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

Understand

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.