Does the CMB prefer a leptonic Universe?

TL;DR

Recent CMB and primordial element data suggest a possible negative lepton asymmetry in the Universe, indicating an excess of anti-neutrinos and challenging standard assumptions about neutrino chemical potentials.

Contribution

This paper reanalyzes observational bounds on neutrino chemical potentials considering neutrino oscillations, revealing a potential negative lepton asymmetry.

Findings

Preference for negative neutrino chemical potentials

Lepton asymmetry may exceed baryon asymmetry

Implications for neutrino physics and cosmology

Abstract

Recent observations of the cosmic microwave background (CMB) at smallest angular scales and updated abundances of primordial elements, indicate an increase of the energy density and the helium-4 abundance with respect to standard big bang nucleosynthesis with three neutrino flavour. This calls for a reanalysis of the observational bounds on neutrino chemical potentials, which encode the number asymmetry between cosmic neutrinos and anti-neutrinos and thus measures the lepton asymmetry of the Universe. We compare recent data with a big bang nucleosynthesis code, assuming neutrino flavour equilibration via neutrino oscillations before the onset of big bang nucleosynthesis. We find a slight preference for negative neutrino chemical potentials, which would imply an excess of anti-neutrinos and thus a negative lepton number of the Universe. This lepton asymmetry could exceed the baryon asymmetry by orders of magnitude.