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Sound horizon

Posted: April 03 2020
by Susana Landau
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)

end function dsound_da_exact

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

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

in stead of

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.

Re: Sound horizon

Posted: April 04 2020
by Antony Lewis
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.