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Preinflationary QCD axion stars after moduli domination

Edward Hardy*, Noelia Sánchez González†, Henry Stubbs‡, and Lorenzo Tranchedone§

  • Rudolf Peierls Centre for Theoretical Physics, Department of Physics, University of Oxford, Parks Road, OX1 3PU, United Kingdom

  • *Contact author: edward.hardy@physics.ox.ac.uk
  • †Contact author: noelia.sanchezgonzalez@physics.ox.ac.uk
  • ‡Contact author: henry.stubbs@physics.ox.ac.uk
  • §Contact author: lorenzo.tranchedone@physics.ox.ac.uk
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Phys. Rev. D 113, 115052 – Published 22 June, 2026

DOI: https://doi.org/10.1103/23zs-8wmn

Abstract

The growth of adiabatic density perturbations during an era of early matter domination induces O(1) fluctuations in preinflationary QCD axion dark matter across a broad, string-theory-motivated parameter space. Remarkably, at ΛCDM matter-radiation equality the scale of these perturbations coincides with the quantum Jeans scale, so they collapse to solitonic “axion stars.” These axion stars have densities up to 104  eV4, and, including their surrounding halos, they contain as much as 50% of dark matter. Direct searches for a smooth axion background can be suppressed, but transient enhancements or indirect astrophysical signals at axion masses ma≲10−5  eV would point to a nonstandard cosmological history.

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Phys. Rev. D 113, 115052– Published 22 June, 2026
  • Received 7 May 2026
  • Accepted 2 June 2026
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