Evidence for cosmic neutrino background from CMB circular polarization

The primordial anisotropies of the cosmic microwave background are linearly polarized via Compton-scattering. On the other hand, a primordial degree of circular polarization of the Cosmic Microwave Background is not observationally excluded. In this work, we discuss the generation of the circular polarization of CMB via their scattering on the cosmic neutrino background since the epoch of recombination. We show that the photon–neutrino interaction can transform plane polarization into circular polarization through processes $$\gamma +\nu \rightarrow \gamma +\nu $$ and the Stokes-$$V$$ parameter of CMB has a linear dependence on the wavelength and square dependence on the average bulk velocity of the cosmic neutrino background and also the maximum value of $$C^V$$ is estimated in range of a few nK square