## Sound horizon

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Susana Landau
Posts: 20
Joined: December 10 2004
Affiliation: Buenos Aires University

### Sound horizon

My question is regarding the calculation of the sound horizon which is encoded in function dsound_da_exact in results.f90 (fortran version of CAMB)

function dsound_da_exact(this,a)
class(CAMBdata) :: this
real(dl) dsound_da_exact,a,R,cs
R = 3*this%grhob*a / (4*this%grhog)
cs=1.0d0/sqrt(3*(1+R))
dsound_da_exact=dtauda(this,a)*cs

end function dsound_da_exact

I checked several papers and books and from this R should be

R = 3*this%grhob / (4*this%grhog)

R = 3*this%grhob*a / (4*this%grhog)

I checked to make the change for a specifici cosmological model and resulting Cl's are the same. Could you confirm this? thank very much in advanced for making this clear.

Antony Lewis
Posts: 1644
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: Sound horizon

The sound horizon calculation is not used to calculate the CL. rhob and rhog are constants, so to the $a$ is needed to calculate the density ratio at the relevant time.