Less isn't more: Cosmological bounds on the neutrino masses are robust to changes in the neutrino abundance

We investigate how neutrino-mass constraints from cosmology depend on the assumed thermal history of the universe. Photon injection after neutrino decoupling would decrease the neutrino abundance inferred from the temperature of the cosmic microwave background (CMB), potentially loosening the upper limit on their masses. We first evaluate how the cosmological neutrino-mass bound is altered by the decay of massive particles into photons and dark radiation after Big Bang nucleosynthesis. To test the robustness of this constraint more generally, we also assess the impact of varying the temperature of the cosmic neutrino background without assuming a specific physical mechanism. We perform MCMC analyses of both frameworks with primary CMB observations from Planck, CMB lensing measurements from Planck and ACT, and baryon acoustic oscillation data from DESI. For the degenerate mass ordering, the 95% credible limit tightens from sum m ν<0.0691 eV in a standard thermal history to sum m ν<0.0652 eV if the massive particles decay only into photons, while the injection of dark radiation in addition to photons slightly relaxes this limit to sum m ν<0.0710 eV. The same pattern holds for the normal and inverted orderings, and the decay scenario shifts the bound on the sum of the neutrino masses by at most 0.004 eV for fixed mass splittings. Allowing model-agnostic changes in the neutrino-to-photon ratio yields a 95% credible limit of sum m ν<0.0724 eV for the degenerate ordering, indicating that the stringency of our neutrino-mass bounds is not driven by constraints on the decay scenario. We find that the neutrino temperature and the sum of the neutrino masses are positively correlated, which implies that reducing the pre-recombination radiation density will only worsen the emerging tension between cosmological bounds on the neutrino masses and the measured mass splittings.