Duality Symmetry and Anomaly for Gravitational Waves in Curved Spacetime

TL;DR

This paper explores the electric-magnetic duality symmetry in linearized gravitational waves, showing it leads to a conserved charge in flat spacetime but is anomalously broken in curved backgrounds, implying possible chiral imbalances.

Contribution

It develops a framework to explicitly identify gravitoelectric and gravitomagnetic components and demonstrates the duality symmetry as a Noether symmetry with a conserved charge, also analyzing its quantum anomaly in curved spacetime.

Findings

Duality symmetry is a Noether symmetry with a conserved charge in flat spacetime.

The symmetry is anomalously broken in curved backgrounds, leading to potential chiral imbalance.

A gravitational chiral anomaly analogous to fermionic and photon cases is identified.

Abstract

The vacuum Einstein equations admit a formulation closely analogous to the source-free Maxwell theory. In particular, the linearized equations exhibit an electric-magnetic duality symmetry. We develop a framework that makes this analogy manifest by explicitly identifying the electric and magnetic components of perturbative gravitational waves. Within this formulation, we show that duality rotations between these gravitoelectric and gravitomagnetic fields constitute a Noether symmetry of the linearized theory, and we derive the associated conserved current. The corresponding conserved charge encodes the difference in intensity between the right- and left-handed circularly polarized components of the gravitational wave - that is, between its self-dual and anti-self-dual parts. Remarkably, this conservation law remains valid even when the gravitational perturbations propagate on generic curved backgrounds. We then investigate whether this symmetry survives quantization. While the duality symmetry is preserved at the quantum level in flat spacetime, we find that it is anomalously broken in curved backgrounds. As a result, an imbalance between right- and left-handed gravitons could be excited from the vacuum. This effect represents a chiral anomaly for massless spin-two fields, generalizing known results for fermions and spin-one photon fields.