Dynamics of Screening in Modified Gravity

Gravitational theories differing from General Relativity may explain the accelerated expansion of Universe without a cosmological constant. However, to pass local gravitational tests, "screening mechanism" is needed suppress, on small scales, fifth force driving acceleration. We consider simplest these theories, i.e. scalar-tensor theory with first-order derivative self-interactions, and study isolated (static spherically symmetric) non-relativistic relativistic stars. produce screened solutions use them as initial data for non-linear numerical evolutions in spherical symmetry. find that are stable under large perturbations, long they do not cause collapse. When collapse triggered, characteristic speeds scalar evolution equation diverge, even before apparent black-hole or sound horizons form. This casts doubts whether dynamical stars be predicted effective field theories.