LERMA UMR8112

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.