A New Window into Stochastic Gravitational Wave Background

TL;DR

This paper explores how gravitational waves can lens the cosmic microwave background, especially affecting polarization spectra, and discusses how future measurements could constrain the energy density of the stochastic gravitational wave background.

Contribution

It demonstrates that gravitational wave lensing of the CMB is more efficient than scalar perturbations and proposes using CMB polarization measurements to constrain the gravitational wave background.

Findings

GW lensing affects all CMB power spectra, especially polarization.

Current constraints on $\\Omega_{GW}$ are mainly from temperature anisotropies.

Future B-mode polarization measurements could tighten bounds on $\\Omega_{GW}$.

Abstract

A stochastic gravitational wave background (SGWB) would gravitationally lens the cosmic microwave background (CMB) photons. We find that the lensing due to gravitational waves(GW) is more efficient as compared to lensing due to scalar density perturbations. Though the effect of lensing due to GW is found to effect all the four CMB power spectra, its effect is most prominently seen in the CMB polarization power spectra. This suggests that the measurements of the CMB angular power spectra could be used to constraining the energy density ($\Omega_{GW}$) of the SGBW. In our analysis we find that the most stringent constraints on $\Omega_{GW}$ are due to measurements of the angular power spectra of CMB temperature anisotropies. We show that in the future it will be possible to place more stringent bounds on $\Omega_{GW}$ using improved upper limits or detections of the angular power spectra of the B-modes of CMB polarization at large multipoles.