Is the CMB telling us that dark matter is weaker than weakly interacting?

TL;DR

This paper explores how decays of heavy particles in the early universe could influence dark matter abundance and interactions, suggesting weaker dark matter interactions than previously thought due to early universe modifications.

Contribution

It introduces a scenario where early universe decays affect dark matter relic abundance, implying lower annihilation cross sections than standard models.

Findings

Decays of heavy states can dilute dark matter density.

Dark matter may need to have weaker interactions to match observations.

Implications for dark matter detection strategies.

Abstract

If moduli, or other long-lived heavy states, decay in the early universe in part into light and feebly interacting particles (such as axions), these decay products could account for the additional energy density in radiation that is suggested by recent measurements of the CMB. These moduli decays will also, however, alter the expansion history of the early universe, potentially diluting the thermal relic abundance of dark matter. If this is the case, then dark matter particles must annihilate with an even lower cross section than required in the standard thermal scenario (sigma v < 3x10^-26 cm^3/s) if they are to make up the observed density of dark matter. This possibility has significant implications for direct and indirect searches for dark matter.