David Breidenbach, age 11, of Bixby, Oklahoma, for his question:

What is the Big Bang Theory of the Universe?

Our galaxy may be called an island universe. The cosmos is the universe of endless space, including all its matter and energy. We know that its teeming galaxies extend at lest five billion light years in all directions. The end, if any, is nowhere in sight and at present we can probe no farther. Cosmologists cannot prove how it all began, 'though they suggest three likely possibilities. The Big Bang Theory suggests that originally all the ingredients were massed in a core    and that the Universe was born with an enormous explosion that sent the scattered fragments expanding outward, on and on forever and evermore.

Stretching exercises are good for the brain, though it tends to boggle when one thinks of the scope of the universe. Light, traveling at 186,000 miles per second, takes roughly 100,000 years to cross our pinwheeling galaxy of 100 billion or so starry suns. Beyond it, the vast reaches of outer space are strewn with uncounted galaxies in various stages of development. Our 200 inch telescope is limited to five billion light years. The light it gathers left galaxies in these outer regions five billion years ago. And from other observations we know that things there have changed between then and now.

When we look at Sirius shining in our winter skies, we see light that left this star more than eight years ago. When we look at any part of the universe we are looking back into the past. Cosmologists must allow for this annoyance when they consider the destiny of the universe, past and future. However, telescopes, spectroscopes and other instruments provide some reliable basic clues.