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Characterizing CMB noise anisotropies from CMB delensing

Louis Legrand1,2 and Julien Carron3

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Phys. Rev. D 112, 083538 – Published 21 October, 2025

DOI: https://doi.org/10.1103/dcj9-hhl4

Abstract

Undoing the effect of gravitational lensing on the cosmic microwave background (CMB) (“delensing”) is essential in shaping constraints on weak signals limited by lensing effects on the CMB, for example on a background of primordial gravitational waves. Removing these anisotropies induced by large-scale structures from the CMB maps also generally helps our view of the primordial Universe by sharpening the acoustic peaks and the damping tail. However, practical implementations of delensing transfer parts of these anisotropies to the noise maps. This will induce a new large scale “mean-field” bias to any anisotropy estimator applied to the delensed CMB, and this bias directly traces large-scale structures. This paper analytically quantifies this delensed noise mean field and its impact on quadratic and likelihood-based lensing estimators. We show that for Simons-Observatory-like surveys, this mean-field bias can reach 15% in cross-correlation with large-scale structures if unaccounted for. We further demonstrate that this delensed noise mean field can be safely neglected in likelihood-based estimators without compromising the quality of lensing reconstruction or B-mode delensing, provided the resulting lensing map is properly renormalized.

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Phys. Rev. D 112, 083538– Published 21 October, 2025
  • Received 12 May 2025
  • Accepted 19 September 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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