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Weak nuclear bosons become massive

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TIME: 10-10 seconds

. At this 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.
. In elementary 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.
. For this 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.

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