A universe with three neutrino species – and no additional light species – provides 3.046 effective relativistic species. Simons Observatory aims to measure σ(Neff) = 0.07. Simons Observatory’s target would exclude at more than 95% confidence any models with three or more additional light non-scalar particles that were in thermal equilibrium with the particles of the Standard Model at any point back to the time of reheating.

Figure (1) shows theoretical targets for Neff , assuming the existence of an additional Beyond-the-Standard- Model relativistic particle that was in thermal equilibrium with the Standard Model at temperatures T > TF=freeze−out. The top panel shows the evolution of the effective degrees of freedom for Standard Model particle density, g∗, as a function of photon temperature in the early universe, Tγ. Vertical bands show the approximate temperature of neutrino decoupling and the QCD phase transition, and dashed vertical lines denote some mass scales at which corresponding particles annihilate with their antiparticles, reducing g∗. The bottom panel shows the expected ∆Neff today for species decoupling from thermal equilibrium as a function of the decoupling temperature, where lines show the prediction from the Borsanyi et al. fit assuming a single scalar boson with g = 1 (dark green), bosons with g = 2 (e.g., a gauge vector boson, light green), a Weyl fermion with g = 2 (green), or fermions with g = 4 (yellow). Shaded regions show the exclusion regions for Planck and SO (baseline). Arrows on the left show the lower limits for specific cases, for example any particle with spin that decoupled after the start of the QCD phase transition would typically be measurable by SO at 2σ. The lower two arrows and corresponding numbers on the right give lower bounds for particles produced any time after reheating (at TR).