A radiative decaying Big Bang relic with a mass ma≃5−25eV, 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 one 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 the 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 1029s or 1027s for the cold dark matter and the hot relic case, respectively.
@article{arxiv.2301.06560,
title = {Probing the Blue Axion with Cosmic Optical Background Anisotropies},
author = {Pierluca Carenza and Giuseppe Lucente and Edoardo Vitagliano},
journal= {arXiv preprint arXiv:2301.06560},
year = {2024}
}
Comments
21 pages, 13 figures. Matches published version, with a note added to show Figure 1 expanded to smaller axion masses (Fig. 14)