270 The Poetry of Physics and The Physics of Poetry
The high temperatures that exist during the first half hour of
the radiation era bring about thermonuclear reactions, which lead to
the formation of helium nuclei principally, as well as the nuclei of
deuterium, carbon, oxygen, nitrogen and heavy elements such as iron and
titanium. Calculation of the relative abundance of helium and hydrogen
due to helium formation during the radiation era agrees exactly with the
abundance of helium presently found in the universe both in stars and
interstellar space. This is one of the great successes of the Big Bang
theory.
The radiation era came to an end after 1 million years when the
temperature had dropped to 3000 K. Below this temperature photons no
longer have enough energy to ionize atoms. Electrons and protons could
finally unite to form neutral hydrogen atoms. During the radiation era
neutral hydrogen atoms would form but their lifetime was extremely
short because the probability of ionization by a photon was so high. Once
neutral matter formed the radiation field became decoupled from matter.
It was the beginning of the stellar era.
The Stellar Era
The universe at this time was one million light years across and had
a density of 10-21 grams per cubic centimeter, which is approximately
1000 times the present density of our galaxy. At this point, matter
began to distribute itself into clusters, galaxies and stars. Today, some
13.7 billion years later, we are still in the stellar era. The universe has
expanded to a radius of 13.7 billion light years corresponding to a
density of 10-30 g/cm. The temperature of the photons has dropped from
3000 K to 2.725 K.
This radiation predicted by the Big Bang theory has been observed
as an almost isotropic or uniform flux of microwave photons whose
average energy is 2.725 K. This result is an important confirmation of
the Big Bang theory. In addition to the photons, it is also believed that
there exists equal numbers of neutrinos and anti-neutrinos also
homogeneously and isotropically distributed. Because of the difficulty of
detecting neutrinos this prediction of the Big Bang theory has not yet
been verified. The total number of photons and neutrinos is very large,
approximately 10^9 for each nucleon in the universe or 10^90 particles
altogether. The energy per particle is quite small but the overall energy
content of all the radiation fields is quite large, approximately 1/7 the