Constraining stochastic gravitational wave background from weak lensing of CMB B-modes

A stochastic gravitational wave background (SGWB) will affect the CMB anisotropies via weak lensing. Unlike weak lensing due to large scale structure which only deflects photon trajectories, a SGWB has an additional effect of rotating the polarization vector along the trajectory. We study the relative importance of these two effects, deflection & rotation, specifically in the context of E-mode to B-mode power transfer caused by weak lensing due to SGWB. Using weak lensing distortion of the CMB as a probe, we derive constraints on the spectral energy density (Omega(GW)) of the SGWB, sourced at different redshifts, without assuming any particular model for its origin. We present these bounds on Omega(GW) for different power-law models characterizing the SGWB, indicating the threshold above which observable imprints of SGWB must be present in CMB.