Why doesn't the Big Bang model account for the isotropy of the CMBR?Date: Tue May 4 17:14:49 1999
Posted by Caylin Mendelowitz
Grade level: undergrad School: The Evergreen State College
City: Olympia State/Province: WA Country: USA
Area of science: Astronomy
I assume you mean that since, at the moment of the Big Bang, every point in the Universe was in contact with every other point, so information was relayed instantly. Any temperature fluctuation would be transmitted to every other point, which would respond. So why do we see fluctuations?
There are a few ways of answering this. One is that the phrase ``the moment of (or just before) the Big Bang'' has no real meaning. The BB marked the starting of time as we know it, so asking what happened at that very instant has no meaning whatsoever!
To avoid that let's start at some arbitrarily small amount of time after the BB. It's possible that, even though the Universe was still ``small'', it had fluctuations in it. There is no guarantee that the creation event was isotropic at all; most explosions that have happened since then aren't symmetric (strictly speaking, the BB was not an explosion, but it has many characteristics of one). Even if all the points in the Universe were in information contact with each other, there still might have existed asymmetries. They may not have had enough time to smooth each other out (for example, a somewhat overly hot spot heating up a spot near it, and therefore cooling itself off) before the general expansion of the Universe made them too far away to effectively communicate.
So as you point out, the observed isotropy is a bit of a problem. To produce the amazing amount of isotropy seen, Alan Guth proposed that the Universe underwent rapid expansion for a very brief moment of time, which forcibly smoothed out any density fluctuations the very early Universe might have had. His ``inflation'' hypothesis is still hotly debated. Not everyone believes in it.
There is still a lot to understand about cosmology. It does have a wonderful way of twisting our minds around!
Incidentally, a lot of question here at the Mad Scientist Network deal with these issues. Try searching on them! Also, you can almost never go wrong by looking at Ned Wright's Cosmology FAQ/Tutorial. He answers many questions just like yours.