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

Abstract

Circular polarization of the Cosmic Microwave Background (CMB) offers thepossibility of detecting rotations of the universe and magneticfields in the primeval universe or in distant clusters of galaxies. We used the MilanoPolarimeter (MIPOL) installed at the Testa Grigia Observatory, on the italian Alps,to improve the existing upper limits to the CMBcircular polarization at large angular scales. We obtain 95% confidence level upper limits to the degree of the CMB circular polarization rangingbetween 5.0⋅10−4 and 0.7⋅10−4at angular scales between 8° and 24°, improvingby one order of magnitude preexisting upper limits at large angular scales.Our results are still far from thenK region where today expectations place the amplitude of the V Stokesparameter used to characterize circular polarization of the CMB but improvethe preexisting limit at similar angular scales.Our observations offered also the opportunity of characterizingthe atmospheric emission at 33 GHz at the Testa Grigia Observatory.