An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853

TL;DR

This paper investigates how nonminimal torsion-electromagnetism coupling could cause spacetime birefringence, leading to depolarization of light from astrophysical sources, and uses white dwarf data to set limits on this effect.

Contribution

It provides the first observational constraints on gravity-induced birefringence from nonminimal torsion coupling using white dwarf polarimetry.

Findings

Constraints on coupling constant: k^2 slim; (19 m)^2

No significant birefringence detected in white dwarf data

Supports the viability of certain gravitational theories with torsion

Abstract

The coupling of the electromagnetic field directly with gravitational gauge fields leads to new physical effects that can be tested using astronomical data. Here we consider a particular case for closer scrutiny, a specific nonminimal coupling of torsion to electromagnetism, which enters into a metric-affine geometry of space-time. We show that under the assumption of this nonminimal coupling, spacetime is birefringent in the presence of such a gravitational field. This leads to the depolarization of light emitted from extended astrophysical sources. We use polarimetric data of the magnetic white dwarf ${RE J0317-853}$ to set strong constraints on the essential coupling constant for this effect, giving $k^2 \lsim (19 {m})^2 $.