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W.R. STOEGER S.J.
67
1 (2008) 64-77
L. Humason. The farther out a galaxy is, the larger is its redshift. This means that the
more distant a galaxy is from us the faster it is travelling away from us. The universe is
systematically expanding in all directions – and very close to the same amount in every
direction. There is really very little doubt about this.
So the universe is expanding? But what evidence is there that it has been cooling –
that as we go back into the past we encounter a succession of ever hotter stages? First of
all, there is the large abundance of helium in the universe, and the trace abundance of
deuterium (the isotope of hydrogen with a proton and a neutron in the nucleus, instead
of just one proton). 24 % by weight of all matter in the universe is helium. That’s a lot!
If we calculate the amount of helium all the stars in cosmic history have produced, it
amounts to only 5 to 7 % at most. So where did all the helium come from? It turns out
that it was very easy to form lots of helium between 1 and 3 minutes after the Big Bang
– the temperature and density of the universe would have been just right for that. And
it’s easy also to produce a certain amount of deuterium at the same time. In some ways
the abundance of deuterium was just as much of a problem as the large amount of
helium – deuterium is a very fragile isotope, and stars are incapable of producing a net
amount of it – rather they destroy it. Thus, the helium and deuterium abundances we
observe throughout the universe strongly indicate that the temperature of the universe
was once above a billion degrees Kelvin.
Secondly, in 1965 Arno Penzias and Robert Wilson discovered the cosmic microwave
background radiation (CMWBR), which is like the “afterglow” of the Big Bang.
They detected a low level of microwave noise coming from all directions and having
exactly the same amplitude in all directions. They also discovered that it was radiation
with an exact blackbody, or equilibrium, spectrum. Thus, with the same amplitude in
every direction, it has almost the same temperature in every direction, too. The CMW-
BR is a bath of microwave radiation that pervades the entire universe which originates
not from discrete sources like stars or galaxies but from the hot gas before the formation
of galaxies, when the temperature of the universe fell to about 4000 K, and the matter
decoupled from the radiation due recombination of electrons with protons and neutrons
to form neutral atoms. This is the first moment at which the universe became
transparent to light. Thus, as we look out farther and farther – between all the galaxies
and stars – and thus back farther and farther in time [Since the velocity of light is finite,
the farther out in space we peer, the farther back in time we are seeing.] we eventually
see a “fog bank” in microwaves where the universe becomes opaque – where our line of
sight encounters the primeval ionized plasma. Fog banks always emit a lot of diffuse
light – we see clearly see the white light coming from it. The CMWBR fog bank is no
different, except that we see microwaves coming from it and not optical light. This is
due to the fact that the original light from the moderately hot plasma has been cosmologically
redshifted into the microwave region of the spectrum by a factor of about 1300,
due to the expansion of the universe since that time. Thus, the microwave background
radiation assures us that the universe was once at least 4000 K, and more importantly
that there was a time when there were no stars or galaxies. This is clear from the almost
perfect smoothness of the CMWBR.