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W.R. STOEGER S.J.
66
1 (2008) 64-77
immediate ancestral primates for about four million years. Multi-cellular organisms have
been around for only about a billion years. Before that there was only primitive plants,
plankton and one-celled organisms. Earlier than about 3.9 billion years there was no life
at all on Earth. Earlier than 5 billion years ago the Sun and the Earth did not exist.
But there were other stars before our Sun which manufactured all the heavy elements
(elements heavier than hydrogen and helium) of which our earth, all life and we ourselves
are composed. So there was an time when there were no elements besides hydrogen,
helium and a very little bit of lithium, the lightest metal. In fact, we know for sure that
there was a time – the first several hundred million years after the Big Bang, when there
were no stars or galaxies at all. Things were very different, and life was not yet possible.
In fact for the first 300,000 years after the Big Bang, the universe was just an expanding,
gradually cooling ball of hot ionized gas – very smooth and with practically no lumps in
it. Fortunately for us, there were very slight over-densities and under-densities – we call
them perturbations – in the hot ionized gas. Once the temperature of the universe fell
below about 4,000 K, the electrons and protons and neutrons in the hot gas combined
to form neutron hydrogen and helium atoms, the gas decoupled from the radiation in
the universe, and for the first time the slight over-densities in the gas (overdensities of
about 1 part in 100,000) began to grow. Only after hundreds of millions of years would
they eventually collapse and fragment to form the first galaxies and stars. Only then
were the carbon, oxygen, copper, phosphorous, iron, nitrogen and all the other heavier
elements produced. Gradually – very gradually – the necessary components for the rich
molecular mediums essential for the emergence of life were being generated.
The picture which emerges from observational cosmology and astronomy is very
simple and straightforward. At the Big Bang, about 13.7 billion years ago, the universe
was very, very hot – T> 1032K – so hot that not even space and time as we know them
could exist, much less particles or any structure. In fact we do not have anything even
approaching an adequate physical description of this extreme state. A quantum theory
of gravity is needed. But immediately after the Big Bang, the universe – as just a homogeneous
spherically symmetric ball of super-hot mass-energy – began to expand and
cool quite rapidly. And as it cooled new things became possible. The lower the temperature,
the more laws of nature themselves and the matter and energy they describe could
differentiate and complexity. As an example, much later, as we have already seen, the
cosmic plasma was cool enough to clump into stars, galaxies and clusters of galaxies,
and to organize itself into particles, atoms and primitive molecules. Thus, at the beginning
the universe is very hot, very dense, very smooth, very simple and very undifferentiated.
As it expands, it cools, become more lumpy, more complex, and more
differentiated. The very fact that it fragments and condenses into hundreds of billions
of galaxies and trillions of stars essentially means that trillions of separate evolutionary
experiments are being carried out at the same time – each star and each planet in the
universe is a separate, relatively isolated location where physical, chemical, and in some
cases possibly biological evolution occurs.
How do we know that this scenario of cosmic evolution is correct? What is the
evidence for it? There are three major independent indications. The first is the systematic
redshifts of distant galaxies, first discovered by Edwin Hubble and his colleague M.