Starts 7 Apr 2016 16:30
Ends 7 Apr 2016 18:00
Central European Time
Leonardo Building - Luigi Stasi Seminar Room
Abstract. The analysis and interpretation of large cosmological surveys increasingly relies on the use of large volume simulations of collisionless dynamics. Since these simulations routinely use Newtonian equations, one must be careful in interpreting their output, particularly when considering statistics on length scales comparable to the Hubble radius where general relativistic (GR) effects are expected to become important. This is potentially relevant for future surveys such as the SKA or LSST which will be able to access these scales observationally. The standard solution to the problem involves a clever use of gauge degrees of freedom in GR to map the variables evolved by standard Newtonian simulations to variables in multiple gauges in GR. We will discuss an alternate approach to this problem in which the Poisson equation for the gravitational potential is replaced with a Helmholtz equation, using a simple but accurate approximation valid in the so-called conformal Newtonian gauge. This Helmholtz equation — which, like the Poisson equation, is fully nonlinear in the density fluctuations — contains a ‘screening’ scale whose value is determined by the expansion of the Universe and the linear growth of structure. Length scales smaller than the screening scale evolve according to Newtonian dynamics, while larger scales pick up relativistic corrections that accurately reproduce known results from perturbation theory in the Newtonian gauge. This approach provides a clear physical understanding of how the expansion of the Universe ‘screens’ small scales from the gauge-dependent power at large scales through a criterion closely resembling that of Jeans stability. In particular, our analysis provides a natural resolution to the ‘Jeans swindle’ problem in the presence of super-horizon fluctuations. Finally, the simplicity of our modification (Poisson to Helmholtz) also leads to an accurate post-processing prescription to convert the outputs of standard simulations into the correctly screened relativistic outputs in the Newtonian gauge. We will discuss how our results might be applied in studying the large scale clustering of biased tracers of matter, as well as extended to the case of multiple fluids. Reference: Hahn & Paranjape, arXiv:1602.07699
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