Before the Big Bang?

One of the most popular questions asked by audiences when I
lecture on cosmology concerns what there was before the Big Bang.
After all, if the Big Bang really did happen 15 billion years ago,
what then caused it to happen? What triggered the birth of our
Universe in the first place? I will briefly state here three standard
answers to the above. I will go through them here in reverse order
of (personal) preference.
The first only applies if our Universe contains enough matter
to eventually stop it expanding. In that case, it will one day in
the very, very distant future begin to contract, ending finally in
a Big Crunch. If this happens, and we think of the collapse into
the Big Crunch as the time-reverse of the initial Big Bang, then
the two events are equivalent. The Big Crunch of our Universe
may therefore serve as a Big Bang for a new universe born out of
the ashes of our own. And if this is the case, then our Universe
may have followed an earlier one that had also expanded then
collapsed. It may have been like this forever; an infinite number
of universes, each expanding then collapsing in turn. Thus the
answer to the question: what was there before the Big Bang? is
that there was another universe, possibly similar to our own.
Since it now looks like the expansion of the Universe is
speeding up, it will never be able to collapse again. Maybe the
Big Bang was a one-off event. In that case, we must look to more exotic answers to the question. One which is gaining in popularity
among the more mathematically inclined physicists is that the
Universe was, until the Big Bang, part of a much grander space
of ten (or eleven depending on who you talk to) dimensions. This
universe is described as being ‘unstable’ as though it were unsure
what to do with itself. The Big Bang came to the rescue causing it to
‘quantum leap’ into a more stable state. When this happened, six
(or seven) of the dimensions curled up into an incredibly tiny ball
leaving the three dimensions of space and one of time that we have
today. This load of theoretical gobbledegook actually emerges
naturally from the most sophisticated, yet at the same time most
obscure, theories in modern physics, known as superstring theory
and M-theory. Time will tell whether they are on the right track.
The final, and standard, answer is the following. If Einstein’s
general theory of relativity is correct, andweare confident that it is,
then the Big Bang not only marked the birth of the Universe but the
beginning of time itself. Asking questions about what happened
before the Big Bang necessitates having time to imbed the word
‘before’ in. Since there simply was no time before the Big Bang,
the question doesn’t make sense.