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

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, $\sum m_ν\lesssim 0.079\,\mathrm{eV}$. 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.

Figures (13)

• Figure 1: Fractional change to the Hubble rate in a cosmology with massive neutrinos and/or DDM with massless products, normalized to that of a massless neutrino universe (reproduced from Ref. Lynch:2025ine). See main text for discussion.
• Figure 2: Bayesian posteriors and profile likelihoods of the neutrino mass sum for the cosmological models under study in the "background-only" analysis. Curves are given for BAO data with knowledge of CMB data (left) and when information from SN1a (in our case Pantheon+) is incorporated as well (right). The bottom plots compare the corresponding $\Delta\chi^{2}$ contours with parabola fits to indicate the best fit position of the neutrino mass sum. Here, filled regions indicate the effect of the chosen fit range, i.e. $\sum m_{\nu}<0.1\,$eV vs. $\sum m_{\nu}<0.9\,$eV.
• Figure 3: Constraints on the sum of neutrino masses as a function of dark sector parameters in the background-only analysis. Left panels: Decaying Cold Dark Matter ($\Lambda$DDM) model showing the fraction of decaying dark matter ($f_{\rm DDM}$) and decay rate ($\log_{10}\Gamma$) vs. $\sum m_\nu$. Right panels: Dynamical dark energy model ($w_{0}w_{a}$CDM) showing $(w_0, w_a)$ vs. $\sum m_\nu$. Colored lines indicate the best fit profile likelihood paths with $\Delta\chi^2$ encoded with color. The color bar is truncated at $\Delta\chi^2 = 4$ for clarity.
• Figure 4: Parameter evolution as a function of the sum of neutrino masses (profiled likelihood) for the models $\Lambda$DDM (left) and $w_{0}w_{a}$CDM (right), respectively, in the background-only analysis. The $\Delta\chi^2$ along the paths is color-coded. Results for BAO+CMB prior are indicated with solid lines, while results including data from SN1a are shown with dashed lines. The degeneracy between $f_{\mathrm{DDM}}$ and $\sum m_{\nu}$ is clearly visible in the left plots. $\omega_c^{\rm tot}$ corresponds to the dark matter density including the decaying fraction. In contrast, larger neutrino masses are compensated by an interplay of multiple parameters in the $w_0w_a$CDM model shown in the right plot.
• Figure 5: Change of the Hubble parameter along the neutrino mass profiles for $\Lambda$DDM and $w_{0}w_{a}$CDM using the combined BAO+SNa1+CMB prior data. For $\sum m_\nu = (0.0, 0.1, 0.2, 0.3)$ eV, $\Delta \chi^{2}$ values of (0.2, 0.0, 0.0, 0.0) for $\Lambda$DDM and (0.0, 2.3, 6.6, 12.7) for $w_{0}w_{a}$CDM are obtained with respect to the models' individual best fit values. Best fit parameters of Planck 2018 TTTEEE+lensing are used as reference.