Complementing the ground-based CMB-S4 experiment on large scales with the PIXIE satellite

Abstract

We present forecasts for cosmological parameters from future cosmic microwave background (CMB) data measured by the stage-4 (S4) generation of ground-based experiments in combination with large-scale anisotropy data from the PIXIE satellite. We demonstrate the complementarity of the two experiments and focus on science targets that benefit from their combination. We show that a cosmic-variance-limited measurement of the optical depth to reionization provided by PIXIE, with error $\ensuremath{\sigma}(\ensuremath{\tau})=0.002$, is vital for enabling a $5\ensuremath{\sigma}$ detection of the sum of the neutrino masses when combined with a CMB-S4 lensing measurement and with lower-redshift constraints on the growth of structure and the distance-redshift relation. Parameters characterizing the epoch of reionization will also be tightly constrained; PIXIE's $\ensuremath{\tau}$ constraint converts into $\ensuremath{\sigma}({z}_{\mathrm{re}})=0.2$ for the mean time of reionization, and a kinematic Sunyaev-Zel'dovich measurement from S4 gives $\ensuremath{\sigma}(\mathrm{\ensuremath{\Delta}}{z}_{\mathrm{re}})=0.03$ for the duration of reionization. Both PIXIE and S4 will put strong constraints on primordial tensor fluctuations, vital for testing early-Universe models, and will do so at distinct angular scales. We forecast $\ensuremath{\sigma}(r)\ensuremath{\approx}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ for a signal with a tensor-to-scalar ratio $r={10}^{\ensuremath{-}3}$, after accounting for diffuse foreground removal and delensing. The wide and dense frequency coverage of PIXIE results in an expected foreground-degradation factor on $r$ of only $\ensuremath{\approx}25%$. By measuring large and small scales PIXIE and S4 will together better limit the energy injection at recombination from dark matter annihilation, with ${p}_{\mathrm{ann}}<0.09\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\text{ }\text{ }{\mathrm{m}}^{3}/\mathrm{s}/\text{kg}$ projected at 95% confidence. Cosmological parameters measured from the damping tail with S4 will be best constrained by polarization, which has the advantage of minimal contamination from extragalactic emission.