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

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 $1\sigma$ constraints on the tensor-to-scalar ratio $r$, albeit with a simplistic foreground treatment, and find constraints of ${\sigma }_r\simeq 7\times 10^{-3}$ from the correlation of CMB-S4-Deep $B$-mode lensing and LSST galaxies, ${\sigma }_r\simeq 5\times 10^{-3}$ from the correlation of CMB-S4-Deep $B$-mode lensing and CMB-S4-Deep CMB lensing, and ${\sigma }_r\simeq 10^{-2}$ 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 $r$.

arXiv comment6 pages, 1 figure, accepted for publication in Physical Review Letters

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Cited by in corpus (1)

• Impact of Galactic non-Gaussian foregrounds on CMB lensing measurements