Effects of Lighter-than-QCD Axions on Neutron Star Tidal Deformability
Abstract
Finite density corrections to the lighter-than-QCD axion can invert the effective axion potential, sourcing a non-trivial axion field inside dense objects. We perform the first numerical study of the complete dynamics of the lighter-than-QCD axion in a neutron star in 1+1 general relativity, extending the region of analysis to low-mass axions with kilometer-scale Compton wavelengths. We calculate gravitational effects of the axion field on the neutron star and show that for a broad range of axion masses and decay constants, neutron star properties, such as the mass, radius, and compactness, are affected at the order-1 level. This result indicates that approximate universal tidal deformability-compactness relation for neutron stars is non-trivially broken and can serve as a probe of lighter-than-QCD axions, independent of the unknown nuclear equation of state. We comment on the potential for axion studies with future gravitational-wave observations of neutron stars and applications of this work to other new physics signatures.
Cite
@article{arxiv.2511.13812,
title = {Effects of Lighter-than-QCD Axions on Neutron Star Tidal Deformability},
author = {Yonatan Kahn and Michael Wentzel and Nicolás Yunes},
journal= {arXiv preprint arXiv:2511.13812},
year = {2025}
}
Comments
18 pages, 10 figures, 5 pages of appendices