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

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 O(1 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 Planck CMB likelihood, and
in particular CMB lensing, restores strong constraints on the neutrino mass in the DDM
scenario, ∑ mν ≲ 0.079 eV. In contrast, neutrino mass constraints in a smooth dark energy
model described by the Chevallier-Polarski-Linder parametrization become merely ∼ 25%
... mehr
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.