Mass-varying Dark Matter from a Phase Transition

TL;DR

This paper introduces a mass-varying dark matter model with a scalar and fermionic field that undergoes a phase transition, evolving from radiation-like behavior to pressureless dark matter, affecting structure formation and observational measurements.

Contribution

It presents a novel scalar-fermion dark matter model with a phase transition mechanism that explains mass variation and impacts cosmological observations.

Findings

Dark matter relic density achieved for fermion masses 1 GeV to 10^9 GeV.

Scalar component reduces structure formation amplitude by a few percent.

Mass-varying fermion causes ~10% discrepancy in dark matter density measurements.

Abstract

We propose a mass-varying dark matter (MVDM) model consisting of a scalar field and a fermionic field interacting via a simple Yukawa coupling, and containing an exponential self-interaction potential for the scalar field. Analyzing the evolution of this coupled scalar-fermion system in an expanding Universe, we find that it initially behaves like radiation but then undergoes a phase transition after which it behaves like pressureless dark matter. The one free parameter of this model is the temperature at which the phase transition occurs; the mass of the dark matter particle, given by the mass of the fermion, is derived from this. For a phase transition temperature between 10 MeV and $10^7$ GeV, the current dark matter relic density is achieved for a fermion mass in the range of 1 GeV to $10^9$ GeV. In this dark matter model, the scalar becomes a sub-dominant unclustered component of dark matter that can lower the amplitude of structure formation by up to a few percent. Another feature is that the mass-varying fermion component can lead to discrepant measurements of the current dark matter density of about ten percent inferred from early and late-time probes assuming LCDM.