Re: Tabulated Power Spectrum and Initial Conditions

From: Volker Springel <volker_at_MPA-Garching.MPG.DE>
Date: Sun, 9 Jun 2013 09:53:43 +0200

On Jun 6, 2013, at 12:01 PM, Antonio Bibiano wrote:

> Hello Everyone,
> I was trying to generate some initial conditions for a cosmological simulation using
> N-genIC and i wanted to use a tabulated power spectrum.
> This is my first experience with simulations so I apologize if my questions are obvious or even stupid!
> My question is, should the redshift of the tabulated power spectrum be the same as the starting
> Redshift in the N-genIC parameter file?

Dear Antonio,

No, this is not really necessary, provided you use ReNormalizeInputSpectrum = 1. The IC code will then only use the shape of the (linear) input spectrum and scale it up or down as needed (by the same factor for all k) to reproduce the specified normalization at z=0 (given by sigma8).

> I got my power spectra from CAMB and CLASS, as far as i understand those powerspectra a normalized to the sigma_8 that the programs give in their output, so ReNormalizeInputSpectrum = 0 should be the right setting, is this right?

I think so, but this question is really one about CAMB and/or CLASS, not about N-GenIC.

> And so the Sigma8 parameter should be initialized as the one given by those ouputs?

If you use ReNormalizeInputSpectrum = 0, the N-GenIC code will ignore the value specified for sigma8 in its parameterfile and assume that the input spectrum is correctly normalized for z=0. It will then scale it back to the starting redshift with the linear growth factor for the specified cosmology.


> Antonio Bibiano.
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Received on 2013-06-09 09:53:20

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