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

Understand

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 $k_*$ = 0.05 Mpc$^{-1}$ by $f_{\rm iso}$, Planck observations of the cosmic microwave background (CMB) yield $f_{\rm iso}$ < 0.31 at the 2$\sigma$-level.