Isocurvature forecast in the anthropic axion window

We explore the cosmological sensitivity to the amplitude of isocurvature fluctuations that would be caused by axions in the “anthropic window” where the axion decay constant fa ≫ 10 GeV and the initial misalignment angle Θi ≪ 1. In a minimal ΛCDM cosmology extended with subdominant scale-invariant isocurvature fluctuations, existing data constrain the isocurvature fraction to α < 0.09 at 95% C.L. If no signal shows up, Planck can improve this constraint to 0.042 while an ultimate CMB probe limited only by cosmic variance in both temperature and E-polarisation can reach 0.017, about a factor of five better than the current limit. In the parameter space of fa and HI (Hubble parameter during inflation) we identify a small region where axion detection remains within the reach of realistic cosmological probes. Isocurvature forecast in the anthropic axion window 2