Hydrogen is the most common element in the universe. More than 90% of the atoms in the universe are hydrogen. Ten times more than helium atoms and a hundred times more than all the other elements combined. It is everywhere, from the waters of our oceans to the earliest reaches of the Cosmic Dawn. Fortunately for astronomers, all that neutral hydrogen can emit a weak streak of radio radiation.

It is known as the HI hydrogen line or the 21 cm line. Hydrogen consists of an electron bound to a proton. When the spins of these two are aligned in the same way, the hydrogen has a slightly higher energy than when the spins are in opposite directions. So the electron can undergo a spin and release some energy in the form of a photon of light. There is no need to overheat or ionize the hydrogen for this. It can happen spontaneously. So wherever there are hydrogen clouds, you can be sure that 21 cm of radio light is emitted.

Since the emission line has a very specific wavelength, we can use it to measure the relative motion or cosmological redshift of hydrogen. One of the first applications of this trick was to measure the motion of hydrogen in the Milky Way and other nearby galaxies, allowing Vera Rubin to discover dark matter. Now a new study shows how a 21-centimeter line could give us the first evidence of dark matter particles.

Research published on the preprint server arXiv He focused on the Age of Hydrogen Re-ionization Array (HERA) in South Africa, a radio telescope particularly suited to observing hydrogen in the early universe.

When HERA comes online, it will map the large-scale structure of hydrogen in the cosmic dawn period, the time between the cosmic dark ages and the destruction of the primordial fireball of the Big Bang and the appearance of the first stars and galaxies. . During this period, the universe was filled with dark matter and clouds of hot hydrogen.

If dark matter is truly neutral and interacts only gravitationally with matter and light, the only light emitted during this period is 21 cm of light. But the most popular model of dark matter contains particles known as WIMPs.

Neutral dark matter particles are much heavier than ordinary matter particles such as protons and electrons. In some dark matter particles, these WIMPs occasionally decay into ordinary matter, creating an energetic burst of positrons and electrons or protons and antiprotons. If so, then these energetic decay particles will interact with 21 cm of light.

Based on observations of the cosmic microwave background and other studies, we know that WIMP will have a very long half-life. So far we have seen no evidence of dark matter decay; which means that WIMPs do not exist or their half-lives exceed one trillion years. This new study shows that HERA can detect its effect early in the 21-centimeter line, even if the half-life of WIMPs is a thousand times longer. And it will have enough data to do that for 1000 hours of observation.

Even if HERA finds no evidence of dark matter decay, it would still be a big step forward. Its constraints on dark matter half-life would be strong enough to exclude some WIMP models and scatter the model range. Source