Title:
Non-linearity of the large scale structure and how it impacts cosmology

Abstract :
In the primordial Universe, density fluctuations were small and had a well-defined statistical distribution. This allowed cosmologist tremendous progresses thanks to the analyses of observations of the Cosmic Microwave Background. In the coming decade however, progresses are going to come from galaxy surveys. These probe later times with huge density contrasts, as high-density clusters are surrounded by huge voids and low-density filaments.
In these conditions, physics is inevitably nonlinear, and I will talk about these implications to cosmological inference. I will discuss how this makes our traditional methods suboptimal. It also provokes the appearance of additional cosmic variance, i.e. errors coming from sampling a finite volume of the Universe. I will focus first on super-sample covariance (SSC), showing its impact on both cluster counts, galaxy clustering and weak lensing. I will show that it cannot realistically be estimated with traditional data- or simulation-based methods, but can be predicted analytically even accounting for complex survey geometry. I will then present a recent approximation that I devised to allow easy implementation of SSC by the community. That approximation further sheds light on the SSC behaviour and impact on the information content of cosmological observables. Finally, I will show that a wealth of other nonlinear effects are present and could be important, and I will give some thoughts on what can or cannot be tackled with suites of N-body simulations.