Circular polarization of the CMB: Foregrounds and detection prospects

Abstract

The cosmic microwave background (CMB) is one of the finest probes of cosmology. Its all-sky temperature and linear polarization (LP) fluctuations have been measured precisely at a level of deltaT/TCMB ~10^{-6}. In comparison, circular polarization (CP) of the CMB, however, has not been precisely explored. Current upper limit on the CP of the CMB is at a level of deltaV/TCMB ~10^{-4} and is limited on large scales. Some of the cosmologically important sources which can induce a CP in the CMB include early universe symmetry breaking, primordial magnetic field, galaxy clusters and Pop III stars (also known as the First stars). Among these sources, Pop III stars are expected to induce the strongest signal with levels strongly dependent on the frequency of observation and on the number, Np, of the Pop III stars per halo. Optimistically, a CP signal in the CMB due to the Pop III stars could be at a level of deltaV/TCMB ~ 2x10^{-7} in scales of 1 degree at 10 GHz, which is much smaller than the currently existing upper limits on the CP measurements. Primary foregrounds in the cosmological CP detection will come from the galactic synchrotron emission (GSE), which is naturally (intrinsically) circularly polarized. We use data-driven models of the galactic magnetic field (GMF), thermal electron density and relativistic electron density to simulate all-sky maps of the galactic CP in different frequencies. This work also points out that the galactic CP levels are important below 50 GHz and is an important factor for telescopes aiming to detect primordial B-modes using CP as a systematics rejection channel. Final results on detectability are summarized in Fig (11-13).