Re: vacuum boundaries

From: Volker Springel <volker_at_MPA-Garching.MPG.DE>
Date: Thu, 09 Apr 2009 11:34:18 +0200

Johan Maes wrote:
> Dear all,
>
> If vacuum boundaries are used, extra terms are added to the particle
> acceleration. In the case of comoving integration, OmegaMatter0 is the
> defining parameter (gravtree.c, line 290). And in the case of physical
> coordinates, it's OmegaLambda: (gravtree.c line 331). Can anyone
> enlighten me about the exact origin is of these terms? Thanks in advance!
>

In physical coordinates, you can apply Birkhoff's theorem for a spherically
symmetric set-up (the one assumed for vacuum boundaries). Then you just have
Newton's equations for the matter that is actually present.

If you transfer this to comoving coordinates, you get equation (55) in the
gadget-1 paper, which involves the term with OmegaMatter0.

OmegaLambda only affects the expansion rate, and in comoving coordinates it is
automatically taken care of that way. Transforming this to physical coordinates,
one get an additional fiducial force proportional to OmegaLambda, but this
cannot be understood from Newtonian theory alone.

Volker



> Cheers,
>
> Johan
>
>
>
>
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Received on 2009-04-09 11:34:19

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