Neutrino scalar mediator in unimodular gravity drives dynamical dark energy with 2-sigma preference for Gs ~10^12 eV^-2

The authors construct one- and two-neutrino realizations where a light scalar couples neutrinos, inducing temperature-dependent masses that source an effective interaction current violating stress-energy conservation only in the unimodular framework. This yields dynamical dark energy whose density either decreases steadily or reaches a maximum near intermediate redshifts before declining. Constraints from late-time probes favor Gs approximately 10^12 eV^-2 at 2 sigma for lightest neutrino masses 0.05-1 meV, with the coupling strength declining as assumed mass increases.

Unimodular gravity supplies the key non-conservation mechanism absent in standard general relativity, allowing neutrino interactions to source dark energy without additional ad hoc fields. The two-neutrino case introduces non-monotonic evolution that can alleviate certain low-redshift tensions while remaining compatible with data. Planck distance priors visibly shrink degeneracies and pull the reconstructed equation of state closer to a cosmological constant.

The analysis remains purely background-level; perturbation evolution and full Boltzmann integration are omitted, limiting claims about structure formation. Future high-precision BAO and supernova surveys can test the predicted peak in dark energy density at z approximately 1 if the two-neutrino scenario holds.

A natural extension would embed the scalar mediator within seesaw mechanisms, predicting correlated signatures in neutrinoless double-beta decay rates testable by next-generation experiments.