Isocurvature bounds on axions revisited

Abstract

The axion is one of the best motivated candidates for particle dark matter. We study and update the constraints imposed by the recent CMB and LSS experiments on the mass of axions produced by the misalignment mechanism, as a function of both the inflationary scale and the reheating temperature. Under some particular although not unconventional assumptions, the axionic field induces too large isocurvature perturbations. Specifically, for inflation taking place at intermediate energy scales, we derive some restrictive limits which can only be evaded by assuming an efficient reheating mechanism, with T>10^{11} GeV. Chaotic inflation with a quadratic potential is still compatible with the axion scenario, provided that the Peccei-Quinn scale f_a is close to 10^{10} or 10^{11} GeV. Isocurvature bounds eliminate the possibility of a larger f_a and a small misalignment angle. We find that isocurvature constraints on the axion scenario must be taken into account whenever the scale of inflation is above 10^{12} GeV; below this scale, axionic isocurvature modes are too small to be probed by current observations.