Wave Propagation in Modified Gravity
  We investigate the propagation of scalar waves induced by matter sources in
the context of scalar-tensor theories of gravity which include screening
mechanisms for the scalar degree of freedom. The usual approach when studying
these theories in the non-linear regime of cosmological perturbations is based
on the assumption that scalar waves travel at the speed of light. Within
General Relativity such approximation is good and leads to no loss of accuracy
in the estimation of observables. We find, however, that mass terms and
non-linearities in the equations of motion lead to propagation and dispersion
velocities significantly different from the speed of light. As the group
velocity is the one associated to the propagation of signals, a reduction of
its value has direct impact on the behavior and dynamics of nonlinear
structures within modified gravity theories with screening. For instance, the
internal dynamics of galaxies and satellites submerged in large dark matter
halos could be affected by the fact that the group velocity is smaller than the
speed of light. It is therefore important, within such framework, to take into
account the fact that different part of a galaxy will see changes in the
environment at different times. A full non-static analysis may be necessary
under those conditions.
