Updating neutrino mass constraints with Background measurements

Low-redshift probes, such as Baryon Acoustic Oscillations (BAO) and Supernovae Ia luminosity distances, have been shown to be crucial for improving the bounds on the total neutrino mass from cosmological observations, due to their ability to break degeneracies among the different parameters. Here, we expand background observations to include H(z) measurements from cosmic chronometers, distance moduli from Gamma Ray Bursts (GRBs), and angular diameter distances from galaxy clusters. For the very first time, we find neutrino mass limits below the minimal expectations from neutrino oscillation probes, suggesting non-standard neutrino and/or cosmological scenarios. The tightening of the neutrino mass bound is due to the slightly higher value of the Hubble constant H_0 preferred by the former three background probes, and also due to the improved errors on H_0 and the matter mass-energy density Ω_ m. All values of H_0 are however in agreement at the 1-2σ level. Interestingly, it is not only the combination of the three background probes that is responsible for the ∑ m_ν <0.06 eV limits, but also each of them independently. The tightest bound we find here is ∑ m_ν<0.043 eV at 2σ after combining Cosmic Microwave Background Planck data with DESI BAO, Supernovae Ia, GRBs, cosmic chronometers, and galaxy clusters, showing a clear tension between neutrino oscillation results and cosmological analyses. In general, removing either one of the two background probes still provides a limit ∑ m_ν≲ 0.06 eV, reassuring the enormous potential of these low-redshift observations in constraining the neutrino mass.