Towards a cosmological neutrino mass detection

TL;DR

Problem: determine the cosmological sum of neutrino masses, Σmν, from upcoming observations. Approach: perform Fisher forecasts combining lensed CMB power spectra (TT/TE/EE) and reconstructed lensing (κκ) with BAO distance ratios (r_s/d_V), across Planck, Stage-3 (S3), Stage-4 (S4) CMB data and DESI, exploring ΛCDM+Σmν and extensions to curvature and dynamical dark energy. Key findings: projected 1σ uncertainties reach about $19$–$22$ meV (and $15$–$19$ meV with optimistic ℓ_min) for ΛCDM+Σmν; degeneracies with Ω_k and w0/wa can widen the constraints to ~64 meV in expanded models, but the combination of CMB lensing and BAO significantly strengthens the neutrino mass signal. Significance: these results indicate a viable path for an indirect detection of the minimal neutrino mass within the next decade, contingent on controlling large-scale polarization systematics and incorporating complementary low-redshift probes to break residual degeneracies.

Abstract

Future cosmological measurements should enable the sum of neutrino masses to be determined indirectly through their effects on the expansion rate of the Universe and the clustering of matter. We consider prospects for the gravitationally lensed Cosmic Microwave Background anisotropies and Baryon Acoustic Oscillations in the galaxy distribution, examining how the projected uncertainty of $\approx15$ meV on the neutrino mass sum (a 4$σ$ detection of the minimal mass) might be reached over the next decade. The current 1$σ$ uncertainty of $\approx 103$ meV (Planck-2015+BAO-15) will be improved by upcoming 'Stage-3' CMB experiments (S3+BAO-15: 44 meV), then upcoming BAO measurements (S3+DESI: 22 meV), and planned next-generation 'Stage 4' CMB experiments (S4+DESI: 15-19 meV, depending on angular range). An improved optical depth measurement is important: the projected neutrino mass uncertainty increases to $26$ meV if S4 is limited to $\ell>20$ and combined with current large-scale polarization data. Looking beyond $Λ$CDM, including curvature uncertainty increases the forecast mass error by $\approx$ 50% for S4+DESI, and more than doubles the error with a two-parameter dark energy equation of state. Complementary low-redshift probes including galaxy lensing will play a role in distinguishing between massive neutrinos and a departure from a $w=-1$, flat geometry.

Figures (10)

• Figure 1: Effect of neutrino mass on CMB power spectra and BAO distance scales. Top: Fractional change of the matter power spectrum today $P(k)$ (left) and CMB convergence power spectrum $C_l^{\kappa \kappa}$ (right) with neutrino mass $\Sigma m_\nu$, for fixed physical dark matter density, $\Omega_{\rm c}h^2 + \Omega_{\nu}h^2$. Suppression of power is due to neutrino free-streaming. Bottom left: Lensed CMB $E$-mode power spectrum with varying amplitudes of the lensing potential $A_{\rm lens}$, approximating and exaggerating the effect that massive neutrinos have on the CMB polarization spectrum. Bottom right: BAO distance ratio $r_s/d_V$ for fixed $\theta_A$ and $\Omega_ch^2$. Massive neutrinos behave like additional matter in the BAO redshift range, decreasing $H_0$ and increasing the volume distance $d_V$.
• Figure 2: Top and middle: Fractional change in the convergence $\kappa$ (top) and $E$-mode (middle) power spectrum with neutrino mass, for fixed $\Omega_{\rm c}h^2 + \Omega_\nu h^2$, with expected uncertainties for S3-wide and S4 CMB data. A higher neutrino mass has less lensing, decreasing the E-mode peak smoothing. Bottom: Fractional change in distance ratio $r_s/d_V$, with uncertainties from current (BAO-15 Anderson:2014) and forecast (DESI, Font-Ribera:2014) BAO data. Here $\Omega_{\rm c}h^2$ is fixed.
• Figure 3: Forecast marginal posterior constraints on the sum of the neutrino masses $\Sigma m_\nu$ within a $\Lambda$CDM+$\Sigma m_\nu$ model, assuming Gaussian error distributions. The current uncertainties (P15+BAO-15) are expected to improve rapidly, with S3 CMB data and DESI BAO data expected by $\sim$2020.
• Figure 4: Expected joint constraint (68% CL) on the neutrino mass sum $\Sigma m_\nu$ and physical cold dark matter density $\Omega_{\rm c}h^2$ within a $\Lambda$CDM+$\Sigma m_\nu$ model. The BAO constraint, sensitive to the total late-time cold dark matter density, is almost orthogonal to the CMB lensing constraint, breaking the degeneracy.
• Figure 5: Top: The neutrino mass $\Sigma m_\nu$ is correlated with the optical depth to reionization $\tau$ (forecast 68% CL). Current data at $\ell <20$ ( WMAP-pol) would leave a degeneracy between $\Sigma m_\nu$ and $\tau$ that could be broken with improved large-scale polarization data. Bottom: the expected neutrino mass constraint as a function of the minimum multipole accessible to S4, indicating the benefit of reaching large scales.