Summary

The Big Bang: Yup, almost sure to have happened. There are
still a few physicists around holding out against it though. They
argue that the Universe did not have a moment of creation but has
been around forever. The theory they subscribe to is known as the
steady state theory. What is interesting is that, despite so much
evidence in support of the Big Bang, the steady state idea has yet
to be satisfactorily laid to rest.
The expansion of the Universe: Like the Big Bang, there is no
longer any real doubt about this.

The age of the Universe: Estimates at the end of the twentieth
century put it at 15 billion years but this figure could yet be revised
upwards (or downwards I suppose).
The shape of the Universe: Looks likely to be either open or flat,
based on current estimates of the amount of matter it contains and
the current rate of expansion. If I had to choose I would say that it
is probably flat (or so nearly flat we would not be able to ever tell).
The size of the Universe: It is still possible for a flat or open
Universe to be finite in size, though much larger than the furthest
out we could ever see. Current theories would prefer it not to be
infinite.
The fate of the Universe: Regardless of what shape or size the
Universe might have, the latest results measuring the expansion
rate from distant supernovae suggest strongly that the Universe
will expand forever ending in a Big Freeze. In a way, this is
easier for many people to cope with, for at least then time will
go on forever. It is one thing to talk about the Big Bang being the
beginning of time, but the Big Crunch would mark the very end of
time. Not only would nothing survive after it, but the word ‘after’
would have no meaning!
Inflation: This theory is looking quite healthy at the moment,
although there are a number of different versions of it. Most
require the Universe to be flat, but a new theory called ‘open
inflation’ is currently being developed. This does away with the
requirement for flatness and allows for the bizarre concept of an
infinite, open universe to fit inside a finite volume ‘bubble’ which
floats in some external space9.
Antigravity: At the moment the cosmological constant is
back in fashion, suggesting that there is a repulsive force of
antigravity pushing matter apart which is driving the expansion
of the Universe. But we still do not understand its origin.