Large-Scale Alignments of Quasar Polarization Vectors: Evidence at Cosmological Scales for Very Light Pseudoscalar Particles Mixing with Photons?

TL;DR

This study finds large-scale, non-random alignments of quasar polarization vectors over cosmological distances, suggesting potential evidence for photon-pseudoscalar particle mixing affecting light polarization.

Contribution

It provides observational evidence of quasar polarization alignments over Gpc scales and proposes photon-pseudoscalar mixing as a possible explanation, a novel interpretation in cosmology.

Findings

Polarization vectors are aligned over large sky regions with less than 0.1% probability of chance.

Different redshift groups show distinct preferred polarization directions.

Photon-pseudoscalar mixing can qualitatively explain the observed polarization alignments.

Abstract

Based on a sample of 355 quasars with significant optical polarization, we found that quasar polarization vectors are not randomly oriented over the sky as naturally expected. The probability that the observed distribution of polarization angles is due to chance is lower than 0.1%. The polarization vectors of the light from quasars are aligned although the sources span huge regions of the sky (~ 1 Gpc). Groups of quasars located along similar lines of sight but at different redshifts (typically z ~ 0.5 and z ~ 1.5) are characterized by different preferred directions of polarization. These characteristics make the observed alignment effect difficult to explain in terms of a local contamination by interstellar polarization in our Galaxy. Interpreted in terms of a cosmological-size effect, we show that the dichroism and birefringence predicted by a mixing between photons and very light pseudoscalar particles within a magnetic field can qualitatively reproduce the observations. We find that circular polarization measurements could help constrain this mechanism.