Using Voids to Unscreen Modified Gravity
  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.
