http://arxiv.org/abs/2301.06560
A radiative decaying Big Bang relic with a mass $m_a\simeq 5-25 \,\rm eV$, which we dub “blue axion”, can be probed with direct and indirect observations of the cosmic optical background (COB). The strongest bounds on blue-axion cold dark matter come from the Hubble Space Telescope (HST) measurements of COB anisotropies at $606$ nm. We suggest that new HST measurements at higher frequencies ($336$ nm and $438$ nm) can improve current constraints on the lifetime up to an order of magnitude, and we show that also thermally produced and hot relic blue axions can be competitively probed by COB anisotropies. We exclude the simple interpretation of the excess in the diffuse COB detected by Long Range Reconnaissance Imager (LORRI) as photons produced by a decaying hot relic. Finally, we comment on the reach of upcoming line intensity mapping experiments, that could detect blue axions with a lifetime as large as $10^{29}\,\rm s$ or $10^{27}\,\rm s$ for the cold dark matter and the hot relic case, respectively.
P. Carenza, G. Lucente and E. Vitagliano
Wed, 18 Jan 23
121/133
Comments: 21 pages, 12 figures