Time-varying neutrino mass from a supercooled phase transition: Current cosmological constraints and impact on the Ωm−σ8 plane

Abstract

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.[1] 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 as of the phase transition to be very large, peaking at as ∼ 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 mν < 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 Ωm-σ8 discrepancies.

Reconstruction of the neutrino mass as a function of redshift

“…On the other hand, the upper cosmological neutrino mass bound is sensitive to a number of model assumptions and can be slightly relaxed, for example when the dark energy equation of state is allowed to vary in time [14][15][16][17][18][19][20], when the curvature of the Universe is not fixed [20], when considering additional relativistic degrees of freedom [4], or when assuming non-standard momentum distributions of the cosmic neutrinos [21]. Moreover, it has been shown that the cosmological neutrino mass bound can be substantially weakened when neutrinos are unstable and thus their lifetime is smaller than the age of the Universe [22][23][24][25][26] or when neutrino masses are varying in time [9,[27][28][29]. Note, however, that the 1 By free-streaming of the cosmic neutrino background before photon decoupling [6,7], by the resulting phase shift in the CMB peaks [8], and by precise CMB measurements of the effective number of species in the early Universe [4].…”

“…strongly relaxed neutrino mass bounds of m ν < 0.9 eV (95% CL) with neutrino decays [23] or m ν < 4.8 eV (95% CL) with time-varying neutrino masses [29] have been derived from previous releases of cosmological data, including the Planck 2015 CMB data. The neutrino mass bound has been substantially tightened with the Planck 2018 release, m ν =0.12 eV (95% CL, Planck 2018 TT-TEEE+lowE+lensing+BAO [4]) and therefore, we expect the above-mentioned bounds also to change.…”

“…In a previous publication [29], some of us investigated how the standard cosmological neutrino mass bound would be affected if neutrino masses were to be generated late in the Universe. This analysis suggested that the combination of current CMB temperature, polarization and lensing data, as well as BAO and supernovae (SN) data prefers neutrino masses to be generated at low redshifts, allowing a significantly larger cosmological neutrino mass upper bound.…”

Time- and space-varying neutrino masses from soft topological defects

“…Thus, almost all neutrinos convert into dark radiation in the late Universe, apart from a negligibly small freezeout density. This late "neutrinoless Universe" scenario can only be evaded in the hypothetical presence of neutrino asymmetries [20]. The absolute neutrino mass scale is constrained by m ν T CMB ∼ 0.3 eV if the phase transition takes place instantaneously at a temperature [1,21] so as not to conflict with CMB observations.…”

“…The absolute neutrino mass scale is constrained by m ν T CMB ∼ 0.3 eV if the phase transition takes place instantaneously at a temperature [1,21] so as not to conflict with CMB observations. However, the phase transition can also be supercooled and thus can give rise to relatively large neutrino masses even at a low apparent transition temperature, T ΛG < Λ G ∼ m ν 1.5 eV at 95% CL [20,22]. Such a supercooling mechanism would allow for substantial energy densities of the φ k bosons and the topological defects after the transition [23].…”