Sterile Neutrino Dark Matter as a Probe of Inflationary Reheating
A new study published in the Journal of Cosmological and Astroparticle Physics finds that allowing a fraction of dark matter to decay into massless dark radiation can, under certain data combinations, permit neutrino masses as large as ~1 eV — far above standard cosmological limits. However, this loosening of constraints disappears once measurements sensitive to the growth of cosmic structure are included, restoring a tight upper bound of roughly 0.079 eV on the sum of neutrino masses. The findings underscore that robust neutrino mass bounds require structure-growth observables, not just background-density measurements.
Cosmological observations have long placed tight upper bounds on the sum of neutrino masses, but those bounds depend on assumptions about the dark sector. This study revisits those constraints by introducing a decaying dark matter (DDM) component that converts into massless dark radiation, which can compensate for the late-time increase in matter density caused by neutrinos becoming non-relativistic. Using only background-level data — baryonic acoustic oscillations (BAO), Type Ia supernovae, and a CMB distance prior — the authors show that neutrino masses up to O(1 eV) become statistically acceptable, and that the DDM model even reduces the need for dynamical dark energy with phantom crossing. In contrast, a smooth dark energy model described by the Chevallier-Polarski-Linder parametrization only weakens neutrino mass constraints by about 25% in background-only analyses. Crucially, once the full Planck CMB likelihood — including CMB lensing, which probes structure growth — is incorporated, the DDM-neutrino mass degeneracy is decisively broken and the neutrino mass bound tightens back to Σmν ≲ 0.079 eV. The results highlight that perturbation-level observables, particularly CMB lensing and structure growth measurements, are essential for obtaining reliable neutrino mass constraints in extended dark sector models.
What's missing
The analysis assumes specific DDM decay channels (to massless dark radiation) and does not explore whether decays to massive or interacting products would alter the conclusions.
What different sources said
- arXiv astro-phCenter
Probing the Perturbative Reheating History of Decaying Oscillatory Inflation with ACT Constraints
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