From fluctuation to polarization: imprints of 𝒪(1-10) Mpc-1 curvature perturbations in CMB B-modes from scalar-induced gravitational waves

Abstract

Probing primordial curvature perturbations on small scales, beyond those accessible using cosmic microwave background (CMB) primary anisotropies and Lyman-α forest data, remains a major open challenge. Current constraints on the scalar power spectrum at these scales are either weak (e.g. 𝒫ℛ ≲ 10-4 from CMB spectral distortions) or rely heavily on model-dependent assumptions about small-scale structure. In this work, we propose a novel method to probe the small-scale primordial power spectrum using scalar-induced tensor perturbations, which are inevitably sourced by curvature perturbations at second order in cosmological perturbation theory. While induced tensor modes have traditionally been studied in the context of the stochastic gravitational wave background, we highlight a complementary observable: the distinctive pattern of B-mode polarization they imprint on the CMB. We compute the angular spectrum of these B-modes arising from enhanced scalar perturbations and show that the resulting signal can be competitive with inflationary predictions for values of the tensor-to-scalar ratio targeted in upcoming CMB experiments, most notably CMB-Stage 4. We map the region of the scalar power spectrum to which these future B-mode experiments will be sensitive and compare with existing constraints, finding it to exceed current sensitivities at k ∼ 𝒪(1-10) Mpc-1. In addition to providing a new CMB-based probe of the small-scale power spectrum, this work also motivates dedicated B-mode searches at higher multipoles (ℓ ≳ 100).