General Relativity and Quantum Mechanics are known to be incompatible. A unified theory that is complete and well-posed, however, is still absent. Several quantum gravitational models have been proposed, such as string theory and loop quantum gravity, but none of this is yet in a complete state of development. Lacking a concrete and complete model, it may be interesting to study particular effective theories that modify General Relativity by violating one of its fundamental principles, such as parity invariance, locality, or general covariance.

The XGI has been studying a particular class of effective theories that break parity invariance to determine how observables are modified. Parity invariance is a fundamental symmetry that states that the laws of physics are the same no matter whether one uses a left- or a right-handed coordinate system. The standard model of elementary particles, however, is known to include parity-violating interactions, which have already been measured in particle accelerators. The inclusion of parity-violating interactions in the gravitational sector induces a plethora of effects that have interesting observational consequences, such as modifications to the dragging of inertial frames and the rate of inspiral of compact objects.