Someone kindly explain,please.My atoms left the spot of the 'BIG BANG' at it's ocurance some 15 billion years ago.They traveled at a relatively slow rate compared to the speed of light.therefore,would not the light generated at BIG BANG time have passed me by and been long forgotten.Yet,we get reports of light from near the time of the BIG BANG just being received as if we had been here all along awaiting it?

Asked by: Joe Thomas

Answer

I believe that the reports of light from near the time of the big bang that you are hearing are not of light that was generated from the big bang itself, but of light from stars that were formed in the early part of the universe (but after the big bang). Since we are a great distance from these early stars, the light which was emmitted from them (after the big bang, but long before we came into existence), is now just reaching us.

Your atoms did not actually leave the 'spot' of the big bang. They were instead made in fusion reactions in stars much later. It is believed that supernova explosions of stars send the higher elements out into space, where they later combine to form planets, etc.

The light from the big bang is in fact all around us, and is essentially uniform. This is referred to as the 'microwave background'. This light is now in the microwave frequency because the universe is cooling as it is expanding.

It is not correct to think of the big bang as having happened at a 'spot' or at some location. It was not that everything, such as your atoms, were squeezed into a little part of the universe that is the present day universe. Instead, the entire universe, occupied a very small volume. Since the big bang it has been in continual expansion. But it is actually space itself that is expanding, not objects moving through space away from some spot. A typical example of this is given by a balloon. Draw spots on an uninflated ballon, which would be like galaxies at an earlier point in the universe. Then blow up the ballon. The spots move farther apart from each other because of the streching of the material of the ballon, not because the spots are actually moving somewhere. The streching of the balloon is analagous to the expansion of space itself, and our galaxy is like a spot on the ballon.
Answered by: David McMahon, Student, University of New Mexico

At the beginning, the 'spot' of the Big Bang embraced the entire universe. Consequently, every point in the current universe was once at, and therefore participated in, the Big Bang.

Depending upon the phase of the expansion, the size of the universe [meaning its radius and the average distance between objects (such as galaxies)] is proportional to the 1/2 or 2/3 power of the time, while the furthest distance from which light emitted during the Big Bang can reach us [the event horizon] is proportional to the time [ct]. Therefore, as time advances, the event horizon encloses a larger and larger fraction [as well as absolute volume] of the universe. That is, the distance to the event horizon grows proportionally faster than the universe itself.

This means that, as time goes on, the event horizon continually expands to encompass new points in the universe that had, up to that time, been beyond the event horizon. Therefore, there continues to be a supply of new points whose light generated in the Big Bang is now just able to reach us.

Of course, you are correct that light already received at an earlier time from points lying closer in than the current event horizon has already passed by, never to be seen again on earth [Until, perhaps, the event horizon has grown so large that it encompasses the given point a second time. In other words, until the light emitted toward earth has travelled past the earth and made a full circuit of the universe and returned to the earth. Recall that the universe is modelled as a sphere.]

For a more detailed discussion, and a useful diagram, see:

'The First Three Minutes,' by Steven Weinberg, Bantam Books, ISBN 0-553-14131-7, pp. 36-38 (1977).
Answered by: Warren F. Davis, Ph.D., Physics, MIT