CAMB: Dark Energy Perturbations

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
Charles Shapiro
Posts: 24
Joined: February 05 2005
Affiliation: University of Portsmouth

CAMB: Dark Energy Perturbations

Post by Charles Shapiro » February 02 2007

Why put people off about changing it or keep it highly depricated ?
Simply using the switch does not give a sensible answer. When DE perturbations are turned off and w!=-1, I see large changes to the small scale power where there should be none.

As Antony says, if you use the switch, you MUST ALSO edit the equations somehow. CAMB should at least come with this warning.

The switch might also come with armed guards and require two high-ranking officials to enter launch codes and turn two separate keys simultaneously, but that's probably too much.

Charles Shapiro
Posts: 24
Joined: February 05 2005
Affiliation: University of Portsmouth

CAMB: Dark Energy Perturbations

Post by Charles Shapiro » February 09 2007

In case anyone is interested, I've used the power spectrum output from CAMB to track the linear growth of a small-scale matter mode with and without dark energy perturbations. I chose the mode k=1 h/Mpc, using a flat universe with \Omega_M=0.27 and constant dark energy EOS, w_0=-0.4.

The graph shows the growth of this mode relative to its amplitude at z=100. The red line is my result obtained from integrating the 2nd order differential equation for the linear growth (posted above).

Image

Antony Lewis
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Joined: September 23 2004
Affiliation: University of Sussex
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Re: CAMB: Dark Energy Perturbations

Post by Antony Lewis » February 09 2007

When I did it the two consistent results agreed very well. Is this because you are normalizing at z=100 rather than z=0? (at z=100 it may be important to keep \Delta_c \ne \Delta_b, so might expect some departure unless you are tracking \Delta_b separately; I assume the plot is \Delta_c?)

Charles Shapiro
Posts: 24
Joined: February 05 2005
Affiliation: University of Portsmouth

CAMB: Dark Energy Perturbations

Post by Charles Shapiro » February 09 2007

My calculation is quick and dirty. I've used the value of the total matter density, but I've assumed it's all cold, whereas the CAMB plot is obtained from the total (correct) power spectrum.

I'm not doing a precision test here - I just wanted to quickly verify what you said about the equations being inconsistent when the DE perturbations are switched off (as opposed to being some other issue, e.g. normalization). And clearly you were right - the usual growth equation does not hold.

Charles Shapiro
Posts: 24
Joined: February 05 2005
Affiliation: University of Portsmouth

CAMB: Dark Energy Perturbations

Post by Charles Shapiro » February 09 2007

By "I've assumed it's all cold" of course I meant "I've assumed it's all dark."

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