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arXiv · 2304.10315

New probe of inflationary gravitational waves: cross-correlations of lensed primary CMB B-modes with large-scale structure

Toshiya Namikawa·Blake D. Sherwin
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Abstract

We propose a new probe of inflationary gravitational waves (IGWs): the cross-correlation of the lensing of inflationary $B$-mode polarization with a large-scale structure (LSS) tracer, which can also be a cosmic microwave background (CMB) lensing map. This is equivalent to measuring a three-point function of two CMB $B$-modes and an LSS tracer. We forecast expected $1\,σ$ constraints on the tensor-to-scalar ratio $r$, albeit with a simplistic foreground treatment, and find constraints of $σ_r \simeq 7 \times 10^{-3}$ from the correlation of CMB-S4-Deep $B$-mode lensing and LSST galaxies, $σ_r \simeq 5 \times 10^{-3}$ from the correlation of CMB-S4-Deep $B$-mode lensing and CMB-S4-Deep CMB lensing, and $σ_r \simeq 10^{-2}$ from the correlation of LiteBIRD $B$-mode lensing and CMB-S4-Wide lensing. Because this probe is inherently non-Gaussian, simple Gaussian foregrounds will not produce any biases to the measurement of $r$. While a detailed investigation of non-Gaussian foreground contamination for different cross-correlations will be essential, this observable has the potential to be a useful probe of IGWs, which, due to different sensitivity to many potential sources of systematic errors, can be complementary to standard methods for constraining $r$.

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Toshiya Namikawa, Blake D. Sherwin. 2023-09-21. New probe of inflationary gravitational waves: cross-correlations of lensed primary CMB B-modes with large-scale structure. https://doi.org/10.1103/physrevlett.131.131001

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