Angular power spectrum dependence in redshift z and k

In summary, the conversation involves a question about the relationship between P(k) and C_l, with the author mentioning the use of spherical Bessel functions and asking for clarification on the meaning of z and z'. It is mentioned that the left hand side may be the angular correlation function rather than C_l, and the current method for calculating power spectra is referenced. A formula for converting between C_l and P(k) is provided, with the clarification that P(k) in this case is still the temperature power spectrum.
  • #1
fab13
320
7
Hi,

I wanted to have a precision about a question that has been post on this relation between P(k) and C_l

The author writes the ##C_\ell## like this :

$$C_\ell(z,z') = \int_0^\infty dkk^2 j_\ell(kz)j_\ell(kz')P(k)$$

I don't undertstand the meaning of ##z## and ##z'## : these are not redshift, are they ?

Normally, it should depend of the multipole ##\ell## but how to make the link between these 2 quantities ##z## and ##z'## and multipole ##\ell##.
Moreover, does the ##P(k)## represent systematically the linear matter power spectrum ? or can we add RSD (Redshift Space Distorsions) like Kaiser or alcock-paczynski effects ?

Thanks for your clarifications and explanations.

Regards
Source https://www.physicsforums.com/threads/relationship-between-the-angular-and-3d-power-spectra.993041/
 
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  • #2
I think there's a mistake there. I suspect the left hand side is not ##C_\ell##, but the angular correlation function, which is a function of the difference between two angles on the sky, with ##z## and ##z'## being unit vectors.

That said, I don't think this is the way things are currently done. Current power spectrum calculations are based on this paper: https://arxiv.org/abs/astro-ph/9603033
 
  • #3
@kimbyd .Thanks for your quick answer.

I have looked at your paper and notice that passing :
Capture d’écran 2021-04-14 à 22.12.04.png


From what you seem to say, the ##C(\theta)## on the left would represent the angular correlation function, wouldn't it ?

and in the right term member would appear ##C_{\ell}## which would be the Angular Power spectrum ?

If this is the case, How could I express ##C_{\ell}## from ##C(\theta)##.

Your help will be precious, thanks in advance.
 
  • #4
Equation 10 there is a reversible transformation. The orthogonality condition is described here:
https://en.wikipedia.org/wiki/Legendre_polynomials#Orthogonality_and_completeness

There's a notational difference there, in part driven by the argument being ##x## rather than ##cos(\theta)##, but it shouldn't be too difficult to take a simple example (e.g. ##P_1##), do the integrals manually, and see what the normalization factor needs to be.
 
  • #5
Just a precision :

in the link that I gave firstly in my post ( https://www.physicsforums.com/threads/relationship-between-the-angular-and-3d-power-spectra.993041 )

Why does the writter say "How to write the 3D power spectrum, P(k), as an integral of the angular power spectrum, C_l ?"

whereas from kimbyd, it is not the direct relation beween angular power spectrum ##C_{\ell}## and matter power spectrum ##P_{k}##

By, the way, could anyone write the complete formula that links these 2 quantities (##C_{\ell}## and ##P_{k}##) ?

I would be grateful since I am a little confused between angualr correlation function and matter power spectrum and spherical Bessel functions.

Any help would be great, I am begin to desperate.

Best regards
 
  • #6
I don't think ##P(k)## there is the matter power spectrum. It's still the temperature power spectrum, just represented differently.

The relationship between ##P(k)## and ##C_\ell## is that ##C_\ell## results from the intersection of the 3D power spectrum ##P(k)## with the surface of last scattering.

I believe the equation was written above.
 

FAQ: Angular power spectrum dependence in redshift z and k

What is the significance of the angular power spectrum in redshift z and k?

The angular power spectrum is a measure of the distribution of fluctuations in the cosmic microwave background (CMB) radiation as a function of angular scale, redshift, and wavenumber. It provides important information about the structure and evolution of the universe.

How does the angular power spectrum vary with redshift z?

The angular power spectrum is expected to have a different shape at different redshifts. This is because the CMB radiation has been traveling through the expanding universe, and its fluctuations have been affected by various physical processes such as gravitational lensing and the growth of large-scale structures.

What is the relationship between the angular power spectrum and wavenumber k?

The angular power spectrum is directly related to the wavenumber k, which represents the spatial frequency of fluctuations in the CMB. As k increases, the angular power spectrum decreases, indicating a decrease in the amplitude of fluctuations on smaller scales.

How does the angular power spectrum depend on the cosmological model?

The shape and amplitude of the angular power spectrum are sensitive to the cosmological parameters that govern the evolution of the universe, such as the density of matter and dark energy. Therefore, the angular power spectrum can be used to constrain and test different cosmological models.

What observational techniques are used to measure the angular power spectrum in redshift z and k?

The angular power spectrum can be measured using various techniques, such as ground-based and space-based telescopes, interferometers, and satellite missions. These observations typically involve measuring the temperature and polarization anisotropies in the CMB and analyzing their statistical properties to extract the angular power spectrum.

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