# Birefringent light propagation on anisotropic cosmological backgrounds

## TL;DR

This paper investigates electromagnetic wave behavior in anisotropic cosmological backgrounds, revealing birefringence, dispersion, and deviations from null geodesics, which could impact understanding of cosmic redshift and horizon issues.

## Contribution

It provides exact solutions for electromagnetic waves in anisotropic Bianchi I spacetimes, highlighting polarization-dependent effects and non-null geodesic propagation.

## Key findings

- Electromagnetic waves exhibit birefringence and dispersion in anisotropic backgrounds.
- Wave propagation deviates from null geodesics, affecting redshift calculations.
- Results may offer insights into the horizon problem in cosmology.

## Abstract

Exact electromagnetic wave solutions to Maxwell equations on anisotropic Bianchi I cosmological spacetime backgrounds are studied. The waves evolving on Bianchi I spacetimes exhibit birefringence (associated to linear polarization) and dispersion. The particular case of a vacuum--dominated anisotropic Universe, which reproduces a Friedmann-Robertson-Walker Universe (for late times) while for earlier times it matches a Kasner Universe, is studied. The electromagnetic waves do not, in general, follow null geodesics. This produces a modification of the cosmological redshift, which is now dependent on light polarization and dispersion and its non-null geodesic behavior. New results presented here may help to tackle some issues related to the "horizon" problem.