Reconstructing Cosmic Polarization Rotation with ResUNet-CMB

Abstract

Cosmic polarization rotation, which may result from parity-violating new physics or the presence of primordial magnetic fields, converts E-mode polarization of the cosmic microwave background (CMB) into B-mode polarization. Anisotropic cosmic polarization rotation leads to statistical anisotropy in CMB polarization and can be reconstructed with quadratic estimator techniques similar to those designed for gravitational lensing of the CMB. At the sensitivity of upcoming CMB surveys, lensing-induced B-mode polarization will act as a limiting factor in the search for anisotropic cosmic polarization rotation, meaning that an analysis which incorporates some form of delensing will be required to improve constraints on the effect with future surveys. In this paper we extend the ResUNet-CMB convolutional neural network to reconstruct anisotropic cosmic polarization rotation in the presence of gravitational lensing and patchy reionization, and we show that the network simultaneously reconstructs all three effects with variance that is lower than that from the standard quadratic estimator nearly matching the performance of an iterative reconstruction method.

Mitigating Foreground Bias to the CMB Lensing Power Spectrum for a CMB-HD Survey

“…We assume that the late-time kSZ can be removed from the largescale gradient map with an overlapping galaxy survey [72,76], such as from the Rubin Observatory [77], and that it can be removed from the small-scale map with machine learning techniques (e.g. [66,[78][79][80][81][82][83]) (see Section V). The reionization kSZ originates at early times (z ≈ 10) and is more Gaussian than the lower redshift foregrounds.…”

“…While the kSZ effect has no frequency dependence, making it hard to remove with multi-frequency techniques [53], the kSZ signal has a characteristic spatial pattern that can be recognized by a convolution neural network (e.g. [66,78,[80][81][82]). In turn, we can use this recognized spatial pattern to remove the kSZ signal, especially on small scales above = 5000 where it is the dominant signal.…”

The Benefits of CMB Delensing

“…Just as is the case for lensing reconstruction, reducing the lensing-induced power, especially on small scales and in B-mode polarization, will reduce the variance of these estimators; see Refs. [90,91] for a quantitative discussion of the benefits of delensing for patchy reionization and cosmic polarization rotation. Delensing also helps to mitigate biases that arise in reconstructing fields whose effects on the CMB are not orthogonal to lensing, though some form of bias-hardening is still required to eliminate the bias due to residual lensing [92].…”