The Beginnings of Big Bang Theory
In the 1920s astronomers were able to show that some nebulae (clouds of
luminous gas and dust) were too far away to be part of the Milky Way - they
were island universes or galaxies in their own right. One of the discoverers of
extragalactic objects, Edwin Hubble, went a stage further. In 1924 he announced
the discovery that light from distant galaxies was systematically redder than
light from nearby galaxies and that the degree of red shift was proportional to
the distance.
The simplest explanation for this red shift is that it is a case of the
Doppler effect. This is the phenomenon that causes the pitch of a train whistle
to vary as the train approaches or recedes. According to this explanation, the
light is reddened because the galaxies are moving away from us. Since the
degree of reddening is also a measure of the speed of recession, Hubble was
able to show that more distant galaxies are receding from us faster than nearby
ones.
At first sight this observation might suggest that the Earth was located at
the centre of some cosmic explosion. However, the fact that all motion is
relative implies that observers elsewhere in the universe would make similar
observations. This observation is consistent with an expanding universe, To
illustrate this one might paint spots on a balloon and blow it up, As the
balloon expands, the spots recede from each other and more distantly separated
spots recede more rapidly.
Extrapolating backwards in time from the observation that the universe is
expanding leads to the suggestion that there might have been a time in the
distant past (about 10-20 billion years ago) when the entire universe was
concentrated into a single point. This point would be unimaginably hot and
dense. At this ‘t=0’ the universe would begin to expand rapidly, if not
violently. As it expands and cools, matter as we now know it begins to appear.
Small variations in the density of that matter lead to condensation and the
eventual formation of stars, galaxies and planets. Gradually the mutual
gravitational attraction of matter slows the expansion of the universe. The
result is the basic picture of the universe as portrayed by modern cosmology.
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Contributed by: Dr. Christopher Southgate
Source: God, Humanity and the
Cosmos (T&T Clark, 1999)
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