Output a scale-dependent growth rate for mu-Sigma runs.

[TessaBaker]

Scale-dependent growth rates (or growth rates * sigma_8) feature a lot in tests of gravity. It would be great to have a feature that could output f(z,k) for a given input mu(z,k) and Sigma(z,k), or similar. In principle this could be pulled out of the Boltzmann solver; difficulty might vary depending on how/if f is calculated internally by the Boltzmann solver.

[c-d-leonard]

Putting this here for reference: discussion of related issues in page 21 of this paper: https://arxiv.org/pdf/1811.12928v1.pdf

[damonge]

Are we still interested in this? @mishakb @c-d-leonard ?

[damonge]

@mishakb @c-d-leonard is this still useful?

[mishakb]

@damonge I think so, being able to produce the growth function for MG is always useful, e.g. to compare to simulation wok, to theoretical models predictions. The only question is that I "recall" that there was a need to go back and check this out as it was not the main point of development when we did mu-sigma (z,k) implementation and this was not completed or validated as it was a sperate thing on its own. This is my recollection. So this might be a good project for someone to look into if there are volunteers.