Anti-lensing: the bright side of voids

TL;DR

This paper reveals that relativistic effects, especially Doppler terms, cause objects behind cosmic voids to appear brighter, challenging traditional weak lensing assumptions and impacting cosmological measurements.

Contribution

It demonstrates that relativistic Doppler effects dominate weak lensing signals around voids, revealing a previously neglected factor that influences observational cosmology.

Findings

Relativistic Doppler terms cause objects behind voids to be brighter.

Nonlinear corrections can exceed 20% for deep voids.

Standard linear theory underestimates the relativistic lensing effect.

Abstract

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.