# Cosmology and the Einstein Toolkit

#### Overview

The study of spacetimes containing compact objects, especially in binary systems, is the traditional application area of Numerical Relativity and has greatly benefitted from the advances in the field in recent years. The properties of other regimes of General Relativity are likewise only accessible through the integration of Einstein's equation in three dimensions: the large-scale behavior of our universe in different stages of its evolution provides many examples.

#### Physical questions

Homogeneous and isotropic models are a simple and effective way to describe the large-scale universe; however, some of the resulting properties, such as the existence of a dark sector, are rather puzzling. In order to evaluate the validity of the assumption of homogeneity and isotropy, one must construct more general models including some degree of asymmetry. This task has only been accomplished in a few simplified scenarios, such as the perturbative regime. For a review of the relevance of constructing generic models with a non-trivial power spectrum of inhomogeneities in the late universe, see this Focus section of CQG. For the role of inhomogeneities in the early universe, see e.g. this paper on gravitational-wave production.

#### Potential workshop topics

- Modelling inhomogeneities via black-hole lattices;
- Modelling inhomogeneities via the Einstein-Klein-Gordon system;
- Solving the initial-data problem on periodic spaces.