268 The Poetry of Physics and The Physics of Poetry
was good.” (Genesis 1). The light immediately was converted by the
mechanism of pair creation into a sea of hadrons, leptons, antihadrons,
antileptons and photons. Because of the high energies involved, this
fluid of particles was dominated by hadrons. Hadrons continued to
be the dominant constituents until the universe expanded to a radius
of 30 kilometers after 10-4 seconds had passed by which time the
temperature of the universe had dropped to 10^12 degrees and the density
was only 10^14 grams per cubic centimeter. At this point the hadron era
came to an end because for temperatures below 10^12 K photons can no
longer make hadron-antihadron pairs. The existing hadron-antihadron
pairs annihilated each other leaving the 10^80 nucleons, which presently
constitute the universe. The reader might wonder how the term era can
be applied to the hadron era since it only loses 10-4 seconds. This instant
of time hardly seems to be an era. I remind my readers, however, that a
nuclear reaction only takes 10-23 seconds so there is sufficient time
during the hadron era for 10^19 separate reactions to take place for each
particle.
There are actually two different descriptions of the hadron era. In the
more widely held version it is postulated that the number of baryons
(nucleons and other heavy particles) is slightly greater than the number
of antibaryons by one part in 10^9. Therefore, after the annihilation of the
hadron and antihadrons pairs takes place only nucleons remained in the
universe and no antinucleons. No explanation is given for the initial
imbalance of baryons and antibaryons other than to describe the
asymmetry as a quantum fluctuation. This version leads to a universe
composed totally of matter.
In the second version the number of nucleons and antinucleons is
exactly the same. When the universe expands out of the hadron era
pockets of matter and antimatter collect due to fluctuations of the
density. A pocket of pure matter or pure antimatter is the only possible
stable structures that can form since a mixture of matter and antimatter
would destroy itself by pair annihilation. This version of the Big Bang
theory predicts the existence of two types of clusters, one type composed
of matter and a second type composed of antimatter. The number of
clusters just equals the number of anticlusters. If a collision were to
occur between a cluster and an anticluster, enormous amounts of energy
would be released. Nothing of this nature has been observed, however.
On the basis of what we presently know of the universe there
is no way of distinguishing between these two versions. Clusters and