Once we understand the big bang, a simple question comes to mind. Will the expansion we see going on around us today continue in the future, or will these outgoing galaxies someday slow down, reverse their motion, and fall together in an event that astronomers, half jest, call the "big crunch"? This question,as simple as it seems, is one of the most fundamental inquires we can make, for it involves nothing less than future of the universe.
We do not, as you might suppose, have to resort to esoteric theory or deep philosophy to answer the question. It can be answered (at least in principle) by observing the universe today. The only force we know that is capable of reducing the speed of a receding galaxy is gravity. If the universe holds enough matter, even those quasars at the edge of the observable universe will someday come falling back in. The question of the fate of the universe comes down to the question of whether the universe contains enough matter to exert that force. if it dose, then we say the universe is closed and the expansion will someday reverse. If it dose not then we sat the universe is open, and it will do on expanding forever. The boundary between these two, in which the expansion slows and approaches zero after infinite time has passed is the flat universe.
If you count only luminous matter (the stuff we ca actually see), then we observe only about 1% of the matter needed to close the universe, and the expansion will go on forever. Dark matter, however, adds to this total. Astronomers so far, found perhaps 20%-30% of the matter required, and the search continues.
One feature of the debate about the future of the universe that you might want to think about is this: The inflationary theories--the same theories that explain the behavior of the universe at 10-35 second--predict that exactly enough matter exists for the universe to be flat. Some theoretical astrophysicists have taken this to mean that the universe must be flat and the observational astronomers should work harder to find the rest of the dark matter. Observers, on the other hand, say that if the universe is open, that's all there is to it.
How much faith do you think should be put in theoretical predictions of this sort? How hard should observers look for the "missing" matter?
