CAMB: Dark Energy Perturbations

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
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Simon DeDeo
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Joined: October 26 2004
Affiliation: Santa Fe Institute
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CAMB: Dark Energy Perturbations

Post by Simon DeDeo » November 04 2004

A second question on CAMB and dark energy perturbations (I apologize if this is answered somewhere -- perhaps someone could point me to a reference?)

When CAMB computes the evolving dark energy perturbations does it:

1. assume that the dark energy was originally homogenous, and that it just falls into the dark matter potential wells?

2. assume that, like dark matter, the dark energy has initial perturbations from inflation? (Which, perhaps, we can take to be adiabatic.)

Does it make a huge difference? It would be interesting some day to see if these two choices had any observational consequences -- would tell us something about reheating and the dark sector, I guess?

Antony Lewis
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Re: CAMB: Dark Energy Perturbations

Post by Antony Lewis » November 05 2004

There are no primoridal dark energy perturbations. For the non-tracking constant w model asssumed by default the adiabatic primordial dark energy perturbations have very little effect I think.

(Also in any model in which w=-1 initially because of Hubble friction - like many scalar field models - they are zero)

Charles Shapiro
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Affiliation: University of Portsmouth

CAMB: Dark Energy Perturbations

Post by Charles Shapiro » January 13 2007

Setting w=-0.9, Omega_DE=0.73, Omega_K=0, I run CAMB to get transfer functions with and without dark energy perturbations. I find that adding the perturbations increases T(k) on large scales and decreases it on small scales. I don't understand why the latter should be true unless there is some subtle normalization going on. I am not using COBE normalization - I am inputting the same scalar amplitude (2.57E-9) each time.

Can anyone explain this?

Christopher Gordon
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Re: CAMB: Dark Energy Perturbations

Post by Christopher Gordon » January 15 2007

Simon DeDeo wrote:A second question on CAMB and dark energy perturbations (I apologize if this is answered somewhere -- perhaps someone could point me to a reference?)

When CAMB computes the evolving dark energy perturbations does it:

1. assume that the dark energy was originally homogenous, and that it just falls into the dark matter potential wells?

2. assume that, like dark matter, the dark energy has initial perturbations from inflation? (Which, perhaps, we can take to be adiabatic.)

Does it make a huge difference? It would be interesting some day to see if these two choices had any observational consequences -- would tell us something about reheating and the dark sector, I guess?
In tracking models any non-adiabatic perturbation quickly decays away. No perturbations in dark energy are an example of a non-adiabatic perturbation.

For non-tracking canonical scalar field dark energy models, if you start out with no perturbations in the dark energy (in the flat gauge) then you rapidly approach adiabatic perturbations.

I think the easiest way of seeing this is with the separate universe approach, see for example
astro-ph/0503680

Chris

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