Adding initial data

Writing an Initial Data thorn for the Einstein Toolkit

This page explains how to write a new initial data thorn for the Einstein Toolkit. These initial data will set up ADMBase or HydroBase variables, so that they are usable for all other thorns that are based Einstein Toolkit.

Introduction

You need:

• a prescription or routine that actually generates the initial data,
• to decide whether this are initial data for the spacetime, for hydrodynamics, or both,
• these instructions.

Example

Let us assume, for the sake of simplicity, that we are going to set up initial data for the spacetime only. Let us pick a concrete example: the Kruskal coordinates. The Kruskal coordinates describe a single, static black hole (the Schwarzschild spacetime), and their advantage over many other coordinate systems is that they cover the whole, extended Schwarzschild spacetime (i.e. including both asymptotically flat ends, the worm hole, the black and white hole horizons), and that they do not introduce coordinate singularities. Of course there are curvature singularities inside the white and black holes, but there are no "artificial" singularities, e.g. at the horizons. The Krukal coordinates are beautifully described in Sean M. Carroll's "A No-Nonsense Introduction to General Relativity", 2001, at http://pancake.uchicago.edu/~carroll/notes/grtinypdf.pdf .