Large-Scale Structure Probes of the Post-Inflationary Axiverse

TL;DR

This paper investigates how post-inflationary axions influence cosmic structure formation, using galaxy luminosity data and other cosmological observations to set new limits on axion properties and related isocurvature perturbations.

Contribution

It introduces a novel method combining UV galaxy data with cosmological probes to constrain post-inflationary axion dark matter and white-noise power spectra.

Findings

Strong bounds on axion mass for dark matter fraction near unity.

Enhanced structure growth from axion-induced isocurvature fluctuations.

New constraints on white-noise spectra from high-redshift galaxy data.

Abstract

We study the cosmology of axions in the post-inflationary scenario, where random initial conditions and the ensuing string-domain-wall network generate an isocurvature power spectrum. Axions radiated from strings behave as warm, wave-like dark matter: when they constitute the full dark matter abundance, free streaming sets the strongest bounds on their mass. For subdominant fractions, despite being warm, they still lead to an overall enhancement of structure growth in the dominant component, seeded by the axion white-noise fluctuations. We search for this effect using the ultraviolet luminosity function (UVLF) of galaxies at $z=4$-$10$, probing $k\simeq0.5$-$10\,\mathrm{Mpc}^{-1}$. Combining the UVLF analysis with Lyman-$\alpha$ and CMB data yields the leading cosmological limits on post-inflationary axion dark matter, sensitive to tiny fractions for $m_a\lesssim10^{-21}\,\mathrm{eV}$. As a byproduct, we obtain new constraints on generic white-noise power spectra from the UVLF. These results apply broadly to scenarios that generate similar isocurvature perturbations, linking early-universe field dynamics to high-redshift structure formation.