BAO and CMB Measurements of Compensated Isocurvature Perturbations and Implications for Tensions

CMB measurements have shown that the Universe started with mostly adiabatic perturbations: different particle species fluctuate spatially in the same way. The differences between fluctuations of species (called isocurvature) is highly constrained by Planck measurements, except for one type of isocurvature called Compensated Isocurvature Perturbations (CIPs), in which baryon density fluctuations are compensated by opposite dark matter density fluctuations and therefore hard to detect. In this talk, I will review various detection techniques with the CMB and propose a new measurement method by searching for a spatial modulations of the baryon acoustic oscillation (BAO) scale. We find that a Euclid-like survey would achieve WMAP CMB sensitivity while a cosmic-variance-limited BAO survey using emission-line galaxies up to z~7 has sensitivity between stage-3 and stage-4 CMB experiments. The results could be improved with a more optimal estimator and future 21cm measurements. Finally, if CIPs exist, they can bias cosmological measurements made assuming no CIPs, by acting as a super-sample fluctuation of the baryon density. I will show how a 1 sigma and isotropic (2 sigma and radial) CIP fluctuation maximally allowed by the current 95% Planck constraints can bias BAO measurements of H(z) at the 1% (6.6%) level, partially reducing the tension with the local measurements.

Speaker: Chen Heinrich, Caltech

Room LBL 50-5132