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Probing inflationary gravitational waves with cross-correlations: Improved forecasting and validation with simulations

Toshiya Namikawa1,2,3, Irene Abril-Cabezas2,3, and Blake D. Sherwin2,3

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Phys. Rev. D 112, 023536 – Published 18 July, 2025

DOI: https://doi.org/10.1103/bwdp-kf8l

Abstract

We present a follow-up study to the method recently proposed by Namikawa and Sherwin (2023) to probe gravitational waves using cross-correlations between two cosmic microwave background (CMB) B-modes and a large-scale structure tracer. We first improve on the previous forecast by including the impact of CMB component separation and find that, if the tensor-to-scalar ratio is r=0, we can achieve σr≃3.6×10−3 by combining upcoming experiments, i.e., LiteBIRD, CMB-S4 and the Advanced Simons Observatory. With a more futuristic experiment, we can achieve even tighter constraints on r if improved delensing can be realized. Using a simulated analysis pipeline, we also explore possible biases from higher-order terms in the lensing potential, which were previously not examined in detail. We find that these bias terms are negligible compared to a detectable signal from inflationary gravitational waves. Our simulated results confirm that this new method is capable of obtaining powerful constraints on r. The method is immune to Gaussian Galactic foregrounds and has a different response to non-Gaussian Galactic foregrounds than the B-mode power spectrum, offering an independent cross-check of r constraints from the standard power spectrum analysis.

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Phys. Rev. D 112, 023536– Published 18 July, 2025
  • Received 9 March 2025
  • Accepted 1 July 2025
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