Can Cosmic Shear Shed Light on Low Cosmic Microwave Background Multipoles?

The Wilkinson Microwave Anisotropy Probe (WMAP) finds that the lowest multipole moments of the cosmic microwave background (CMB) are smaller than expected for a scale-invariant power spectrum. One possible explanation is a cutoff in the primordial power spectrum $P(k)$ below a comoving scale of $k_c \simeq 5.0 \times 10^{-4}$ Mpc$^{-1}$. Such a suppression in the large-scale power would affect not only the primordial CMB but also the cosmic shear of the CMB, the weak-lensing deflection that CMB photons experience as they propagate from the last-scattering surface to the observer. We calculate the effects of a cutoff in $P(k)$ on the cosmic-shear power spectrum and its cross-correlation with the CMB. We find that cosmic shear may reduce the error to $k_c$ by roughly 50%, an improvement that may tilt the balance between a $\sim2\sigma$ discrepancy and a $>3\sigma$ detection of a large-scale cutoff in the power spectrum.