Separation of Gravitational-Wave and Cosmic-Shear Contributions to Cosmic Microwave Background Polarization

Inflationary gravitational waves generate a distinct signature in the cosmic microwave background (CMB) through a contribution to the curl, or magnetic-like, component in the polarization pattern. Though there is no direct contribution to these curl modes from scalar (density) perturbations, cosmic shear--gravitational lensing of the CMB by large-scale structure along the line of sight--converts a fraction of the polarization in the dominant gradient, or electric-like, component to the curl component. Measurements of higher-order correlations in the CMB temperature and polarization can be used to map the cosmic shear as a function of position on the sky, and with this cosmic-shear map, the lensing contribution to the curl can in principle be subtracted. We study how well this cosmic-shear subtraction can be accomplished. We find that CMB information down to arcminute resolution will be required to pursue gravitational-wave amplitudes significantly smaller than those accessible with the Planck satellite. The existence of a finite cutoff in the CMB power spectrum at small scales leads to a minimum detectable gravitational-wave-background amplitude that corresponds to an inflationary energy scale near 10^15 GeV