Abstract
We show that a parity-breaking uniform (averaged over all directions on the sky) circular polarization of amplitude can be induced by a chiral gravitational-wave (GW) background with a tensor-to-scalar ratio and chirality parameter (which is for a maximally chiral background). We also show, however, that a uniform circular polarization can arise from a realization of a nonchiral GW background that spontaneously breaks parity. The magnitude of this polarization is drawn from a distribution of root variance , implying that the chirality parameter must be to establish that the GW background is chiral. Although these values are too small to be detected by any experiment in the foreseeable future, the calculation is a proof of principle that cosmological parity breaking in the form of a chiral gravitational-wave background can be imprinted in the chirality of the photons in the cosmic microwave background. It also illustrates how a seemingly parity-breaking cosmological signal can arise from parity-conserving physics.
- Received 26 November 2018
- Revised 16 May 2019
DOI:https://doi.org/10.1103/PhysRevLett.123.031305
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