Constraints on Global Symmetry Breaking in Quantum Gravity from Cosmic Birefringence Measurements

TL;DR

This paper discusses how cosmic birefringence measurements could indicate the existence of a well-preserved global symmetry in nature, with implications for quantum gravity and particle physics.

Contribution

It proposes that cosmic birefringence may signal a nearly preserved global symmetry and derives bounds on gravitational symmetry breaking effects affecting light pseudoscalar fields.

Findings

Cosmic birefringence detection suggests a light pseudoscalar field interacting with electromagnetism.

Gravitational effects on global symmetry breaking are constrained to be very small.

Implications for quantum gravity theories and particle physics models are discussed.

Abstract

All global symmetries are expected to be explicitly broken by quantum gravitational effects, and yet may play an important role in Particle Physics and Cosmology. As such, any evidence for a well-preserved global symmetry would give insight into an important feature of gravity. We argue that a recently reported $2.4\sigma$ detection of cosmic birefringence in the Cosmic Microwave Background could be the first observational indication of a well-preserved (although spontaneously broken) global symmetry in nature. A compelling solution to explain this measurement is a very light pseudoscalar field that interacts with electromagnetism. In order for gravitational effects not to lead to large corrections to the mass of this scalar field, we show that the breaking of global symmetries by gravity should be bounded above. Finally, we highlight that any bound of this type would have clear implications for the construction of theories of quantum gravity, as well as for many particle physics scenarios.