Date: Prior to 1993
What occurred before the big bang?
We do not know. There is nothing that we can now see that we
have identified as showing us something from before the big bang. The big
bang followed a time when matter was so dense that we have no good theory
about what it was like. We have no idea about what was happening before the
big bang itself. The one thing we can measure is the background infrared
radiation (which was recently seen to be slightly non-uniform) and this
radiation only dates back toward the big bang to a time when matter became low
enough density for light to escape it. This was a long time after the big
bang itself. There is a good book called the First Three Minutes, by Weinberg
that describes some of this. I do not know if it is appropriate for your
level and I do not know if it is easily available.
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Update: June 2012