Title: Poisson is not a red herring: Modified gravity on non-linear scales
Linear theory parameterisations for testing gravity in a model independent way are only valid on larger scales in the Universe. Upcoming surveys will have a lot of data on smaller, non-linear scales, so in order to test gravity in a model independent way using all of the data, we need new approaches. I will present such an approach, which contains a set of parameterised equations analogous to those used in linear theory, but which is valid on all cosmological scales. The equations are derived from a set of Post-Friedmann “master” equations, that are higher order in the senses of containing both non-linear and post-Newtonian effects. I will explain under what conditions these parameterised equations provide a good description of gravitational phenomenology in cosmology, and present a set of explicit tests that can be used to determine if any particular modified gravity theory fits under this approach. These parameterised equations have now been used to run N-body simulations. I will present results from these simulations, and show the performance of various fitting functions in capturing the simulation phenomenology. I will also show some consequences of this phenomenology for weak gravitational lensing studies. These results pave the way for all of the data in future surveys to be used to test modified gravity in a model independent way.