A new study has investigated the robustness of cosmological constraints on the sum of neutrino masses (Σmν) against variations in the early thermal history of the universe. The results indicate that, even under scenarios that alter the inferred neutrino abundance from the cosmic microwave background (CMB), the upper limits on their masses remain stringent, with small variations that do not significantly relax current bounds. This is crucial for precision cosmology and particle physics, as neutrino mass is one of the few Standard Model parameters not yet precisely determined.

The researchers evaluated how the injection of photons or dark radiation after neutrino decoupling, due to the decay of massive particles, could affect the inference of neutrino abundance and, consequently, their masses. Using MCMC analyses with data from the Planck satellite (primary CMB and CMB lensing), ACT (CMB lensing), and DESI (baryon acoustic oscillations), they found that, for a degenerate mass ordering, the 95% credible limit for Σmν tightens slightly from 0.0691 eV in the standard model to 0.0652 eV if massive particles decay only into photons. If dark radiation is also injected, the limit relaxes marginally to 0.0710 eV. These changes are approximately 0.004 eV, demonstrating the stability of the constraints.

To test the generality of this finding, the study also explored model-agnostic changes in the neutrino-to-photon temperature ratio, without assuming a specific physical mechanism. In this case, the 95% credible limit for Σmν in the degenerate ordering was 0.0724 eV. The positive correlation found between neutrino temperature and the sum of their masses suggests that reducing the pre-recombination radiation density would only worsen the emerging tension between cosmological bounds on neutrino masses and measured mass splittings. These results reinforce the robustness of current cosmological constraints on neutrino mass, a fundamental parameter for understanding the evolution of the universe and particle physics beyond the Standard Model.