Isocurvature bounds on axion-like particle dark matter in the post-inflationary scenario

Abstract

We assume that dark matter is comprised of axion-like particles (ALPs) generated by the realignment mechanism in the post-inflationary scenario. This leads to isocurvature fluctuations with an amplitude of order one for scales comparable to the horizon at the time when the ALP field starts oscillating. The power spectrum of these fluctuations is flat for small wave numbers, extending to scales relevant for cosmological observables. Denoting the relative isocurvature amplitude at $k_{\ast }$ = 0.05 Mpc${}^{-1}$ by $f_{\mathrm{iso}}$, Planck observations of the cosmic microwave background (CMB) yield $f_{\mathrm{iso}}$ < 0.31 at the 2$\sigma$-level. This excludes the hypothesis of post-inflationary ALP dark matter with masses $m_a$ < 10${}^{-20}-$10${}^{-16}$ eV, where the range is due to details of the ALP mass-temperature dependence. Future CMB stage IV and 21-cm intensity mapping experiments may improve these limits by 1$-$2 orders of magnitude in $m_a$.

24 pages, 6 figures, 2 tables; minor changes, references added

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• Constraints on interacting dark energy revisited: implications for the Hubble tension
• 21cm forest probes on the axion dark matter in the post-inflationary Peccei-Quinn symmetry breaking scenarios
• Cornering Extended Starobinsky Inflation with CMB and SKA
• Starlight from JWST: Implications for star formation and dark matter models
• Probing axion dark matter with 21cm fluctuations from minihalos
• Early universe dynamics of PQ field with very small self-coupling and its implications for axion dark matter
• How light can ALP dark matter be?
• Probing the Inflaton Potential with SKA