This new telescope could detect decaying dark matter in the early Universe
Hydrogen is the most abundant element in the universe. By far. More than 90% of the atoms in the universe are hydrogen.
Ten times as many helium atoms and a hundred times more than all other elements combined.
It is everywhere, from the waters of our oceans to the earliest 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 hydrogen line or the 21 centimeter line. Hydrogen consists of a single electron bonded to a single proton.
When the spins of these two are aligned, hydrogen has a slightly higher energy than when the spins are opposite. So the electron can go through a spin flip and release some energy as a light photon.
To do this, the hydrogen does not have to be overheated or ionized. It can happen spontaneously. So where there are clouds of hydrogen, you can be sure it’s emitting 21 centimeters of radio light.
Because 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 uses of this trick was to measure the movement of hydrogen in the Milky Way and other nearby galaxies, allowing Vera Rubin to detect dark matter.
Now a new study shows how the 21-centimeter line could give us the first hint of dark matter particles.
The study focuses on the Hydrogen Epoch of Reionization Array (HERA), a radio telescope in South Africa particularly suited to observing hydrogen in the early Universe.
When it goes online, HERA will map the large-scale structure of hydrogen during the cosmic dark ages and cosmic dawns, the time between the fading of the primeval Big Bang fireball and the appearance of the first stars and galaxies.
During this time, the cosmos was filled with dark matter and warm clouds of hydrogen gas.
If dark matter is truly neutral and only interacts with matter and light through gravity, then the 21-centimetre light is essentially the only light emitted during this period.
But the most popular model for dark matter are particles known as WIMPs.
Neutral dark matter particles are much heavier than normal matter particles like protons and electrons.
In some dark matter particles, these WIMPs occasionally decay into normal matter, producing a burst of energetic positrons and electrons or protons and antiprotons. If that were the case, these energetic decay particles would interact with the 21-centimeter light.
Based on cosmic microwave background observations and other studies, we know that WIMPs would have a very long decay half-life.
We haven’t seen any evidence of dark matter decay so far, which means WIMPs either don’t exist or their half-life is much longer than a trillion years.
This new study shows that even with a half-life of WIMPs a thousand times longer, HERA would be able to detect their effect at the early 21-centimeter line. And it would have enough data to do so within 1,000 hours of observation.
Even if HERA finds no evidence of dark matter decay, it would still be a major advance.
Its limitations on the dark matter half-life would be strong enough to exclude some WIMP models and limit the range of models.
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Written by Brian Koberlein, universe today.
