# The Cosmology of Sub-MeV Dark Matter

## TL;DR

This paper investigates sub-MeV dark matter models, highlighting their cosmological constraints, potential signals in future CMB observations, and differences in detectability via nuclear interactions versus absorption signatures.

## Contribution

It explores the viable parameter space for sub-MeV dark matter models, considering cosmological, astrophysical, and laboratory constraints, and predicts observable effects on neutrino counts.

## Key findings

- Nuclear cross-sections above the neutrino floor imply a $ riangle N_{eff} = 0.09$ detectable by future CMB experiments.
- Models with dark matter absorption signatures are less constrained by cosmology.
- The parameter space for electron-coupled dark matter is highly restricted, while nuclear-coupled models are less so.

## Abstract

Light dark matter is a compelling experimental target in light of stringent constraints on heavier WIMPs. However, for a sub-MeV WIMP, the universe is sufficiently well understood at temperatures below 10 MeV that there is no room for it to be a thermal relic. Avoiding thermalization is itself a strong constraint with significant implications for direct detection. In this paper, we explore the space of models of sub-MeV dark matter with viable cosmologies. The parameter space of these models that is also consistent with astrophysical and lab-based limits is highly restricted for couplings to electrons but somewhat less constrained for nuclei. We find that achieving nuclear cross-sections well-above the neutrino floor necessarily predicts a new contribution to the effective number of neutrino species, $\Delta N_{\rm eff} = 0.09$ that will be tested by the next generation of CMB observations. On the other hand, models with absorption signatures of dark matter are less restricted by cosmology even with future observations.