stage of expansion and cooling of the Universe, the average particle energy
is dropping to the typical energy scale of the weak nuclear force, and something
dramatic happens to the particles that transmit the weak nuclear force.
particle physics, we have learned that the bosons that transmit the weak nuclear
force (as in nuclear fission) are very heavy, and that they gain their large
mass through a process known as spontaneous symmetry breaking.
This process occurs at some definite energy scale, at the energy of the weak
nuclear force. Above that energy scale, the weak nuclear bosons are massless
like the photon that transmits the electromagnetic force between electrons and
protons and the gluon that transmits the strong nuclear force between quarks.
Below that energy scale, the weak bosons are big and heavy, and so the weak
nuclear force only acts over a very small distance scale, about 10-16
centimeters, about one thousandth the size of a nucleus.
reason, cosmologists believe that when the Universe was so hot that the average
energy of the radiation is above the energy of the weak nuclear force, the weak
nuclear bosons were massless and the weak nuclear force had an infinite range
like that of the photons and gluons. But as the Universe expanded and cooled,
the average energy dropped to the level where spontaneous symmetry breaking
occurred, and weak nuclear bosons gained mass. This slowed them down and restricted
their force to a small range.