Cosmic Birefringence Triggered by Dark Matter Domination

TL;DR

This paper proposes a mechanism where axion-like particles coupled to dark matter induce cosmic birefringence after matter-radiation equality, with implications for dark matter composition and the QCD axion.

Contribution

It introduces a model where dark matter's hidden monopoles give ALPs a large effective mass, explaining cosmic birefringence without fine-tuning.

Findings

ALPs acquire large effective mass post-matter-radiation equality

Hidden monopoles can constitute a fraction of dark matter

The model addresses the QCD axion domain wall problem

Abstract

Cosmic birefringence is predicted if an axion-like particle (ALP) moves after the recombination. We show that this naturally happens if the ALP is coupled to the dark matter density because it then acquires a large effective mass after the matter-radiation equality. Our scenario applies to a broad range of the ALP mass $m_\phi \lesssim 10^{-28}$ eV, even smaller than the present Hubble constant. We give a simple model to realize this scenario, where dark matter is made of hidden monopoles, which give the ALP such a large effective mass through the Witten effect. The mechanism works if the ALP decay constant is of order the GUT scale without a fine-tuning of the initial misalignment angle. For smaller decay constant, the hidden monopole can be a fraction of dark matter. We also study the implications for the QCD axion, and show that the domain wall problem can be solved by the effective mass.