Why Does Cosmology Give Us A Negative Neutrino Mass As A Best Fit Value?

The apparent preference for a best fit value of the neutrino masses from cosmology measurements is probably a matter of some fine methodological adjustments that weren't made for gravitational lensing.

Recent analyses combining cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) challenge particle physics constraints on the total neutrino mass, pointing to values smaller than the lower limit from neutrino oscillation experiments. To examine the impact of different CMB likelihoods from Planck, lensing potential measurements from Planck and ACT, and BAO data from DESI, we introduce an effective neutrino mass parameter (∑m̃ ν) which is allowed to take negative values. 

We investigate its correlation with two extra parameters capturing the impact of gravitational lensing on the CMB: one controlling the smoothing of the peaks of the temperature and polarization power spectra; one rescaling the lensing potential amplitude. In this configuration, we infer ∑m̃ ν=−0.018+0.085−0.089 eV (68% C.L.), which is fully consistent with the minimal value required by neutrino oscillation experiments. 

We attribute the apparent preference for negative neutrino masses to an excess of gravitational lensing detected by late-time cosmological probes compared to that inferred from Planck CMB angular power spectra. We discuss implications in light of the DESI BAO measurements and the CMB lensing anomaly.

Andrea Cozzumbo, et al., "A short blanket for cosmology: the CMB lensing anomaly behind the preference for a negative neutrino mass" arXiv:2511.01967 (November 3, 2025).

A Dark Energy Alternative

There are multiple possible alternatives to a cosmological constant. This is one of the better attempts.

In our local-to-global cosmological framework, cosmic acceleration arises from local dynamics in an inhomogeneous Einstein-de Sitter (iEdS) universe without invoking dark energy. 

An iEdS universe follows a quasilinear coasting evolution from an Einstein-de Sitter to a Milne state, as an effective negative curvature emerges from growing inhomogeneities without breaking spatial flatness. Acceleration can arise from structure formation amplifying this effect. 

We test two realizations, iEdS(1) and iEdS(2), with H(0) = {70.24,74.00} km s^−1 Mpc^−1 and Ω(m,0) = {0.290,0.261}, against CMB, BAO, and SN Ia data. 

iEdS(1) fits better than ΛCDM and alleviates the H0 tension, whereas iEdS(2) fully resolves it while remaining broadly consistent with the data. Both models yield t0≃13.64 Gyr, consistent with globular-cluster estimates.

Peter Raffai, et al., "A Case for an Inhomogeneous Einstein-de Sitter Universe" arXiv:2511.03288 (November 5, 2025).