Constraints on Neutrino Physics from DESI DR2 BAO and DR1 Full Shape

TL;DR

DESI DR2 BAO and DR1 full-shape analyses, combined with Planck and ACT CMB data, yield the strongest cosmological constraints on neutrino properties to date, notably ∑mν < 0.0642 eV (95%) in ΛCDM and N_eff ≈ 3.23, while a dynamical dark-energy extension relaxes the bound to ∑mν < 0.163 eV. A persistent tension emerges between these cosmological upper limits and lower bounds from neutrino oscillations, which persists even when employing an effective neutrino mass parameter that allows negative values, suggesting either unidentified systematics or new physics, possibly linked to dark-energy dynamics. The analysis demonstrates that the free-streaming imprint on the small-scale power spectrum and the late-time expansion history jointly drive the neutrino constraints, and highlights the value of multiple parametrizations to diagnose biases. These results motivate further DESI analyses, including DR2 full-shape clustering, to break degeneracies and clarify the neutrino mass and dark-energy interplay with upcoming data. Overall, the work underscores the sensitivity of cosmological neutrino inferences to the assumed background model and external priors, and its potential to reveal new physics at the intersection of particle physics and cosmology.

Abstract

The Dark Energy Spectroscopic Instrument (DESI) Collaboration has obtained robust measurements of baryon acoustic oscillations (BAO) in the redshift range, $0.1 < z < 4.2$, based on the Lyman-$α$ forest and galaxies from Data Release 2 (DR2). We combine these measurements with external cosmic microwave background (CMB) data from Planck and ACT to place our tightest constraints yet on the sum of neutrino masses. Assuming the cosmological $Λ$CDM model and three degenerate neutrino states, we find $\sum m_ν<0.0642$ eV (95%) with a marginalized error of $σ(\sum m_ν)=0.020$ eV. We also constrain the effective number of neutrino species, finding $N_\rm{eff} = 3.23^{+0.35}_{-0.34}$ (95%), in line with the Standard Model prediction. When accounting for neutrino oscillation constraints, we find a preference for the normal mass ordering and an upper limit on the lightest neutrino mass of $m_l < 0.023$ eV (95%). However, we determine using frequentist and Bayesian methods that our constraints are in tension with the lower limits derived from neutrino oscillations. Correcting for the physical boundary at zero mass, we report a 95% Feldman-Cousins upper limit of $\sum m_ν<0.053$ eV, breaching the lower limit from neutrino oscillations. Considering a more general Bayesian analysis with an effective cosmological neutrino mass parameter, $\sum m_{ν,\rm{eff}}$, that allows for negative energy densities and removes unsatisfactory prior weight effects, we derive constraints that are in $3σ$ tension with the same oscillation limit. In the absence of unknown systematics, this finding could be interpreted as a hint of new physics not necessarily related to neutrinos. The preference of DESI and CMB data for an evolving dark energy model offers one possible solution. In the $w_0w_a$CDM model, we find $\sum m_ν<0.163$ eV (95%), relaxing the neutrino tension. [Abridged]

Figures (17)

• Figure 1: Constraints on $\Omega_\mathrm{m}$ and $\sum m_{\nu,\mathrm{eff}}$ obtained from a BAO analysis of two large Peregrinus mocks, combined with a correlated CMB prior on the parameters $(\theta_*,\omega_\mathrm{cdm},\omega_\mathrm{b})$. The DESI_M060 simulation has a DESI-like cosmology with $\sum m_\nu=0.059\eV$DESI2024.VI.KP7A and the PLANCK_M240 simulation has a Planck-based cosmology with $\sum m_\nu=0.24\eV$, the largest mass allowed by Planck at 95% PlanckCosmology2020. The dashed lines indicate the true values in the simulations. The contours enclose 68% and 95% of the posterior volume.
• Figure 2: Constraints on the sum of effective neutrino masses, $\sum m_{\nu,\mathrm{eff}}$, and $\Omega_\mathrm{m}$ for DESI DR2 BAO combined with different BBN and CMB priors ('nl' stands for no CMB lensing). The contours enclose 68% and 95% of the posterior volume. The figure demonstrates that BAO alone do not constrain $\sum m_{\nu,\mathrm{eff}}$. The addition of BBN and geometric CMB information, in the form of a prior on $\theta_*$, only improves the constraint on $\Omega_\mathrm{m}$. CMB priors on $\omega_\mathrm{b}$ and $\omega_\mathrm{cdm}$ help to break the degeneracy between the CDM and neutrino densities. When BAO is combined with a prior on $(\theta_*,\omega_\mathrm{b},\omega_\mathrm{cdm})$, the posterior approaches that of the full DESI BAO + CMB-nl combination.
• Figure 3: The neutrino effect at $z=0.3$ on the linear power spectrum of cold dark matter and baryons, $P_\mathrm{cb}(k)$, compared to the massless case, for fixed cosmological parameters, $(h,\omega_\mathrm{b},\omega_\mathrm{cdm},A_\mathrm{s},n_\mathrm{s},\tau)$. The range of scales used in the DESI full-shape power spectrum analysis is shown as a gray band.
• Figure 4: Constraints on $\sum m_\nu$ from different combinations of DESI BAO and CMB data. The CMB dataset includes Planck low-$\ell$ TTEE PlanckLikelihood2020 and high-$\ell$ TTTEEE (PR4 CamSpec) Rosenberg22Efstathiou2021 and PlanckPlanckLensing2022 and ACT ACTDR62024 lensing. Shown are results for the CMB alone, CMB combined with DESI DR1 BAO or DR2 BAO, where the latter is our baseline result. Also shown are variations from the baseline for alternative CMB likelihoods: PR3 plikPlanckLikelihood2020 or PR4 L-HTristram2021Tristram2024. The vertical dashed lines and shaded regions indicate, from left to right, the minimum masses corresponding to the normal and inverted mass ordering scenarios.
• Figure 5: Constraints on the parameter combination $H_0r_\mathrm{d}$ and on the matter density, $\Omega_\mathrm{m}$, both of which are measured to high precision by DESI BAO, and on the sum of neutrino masses, $\sum m_\nu$, from the combination of DESI BAO and CMB data (both for DR1 and the new DR2) and from CMB data alone. The contours represent the 68% and 95% probability regions. The figure shows how the DESI preference for lower $\Omega_\mathrm{m}$ and higher $H_0r_\mathrm{d}$, compared to the CMB, leads to upper limits on the sum of neutrino masses that are stronger than expected from forecasts.