Instabilities of Scalar Fields around Oscillating Stars.

The behavior of fundamental fields in strong gravity or nontrivial environments is important for our understanding of nature. This problem has interesting applications in the context of dark matter, of dark energy physics, or of quantum field theory. The dynamics of fundamental fields has been studied mainly in static or stationary backgrounds, whereas most of our Universe is dynamic.

Collective Scalarization or Tachyonization: When Averaging Fails.

Certain scalar-tensor theories of gravity provide negative-energy, tachyonic modes to a fundamental scalar inside matter, giving rise to nonperturbative phenomena around compact stars. Studies of this and other tachyonic instabilities always average over local matter properties. We use elementary, flat space models to understand possible collective effects and the accuracy of the averaging procedure.

Holographic Collisions across a Phase Transition.

We use holography to analyze relativistic collisions in a one-parameter family of strongly coupled gauge theories with thermal phase transitions. For a critical value of the parameter, the transition is second order, for subcritical values it is first order, and for supercritical values it a smooth crossover. We extract the gauge theory stress tensor from collisions of gravitational shock waves on the dual geometries.