Cosmic Birefringence from the Axiverse

TL;DR

This paper explores how cosmic birefringence measurements can constrain the statistical distribution of axion parameters in the Axiverse, using various axion potential models and predicting future tomography prospects.

Contribution

It introduces a method to relate observed birefringence to axion parameter PDFs, analyzing different potential types and forecasting the impact of future measurements.

Findings

Birefringence constrains axion parameter distributions.

Different axion potentials produce distinct birefringence signatures.

Future tomography can rule out simple Axiverse models.

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 the cosmic birefringence angle and investigate its properties. By relating the observed value of the birefringence angle to the mean or standard deviation of the constructed PDF, we constrain the shape of the input PDFs, providing insights into the statistical distribution of the Axiverse. We focus on three different types of axion potentials: cosine, quadratic, and asymptotically linear axion monodromy. Our analysis showcases the potential of cosmic birefringence in constraining the distribution of axion parameters and uncovering possible correlations among them. We additionally offer predictions for "birefringence tomography," anticipating future measurements of birefringence from lower multipoles, and show how it can be used to rule out simpler versions of the Axiverse. Our findings contribute to the ongoing exploration of the Axiverse and its implications for cosmic birefringence.