Expert: Jayendra Upadhye - 8/11/2006

Question
Hi--

Years ago I read that the universe is homogeneous and isotropic? I seem to remember that isotropism implies homogeneousness, but that homgeneousness does not necessarily imply isotroism. (i.e., the universe may be both, but we can't conclude that it is isotropic simply becasue it is homogeneous).

Why not?

Many thanks,

Steve  

Answer
Hi steve,
Only recently have astronomers decided that the universe is anisotropic. (not isotropic). (thanks to the chandra & kobe experiments)

But the universe is greately isotropic too. It all depends on the scale of reference.Sensitivity.

It is generally isotropic for all practical purposes, but at the level of the 3rd decimal point, it shows graininess.

It is this minute lumpiness that triggered the original star formation and galaxy building processes, and that led to the current foamy large scale structure of the universe.

A reflection of this is seen in the lumpiness of the cosmic background radiation. But lumpiness only in the decimal 0.00 level! [lumpy nevertheless]

>>I seem to remember that isotropism implies >>homogeneousness, but that homgeneousness does not >>necessarily imply isotroism.

True..take the universe.
Look at the planets ..all lumpy with space between.
But no matter where else you look, it is all the same..more planets and stars with space between.
the geater picture is homogenous (average over huge volume) all over.

Regarding homogeity (allow me that word) not implying isotropy..well homogeneity implies structural sameness..and isotropy means similar properties in all directions...
you could have a matrix of polarised cells all homogenous, but obviously having anisotropic (directional) behaviour with respect to light!

With the universe, we conclude it to be isotropic, as we see the SAME laws governing its subregions in all directions..
mainly gravitation and the Cosmic background radian intensity.
And the upper limit to speed, C (that of light in absolute vacuum).
Gravitation envelops the universe, it is all pervasive.
So is the CMB.
And so is Light! (there is no part of the universe that is wholly devoid of either of these.

And therin lies the isotropy.

hope that suffices.
Jayen