### Cobaya: theory module camb - getting growth D(z) and growth rate f(z)

Posted:

**September 17 2020**Hi, I am writing a new theory class with Cobaya where I need the following ingredients:

1) D(z) (normalized to z = 0 or 1/(1+z) at matter domination)

2) f(z) for calculating the RSD parameter

I wonder what's the best way of getting them from the camb theory module in cobaya?

1) Would sigma8(z)/sigma8(z=0) suffice to get the unnormalized D(z) or are there caveats around doing that? In particular, I found that he following code could work for getting sigma8(z) but maybe there are dangers with going into the matter transfer data by had (e.g. z is flipped)? If so, would using the Cobaya interface self.provider.get_sigma_R() be the preferred option? Also, if there is a way to directly get D(z), that's probably preferable.

where provider here is the camb module.

2) The closest thing I can come up with is f = fsigma8(z)/sigma8(z) where get_fsigma8() is provided by the theory module camb and sigma8(z) like above. But I wonder if that's safe and if there is a more direct way to get f(z). Thank you!

1) D(z) (normalized to z = 0 or 1/(1+z) at matter domination)

2) f(z) for calculating the RSD parameter

I wonder what's the best way of getting them from the camb theory module in cobaya?

1) Would sigma8(z)/sigma8(z=0) suffice to get the unnormalized D(z) or are there caveats around doing that? In particular, I found that he following code could work for getting sigma8(z) but maybe there are dangers with going into the matter transfer data by had (e.g. z is flipped)? If so, would using the Cobaya interface self.provider.get_sigma_R() be the preferred option? Also, if there is a way to directly get D(z), that's probably preferable.

Code: Select all

```
params, results = self.provider.get_CAMB_transfers()
data = results.get_matter_transfer_data()
sigma8 = np.flip(data.sigma_8)
sigma8_z0 = self.provider.get_param('sigma8’)
D = sigma8 / sigma8_z0
```

2) The closest thing I can come up with is f = fsigma8(z)/sigma8(z) where get_fsigma8() is provided by the theory module camb and sigma8(z) like above. But I wonder if that's safe and if there is a more direct way to get f(z). Thank you!