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

Damien Hutsemékers, Alexandre Payez+5 more|arXiv (Cornell University)|Sep 18, 2008

Cosmology and Gravitation Theories2 references8 citations

TL;DR

The paper presents strong evidence for large-scale alignments of quasar polarization vectors across cosmological distances (up to ~1 Gpc), with a statistical significance exceeding 99.9%. It proposes that this alignment arises from photon-pseudoscalar mixing in cosmic magnetic fields, inducing dichroism and birefringence that modulate polarization during propagation, with circular polarization as a key observational signature.

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.

Motivation & Objective

• To investigate the statistical significance and spatial coherence of quasar polarization vector orientations across the sky.
• To rule out local contamination from interstellar or instrumental polarization as the origin of observed alignments.
• To explore cosmological-scale mechanisms that could explain large-scale polarization coherence over gigaparsec distances.
• To test whether photon-pseudoscalar mixing in extragalactic magnetic fields can quantitatively reproduce the observed alignment patterns.
• To identify observable signatures, such as induced circular polarization, that could confirm the proposed mechanism.

Proposed method

• Analysis of a sample of 355 quasars with reliable optical polarization measurements and high galactic latitude (>30°), ensuring minimal interstellar contamination.
• Application of circular statistics and nearest-neighbor analysis to test the null hypothesis of random polarization angle distribution.
• Simulation of photon-pseudoscalar mixing effects in external magnetic fields using the effective Lagrangian formalism, including birefringence and dichroism.
• Modeling the evolution of Stokes parameters (q, u, v) along the line of sight, with initial conditions set to simulate intrinsic random polarization and small systematic offsets.
• Use of wavelength-dependent oscillatory effects in polarization to explain the absence of similar alignments at radio wavelengths.
• Comparison of simulated polarization patterns with observed data, particularly focusing on alternating regions of aligned and random polarization vectors.

Experimental results

Research questions

• RQ1 Is the observed alignment of quasar polarization vectors statistically significant beyond random chance?
• RQ2 Can interstellar or instrumental polarization alone explain the observed large-scale coherence in polarization angles?
• RQ3 Do the differences in mean polarization angles between low- and high-redshift quasar groups point to a cosmological-scale physical mechanism?
• RQ4 Can photon-pseudoscalar mixing in cosmic magnetic fields reproduce the observed alternation of aligned and random polarization regions?
• RQ5 What observable signatures, such as induced circular polarization, could confirm the photon-pseudoscalar mixing hypothesis?

Key findings

• The probability that the observed polarization angle distribution is due to chance is less than 0.1%, indicating strong statistical significance.
• Low-redshift quasars (z ~ 0.5) show a mean polarization angle of ~79°, while high-redshift quasars (z ~ 1.5) show a distinct mean angle of ~8°, indicating different preferred polarization directions.
• The alignment effect persists over cosmological baselines of ~1 Gpc, ruling out local contamination as the primary cause.
• Simulations show that photon-pseudoscalar mixing in external magnetic fields can produce oscillatory polarization patterns consistent with the observed alternation of aligned and random regions.
• The model predicts a non-zero circular polarization degree (v ≈ u) as a direct signature of the mixing mechanism, which could be tested with future observations.
• The required magnetic field strength (~4×10⁻¹¹ G) and coupling constant (g ~ 7×10⁻¹² GeV⁻¹) are consistent with current upper limits for cosmological magnetic fields and axion-like particles.