[dreadgeek]

So since I mentioned dark energy I thought I would go ahead and talk about this subject as well. There's another problem in cosmology and it is this: the expansion of the Universe isn't slowing down. To understand why this is a problem it's necessary to go back to the very beginning: the Big Bang. So according to the prevailing model, at one point the whole of the Universe was contained in a space about the size of what is called the Planck length (10^-43 or 1 with 43 zeroes in front of it) which is the smallest anything existing can be said to be. For reasons that may forever be beyond our understanding this fantastically small object suddenly became *much* larger. This is the Big Bang. In the first few moments of the Universe spacetime grew hugely. Things settled down after a while, the balance of matter-antimatter tipped in favor of matter and it started to get cold enough for atoms to start to form. After the rapid inflation expansion started to slow down a bit and matter started clumping together to form stars and galaxies and all the rest of the things observable in the Universe.

Now, here's where things get interesting. Gravity is *always* attractive and is caused by the warping of spacetime by the presence of mass. The consequence of this is that what we should see is that over time, the expansion of the Universe should slow down and then, possibly, reverse coming together in a Big Crunch (or as Douglas Adams put it in Restaurant at the End of the Universe, a gnab gib). The problem is that we aren't observing this. In fact, not only has the expansion continued but it is still accelerating. This cries out for explanation.

That explanation is dark energy. *Some* form of energy, that we lack the means to detect at present levels of technology, is pushing the Universe apart. We don't know precisely what it is but there is a damn lot of the stuff whatever it is. This more or less seals the fate of the universe. Eventually what will happen is that all of the galaxies will be so far apart from one another that they will be too faint to see.

How do we know that the galaxies are all rushing away from one another? (on the whole, the Milky Way is going to eventually collide with the Andromeda galaxy because the greater mass of our galaxy is pulling the Andromeda galaxy toward us) We know because of what is called red shift. This is simply the familiar Doppler effect applied to light. When a siren is approaching you the pitch increases and as it speeds away from you the pitch decreases. The same thing happens to light (although we can't detect it because the distances light travels here on Earth can never be far enough to see any kind of shifting) but at the astronomical level stars or galaxies moving away from us have their light shifted toward the red part of the spectrum. The more red shift the farther away you are from the observed object.

Cheers
Aj