Neutrino mass limits and decaying dark matter: background evolution versus perturbations

TL;DR

This paper investigates how decaying dark matter affects cosmological neutrino mass bounds, showing that including perturbation data is crucial for robust constraints and highlighting the importance of structure-growth measurements.

Contribution

It demonstrates that background-only analyses can underestimate neutrino mass bounds in decaying dark matter models, emphasizing the role of perturbation data in constraining neutrino masses.

Findings

Decaying dark matter can mask neutrino mass effects in background data.

Including perturbation data restores strong neutrino mass constraints.

Structure-growth measurements are essential for robust dark sector and neutrino mass bounds.

Abstract

We revisit cosmological neutrino mass bounds when a fraction of dark matter is allowed to decay to massless dark radiation. By compensating the late-time increase in the matter density induced by neutrinos becoming non-relativistic, decaying dark matter (DDM) can render datasets solely sensitive to the background density effectively insensitive to neutrino masses. Using data from baryonic acoustic oscillations (BAO) and Type Ia supernovae together with a distance prior from the cosmic microwave background (CMB), we find that neutrino masses as large as ${\cal O}(1\,\mathrm{eV})$ are allowed without degrading the fit. Moreover, the combination of BAO data with the CMB distance prior yields a preference for a non-zero DDM fraction, and alleviates the need for dynamical dark energy with phantom crossing. However, the degeneracy introduced by DDM is decisively broken once perturbation observables are included. Incorporating the full $\textit{Planck}$ CMB likelihood, and in particular CMB lensing, restores strong constraints on the neutrino mass in the DDM scenario, $\sum m_\nu \lesssim 0.079\,\mathrm{eV}$. In contrast, neutrino mass constraints in a smooth dark energy model described by the Chevallier-Polarski-Linder parametrization become merely $\sim 25\%$ stronger compared to background-only analyses. Our results highlight the essential role of structure-growth measurements in assessing extensions of the dark sector and to obtain robust cosmological neutrino mass bounds.