  We investigate the potential of weak lensing by voids to test for deviations
from General Relativity. We calculate the expected lensing signal of a scalar
field with derivative couplings, finding that it has the potential to boost the
tangential shear both within and outside the void radius. We use voids traced
by Luminous Red Galaxies in SDSS to demonstrate the methodology of testing
these predictions. We find that the void central density parameter, as inferred
from the lensing signal, can shift from its GR value by up to 20% in some
galileon gravity models. Since this parameter can be estimated independently
using the galaxy tracer profiles of voids, our method provides a consistency
check of the gravity theory. Although galileon gravity is now disfavoured as a
source of cosmic acceleration by other datasets, the methods we demonstrate
here can be used to test for more general fifth force effects with upcoming
void lensing data.
