Vainshtein mechanism beyond the quasistatic approximation

Theories of modified gravity, in both the linear and fully nonlinear regimes, are often studied under the assumption that the evolution of the new (often scalar) degree of freedom present in the theory is quasistatic. This approximation significantly simplifies the study of the theory, and one often has good reason to believe that it should hold. Nevertheless it is a crucial assumption that should be explicitly checked whenever possible. In this paper we do so for the Vainshtein mechanism in a cosmological setting. By solving for the full spatial and time evolution of the Dvali-Gabadadze-Porrati and the cubic Galileon models, in an expanding spherical symmetric spacetime, we are able to demonstrate that the Vainshtein solution is a stable attractor and forms no matter what initial conditions we take for the scalar field. Furthermore, the quasistatic approximation is also found to be a very good approximation whenever it exists. For the best-fit cubic Galileon model, however, we find that for deep voids at late times the numerical solution blows up at the same time as the quasistatic solution ceases to exist. We argue that this phenomenon is a true instability of the model.