Cosmological Constant and the Colapse of the Universe

[NWH]

I was wondering if someone would mind talking a little about why the cosmological constant was introduced and what it actually means for the universe to collapse according to Einstein's Universe. I was reading up on Wikiepdia (not the greatest source, I know) in the Static Universe entry and it got me interested. Although it didn't go into too much detial on why this actually happened and what it meant. Any input is appreciated...

 

[mathman]

The essential point (when Einstein introduced the cosmological constant) was that a static universe would be an unstable equilibrium. To avoid either a collapse or an expansion he put it in. Later on Hubble discovered that the universe is expanding and more recently it was discovered that the expansion is speeding up. To explain this speeding up, most cosmologists have put the idea of a cos. constant back. This whole area is still open to further analysis.