Pre-inflationary QCD axion stars after moduli domination
Edward Hardy, Noelia Sánchez González, Henry Stubbs, Lorenzo Tranchedone
TLDR
Early matter domination leads to pre-inflationary QCD axion dark matter forming dense axion stars, potentially comprising 50% of dark matter.
Key contributions
- Early matter domination induces significant fluctuations in pre-inflationary QCD axion dark matter.
- These fluctuations collapse into dense solitonic "axion stars" at matter-radiation equality.
- Axion stars, including halos, can comprise up to 50% of dark matter with high densities.
- This could suppress direct axion searches but enable indirect astrophysical detection.
Why it matters
This paper proposes a novel mechanism for axion dark matter formation, explaining how early universe conditions lead to dense axion stars. It suggests new avenues for detecting axions, shifting focus from smooth backgrounds to transient or indirect signals, which is crucial for understanding dark matter.
Original Abstract
The growth of adiabatic density perturbations during an era of early matter domination induces $\mathcal{O}(1)$ fluctuations in pre-inflationary QCD axion dark matter across a broad, string-theory-motivated parameter space. Remarkably, at $Λ$CDM 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 $10^4\,\mathrm{eV}^4$, 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 $m_a\lesssim 10^{-5}\,{\rm eV}$ would point to a non-standard cosmological history.
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