Dark matter relic density in scalar-tensor gravity revisited

TL;DR

This paper reevaluates dark matter relic density calculations within scalar-tensor gravity, showing BBN constraints limit enhancements to relic density to about a factor of three, much lower than previously thought.

Contribution

It provides a revised analysis of dark matter relic abundance in scalar-tensor gravity models considering BBN constraints, reducing the maximum enhancement estimate.

Findings

BBN constraints restrict the expansion rate to be nearly standard during dark matter decoupling.

Maximum relic density enhancement is about a factor of 3, much lower than earlier estimates.

Scalar-tensor models can still significantly modify relic densities within observational bounds.

Abstract

We revisit the calculation of dark matter relic abundances in scalar-tensor gravity using a generic form $A(\varphi_*) = e^{\beta\varphi_*^2/2}$ for the coupling between the scalar field $\varphi_*$ and the metric, for which detailed Big Bang Nucleosynthesis constraints are available. We find that BBN constraints restrict the modified expansion rate in these models to be almost degenerate with the standard expansion history at the time of dark matter decoupling. In this case the maximum level of enhancement of the dark matter relic density was found to be a factor of $\sim 3$, several orders of magnitude below that found in previous investigations.