A new telescope can see dark matter decaying in the early universe

Hydrogen is the most abundant element in the universe, by far. Over 90% of the atoms in the universe are hydrogen. Ten times the number of helium atoms, and a hundred times more than all the other elements combined. Everywhere, from the waters of our oceans to the very cores of the Cosmic Dawn. Fortunately for astronomers, all this neutral hydrogen can emit a faint emission line of radio light.

It is known as the HI hydrogen line, or the 21 centimeter line. Hydrogen contains one electron bound to one proton. When the spins of the pairs are aligned in the same direction, hydrogen has slightly more energy than when the spins are opposite. So an electron can go into a spin flip and emit as little energy as a photon of light. Hydrogen does not need to be very hot or ionized to do this. It can happen automatically. So wherever there are hydrogen clouds, you can be sure it’s emitting a 21-centimeter radio beam.

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

Review, from the preprint server arXivfocuses on the Hydrogen Epoch of Reionization Array (HERA), a radio telescope in South Africa that is uniquely suited to observe hydrogen in the early universe.

When it comes online, HERA will map the superstructure of hydrogen during the cosmic dark and the cosmic dawn, which is the time between the end of the fireball before the Big Bang and the appearance of the first stars and galaxies. At this time the cosmos was filled with dark matter and hot clouds of hydrogen gas.

If black matter is neutral, and only interacts with matter and light by gravity, then the 21-centimeter light is the only light that comes out at this time. But the most popular model of dark matter involves particles known as WIMPs.

Neutral dark particles are much heavier than ordinary particles such as protons and electrons. For other dark particles, these WIMPs sometimes decay into normal matter, creating bursts of energetic positrons and electrons, or protons and anti-protons. If so, these particles of decay would interact with light 21 centimeters away.

Based on observations of the cosmic microwave background and other studies, we know that WIMPs will have very long half-lives. We have seen no evidence of dark matter decay so far, meaning that WIMPs either do not exist or their half-lives are longer than a trillion years. This new study shows that even if WIMPs have half-lives a thousand times longer, HERA will be able to detect their effect on the first line of 21 centimeters. And it can have enough data to do that in 1,000 hours of observation.

Even if HERA doesn’t detect any evidence of dark matter decay, it could be a big step forward. Its constraints on the dark matter half-life will be strong enough to outclass other WIMP models and win the model range.