CAMB: Dark Energy Perturbations
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CAMB: Dark Energy Perturbations
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?
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?
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Re: CAMB: Dark Energy Perturbations
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)
(Also in any model in which w=-1 initially because of Hubble friction - like many scalar field models - they are zero)
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CAMB: Dark Energy Perturbations
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?
Can anyone explain this?
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Re: CAMB: Dark Energy Perturbations
In tracking models any non-adiabatic perturbation quickly decays away. No perturbations in dark energy are an example of a non-adiabatic perturbation.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?
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