Constraining circular polarization of high-frequency gravitational waves with CMB

TL;DR

This paper proposes a new method to constrain high-frequency gravitational wave circular polarization using CMB V-mode polarization measurements, providing the most stringent limits to date on the GW background in the GHz range.

Contribution

It introduces a novel approach linking graviton-photon conversion in magnetic fields to CMB polarization, enabling constraints on primordial GW circular polarization at GHz frequencies.

Findings

Set new upper limits on GW strain at 40 GHz.

Established constraints on GW circular polarization at 150 GHz.

Demonstrated the potential of CMB V-mode polarization to probe high-frequency GWs.

Abstract

Circular polarization in the cosmic microwave background (CMB) offers a promising probe of the parity-violating physics of the early universe. In this paper, we propose a novel method to constrain the primordial circular polarization of high-frequency gravitational waves (GW) in the GHz range. An efficient conversion of gravitons to photons in a transverse cosmological magnetic field at the epoch of last scattering can generate excess chiral photons if the GW background is chiral in nature. This excess radiation distorts the CMB thermal black-body spectrum, which can be estimated by measuring the V-Stokes parameter in the CMB polarization. Using current upper limits on the angular power spectrum of circular polarization $C_l^{VV}$ from the CLASS, MIPOL, and SPIDER experiments, we obtain the most stringent constraints on the characteristic strain and circular polarization of the isotropic background of stochastic GWs at ${40\,\rm GHz}$ and ${150\,\rm GHz}$, respectively. Our work, therefore, provides an interesting possibility to constrain the circular polarization of high-frequency GWs using the V-mode polarization measurements of CMB.