Gravitational Lensing of the Cosmic Microwave Background: Flux Conservation and Growth of Inhomogeneities
131A Campbell Hall
John Peacock (ROE)
Gravitational lensing is a rich source of cosmological information: it probes both the amplitude and evolution of metric fluctuations, plus the distance-redshift relation via the geometry of the distortion of background images. But in 2014, Clarkson et al. suggested that lensing had an average non-Newtonian "back-reaction" effect, which altered the effective distance to the CMB last-scattering surface, potentially by several percent - enough to cause substantial change in the cosmological parameters we infer. The first part of this talk discusses this issue from the point of view of flux conservation theorems due to Weinberg and to Kibble & Lieu, and a possible loophole in these theorems is identified. This loophole relates to the area of constant-redshift surfaces, and the solution to this puzzle is critical in order to decide whether the Clarkson et al. claims should be taken seriously. The second part of the talk presents some new results on cross-correlation between galaxy surveys (based on WISE & SuperCOSMOS) and CMB lensing data. These yield interesting measurements of the evolution of cosmological perturbations at redshift z<0.35.