We consider the possibility that the cosmic neutrino background might have a nonthermal spectrum, and investigate its effect on cosmological parameters relative to standard Λ-CDM cosmology. As a specific model, we consider a thermal y-distortion, which alters the distribution function of the neutrino background by depleting the population of low-energy neutrinos and enhancing the high-energy tail. We constrain the thermal y-parameter of the cosmic neutrino background using Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillation (BAO) measurements. The y-parameter increases the number of effective relativistic degrees of freedom, reducing the sound horizon radius and increasing the best-fit value for the Hubble constant H0, substantially reducing the tension between CMB/BAO constraints and direct measurement of the expansion rate from Type-Ia supernovae. Including a spectral distortion also allows for a higher value of the spectral index of scalar fluctuations and relaxes the bound on the sum of the neutrino masses. We note a tension between the Λ-CDM-derived value for the primordial helium abundance and the directly measured value obtained by Pitrou et al from BBN which favors a nonzero neutrino thermal y-parameter.
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https://zoom.us/j/92016034244?pwd=b3FpeFRYSWQrb0QreFR4OTVVNXg4Zz09