Anti-lensing: the bright side of voids

Understand

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.