

Hello, dark matter

This is not the end of the story. The scientists were surprised to find that the amplitude of the signal 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 abandoned the hook with a shine for theoretical physicists. All because from physics it becomes clear that at this time of existence of the universe the gas was easy to heat, but it is 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 that was colder than the cosmic gas in the early universe is dark matter. Theorists should now decide whether they can extend the standard model of cosmology and particle physics to explain this phenomenon.

We know that dark matter is five times larger than normal matter, but we do not know what it is made of. Several variants of particles were proposed that could make up dark matter, and the favorite among them is a weakly interacting massive particle (WIMP).

A new study, however, suggests that a particle of dark matter should not be much heavier than a proton (which enters the atomic nucleus together with a neutron). This is well below the masses predicted for WIMP. The analysis also suggests that dark matter is colder than expected and offers an exciting opportunity to use “21 cm cosmology” as a probe of dark matter in the universe. Further discoveries with more sensitive receivers and less interference from terrestrial radio can reveal more details about the nature of dark matter and perhaps even indicate the speed with which it moves.

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