Neutrino Dark Energy -- Revisiting the Stability Issue

TL;DR

This paper examines the stability of models linking neutrino masses to dark energy via scalar fields, identifying conditions under which these models remain stable or become unstable based on perturbation analysis.

Contribution

It provides a model-independent analysis of the stability criteria for scalar-neutrino coupling models of dark energy, including explicit stability constraints.

Findings

Stability depends on the scalar-neutrino coupling and energy density ratios.

Certain parameter regimes lead to unstable hydrodynamic perturbations.

The paper offers examples illustrating stable and unstable models.

Abstract

A coupling between a light scalar field and neutrinos has been widely discussed as a mechanism for linking (time varying) neutrino masses and the present energy density and equation of state of dark energy. However, it has been pointed out that the viability of this scenario in the non-relativistic neutrino regime is threatened by the strong growth of hydrodynamic perturbations associated with a negative adiabatic sound speed squared. In this paper we revisit the stability issue in the framework of linear perturbation theory in a model independent way. The criterion for the stability of a model is translated into a constraint on the scalar-neutrino coupling, which depends on the ratio of the energy densities in neutrinos and cold dark matter. We illustrate our results by providing meaningful examples both for stable and unstable models.