Abstract

A long standing question in cosmology is whether gravitational lensing by structure biases the apparent distance or the mean flux density of sources. Interest in this has been rekindled by recent calculations in 2nd order relativistic perturbation theory which suggest significant implications for cosmological parameter estimation from both supernovae and the microwave background. In this talk I shall first review the somewhat confusing history of the subject, going back to the early ‘60s and including both Weinberg’s 1976 argument that there should be no effect on grounds of flux conservation and the general relativistic “focusing theorem” of the ‘80s that seems to contradict this and which foreshadows the more recent results. I then describe recent work with John Peacock where we have shown how these conflicts are resolved. Regarding Weinberg we show that there is a loophole in his argument, and lensing does in fact bias the apparent distance, but it is a very small effect. We show that the apparent intrinsic tendency for structure to cause focusing of light rays that emerges from the focusing theorem is a statistical rather than a physical effects.