Cosmology and Astrophysics of CP-Violating Axions

TL;DR

This paper investigates the behavior of CP-violating axion-like particles in astrophysical and cosmological contexts, revealing how their effective potential depends on nucleon density and impacts neutron star physics and early universe dynamics.

Contribution

It introduces a density-dependent effective potential for ALPs with CP violation, identifying a critical density that alters their stability and cosmological evolution.

Findings

Neutron stars can source a radial ALP field, offering a new probe.

ALPs may have destabilized in the early universe, enabling large field excursions.

The effective potential's shape depends on nucleon density, affecting ALP dynamics.

Abstract

We study the cosmology and astrophysics of axion-like particles (ALPs) with CP-violating Yukawa couplings to nucleons. At finite nucleon density, the ALP's dynamics is governed by an effective potential which is the sum of the bare periodic potential and a linear potential whose strength depends on the nucleon density. We identify a critical nucleon density $\rho_c$ controlling the dynamics. At densities smaller than $\rho_c$ the effective potential is a tilted sinusoidal curve and the field is displaced from its zero-density minimum. At densities larger than $\rho_c$ the minima (and maxima) are absent, and the ALP is destabilized. Astrophysically, this implies that neutron stars can source a radial ALP field, providing a complementary probe to equivalence principle tests. Cosmologically, the ALP may have been destabilized in the early Universe and could have made large field excursions. We discuss model-building applications of our results for such early universe scenarios.