CAMB: GR correction?

[Minseong Kwon]

Hi,

I'm trying to calculate the GR correction for an N-body simulation as defined below.

[math] k^2 \gamma_{GR} = -\left(\partial_\tau + \frac{\dot{a}}{a}\right)\dot{H}_T + 8\pi G a^2 \Sigma
(https://arxiv.org/pdf/1811.00904.pdf)

Is there a general routine to calculate such a quantity in the CAMB code?

Thanks for any help!
Minseong

[Antony Lewis]

It's not pre-coded. You can use the tools of
https://camb.readthedocs.io/en/latest/ScalEqs.html
to relate to CAMB synchornous-gauge variables, e.g. I think you basically want A(t) (acceleration) in the gauge in which h(t)=0 (perturbation to scale factor), and subtract the acceleration you get from the Poisson equation.