Speaker: Marco Bruni
Location: invited plenary talk at the XX SIGRAV COnference, Naples Observatory
The LCDM is the successful standard model of cosmology. Alternatives to the cosmological constants include models of dark energy, modified theories of gravity, as well as general relativistic (GR) models that either weaken the symmetry assumptions of the cosmological principle or try to construct an average universe to explain acceleration as a back-reaction effect. At a time were we are going to have observational data allowing measurements with unprecedented precision, it is however also worth reconsidering the fine details of how we model structure formation in LCDM cosmology, and how this may affect how we interpret observations. By and large, we model very large scales with relativistic perturbation theory, small scales where non-linearity is important with Newtonian N-body simulations, and we interpret observations (e.g. supernovae) as if light was propagating in a homogeneous-isotropic background. In this talk I will present a new non-linear post-Friedmannian scheme, which is a sort of generalisation to cosmology of the well known post-Newtonian approximation. Using a 1/c expansion of Einstein equations, a set of non-linear approximate equations are obtained in the Newtonian-Poisson gauge, which include the full non-linearity of the Newtonian regime at small scales, and when linearised give the usual scalar and vector linear relativistic perturbation dynamics. The scheme thus provides a unified framework to deal with small as well as large scales, to the horizon and beyond. Just analysing these equations two main results emerge: in the Newtonian regime, consistency of Einstein equations requires that the frame-dragging (gravito-magnetic) vector potential cannot be neglected in the metric; in the post-Friedmannian non-linear regime, there are two gravitational scalar potentials. I will end the talk with a brief overview of current work on non-linear simulations and an outlook to future applications.
Speaker: Marco Bruni