| MadSci Network: Astronomy |
Hello -
We know that the universe exhibits structure - galaxies, clusters of galaxies, superclusters, voids, and so on - on scales up to about 200 megaparsecs, or about 640 million light years. On scales larger than this, the universe is homogenous. Detailed measurements have recently been done of the cosmic microwave background - the remnant of the Big Bang, now cooled to 2.7 degrees above absolute zero - which show that it is remarkably uniform. Here is a Web page which gives more details about this.
The main open question is still whether the universe is open, critical, or closed; that is, will it keep expanding forever, or will it eventually stop expanding and begin to contract? (The critical option in between is where the universe keeps expanding forever but more and more slowly as time goes on, with the expansion rate going to zero at infinitely long times).
The answer is that we still don't know; there is certainly not enough visible matter in the universe to close it (to cause it to stop expanding and begin to contract) - in fact the visible matter is only about 5 to 10% of what is needed to close the universe - but what about invisible matter? This is not as silly a question as it might seem at first; we know from measurements of the rotation of galaxies, and of the motions of galaxies within clusters, that there must be more matter than we can see. But must there be enough to close the universe (or more interestingly, to make the density of the universe exactly equal to the critical density)? This we do not know.
How can we find out? This requires measuring the oldest and most distant objects we are able to observe, and is an area of active research. Here are some Web pages that give details about what is known of the large- scale structure of the universe, and what its ultimate fate is likely to be:
Try the links in the MadSci Library for more information on Astronomy.