Optimal constraints on primordial gravitational waves from the lensed CMB

Abstract

We demonstrate how to obtain optimal constraints on a primordial gravitational wave component in lensed Cosmic Microwave Background (CMB) data under ideal conditions. We first derive an estimator of the tensor-to-scalar ratio, $r$, by using an error-controlled close approximation to the exact posterior, under the assumption of Gaussian primordial CMB and lensing deflection potential. This combines fast internal iterative lensing reconstruction with optimal recovery of the unlensed CMB. We evaluate its performance on simulated low-noise polarization data targeted at the recombination peak. We carefully demonstrate our $r$-posterior estimate is optimal and shows no significant bias, making it the most powerful estimator of primordial gravitational waves from the CMB. We compare these constraints to those obtained from $B$-mode band-power likelihood analyses on the same simulated data, before and after map-level quadratic estimator delensing, and iterative delensing. Internally, iteratively delensed band powers are only slightly less powerful on average (by less than 10\%), promising close-to-optimal constraints from a stage IV CMB experiment.

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• CMB-S4: Iterative Internal Delensing and $r$ Constraints
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• Impact of Polarized Galactic Foreground Emission on CMB Lensing Reconstruction and Delensing of B-Modes
• Measurement of the Cosmic Microwave Background Polarization Lensing Power Spectrum from Two Years of POLARBEAR Data
• Lensing power spectrum of the Cosmic Microwave Background with deep polarization experiments
• LiteBIRD Science Goals and Forecasts: Improving Sensitivity to Inflationary Gravitational Waves with Multitracer Delensing
• The impact of extragalactic foregrounds on internal delensing of CMB B-mode polarization
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• A numerical study of observational systematic errors in lensing analysis of CMB polarization
• Reconstructing Cosmic Polarization Rotation with ResUNet-CMB
• Impact of nonlinear growth of the large-scale structure on CMB B-mode delensing
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• Lensing reconstruction from the cosmic microwave background polarization with machine learning
• Cosmoglobe: Towards end-to-end CMB cosmological parameter estimation without likelihood approximations
• Delensing CMB B-modes using galaxy surveys: the effect of galaxy bias and matter clustering non-linearities