Chiral anomalies induced by gravitational waves

Chiral symmetries in field theory are typically affected by an anomaly in the quantum theory. This anomaly emerges when one introduces an interaction with a Yang-Mills or gravitational background. Physical applications of this quantum effect have been traditionally connected to topological questions of the background field and the study of instantons. We show here how one can alternatively find situations of physical interest that only involve ordinary, but dynamical solutions of the background field equations. More precisely, we show that solutions to the Einstein (Maxwell) equations are able to trigger the chiral anomaly if and only if they admit a flux of gravitational (electromagnetic) radiation with net circular polarization. As a consequence, astrophysical systems that admit such radiation spontaneously generate a flux of particles with net helicity from the quantum vacuum.