Dark Matter Relics and the Expansion Rate in Scalar-Tensor Theories

TL;DR

This paper explores how scalar-tensor theories, motivated by string theory, affect dark matter relic abundance by modifying the universe's expansion rate through conformal and disformal couplings, revealing new dependencies on initial conditions and coupling functions.

Contribution

It analyzes the impact of conformal and disformal couplings in scalar-tensor theories on dark matter relic abundance, highlighting the importance of initial conditions and specific functional relations.

Findings

Conformal coupling alters the required annihilation cross-section for dark matter.

Disformal coupling effects depend on the relation between conformal and disformal functions.

Explicit quadratic disformal function example demonstrates the model's implications.

Abstract

We study the impact of a modified expansion rate on the dark matter relic abundance in a class of scalar-tensor theories. The scalar-tensor theories we consider are motivated from string theory constructions, which have conformal as well as disformally coupled matter to the scalar. We investigate the effects of such a conformal coupling to the dark matter relic abundance for a wide range of initial conditions, masses and cross-sections. We find that exploiting all possible initial conditions, the annihilation cross-section required to satisfy the dark matter content can differ from the thermal average cross-section in the standard case. We also study the expansion rate in the disformal case and find that physically relevant solutions require a nontrivial relation between the conformal and disformal functions. We study the effects of the disformal coupling in an explicit example where the disformal function is quadratic.