Cross-Correlating Probes of Primordial Gravitational Waves

TL;DR

This paper explores how cross-correlating polarization and lensing B-modes in the cosmic microwave background can improve the confirmation of primordial gravitational waves, addressing systematic issues in detection.

Contribution

It demonstrates that cross-correlating polarization and lensing B-modes can enhance the evidence for primordial gravitational waves beyond previous methods.

Findings

Cross-correlation can strengthen detection confidence.

Lensing surveys may not significantly constrain amplitude.

Systematic issues in large-scale detection are addressed.

Abstract

One of the most promising ways of detecting primordial gravitational waves generated during inflation is to observe B-modes of polarization, generated by Thomson scattering after reionization, in the cosmic microwave background (CMB). Large scale foregrounds though are expected to be a major systematic issue, so -- in the event of a tentative detection -- an independent confirmation of large scale gravitational waves would be most welcome. Previous authors have suggested searching for the analogous mode of cosmic shear in weak lensing surveys but have shown that the signal to noise of this mode is marginal at best. This argument is reconsidered here, accounting for the cross-correlations of the polarization and lensing B-modes. A lensing survey can potentially strengthen the argument for a detection of primordial gravitational waves, although it is unlikely to help constrain the amplitude of the signal.