d.sullivan wrote:Recent observations of the cosmic backround radiation indicate that the universe is not going to have a "Big Crunch," but will instead expand infinitely. In fact, astronomers are pretty certain that not only are we expanding indefinitely, but that the rate at which the universe is expanding is increasing, which has astronomers pretty flustered, 'cause they can't figure out why that would be the case.
I think this last comment fails to give astronomers enough credit.
Cosmology is built on Einstein's theory of gravitation (general relativity). In the original version of it, Einstein had a term in it (i.e. something added on one side of the equation) which has been dubbed the "cosmological constant". It makes a certain logical sense in the theory. It's as if you had to decide how much energy there is in a vacuum. It tells you that even if you have no matter, you will still have a (non-zero) gravitational field. He wanted it partly to make it possible to have a model of the universe in which there was no expansion or contraction. He took it back, though, saying he shouldn't have included it. This is equivalent to saying that it is exactly zero. It turns out that his model with no expansion or contraction would be unstable, and besides, astronomers have observed that expansion is taking place. At that point, the model with no cosmological constant was a simpler explanation of the facts.
More recently, however, astronomers have attempted to use observations of the expansion of the universe to measure the cosmological constant (and see for one thing whether it could be simply zero). The increasing rate of expansion is well explained by having a non-zero (and repulsive) cosmological constant. When people make forecasts like, by a certain time in the future, the local cluster of galaxies will be all by itself without any visible stars outside of it, they're forecasting based on Einstein's theory with the measured cosmological constant plugged into it. So in spite of his having considered it a mistake, it seems Einstein's original theory works better.
Now it's true that there's a sense in which nobody understands fully why there is a cosmological constant and why it has the value that it does. All kinds of different features of fundamental physics could make a contribution to the value of the constant. In quantum physics, the vacuum is not just a simple blank space with nothing in it; it has subtle potentials in it, that one might expect to contribute to the cosmological constant. It's not assumed that the cosmological constant is just the way things are, with nothing more basic to explain why it's the way it is. On the other hand, a complete explanation seems likely to require a pretty comprehensive theory of physics. Like, one would have to know whether string theory is an accurate theory or not.
As far as I know, however, not knowing why the cosmological constant is what it is, is not a source of embarrassment for anybody. The cause of the accelerating expansion is believed pretty confidently to be the cosmological constant; it's just that we don't know all the factors that give rise to it.