Probing the Blue Axion with Cosmic Optical Background Anisotropies

TL;DR

This paper investigates the potential of cosmic optical background anisotropies to detect or constrain blue axions, a hypothesized relic particle, by analyzing existing and future observational data across different wavelengths.

Contribution

It proposes new observational strategies using HST data at multiple wavelengths to improve constraints on blue axion properties and discusses the potential of upcoming experiments to detect these particles.

Findings

Current HST measurements constrain blue axion lifetime.

Future HST observations can improve constraints by an order of magnitude.

Upcoming line intensity mapping experiments could detect blue axions with extremely long lifetimes.

Abstract

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 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 $10^{29}\,\rm s$ or $10^{27}\,\rm s$ for the cold dark matter and the hot relic case, respectively.