Cosmic birefringence from the Axiverse

Abstract

We revisit the evidence for CMB birefringence in the context of a rich Axiverse. Using probability density functions (PDFs) for various axion parameters,such as the mass and axion decay constant, we construct the PDF for thecosmic birefringence angle and investigate its properties.By relating the observed value of the birefringence angle to the mean or standard deviationof the constructed PDF, we constrain the shape of the input PDFs, providing insightsinto the statistical distribution of the Axiverse. We focus on three different types of axionpotentials: cosine, quadratic, and asymptotically linear axion monodromy.Ouranalysis showcases the potential of cosmic birefringence in constraining the distributionof axion parameters and uncovering possible correlations among them.We additionally offerpredictions for “birefringence tomography”, anticipating future measurements of birefringence fromlower multipoles, and show how it can be used to rule out simpler versions of the Axiverse. Our findings contribute to theongoing exploration of the Axiverse and its implications for cosmic birefringence.