Cosmology 195
bang, the universe had cooled to an energy of 100 Gev (equivalent to a temperature
of 10
15
degrees Kelvin). As we discussed in Section 14.5, for energies above this, the
Z
0
boson could be easily produced and the weak nuclear and electromagnetic forces
were uni ed, acting as a single force. For energies below 100 GeV, the Z
0
could no
longer be produced and the electromagnetic and weak nuclear forces separated, and
there were the four fundamental forces we see today.
15.4.4 THE FORMATION OF PARTICLES
Right after the electroweak time, there was still enough energy such that quarks
could remain free. But at about 10
−3
s, the universe had cooled enough so that quarks
could form into the elementary particles, such as protons and neutrons. ...