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arXiv · 1409.6294

Separate Universe Simulations

Christian Wagner·Fabian Schmidt·Chi-Ting Chiang·Eiichiro Komatsu
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Abstract

The large-scale statistics of observables such as the galaxy density are chiefly determined by their dependence on the local coarse-grained matter density. This dependence can be measured directly and efficiently in N-body simulations by using the fact that a uniform density perturbation with respect to some fiducial background cosmology is equivalent to modifying the background and including curvature, i.e., by simulating a "separate universe". We derive this mapping to fully non-linear order, and provide a step-by-step description of how to perform and analyse the separate universe simulations. This technique can be applied to a wide range of observables. As an example, we calculate the response of the non-linear matter power spectrum to long-wavelength density perturbations, which corresponds to the angle-averaged squeezed limit of the matter bispectrum and higher $n$-point functions. Using only a modest simulation volume, we obtain results with percent-level precision over a wide range of scales.

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Christian Wagner, Fabian Schmidt, Chi-Ting Chiang, Eiichiro Komatsu. 2014-09-22. Separate Universe Simulations. https://doi.org/10.1093/mnrasl/slu187

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