University of Cambridge > > Cosmology Lunch > Steps toward relativistic structure formation: a nonlinear post-Friedmann framework for small and large scales

Steps toward relativistic structure formation: a nonlinear post-Friedmann framework for small and large scales

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Deviations from homogeneity in cosmology are traditionally studied with different methods at small and large scales. In studies of the early Universe and at scales of the order of the Hubble horizon today, small fluctuations are investigated with relativistic perturbation theory. Non-linear structure formation at small scales is instead studied with Newtonian methods, notably N-body simulations. In view of new large-scale galaxy surveys that will provide data with an unprecedented accuracy, it seems timely to go beyond the Newtonian approximation, unifying the study of the very large scales and the small non-linear scales in a single theoretical framework. In this talk I will describe a sort of post-Minkowskian (weak field) approach to cosmology, such that at leading order in a 1/c expansion Newtonian cosmology is recovered as a consiste approximate solution of Einstein equations, on top of a Friedmannian background. In this post-Friedmann framework, linear and non-linear relativistic contributions appear at next order. Resumming variables and linearising the equations one recovers first-order relativistic perturbation theory, i.e. the framework is valid on horizon scales and beyond. I will illustrate the first practical application, i.e. the extraction of the frame-dragging gravitomagnetic potential from N-body simulations, and its power spectrum. Similarly, the difference between the two scalar potential, known as “slip” in cosmology, can also in principle be computed, sourced at leading order by purely Newtonian non-linear terms. I will conclude with an outlook on possible further developments.

This talk is part of the Cosmology Lunch series.

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