RADIATION PROTECTION - ILEA

Historical Prologue 3
of the electrons and positrons began the radiation era. The universe was
flooded with electromagnetic radiation for about half a million years, during
which the temperature and density continued to drop as the universe
expanded and matter existed only as a faint precipitate.
Although the relatively small number of neutrons and protons remaining
were capable of combining with each other and forming stable configurations,
such as heavy hydrogen nuclei (neutron + proton) and helium
nuclei (2 neutrons + 2 protons), these combinations were prevented
by the presence of the very energetic photons, which had enough energy
to break any bonds that were established. But as the radiation era progressed,
and the temperature continued to fall, the mean photon energy
dropped below the binding energy of the nucleons, and heavy hydrogen,
helium, and lithium built up. (All heavier elements were produced
much later, in the interiors of stars, and ejected into space by a variety of
processes.)
As the universe expanded, the radiation density dropped more rapidly
than the density of matter. When the temperature had dropped below
3000 K, protons and electrons were able to combine, and atoms of matter
began to appear. By the time the mean density of the expanding universe
had dropped to 10 −21 g/cm 3 , one million years after time zero, matter began
to emerge as the dominant constituent of the universe. Figure P.1 illustrates
this saga of creation, annihilation, survival, and evolution.
Today, our universe contains about 10 78 protons and neutrons, a good
portion of them aggregated, under the force of gravity, into celestial structures,
galaxies, stars, and planets all flying through space at high speeds relative
to each other. Extremely energetic cosmic rays, both material particles
and electromagnetic radiation, also stream through space, with about one
billion photons of electromagnetic radiation and neutrinos for every neutron
or proton, and with photon and neutrino densities of the order of
1,000/cm 3 . In addition, microwave radiation, left over from the Big Bang,
is streaming freely through space at energies corresponding to a temperature
of a little less than three degrees above absolute zero. 2
2 The Discovery of Invisible, Unbelievably
Energetic Radiations
We shall dispense with the billions of years of evolution that resulted in the
establishment of the solar system, the origin of life, and the emergence of
modern man. It took our own species, Homo sapiens, at least 25,000 years
2. Great reading on the evolution of the universe includes Guth (1997), Lidsey (2000),
Silk (1994), Ronan (1991), and Weinberg (1977).