Tuesday, February 07, 2006

Q. Is the Universe Collapsing?


The consensus among astrophysicists, astronomers and cosmologists is that the universe is definitely expanding.

This was first observed by Edwin Hubble in the 1920s when he noticed that ever galaxy he could see was racing away from the Earth, with a speed that increased (roughly linearly) with distance. This was identified as an effect of "Universal Expansion" - the space between the galaxies is literally stretching!

While most of us agree that the universe is expanding, there is still some controversy about whether the rate of expansion is increasing, decreasing or remaining constant. Recent observations of a number of supernovae have hinted at an increasing expansion rate due to some sort of unknown energy field - often reffered to as "dark energy" or "quintessence" - that is driving the increasing expansion.

However, many fundamental theories that attempt to explain the early universe and many cosmological observations, like the inflationary theory, do not include quintessence and require the universe to be expanding at a gradually slowing rate untill after an infinite time it reaches a constant size. This is known as a "Flat" universe, due to its overall geometry.

On the other hand, by simply counting the amount of matter we can see through telescopes and comparing the gravity that matter can generate with the speed the galaxies are flying apart at, we would assume a slightly different fate - continued expansion that never approaches a constant size (this is subtly different to the previous possibility). This is known as an open universe. However, there are various reasons to believe that dark matter that we cannot count in this way may exist, perhaps in quantities enough to turn this option into the theoretically predicted flat universe.

Another option (that is currently considered very unlikely) is the closed universe - where gravity will one day overcome the expansion and draw all the matter backwards into a reversal of the big bang, colorfully known as the big crunch.

These are just the "relativity" models of the universe - there also exist "topological" models that deal with ideas about the universe being "spherical" or a series of repeating cubes of space that are exact copies of each other. Or perhaps it is a "finite soccerball?. What these could mean for the universe's expansion is not entirely clear

So, in answer to the question: No, we are as certain as we are about anything that the universe is not collapsing right now. We are also pretty certain (but by no means 100%) that the universe will never collapse. However, the future of infinite expansion is not much more comforting than the big crunch as we find that all the stars will eventually stop and the galaxies will dissapear from view and we will be left (if we last that long) in an isolated galaxy with a finite amount of resources to last us the rest of the infinite time.

Still, the universe is not a democracy - just because most physicists think the universe is expanding doesn't make it so - there are other options, like a collapsing universe.

No comments: