Using CMB spectral distortions to distinguish between dark matter solutions to the small-scale crisis

TL;DR

This paper proposes using cosmic microwave background spectral distortions, specifically $ ext{mu}$-distortions, to probe dark matter microphysics and distinguish between different solutions to the small-scale crisis, such as interactions with neutrinos or photons.

Contribution

It introduces a method to use CMB spectral distortions to differentiate dark matter models involving kinetic coupling to neutrinos or photons, highlighting their potential observational signatures.

Findings

$ ext{mu}$-distortion is enhanced for neutrino interactions.

Distortions can distinguish dark matter interactions from warm dark matter.

Potential detectability with future experiments like PRISM.

Abstract

The dissipation of small-scale perturbations in the early universe produces a distortion in the blackbody spectrum of cosmic microwave background photons. In this work, we propose to use these distortions as a probe of the microphysics of dark matter on scales $1\,\textrm{Mpc}^{-1}\lesssim k \lesssim 10^{4}\,\textrm{Mpc}^{-1}$. We consider in particular models in which the dark matter is kinetically coupled to either neutrinos or photons until shortly before recombination, and compute the photon heating rate and the resultant $\mu$-distortion in both cases. We show that the $\mu$-parameter is generally enhanced relative to $\Lambda$CDM for interactions with neutrinos, and may be either enhanced or suppressed in the case of interactions with photons. The deviations from the $\Lambda$CDM signal are potentially within the sensitivity reach of a PRISM-like experiment if $\sigma_{\textrm{DM}-\gamma} \gtrsim 1.1\times10^{-30} \left(m_{\textrm{DM}}/\textrm{GeV}\right) \textrm{cm}^{2}$ and $\sigma_{\textrm{DM}-\nu} \gtrsim 4.8\times 10^{-32} \left(m_{\textrm{DM}}/\textrm{GeV}\right) \textrm{cm}^{2}$ for time-independent cross sections, and $\sigma^{0}_{\textrm{DM}-\gamma} \gtrsim 1.8 \times 10^{-40} \left(m_{\textrm{DM}}/\textrm{GeV}\right) \textrm{cm}^{2}$ and $\sigma^{0}_{\textrm{DM}-\nu} \gtrsim 2.5 \times 10^{-47} \left(m_{\textrm{DM}}/\textrm{GeV}\right) \textrm{cm}^{2}$ for cross sections scaling as temperature squared, coinciding with the parameter regions in which late kinetic decoupling may serve as a solution to the small-scale crisis. Furthermore, these $\mu$-distortion signals differ from those of warm dark matter (no deviation from $\Lambda$CDM) and a suppressed primordial power spectrum (strongly suppressed or a negative $\mu$-parameter), demonstrating that CMB spectral distortion can potentially be used to distinguish between solutions to the small-scale crisis.