Hydrogen is the most abundant element in the universe. Until now. More than 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. It’s everywhere, from the waters in our oceans to the oldest regions of the cosmic dawn. Fortunately for astronomers, all that neutral hydrogen can emit a faint emission line of radio light.

It is known as the HI line, or 21 centimeter line. Hydrogen consists of one electron bonded to one 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 oriented oppositely. So the electron can undergo a spin and release a little bit of energy in the form of a photon of light. Hydrogen does not need to be superheated or ionized to do this. It can happen spontaneously. So wherever there are clouds of hydrogen, you can be sure that they are emitting 21cm radio light.

Spin and face decay of neutral hydrogen. Credit: Wikipedia user Tiltec

Because the emission line has a very specific wavelength, we can use it to measure the relativistic motion or cosmic redshift of hydrogen. One of the first uses of this trick was to measure the motion of hydrogen in the Milky Way and other nearby galaxies, which allowed Vera Rubin to detect dark matter. A new study now shows how the 21-centimetre line might give us the first evidence of dark matter particles.

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The study focuses on the Hydrogen Reionization Age Array (HERA), a radio telescope in South Africa particularly suited to observing hydrogen in the early universe. When it is published online, HERA will map large-scale large-scale structure of hydrogen during the cosmic dark ages and cosmic dawn, the time between the fading of the primordial fireball of the Big Bang and the emergence of the first stars and galaxies. During this period, the universe was filled with dark matter and warm clouds of hydrogen gas.

How Weakly Interacting Massive Particles (WIMPs) Decay Credit: Gao Linking and Lin Sujie

If dark matter is truly neutral, and only interacts with matter and light via gravity, then the 21-centimeter light is essentially the only light emitted during this period. But the most common model of dark matter involves 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 particles sometimes decay into ordinary matter, creating a wave of energetic positrons and electrons, or protons and antiprotons. If that were the case, these energetic decay particles would interact with light with a diameter of 21 cm.

HERA would further limit the lifespan of dark matter. Credit: Facchinetti, et al

Based on observations of the cosmic microwave background and other studies, we know that weakly interacting massive particles (WIMPs) will have very long decay half-lives. We haven’t seen any evidence of dark matter decay yet, which means either that weakly interacting particles (WIMPs) don’t exist or that their half-lives are well over a trillion years. This new study shows that even if the half-life of WIMPs was a thousand times longer, HERA would be able to detect its effect on the early 21 centimeter line. And it will have enough data to do so over 1,000 hours of observation.

Even if HERA didn’t detect any evidence of dark matter decay, it would still be a huge step forward. Its limitations on the half-life of dark matter would be strong enough to rule out some WIMP models and sift through the pool of models.

reference: Facchinetti, Gaitan, et al. “21-cm Signal Sensitivity for Dark Matter Decay.” arXiv Advance edition arXiv:2308.16656 (2023).

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