Did the Big Bang really happen?

We are now very confident that our Universe was born about
15 billion years ago in a state of incredibly high temperature and
density. What evidence do we have of this? The subfield of
cosmology devoted to studying the birth of the Universe is known
as cosmogony and is one of the most exciting areas of physics
research. The most compelling evidence that our Universe was
created in a Big Bang comes, of course, from the observation that
it is expanding. As I have mentioned before, if the Universe is
getting bigger, with the galaxies flying apart, then at some point
in the past all the matter in the Universe must have been squeezed
together.
Apart from the expansion of the Universe, the Big Bang
model is also supported by two other crucial observations. The
first is the observed abundances of the light elements. The fact
that roughly three quarters of all the atoms in the Universe are
hydrogen atoms and one quarter helium, the lightest and easiest
elements to make, with only a tiny amount of all the other elements,
requires a universe that was initially hot and dense but which
rapidly cooled as it expanded. At the moment of the Big Bang, long
before stars and galaxies had had a chance to form, all the matter
in the Universe was squeezed together and there was no empty space. Immediately after the Big Bang (much less than a second),
subatomic particles began to form and, as the Universe expanded
and began to cool, these particles were able to stick together to
make atoms. The conditions of temperature and pressure had
to be just right for these atoms to form. If the temperature was
too high the atoms would not have been able to remain intact.
They would have been smashed apart in the hectic maelstrom
of high speed particles and radiation. On the other hand, once
the Universe had expanded a little, the temperature and pressure
would have become too low to enable the atoms of hydrogen
and helium to be squeezed together to form any other (heavier)
elements. This is why mainly hydrogen and helium formed in the
early Universe, a process that would have happened in the first
five minutes after the Big Bang. Almost all the other elements had
to wait until they could be cooked inside stars. The Big Bang model
predicts the correct proportions of hydrogen and helium observed
by astronomers.
The other piece of evidence in support of the Big Bang which,
like the expansion of the Universe, was predicted theoretically
before it was confirmed experimentally, is known as the cosmic
background radiation. It is the ‘afterglow’ of the Big Bang
explosion and is in the form of microwave radiation that permeates
the whole of space and has a temperature today of about three
degrees above absolute zero (or minus 270 degrees centigrade). To
measure the temperature of this radiation experimentally, we do
not need to stick a thermometer out in space. Instead, we use what
look like giant satellite dishes which are called radio telescopes and
which are so sensitive they can ‘hear’ this radiation’s faint signal
from deep space. This was done for the first time in the 1960s and
has been confirmed many times since then with ever increasing
sensitivity. If you find this hard to believe, I was impressed when
someone informed me recently that we are even able to hear the
hiss of faint radio waves given off by the planet Jupiter using a
long-wave radio.
Today, there cannot be much doubt that the Big Bang did
happen. There are other issues, however, that have yet to be
resolved. For instance, a few years ago we did not know whether
gravity would one day halt the Universe’s expansion and cause it
to recollapse on itself, ending with all the matter rushing together
in a cataclysmic implosion known as the Big Crunch, or whether
the expansion would continue forever, with the Universe steadily
getting colder and colder and ending in what is known as the heat
death or the Big Freeze. Today we think we have the answer. It
turns out that the fate of the Universe depends not only on how
much matter it contains, but on the role of Einstein’s cosmological
constant. This makes cosmology a bit more complicated than we
would have hoped. So I will wade through some of these big issues
carefully, beginning with the shape of the Universe.