Was the Universe Not Perfectly Homogeneous at Photon-Decoupling?

[Niles]

Hi guys

Please take a look at this familiar picture: http://www.astr.ua.edu/keel/galaxies/wmap_map.jpg

Does this imply that the universe was not perfectly homogeneous or that it was anisotropic at the time of photon-decoupling?

Since the CMB-fluctuations are because of the different dark-matter densities, I would say that it is because of lack of homogenity. Am I corret?

 

[avalonme]

Strictly speaking, it is neither homogeneous nor isotropic.
The Universe is (more or less) statistically homogeneous and statistically isotropic averaged over all small scale fluctuations of density, temperature, velocity and grav.potential.
Those fluctuations are actually small - only about 1 part in 100,000 (they are enhanced for the picture).

 

[Niles]

But if to be very correct, the fluctuation are because of different densities in matter, so it must be lack of homogeneity?

 

[avalonme]

It's a matter or your personal preference.
Spatial dispersion is usually being related to inhomogeneity,
angular dispersion - to anisotropy. Thus people usually prefer to say that anisotropy of CMB is caused by inhomogeneity of matter. You can say, as well, that inhomogeneity of CMB is caused by anisotropy of matter. Unfortunately, we cannot experimentally prove the second case, since we cannot travel in space far enough to measure CMB inhomogeneity.
Mathematically:
Inhomogeneous=>anisotropic, isotropic in every point => homogeneous.

 

[Niles]

Excellent, thanks!