Wave Propagation in Modified Gravity

TL;DR

This paper studies how scalar waves in modified gravity theories propagate at speeds different from light, affecting the dynamics of cosmic structures and requiring non-static analysis for accurate modeling.

Contribution

It reveals that mass terms and non-linearities cause scalar wave velocities to differ from light speed, impacting the understanding of structure formation in screened modified gravity theories.

Findings

Scalar waves propagate at subluminal speeds due to mass and non-linear effects.

Propagation velocities influence the internal dynamics of galaxies and dark matter halos.

Different parts of a galaxy experience environmental changes at different times.

Abstract

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.