FORECASTED CONSTRAINTS ON ISOCURVATURE INITIAL CONDITIONS OF THE UNIVERSE

Abstract

The next generation of CMB experiments will be significantly more sensitive than the current state of the art. In this work, we study the power of the upcoming Simons Observatory (SO) to place constraints on isocurvature modes of primordial perturbations, and extend the discussion to the inclusion of data from the LiteBIRD satellite. We begin with an introduction into how different types and admixtures of primordial curvature and entropy perturbations generate distinct signatures in the Cosmic Microwave Background (CMB). We outline two statistical methods for obtaining constraints on primordial cosmological parameters and discuss the best parameter values that have been obtained to date. We detail our procedure for constructing a set of mock power spectra from the two surveys conducted by SO, with the small and large aperture telescopes (SATs and LAT) in six frequency bands (27, 39, 93, 145, 225 and 280 GHz), and the survey conducted by the LiteBIRD satellite in fifteen frequency bands. We use these multi-frequency datasets, along with data from the Planck satellite, to construct approximate combined datasets spanning ℓ ∈ [2, 3000]. We assign the measurement with the smallest error among the four experiments to each datapoint in order to maximize sensitivity across scales and estimate their covariance; while our likelihood is not perfectly reflective of the entire inter-experiment covariance, it allows for the identification of factors that influence constraining power. We employ a Fisher forecast formalism and apply it to ten different cosmologies with different types of isocurvature, parametrized through the amplitude of the primordial isocurvature power spectrum at two scales, k1 = 0.002Mpc−1 and k2 = 0.1Mpc−1 to obtain constraints on primordial power spectrum amplitudes. We find limited improvement in constraining power on isocurvature parameters when combining only LAT data alone, but a significant improvement— of order a factor of two, on average, but as high as a factor of ten for neutrino velocity isocurvature—when including LiteBIRD, driven primarily by reduced uncertainty in measurements of the large-scale polarization, and improved constraints on the reionization optical depth.