Probing Axions through Tomography of Anisotropic Cosmic Birefringence

TL;DR

This paper introduces a tomographic method to analyze anisotropic cosmic birefringence caused by axion-like fields, revealing how different epochs and axion masses influence the birefringence signal, thus providing new insights into fundamental physics.

Contribution

It pioneers a tomographic analysis of anisotropic cosmic birefringence, linking it to axion-like fields and their mass-dependent effects across cosmic epochs.

Findings

An increase in axion mass enhances anisotropic birefringence amplitude.

Reionization and recombination contributions to birefringence vary with axion mass.

Low multipole birefringence signals are dominated by reionization for light axions.

Abstract

Cosmic birefringence is the in-vacuo rotation of the linear polarization plane experienced by photons of the Cosmic Microwave Background (CMB) radiation when theoretically well-motivated parity-violating extensions of Maxwell electromagnetism are considered. If the angle, parametrizing such a rotation is dependent on the photon's direction, then this phenomenon is called Anisotropic Cosmic Birefringence (ACB). In this paper, we perform for the first time a tomographic treatment of the ACB, by considering photons emitted both at the recombination and reionization epoch. This allows one to extract additional and complementary information about the physical source of cosmic birefringence with respect to the isotropic case. We focus here on the case of an axion-like field $\chi$, whose coupling with the electromagnetic sector induces such a phenomenon, by using an analytical and numerical approach (which involves a modification of the CLASS code). We find that the anisotropic component of cosmic birefringence exhibits a peculiar behavior: an increase of the axion mass implies an enhancement of the anisotropic amplitude, allowing to probe a wider range of masses with respect to the purely isotropic case. Moreover, we show that at large angular scales, the interplay between the reionization and recombination contributions to ACB is sensitive to the axion mass, so that at sufficiently low multipoles, for sufficiently light masses, the reionization contribution overtakes the recombination one, making the tomographic approach to cosmic birefringence a promising tool for investigating the properties of this axion-like field.