### Sound horizon

Posted:

**April 03 2020**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)

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.

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)

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.