PROBLEMS WITH THE BIG BANG

Introduced by Dr C A Collins, Reader, Astronomy Research Institute, Liverpool on 18 April 1977

Understanding the origin of the Universe is one of the most profound questions facing modern
science. The currently accepted view, the Big Bang Theory, is that it was formed some 10 thousand million
years ago, starting out with a temperature of many millions of degrees and with a size no larger than the
smallest atomic particle. It rapidly expanded, cooling as it did so and eventually fluctuations in the density
allowed the formation of stars and galaxies. (This is a very brief account; for a good description read "The
First Three Minutes" by Steven Weinberg).
Evidence for the Big Bang Theory is provided by a number of key observations:
1) The uniformity of the spatial distribution of galaxies in different parts of the sky. This leads to
the assumption that the universe is isotropic and homogeneous on large scales, an idea whose origin can be
traced back to Copernicus. Einstein showed that it was difficult for such a universe to be static.
2) The night sky is dark. This simple observation immediately tells us that a universe populated by
stars and galaxies cannot be both static and infinite. If it was, any line-of-sight would intersect a star and
the whole night sky would be as bright as the sun.
3) Observations of distant galaxies indicate that they are moving rapidly away from us at speeds
close to that of light.
4) Cosmic background microwave radiation, discovered in 1963 by Penzias and Wilson and
investigated by the COBE satellite in 1991, is uniformly isotropic to one part per million. This is explained
by the Big Bang theory as the faint "hiss" of the early hot universe
Despite the success of the Big Bang Theory there are a number of challenges which it faces and
these are the subject of modern cosmological research.
a) from observations of the microwave background and distant galaxies using the Hubble Space
Telescope, the universe appears to be very smooth and homogeneous on the largest scales. This is a
problem because the most distant regions of the universe have only known about each other relatively
recently. In the early universe, signals even at the speed of light, would have been too slow to carry
information across these distances; so how did the different parts come into thermal equilibrium?
b) Where do galaxies come from? There must have been "seed" fluctuations in density at the
beginning of the Big Bang to enable them to form.
c) The oldest galaxies observed by the Hubble Telescope appear to be older than the age of the
universe based on the Big Bang picture.
Modern cosmology attempts to find answers but the solution of one problem makes it more
difficult to find answers to the others. For example, the first two problems can be answered by "inflation", a
process where the universe undergoes rapid expansion early in its history, causing large regions to come
into thermal contact with each other. However, this solution makes the timescale problem even worse since
it makes the universe younger. Other possibilities include: introducing a cosmological constant ( a "fudge
factor"), as favoured by Einstein; perhaps our notion of the expansion is wrong or our estimates of stellar
ages; or, more controversially, maybe the cosmic background radiation is not cosmological in origin, in
which case the Big Bang Theory would have to be radically changed.
Dr C A Collins.
[A month after Dr Collins lecture, a paragraph in The Independent reported a paper at a meeting then in
progress which corrected the interpretation of observations on which the age of the universe had been
found to be less than that of stars- Editor.]