Cosmic Birefringence Test of the Hubble Tension

TL;DR

This paper proposes that an axion-like early dark energy component could cause observable cosmic birefringence in the CMB, providing a test for resolving the Hubble tension with future experiments.

Contribution

It introduces the idea that axion-like early dark energy may induce detectable birefringence signals in CMB polarization, linking dark energy models to observable electromagnetic effects.

Findings

CMB polarization rotation can reveal the presence of early dark energy.

Future CMB-S4 data will significantly enhance detection sensitivity.

Birefringence measurements can distinguish between Hubble tension solutions.

Abstract

An early dark energy component consisting of a cosmic pseudo Nambu-Goldstone boson has been recently proposed to resolve the Hubble tension -- the four-sigma discrepancy between precision measurements of the expansion rate of the universe. Here we point out that such an axion-like component may be expected to couple to electromagnetism by a Chern-Simons term, and will thereby induce an anisotropic cosmic birefringence signal in the polarization of the cosmic microwave background (CMB). We show that observations of the rotation-angle power spectrum and cross-correlation with CMB temperature anisotropy can confirm the presence of this early dark energy component. Future CMB data as expected from the CMB-S4 experiment will improve sensitivity to this effect by two orders of magnitude and help in discriminating between different Hubble tension scenarios.