In this paper we investigate a time-varying neutrino mass model , motivated by the mild tension between cosmic microwave background ( CMB ) measurements of the matter fluctuations and those obtained from low-redshift data .
We modify the minimal case of the model proposed in Ref . [ ] that predicts late neutrino mass generation in a post-recombination cosmic phase transition , by assuming that neutrino asymmetries allow for the presence of relic neutrinos in the late-time Universe .
We show that , if the transition is supercooled , current cosmological data ( including CMB temperature , polarization and lensing , baryon acoustic oscillations , and Type Ia supernovae ) prefer the scale factor a _ { s } of the phase transition to be very large , peaking at a _ { s } \sim 1 , and therefore supporting a cosmological scenario in which neutrinos are almost massless until very recent times .
We find that in this scenario the cosmological bound on the total sum of the neutrino masses today is significantly weakened compared to the standard case of constant-mass neutrinos , with \sum m _ { \nu } < 4.8 eV at 95 % confidence , and in agreement with the model predictions .
The main reason for this weaker bound is a large correlation arising between the dark energy and neutrino components in the presence of false vacuum energy that converts into the non-zero neutrino masses after the transition .
This result provides new targets for the coming KATRIN and PTOLEMY experiments .
We also show that the time-varying neutrino mass model considered here does not provide a clear explanation to the existing cosmological \Omega _ { m } - \sigma _ { 8 } discrepancies .