Speaker
Description
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 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 GHz and 150 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. Moreover, conversion of axions or axion-like particles (ALPs) to photons in a helical magnetic field existing prior to the CMB epoch can generate an excess population of photons carrying net circular polarization. Consequently, measurements of the CMB circular polarization can be used to constrain the parameter space of ALP mass and its coupling to photons.