# Using Voids to Unscreen Modified Gravity

## TL;DR

This paper explores how cosmic voids can serve as effective probes for testing modified gravity models with Vainshtein screening, which are otherwise difficult to detect in dense regions.

## Contribution

It demonstrates that cosmic voids are unscreened regions where deviations from general relativity can be observed, offering a new way to test modified gravity theories.

## Key findings

- Voids are completely unscreened in Vainshtein models.
- Velocity profiles in voids deviate from ΛCDM predictions.
- Void-based measurements can effectively test gravity on large scales.

## Abstract

The Vainshtein mechanism, present in many models of gravity, is very effective at screening dark matter halos such that the fifth force is negligible and general relativity is recovered within their Vainshtein radii. Vainshtein screening is independent of halo mass and environment, in contrast to e.g. chameleon screening, making it difficult to test. However, our previous studies have found that the dark matter particles in filaments, walls, and voids are not screened by the Vainshtein mechanism. We therefore investigate whether cosmic voids, identified as local density minima using a watershed technique, can be used to test models of gravity that exhibit Vainshtein screening. We measure density, velocity, and screening profiles of stacked voids in cosmological $N$-body simulations using both dark matter particles and dark matter halos as tracers of the density field. We find that the voids are completely unscreened, and the tangential velocity and velocity dispersion profiles of stacked voids show a clear deviation from $\Lambda$CDM at all radii. Voids have the potential to provide a powerful test of gravity on cosmological scales.