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The delensing of cosmic microwave background (CMB) maps will be increasingly valuable for extracting as much information as possible from future CMB surveys. Delensing provides many general benefits, including sharpening of the acoustic peaks, more accurate recovery of the damping tail, and reduction of lensing-induced $B$-mode power. In this paper we present several applications of delensing focused on testing theories of early-universe inflation with observations of the CMB. We find that delensing the CMB results in improved parameter constraints for reconstructing the spectrum of primordial curvature fluctuations, probing oscillatory features in the primordial curvature spectrum, measuring the spatial curvature of the universe, and constraining several different models of isocurvature perturbations. In some cases we find that delensing can recover almost all of the constraining power contained in unlensed spectra, and it will be a particularly valuable analysis technique to achieve further improvements in constraints for model parameters whose measurements are not expected to improve significantly when utilizing only lensed CMB maps from next-generation CMB surveys. We also quantify the prospects of testing the single-field inflation tensor consistency condition using delensed CMB data; we find it to be out of reach of current and proposed experimental technology and advocate for alternative detection methods., Comment: 22 pages, 10 figures

Weak gravitational lensing of the Cosmic Microwave Background (CMB) changes CMB statistics in a nontrivial way, allowing for reconstruction of the lensing potential and the use of these reconstructed maps in determining cosmological parameters that affect the formation of intervening large-scale structures. Although in principle there are correlations between the primary CMB and the reconstructed lensing potential due to the lensing procedure itself, in practice CMB analyses treat these as negligible when combining these band powers in likelihoods. In this paper we quantify explicitly the impact on parameter constraints due to these cross-covariances between the lensed CMB and reconstructed lensing power, and we compare to the effect of including all lensing-induced non-Gaussian covariances, which have previously shown to impact parameter constraints on the order of 10%. We perform our analysis for a range of experimental setups, scanning over instrumental noise levels of 0.5 to 10.0 $\mu$K-arcmin in temperature assuming fully polarized detectors, and using a fixed beam size of 1.4 arcmin. When the correlations between the lensed CMB and lensing power are neglected, we find that forecasted constraints shift by at most 3% of the error bar for a 6-parameter $\Lambda$CDM model, and for the noise levels considered in this paper. For some of the $\Lambda$CDM extensions considered here, however, these correlations have a nontrivial impact, in some cases more than 10% of the error bar, even for current experimental noise levels., Comment: 14 pages, 6 figures

The planning and design of future experiments rely heavily on forecasting to assess the potential scientific value provided by a hypothetical set of measurements. The Fisher information matrix, due to its convenient properties and low computational cost, provides an especially useful forecasting tool. However, the Fisher matrix only provides a reasonable approximation to the true likelihood when data are nearly Gaussian distributed and observables have nearly linear dependence on the parameters of interest. Also, Fisher forecasting techniques alone cannot be used to assess their own validity. Thorough sampling of the exact or mock likelihood can definitively determine whether a Fisher forecast is valid, though such sampling is often prohibitively expensive. We propose a simple test, based on the Derivative Approximation for LIkelihoods (DALI) technique, to determine whether the Fisher matrix provides a good approximation to the exact likelihood. We show that the Fisher matrix becomes a poor approximation to the true likelihood in regions where two-dimensional slices of level surfaces of the DALI approximation to the likelihood differ from two-dimensional slices of level surfaces of the Fisher approximation to the likelihood. We demonstrate that our method accurately predicts situations in which the Fisher approximation deviates from the true likelihood for various cosmological models and several data combinations, with only a modest increase in computational cost compared to standard Fisher forecasts., Comment: Published in Physical Review D

This is a solicited whitepaper for the Snowmass 2021 community planning exercise. The paper focuses on measurements and science with the Cosmic Microwave Background (CMB). The CMB is foundational to our understanding of modern physics and continues to be a powerful tool driving our understanding of cosmology and particle physics. In this paper, we outline the broad and unique impact of CMB science for the High Energy Cosmic Frontier in the upcoming decade. We also describe the progression of ground-based CMB experiments, which shows that the community is prepared to develop the key capabilities and facilities needed to achieve these transformative CMB measurements., Comment: contribution to Snowmass 2021

This Snowmass 2021 White Paper describes the Cosmic Microwave Background Stage 4 project CMB-S4, which is designed to cross critical thresholds in our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. We provide an overview of the science case, the technical design, and project plan., Comment: Contribution to Snowmass 2021. arXiv admin note: substantial text overlap with arXiv:1908.01062, arXiv:1907.04473

The effects of gravitational lensing of the cosmic microwave background (CMB) have been measured at high significance with existing data and will be measured even more precisely in future surveys. Reversing the effects of lensing on the observed CMB temperature and polarization maps provides a variety of benefits. Delensed CMB spectra have sharper acoustic peaks and more prominent damping tails, allowing for improved inferences of cosmological parameters that impact those features. Delensing reduces $B$-mode power, aiding the search for primordial gravitational waves and allowing for lower variance reconstruction of lensing and other sources of secondary CMB anisotropies. Lensing-induced power spectrum covariances are reduced by delensing, simplifying analyses and improving constraints on primordial non-Gaussianities. Biases that result from incorrectly modeling nonlinear and baryonic feedback effects on the lensing power spectrum are mitigated by delensing. All of these benefits are possible without any changes to experimental or survey design. We develop a self-consistent, iterative, all-orders treatment of CMB delensing on the curved sky and demonstrate the impact that delensing will have with future surveys., Comment: 38 pages, 10 figures. Delensing implementation in CLASS: https://github.com/selimhotinli/class_delens. Fisher forecasting code: https://github.com/ctrendafilova/FisherLens

We describe the design of a gravitational wave timing array, a novel scheme that can be used to search for low-frequency gravitational waves by monitoring continuous gravitational waves at higher frequencies. We show that observations of gravitational waves produced by Galactic binaries using a space-based detector like LISA provide sensitivity in the nanohertz to microhertz band. While the expected sensitivity of this proposal is not competitive with other methods, it fills a gap in frequency space around the microhertz regime, which is above the range probed by current pulsar timing arrays and below the expected direct frequency coverage of LISA. The low-frequency extension of sensitivity does not require any experimental design change to space-based gravitational wave detectors, and can be achieved with the data products that would already be collected by them., Comment: 18 pages, 6 figures, comments welcome. Matches version accepted for publication in PhysRevD

Since their serendipitous discovery, Fast Radio Bursts (FRBs) have garnered a great deal of attention from both observers and theorists. A new class of radio telescopes with wide fields of view have enabled a rapid accumulation of FRB observations, confirming that FRBs originate from cosmological distances. The high occurrence rate of FRBs and the development of new instruments to observe them create opportunities for FRBs to be utilized for a host of astrophysical and cosmological studies. We focus on the rare, and as yet undetected, subset of FRBs that undergo repeated bursts and are strongly gravitationally lensed by intervening structure. An extremely precise timing of burst arrival times is possible for strongly lensed repeating FRBs, and we show how this timing precision enables the search for long wavelength gravitational waves, including those sourced by supermassive black hole binary systems. The timing of burst arrival for strongly lensed repeating FRBs is sensitive to gravitational wave sources near the FRB host galaxy, which may lie at cosmological distances and would therefore be extremely challenging to detect by other means. Timing of strongly lensed FRBs can also be combined with pulsar timing array data to search for correlated time delays characteristic of gravitational waves passing through the Earth., Comment: 13 pages, 4 figures. Updated to match published content. Comments welcome