Lensing Bias to CMB Measurements of Compensated Isocurvature Perturbations

TL;DR

This paper investigates how gravitational lensing biases affect the detection of compensated isocurvature perturbations (CIPs) in the CMB, revealing that lensing noise and correlations can significantly hinder CIP measurements.

Contribution

It identifies the lensing bias as a major obstacle in CIP detection and quantifies its impact on the measurement sensitivity in CMB temperature data.

Findings

Lensing introduces nearly Gaussian noise that doubles CIP estimator variance.

Cross-correlation between lensing and ISW affects CIP-temperature correlations.

Subtracting lensing bias reduces detection sensitivity by about 30%."

Abstract

Compensated isocurvature perturbations (CIPs) are modes in which the baryon and dark matter density fluctuations cancel. They arise in the curvaton scenario as well as some models of baryogenesis. While they leave no observable effects on the cosmic microwave background (CMB) at linear order, they do spatially modulate two-point CMB statistics and can be reconstructed in a manner similar to gravitational lensing. Due to the similarity between the effects of CMB lensing and CIPs, lensing contributes nearly Gaussian random noise to the CIP estimator that approximately doubles the reconstruction noise power. Additionally, the cross correlation between lensing and the integrated Sachs-Wolfe (ISW) effect generates a correlation between the CIP estimator and the temperature field even in the absence of a correlated CIP signal. For cosmic-variance limited temperature measurements out to multipoles $l \leq 2500$, subtracting a fixed lensing bias degrades the detection threshold for CIPs by a factor of $1.3$, whether or not they are correlated with the adiabatic mode.