Chiral photons from chiral gravitational waves

Abstract

We show that a parity-breaking uniform (averaged over all directions on the sky) circular polarization of amplitude $V_{00}\simeq 2.6\times 10^{-17}\Delta \chi (r/0.06)$ can be induced by chiral gravitational-wave (GW) background with tensor-to-scalar ratio $r$ and chirality parameter $\Delta \chi$ (which is $\pm 1$ for a maximally chiral background). We also show, however, that a uniform circular polarization can arise from a realization of a non-chiral GW background that spontaneously breaks parity. The magnitude of this polarization is drawn from a distribution of root-variance $\sqrt{<V_{00}^2>}\simeq 1.5\times 10^{-18}(r/0.06{)}^{1/2}$ implying that the chirality parameter must be $\Delta \chi \gtrsim 0.12(r/0.06{)}^{-1/2}$ 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.

5 pages, v2: minor changes, version accepted for publication in PRL

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• Geometric Inflation and Dark Energy with Axion $F(R)$ Gravity
• Propagation of the gravitational waves in a cosmological background
• Polarized primordial gravitational waves in the ghost-free parity-violating gravity
• Aspects of Axion $F(R)$ Gravity
• Propagation of Gravitational Waves in Chern-Simons Axion Einstein Gravity
• Measuring Parity Asymmetry of Gravitational Wave Backgrounds with a Heliocentric Detector Network in the mHz Band
• CMB circular and B-mode polarization from new interactions
• CMB $V$ modes from photon-photon forward scattering revisited
• Physics Beyond The Standard Model with Circular Polarization in the CMB and CMB-21cm Cross-Correlation
• Gravity waves in parity-violating Copernican Universes
• Primordial Circular Polarization in the Cosmic Microwave Background
• B-mode Power Spectrum of CMB via Polarized Compton Scattering
• Cosmology of Gravi-Axions
• Unifying an asymmetric bounce to the dark energy in Chern-Simons F(R) gravity