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