Anti-lensing: the bright side of voids
  More than half of the volume of our Universe is occupied by cosmic voids. The
lensing magnification effect from those under-dense regions is generally
thought to give a small dimming contribution: objects on the far side of a void
are supposed to be observed as slightly smaller than if the void were not
there, which together with conservation of surface brightness implies net
reduction in photons received. This is predicted by the usual weak lensing
integral of the density contrast along the line of sight. We show that this
standard effect is swamped at low redshifts by a relativistic Doppler term that
is typically neglected. Contrary to the usual expectation, objects on the far
side of a void are brighter than they would be otherwise. Thus the local
dynamics of matter in and near the void is crucial and is only captured by the
full relativistic lensing convergence. There are also significant nonlinear
corrections to the relativistic linear theory, which we show actually
under-predicts the effect. We use exact solutions to estimate that these can be
more than 20% for deep voids. This remains an important source of systematic
errors for weak lensing density reconstruction in galaxy surveys and for
supernovae observations, and may be the cause of the reported extra scatter of
field supernovae located on the edge of voids compared to those in clusters.
