Can axion-like particles explain the alignments of the polarisations of light from quasars?

TL;DR

This paper evaluates whether axion-like particles can explain quasar polarisation alignments, concluding that recent measurements and more complex models challenge this hypothesis.

Contribution

It introduces a wave-packet formalism and assesses the impact of decoherence and extragalactic fields on axion-like particle explanations.

Findings

Standard axion-like particle models are incompatible with recent circular polarisation measurements.

Decoherence effects reduce circular polarisation but do not support axion-like particles as the cause.

More sophisticated extragalactic field models do not produce polarisation alignments.

Abstract

The standard axion-like particle explanation of the observed large-scale coherent orientations of quasar polarisation vectors is ruled out by the recent measurements of vanishing of circular polarisation. We introduce a more general wave-packet formalism and show that, although decoherence effects between waves of different frequencies can reduce significantly the amount of circular polarisation, the axion-like particle hypothesis is disfavoured given the bandwidth with which part of the observations were performed. Finally, we show that a more sophisticated model of extragalactic fields does not lead to an alignment of polarisations.