Towards a cosmological neutrino mass detection

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 (CMB) anisotropies and baryon acoustic oscillations (BAOs) in the galaxy distribution, examining how the projected uncertainty of approximate to 15 meV on the neutrino mass sum (a 4 sigma detection of the minimal mass) might be reached over the next decade. The current 1 sigma uncertainty of approximate to 103 meV (Planck-2015 + BAO-15) will be improved by upcoming Stage-3 (S3) CMB experiments (S3 + BAO-15: 44 meV), then upcoming BAO measurements (S3 + DESI: 22 meV), and planned next-generation Stage 4 (S4) 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 l > 20 and combined with current large-scale polarization data. Looking beyond Lambda CDM, including curvature uncertainty increases the forecast mass error by approximate to 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.