CAMB: Modifications for Altered Background Dynamics

[Dror Berechya]

Hello everyone,

I'm working on a project where I need to modify CAMB to account for a model in which the Omegas are factored differently (adding more parameters), but only in the context of background dynamics. That is, I want to modify the form of the Hubble parameter without affecting the actual fluid densities.

After some initial research, it seems that the `equation.f90` file might be relevant to this modification. However, I'm unsure about where I need to change this file and whether there are other parts of the code that should also be adjusted.

I would greatly appreciate any guidance or suggestions.

Thank you very much for your help!

[Antony Lewis]

The help assistant can answer this one (help_assist.php)

Modify dtauda Subroutine: The subroutine dtauda(a) in equations.f90 returns dt/da and is used wherever the background evolution is needed. You can modify this subroutine to implement your new model for the Hubble parameter【4:0†source】.

[Dror Berechya]

Thank you very much! :)

[Dror Berechya]

Hi Antony,

I hope you're doing well.

I've been exploring the `dtauda(a)` subroutine in `equations.f90`, and I’m having some trouble understanding how to proceed with my modifications. Specifically, I’m looking to define effective densities for radiation and matter, relevant only to background dynamics.

From what I can gather, it seems like I might need to adjust `grhoa2` by modifying `this%grho_no_de(a)`. However, I’m unclear on the exact meaning of `this%grho_no_de(a)`, and I couldn’t find where `grho_no_de(a)` is defined.

If you could provide any guidance or point me in the right direction, I would be very grateful. To make it easier, I’ve included the part of `equations.f90` where the `dtauda(a)` subroutine is defined:

Code: Select all

function dtauda(this,a)
use results
use DarkEnergyInterface
implicit none
class(CAMBdata) :: this
real(dl), intent(in) :: a
real(dl) :: dtauda, grhoa2, grhov_t

call this%CP%DarkEnergy%BackgroundDensityAndPressure(this%grhov, a, grhov_t)

!  8*pi*G*rho*a**4.
grhoa2 = this%grho_no_de(a) +  grhov_t * a**2
if (grhoa2 <= 0) then
    call GlobalError('Universe stops expanding before today (recollapse not supported)', error_unsupported_params)
    dtauda = 0
else
    dtauda = sqrt(3 / grhoa2)
end if

end function dtauda

Thank you so much for your time and assistance.

Best regards,
Dror

P.S. The comment before `dtauda(a)` in `equations.f90` says:

Code: Select all

! Background evolution, return d tau/ d a, where tau is the conformal time

Is `dtauda(a)` calculates dt/da or d[math]\tau/da?

[Antony Lewis]

it is $d\tau/d a$. The grho_no_de function definition is commented in the source to describe what it is.