Hi Mehran- good to hear from you again. Sorry for reformatting your question; our program was having trouble and it took a while to figure out that the problem was that it couldn’t handle an apostrophe.
1. Although there is no center in any standard picture, you can think of gravity as pulling the parts together. Just like in the classical problem where pieces of matter fly away from an actual center, if the relative velocities are big enough the matter will escape rather than recollapse. So it’s a quantitative issue depending on the relative velocities vs the mass density.
2.There appears to have been a very large density of mass-energy ’stuck’ in space itself at an early stage. Oddly, given the ordinary equations for gravity a fixed density causes accelerating expansion, just the opposite of gravity’s effect when the mass itself is fixed.
3. The generation of ordinary matter occurred at a later, less mysterious stage.
4. In our current working picture, the properties of matter cannot be accounted for by spacetime curvature alone. It is possible, however, that if string theory succeeds all the features of matter will be understood as geometrical properties, but in a higher dimensional space.
For more information than you might want, see:
For information on the intriguing idea of an oscillating unirverse, see:
(published on 12/02/2007)