The benefits of CMB delensing

Hotinli, Selim C.; Meyers, Joel; Trendafilova, Cynthia; Green, Daniel; Engelen, Alexander van

doi:10.1088/1475-7516/2022/04/020

Cited by 33 publications

(31 citation statements)

References 160 publications

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“…Notice that no BBN constraints are applied in this CMB-alone case to break the degeneracy between N S4 eff and Y S4 p in advance. For upcoming surveys like CMB-S4, delensing will help to break their degeneracy and improve their constraints [201,202]. JCAP10(2022)046 push below the neutrino heating limit is somewhat less demanding of 4 He observations, albeit challenging, requiring σ(Y p ) < 0.001 when f S4 sky = 0.3.…”

Section: He Observations and The Hunt For Neutrino Heatingmentioning

confidence: 99%

Probing physics beyond the standard model: limits from BBN and the CMB independently and combined

Yeh

Shelton²,

Olive³

et al. 2022

J. Cosmol. Astropart. Phys.

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We present new Big Bang Nucleosynthesis (BBN) limits on the cosmic expansion rate or relativistic energy density, quantified via the number Nν of equivalent neutrino species. We use the latest light element observations, neutron mean lifetime, and update our evaluation for the nuclear rates d + d ⟶ 3He + n and d + d ⟶ 3H+ p. Combining this result with the independent constraints from the cosmic microwave background (CMB) yields tight limits on new physics that perturbs Nν and η prior to cosmic nucleosynthesis: a joint BBN+CMB analysis gives Nν = 2.898 ± 0.141, resulting in Nν < 3.180 at 2σ. We apply these limits to a wide variety of new physics scenarios including right-handed neutrinos, dark radiation, and a stochastic gravitational wave background. The strength of the independent BBN and CMB constraints now opens a new window: we can search for limits on potential changes in Nν and/or the baryon-to-photon ratio η between the two epochs. The present data place strong constraints on the allowed changes in Nν between BBN and CMB decoupling; for example, we find -0.708 < Nν CMB - Nν BBN < 0.328 in the case where η and the primordial helium mass fraction Yp are unchanged between the two epochs; we also give limits on the allowed variations in η or in (η, Nν ) jointly. We discuss scenarios in which such changes could occur, and show that BBN+CMB results combine to place important constraints on some early dark energy models to explain the H0 tension. Looking to the future, we forecast the tightened precision for Nν arising from both CMB Stage 4 measurements as well as improvements in astronomical 4He measurements. We find that CMB-S4 combined with present BBN and light element observation precision can give σ(Nν ) ≃ 0.03. Such future precision would reveal the expected effect of neutrino heating (Neff -3 = 0.044) of the CMB during BBN, and would be near the level to reveal any particle species ever in thermal equilibrium with the standard model. Improved Yp measurements can push this precision even further.

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Section: He Observations and The Hunt For Neutrino Heatingmentioning

confidence: 99%

Probing physics beyond the standard model: limits from BBN and the CMB independently and combined

Yeh

Shelton²,

Olive³

et al. 2022

J. Cosmol. Astropart. Phys.

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“…This noise estimation procedure differs slightly from the method usually used to estimate delensed B-mode noise using Wiener-filtered E modes [44] (also different from Ref. [46]), but in practice the forecast results are very similar.…”

Section: Discussionmentioning

confidence: 96%

How to detect lensing rotation

Robertson¹,

Lewis²

2023

Preprint

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Gravitational lensing rotation of images is predicted to be negligible at linear order in density perturbations, but can be produced by the post-Born lens-lens coupling at second order. This rotation is somewhat enhanced for Cosmic Microwave Background (CMB) lensing due to the large source path length, but remains small and very challenging to detect directly by CMB lensing reconstruction alone. We show the rotation may be detectable at high significance as a cross-correlation signal between the curl reconstructed with Simons Observatory (SO) or CMB-S4 data, and a template constructed from quadratic combinations of large-scale structure (LSS) tracers. Equivalently, the lensing rotation-tracer-tracer bispectrum can also be detected, where LSS tracers considered include the CMB lensing convergence, galaxy density, and the Cosmic Infrared Background (CIB), or optimal combinations thereof. We forecast that an optimal combination of these tracers can probe post-Born rotation at the level of 5.7σ-6.1σ with SO and 13.6σ-14.7σ for CMB-S4, depending on whether standard quadratic estimators or maximum a posteriori iterative methods are deployed. We also show possible improvement up to 21.3σ using a CMB-S4 deep patch observation with polarization-only iterative lensing reconstruction. However, these cross-correlation signals have non-zero bias because the rotation template is quadratic in the tracers, and exists even if the lensing is rotation free. We estimate this bias analytically, and test it using simple nullhypothesis simulations to confirm that the bias remains subdominant to the rotation signal of interest. Detection and then measurement of the lensing rotation cross-spectrum is therefore a realistic target for future observations.

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“…Gravitational lensing can convert a small part of E-mode polarization into B-mode polarization, which acts as a source of confusion in searches for PGWs. Furthermore, lensing also smooths the acoustic peaks of the CMB power spectra and produces nonstationary statistics of CMB fluctuations (Zaldarriaga & Seljak 1998;Lewis et al 2001;Hotinli et al 2022). In order to improve constraints on the tensor-to-scalar ratio, we must remove the effects of lensing (delensing) from observed CMBs.…”

Section: Introductionmentioning

confidence: 99%

Delensing of Cosmic Microwave Background Polarization with Machine Learning

Yan

Wang

et al. 2023

ApJS

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Primordial B-mode detection is one of the main goals of next-generation cosmic microwave background (CMB) experiments. Primordial B-modes are a unique signature of primordial gravitational waves (PGWs). However, the gravitational interaction of CMB photons with large-scale structures will distort the primordial E modes, adding a lensing B-mode component to the primordial B-mode signal. Removing the lensing effect (“delensing”) from observed CMB polarization maps will be necessary to improve the constraint of PGWs and obtain a primordial E-mode signal. Here, we introduce a deep convolutional neural network model named multi-input multi-output U-net (MIMO-UNet) to perform CMB delensing. The networks are trained on simulated CMB maps with size 20° × 20°. We first use MIMO-UNet to reconstruct the unlensing CMB polarization (Q and U) maps from observed CMB maps. The recovered E-mode power spectrum exhibits excellent agreement with the primordial EE power spectrum. The recovery of the primordial B-mode power spectrum for noise levels of 0, 1, and 2 μK-arcmin is greater than 98% at the angular scale of ℓ < 150. We additionally reconstruct the lensing B map from observed CMB maps. The recovery of the lensing B-mode power spectrum is greater than roughly 99% at the scales of ℓ > 200. We delens the observed B-mode power spectrum by subtracting the reconstructed lensing B-mode spectrum. The recovery of tensor B-mode power spectrum for noise levels of 0, 1, and 2 μK-arcmin is greater than 98% at the angular scales of ℓ < 120. Even at ℓ = 160, the recovery of tensor B-mode power spectrum is still around 71%.

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The benefits of CMB delensing

Cited by 33 publications

References 160 publications

Probing physics beyond the standard model: limits from BBN and the CMB independently and combined

Probing physics beyond the standard model: limits from BBN and the CMB independently and combined

How to detect lensing rotation

Delensing of Cosmic Microwave Background Polarization with Machine Learning

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