Compensated isocurvature perturbations in the curvaton model

TL;DR

This paper investigates how correlated compensated isocurvature perturbations (CIPs) in the curvaton model can be detected through CMB measurements, potentially distinguishing different curvaton-decay scenarios with future experiments.

Contribution

It demonstrates that correlated CIPs can be more sensitively probed via the squeezed bispectrum of the CMB and assesses the detectability with upcoming CMB experiments like CMB Stage 4.

Findings

CMB Stage 4 can detect large CIPs in curvaton scenarios with >3σ significance.

Cross-correlation with primary CMB improves CIP detection signal-to-noise ratio by 2-3 times.

Future observations can differentiate curvaton-decay scenarios based on CIP signatures.

Abstract

Primordial fluctuations in the relative number densities of particles, or isocurvature perturbations, are generally well constrained by cosmic microwave background (CMB) data. A less probed mode is the compensated isocurvature perturbation (CIP), a fluctuation in the relative number densities of cold dark matter and baryons. In the curvaton model, a subdominant field during inflation later sets the primordial curvature fluctuation $\zeta$. In some curvaton-decay scenarios, the baryon and cold dark matter isocurvature fluctuations nearly cancel, leaving a large CIP correlated with $\zeta$. This correlation can be used to probe these CIPs more sensitively than the uncorrelated CIPs considered in past work, essentially by measuring the squeezed bispectrum of the CMB for triangles whose shortest side is limited by the sound horizon. Here, the sensitivity of existing and future CMB experiments to correlated CIPs is assessed, with an eye towards testing specific curvaton-decay scenarios. The planned CMB Stage 4 experiment could detect the largest CIPs attainable in curvaton scenarios with more than 3$\sigma$ significance. The significance could improve if small-scale CMB polarization foregrounds can be effectively subtracted. As a result, future CMB observations could discriminate between some curvaton-decay scenarios in which baryon number and dark matter are produced during different epochs relative to curvaton decay. Independent of the specific motivation for the origin of a correlated CIP perturbation, cross-correlation of CIP reconstructions with the primary CMB can improve the signal-to-noise ratio of a CIP detection. For fully correlated CIPs the improvement is a factor of $\sim$2$-$3.