RaNae Pearce, age 11, of Salt Lake City, Utah, for her question:

What is the Big Bang Theory?

Sooner or later, every young person wonders where space ends or when the Universe began. Of course, we cannot prove any answers to these questions, but this does not stop cosmologists from trying to figure out the story of the entire Universe. One of their suggestions is the Big Bang Theory.

It may seem strange, but this question is very important to the human mind. 'The old brain needs sensible stretching exercises, otherwise it gets set in its ways and dozes when faced with a new idea. The best possible exercise of this sort is thinking about the entire Universe, even though we can never grasp its stupendous dimensions.

Nobody can prove how it all began or when, if ever, it will end, but there are theories. A theory, of course, is no more than a possible explanation, backed up by some reasonable evidence. Someday we may have enough factual evidence to prove that one of these theories is true    or perhaps some other theory may lead to the truth.

At present, cosmologists are debating the merits of three different theories that might explain the origin and possible future of the Universe. Each theory is based on certain known facts. The stars, for example, are born, grow old and die. Also, there is evidence that the starry galaxies are spreading apart in an expanding Universe. Each theory suggests a different explanation of these known facts.

Two theories suggest that originally all the cosmic material was packed together in a dense wad. The Universe was born when this cosmic egg exploded with a Big Bang, spattering the pieces in all directions throughout the endless reaches of space. Both theories agree that the starry galaxies may be part of the original cosmic egg, still spreading apart in the expanding Universe.

However, the original Big Bang Theory expects the galaxies to go on expanding for ever and ever. The alternate Pulsating Theory suggests that this cosmic expansion is merely one phase of a repeating pattern. It assumes certain cosmic forces cause the spreading explosion to slow down. Eventually, expansion will stop and all the material will be drawn together again. Later, for some unknown reason, this cosmic egg will explode and a new expanding Universe is expected to start out again.

Both these theories assume that the Universe has so much material and no more, that no new material is created. The Steady State Theory suggests that new material is being created all the time, and that things did not start with a big bang. Instead, new material forms new galaxies    as all galaxies expand apart at a steady rate.

These cosmic theories tend to boggle the mind    which gives the old brain the stretching exercise it needs to stay alert. The size of the Universe may be limitless and its history may be endless. Certainly our human minds cannot grasp its billions of galaxies and zillions of stars. But now and again it's a good idea to think about these wondrous things, and to realize that they do indeed exist beyond our widest imaginations.