Isocurvature forecast in the anthropic axion window

TL;DR

This paper assesses how well future cosmological observations can detect or constrain isocurvature fluctuations caused by axions in the high decay constant window, improving current limits significantly.

Contribution

It provides updated constraints on axion-induced isocurvature fluctuations and identifies parameter regions where future observations could detect axions.

Findings

Current data constrain isocurvature fraction to < 0.09

Planck can improve this to 0.042

Ultimate cosmic variance limited probe can reach 0.017

Abstract

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 f_a >> 10^12 GeV and the initial misalignment angle Theta_i << 1. In a minimal Lambda-CDM cosmology extended with subdominant scale-invariant isocurvature fluctuations, existing data constrain the isocurvature fraction to alpha < 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 f_a and H_I (Hubble parameter during inflation) we identify a small region where axion detection remains within the reach of realistic cosmological probes.