A short blanket for cosmology: the CMB lensing anomaly behind the preference for a negative neutrino mass

Abstract

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 $\mathit{Planck}$, lensing potential measurements from $\mathit{Planck}$ and ACT, and BAO data from DESI, we introduce an effective neutrino mass parameter ($\sum \tilde{m}_{\nu}$) 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 $\sum \tilde{m}_{\nu}=-0.018^{+0.085}_{-0.089}~\text{eV}~(68\% ~\text{C.L.})$, 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 $\mathit{Planck}$ CMB angular power spectra. We discuss implications in light of the DESI BAO measurements and the CMB lensing anomaly.