Re: If Omega = 1

Hi Niall,

When astronomers talk about Omega = 1, they are talking about the *average* density of the universe. By that I mean the total amount of matter in a very large volume, divided by that volume.

On small scales, of course, the universe isn't terribly homogeneous. Instead, it has quite large density fluctuations, both high and low (without them, we wouldn't be here). If the fluctuation in density is high enough, then that particular region of the universe is gravitationally bound, and will not expand along with the rest of the universe. Clusters of galaxies, such as the Local Group, are examples where this happens.

So, in an Omega=1 universe, small pieces of the universe that have more matter than average will be bound together and not expand. But, the universe as a whole is governed by the average density of matter, and will tend towards a standstill. Actually, if Omega is exactly 1, then the universe will expand forever, but at an ever slower rate.

There are several sites on the web that discuss the Great Attractor. A couple that I've found are: http://library.advanced.org/12659/ outer_space/great_attractor.html, and http:// www.ncsa.uiuc.edu/Cyberia/Cosmos/GtAttractor.html. You can find some more sites using inference find. I believe that it was also discussed in the August 1991 issue of Sky & Telescope.

Make sure to avoid the common misconception that the Great Attractor is some huge mass that is "sucking in" all of the matter around it. It was found by looking at the motion of many galaxies near us, after SUBTRACTING out their expansion velocities. Galaxies are still expanding away from the Great Attractor, due to the overall expansion of the universe, but its gravity is making them expand away more slowly than average. There is a galaxy cluster at about the position where astronomers think the Great Attractor is located, but whether it is the Great Attractor, or just happens to be at the same spot isn't known.