## CAMB: scale factor and adotrad

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Dahee Lee
Posts: 16
Joined: April 16 2018
Affiliation: Kyungpook National University

### CAMB: scale factor and adotrad

Hi.
I want to know reference about exact relation between scale factor and conformal time.
In subroutine initial, a=tau*adotrad*(1+omtau/4).
In CAMB note 12p, regular initial condition series solution S.
In this solution, there is a sentense ‘Following [20]’, but [20] is not found.
What is ‘adotrad’ in module.f90 meaning?
I think ‘adotrad’ is H0*dsqrt(omega_R0), but it’s different value from adotrad.
And, does anyone know reference for scale factor S?

Any help would be appreciated.

Antony Lewis
Posts: 1470
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: CAMB: scale factor and adotrad

These are just series expansions, you can see some maple code at https://camb.info/maple/. adotrad should indeed just be da/dtau_{tau=0} where tau is conformal time, in Mpc units. You can get it from Omega_R0 as long as you define the latter to include photons and all neutrinos as though they were massless (since they will be relativistic at early times).