From: "Newton W."
Date: 16 Nov 2006
Q Was the universe expanding at a constant rate before a billion years ago? At the current increasing rate, how long would it take for the universe's density to effectively approach "zero?"
A Based on observations from the past few years, our understanding is that the universe's expansion rate has been slowing down for much of its history. About a billion years ago, however, this acceleration reversed itself and became deceleration - the expansion began to speed up. This acceleration was discovered in 1998, and its mysterious cause is called "dark energy".
One way to see this in a little more detail is to imagine a bunch of galaxies zooming apart from one another through space it's actually space itself that's expanding, which is subtly different, but this gives the right idea here). Each of the galaxies will exert a gravitational pull on all the others, pulling them towards one another. This will tend to slow the expansion rate with time. in a sense the Big Bang gave us an initial "kick", and normally we'd expect everything to slow down after that. For many years the big question was whether the universe would slow itself down enough to eventually recollapse, but we have now measured things well enough to predict that this won't happen.
Dark energy complicates things by providing in effect a very weak repulsive force. Early on the universe's matter is fairly dense, so its mutual attraction is stronger than this repulsion - the expansion rate slows down over time. As the universe thins out, however, the attractive force weakens while the repulsive force remains constant. At some point the repulsive force wins out over the weakening attractive one and the expansion begins to speed up.
The universe's density will keep decreasing (quite rapidly) but never quite go to zero. One piece of good news is that this acceleration won't change our own night sky anytime soon. Our galaxy and its nearby neighbors are gravitationally bound to one another, so they'll remain together even as the distant galaxies recede into blackness.
UC Berkeley Cosmology Group