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arXiv · 1808.09456

Axion absorption and the spin temperature of primordial hydrogen

Adrien Auriol·Sacha Davidson·Georg Raffelt
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Abstract

An absorption dip in the spectrum of the cosmic microwave background observed by the EDGES experiment suggests an unexplained reduction of the hydrogen spin temperature at cosmic redshift z ~ 17. The mass of dark-matter axions could correspond to the hyperfine splitting of 5.9 micro-eV, between the triplet (H1) and singlet (H0) state. We calculate the rate for a+ H0 <-> H1 in two ways, and find that it is orders of magnitude smaller than the CMB-mediated transition rate, so irrelevant. As a result, this process cannot be used to rule in or out dark matter axions of mass = hyperfine splitting. The axion rate nonetheless has interesting features, for example, on balance it heats the spin temperature, and the axion couplings to protons and electrons contribute on equal footing.

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Adrien Auriol, Sacha Davidson, Georg Raffelt. 2018-08-28. Axion absorption and the spin temperature of primordial hydrogen. https://arxiv.org/abs/1808.09456

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