Hydrogen is the most abundant element in the universe, by far. 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 other elements combined. It's everywhere, from the water in our oceans to the earliest regions of the Cosmic Dawn. Fortunately for astronomers, all this neutral hydrogen can emit a faint emission line of radio light.
single electron
bound to a single proton. When the spins of these two are aligned in the same way, hydrogen has a slightly higher energy than when the spins are oppositely directed. So the electron can undergo a spin flip and release a bit of energy as a photon of light. The hydrogen doesn't need to be super-heated or ionized to do this. It can happen spontaneously. So wherever there are clouds of hydrogen, you can be sure it's emitting 21-centimeter radio light.
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 uses of this trick was to measure the motion of hydrogen in the Milky Way and other nearby galaxies, which allowed Vera Rubin to discover dark matter. Now a new study shows how the 21-centimeter line might give us the first evidence of dark matter particles.
When it comes online, HERA will map the large-scale structure of hydrogen during the cosmic dark ages and cosmic dawn period, which is the time between the fading of the primeval fireball of the Big Bang and the appearance of the first stars and galaxies. During this period the cosmos was filled with dark matter and warm clouds of
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