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W.R. STOEGER S.J.
68
1 (2008) 64-77
However, there are very small perturbations or fluctuations in the CMWBR – of the
order of 1 part in 100,000. These temperature fluctuations mean that the density of the
plasma at that time (300,000 years after the Big Bang) had slight overdensities and underdensities
(perturbations) in it of the same magnitude. As I have already mentioned, these
overdensities are the seeds of clusters of galaxies and of galaxies. After decoupling they
gradually grew in strength, under the influence of their local gravitational fields, and then
collapsed and fragmented to form the structures we now see throughout the cosmos.
But where did these perturbations, or seeds, in the cosmic plasma come from? Without
them we would not be here! We are not completely sure. But it is very likely that they
were generated in the first split second after the Big Bang by what cosmologists refer to
as “inflation.” Inflation is extremely rapid expansion of the universe – exponentially
rapid expansion – during which the universe expanded by at least 30 orders of magnitude
in an extremely short time. Inflation, if it occurred, began and ended well before the
universe was 1 second old. Super-rapid expansion also means supercooling, of course.
Thus, when inflation slowed down and stopped, much of the energy from the inflationary
field was transformed into reheating the universe back to the temperature it had
just before inflation. Unfortunately, we do not have definitive evidence for an inflationary
epoch; we don’t have any evidence against it, either. It turns out that it is very
difficult – if not impossible – to explain a number of facts about the universe without it.
In particular, inflation can easily and quickly generate and preserve these perturbation
seeds we need for later galaxy formation. Also, without inflation it is almost impossible
to account for why the CMWBR in every direction should be of almost the same temperature
and the same amplitude. Without inflation the different regions of the cosmic fog
bank in slightly different directions would have been causally disconnected – not enough
time would have elapsed from the Big Bang for them to communicate information to
one another – and there would therefore be no explantion why they should then be of
the same temperature, as they indeed are.
We already indicated that one of the first transitions that occurred immediately after
the Big Bang was the emergence of gravity and space-time as we know them. It is very
interesting to go back for a minute to this period right after the time of the Big Bang
itself and consider the many important transitions that occurred in the first second.
Before doing this is it important to remember that all of physics can be summarized in
four forces or interactions: gravity, electro-magnetism, the strong nuclear force, which
holds nuclei together, and the weak nuclear force, which explains certain types of nuclear
transitions and decays. The key idea is that these four interactions were almost certainly
not always separate – that at very, very high temperatures they were unified, or
indistinguishable from one another. Thus, at the very beginning – at the Big Bang itself
– there was probably just one superforce, which included gravity, electromagnetism,and
the strong and weak nuclear forces. But when the temperature fell in the first split
second to below 1032 K, gravity – and classical space and time with it – separated from
the combination of the other three (which is called the Grand Unified Theory (GUT)
interaction). A short time later, when the temperature had fallen to 1027 K, the strong
nuclear force became distinct from the remaining electroweak interaction, and at a much
lower temperature, 1015 K – which still corresponds to only a millionth of a second