An improved upper limit to the CMB circular polarization at large angular scales

TL;DR

This study used the Milano Polarimeter to significantly improve the upper limits on the cosmic microwave background's circular polarization at large angular scales, providing tighter constraints for cosmological models.

Contribution

It presents the first one-order-of-magnitude improvement in upper limits of CMB circular polarization at large scales using ground-based observations.

Findings

Upper limits range between 5.0x10^{-4} and 0.7x10^{-4} at 8-24 degrees

Results are still above the nK level expected by theories

Atmospheric emission at 33 GHz was characterized

Abstract

Circular polarization of the Cosmic Microwave Background (CMB) offers the possibility of detecting rotations of the universe and magnetic fields in the primeval universe or in distant clusters of galaxies. We used the Milano Polarimeter (MIPOL) installed at the Testa Grigia Observatory, on the italian Alps, to improve the existing upper limits to the CMB circular polarization at large angular scales. We obtain 95% confidence level upper limits to the degree of the CMB circular polarization ranging between 5.0x10^{-4} and 0.7x10^{-4} at angular scales between 8 and 24 deg, improving by one order of magnitude preexisting upper limits at large angular scales. Our results are still far from the nK region where today expectations place the amplitude of the V Stokes parameter used to characterize circular polarization of the CMB but improve the preexisting limit at similar angular scales. Our observations offered also the opportunity of characterizing the atmospheric emission at 33 GHz at the Testa Grigia Observatory.