Light axion-like dark matter must be present during inflation
AbstractAxion-like particles (ALPs) might constitute the totality of the cold dark matter (CDM) observed. The parameter space of ALPs depends on the mass of the particle m and on the energy scale of inflation HI, the latter being bound by the nondetection of primordial gravitational waves. We show that the bound on HI implies the existence of a mass scale m¯χ=10 neV–0.5 peV, depending on the ALP susceptibility χ, such that the energy density of ALPs of mass smaller than m¯χ is too low to explain the present CDM budget, if the ALP field has originated after the end of inflation. This bound affects ultra-light axions (ULAs), which have recently regained popularity as CDM candidates. Light (m<mχ) ALPs can then be CDM candidates only if the ALP field has already originated during the inflationary period, in which case the parameter space is constrained by the nondetection of axion isocurvature fluctuations. We comment on the effects on these bounds from additional physics beyond the standard model, besides ALPs.