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Nick Kaiser

(Institute for Astronomy, University of Hawaii)

*The Effect of Gravitational Lensing on Cosmological Parameter
Estimation*

A very long standing question in cosmology is whether gravitational
lensing biases the distance-redshift relation D(z) or the mean flux
density of sources. Interest in this has been rekindled by recent
results in 2nd order relativistic perturbation theory which, if
correct, would have profound implications for cosmological parameter
estimation from both SN1a and the CMB. 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 foreshadows
the more recent results. I then show how the claimed subtle
relativistic effects actually result from confusion between different
types of averaging (specifically between averaging over sources and
averaging over directions on the sky). I also show that while
Weinberg's argument conceals a hidden loop-hole, in that he assumes
that the area of a surface of constant redshifts is unaffected by
lensing, the error incurred is only a one part in a million effect.
This result validates conventional methods for analysis of the CMB
and shows that, counter to claims in the literature, the focusing
theorem does not in fact reveal an intrinsic tendency for structure
to cause focusing of light rays.