Tuesday, January 25, 2011

Our Universe Part 6: Electroweak Epoch

A lot is going in the very young universe right now. By the end of this epoch all four fundamental forces will be operating and the first particles of matter will make their appearance. The Higgs field will be ushered in, imparting mass on matter particles as well as some force particles.

The electroweak epoch begins at the same time as cosmic inflation is triggered, around 10-36 seconds. This is also the time when the strong force breaks from the grand unified force. The potential energy released from this symmetry breaking is believed by most researchers to be the trigger for inflation. This epoch will continue until 10-12 seconds, when another phase transition will occur in which the weak interaction and electromagnetism break from the electroweak force. We begin with the universe operating under three kinds of force: gravity, strong and electroweak. 

By the end of this epoch a single electroweak force will break into two fundamental forces operating in the universe today. This means that the electroweak boson itself will break into the photon of electromagnetism and three bosons of the weak force - two charged W bosons and a neutral Z boson. A single force particle will break into a massless particle with infinite range (photon) and massive particles with a range less than 10-17 m (W and Z bosons). It implies a still present but hidden symmetry between electromagnetism and the weak force in our current universe. A single force with a particular potential energy breaks into two forces, each existing at new but different lower energy potentials that can recombine into one force if enough energy (100 GeV) is put back into the system.

The electroweak force is carried by a boson, much like the gluon boson, the force carrier for the strong force. In the universe today, bosons mediate both the weak force and the electromagnetic force. These bosons come in two general kinds. The first kind are the high-mass bosons, called W and Z bosons, which interact with something called the Higgs field. They get their mass by interacting with the Higgs boson, yet another kind of boson, and they mediate the weak force. The W and Z bosons were predicted many years before they were discovered in 1979 by experiments done at CERN, earning the scientists involved a Nobel prize. Because of their mass, these bosons can operate only within short distances confined to the atom's nucleus. The second kind is a massless boson called a photon and it carries out the electromagnetic force. Photons can essentially travel forever as different kinds of radiation.

Experimental evidence suggests that the single electroweak interaction breaks into two different force fields. This field has potential energy with four degrees of freedom. Each degree of freedom represents a boson, one of four massless precursor gauge bosons called Goldstone bosons. Each of these four Goldstone bosons becomes one of two W bosons, the Z boson and one another massive boson, the Higgs boson. The other field has a different potential energy with one degree of freedom and it becomes a massless boson, the photon. When the electroweak phase transition occurs at the end of this epoch, the symmetry of the Goldstone bosons, and the fields they mediate, will break into bosons that interact with the Higgs mechanism (W+, W- and Z bosons of the weak interaction) and the Higgs boson. Another field is also created which does not interact with the Higgs boson. The particle that carries it out remains massless as the photon of the electromagnetic interaction. I explore the recent Higgs boson discovery in detail in the article, The Higgs Boson.

This is an epoch bound by two separate phase transitions, during which the fundamental forces are breaking away from an earlier grand unified force. The first mass to appear in the universe is in the form of massive energy-mediating bosons - the W and Z bosons and the Higgs boson itself. The Higgs field conveys mass to particles and it has just broken from the electroweak force, along with the weak force and electromagnetic force The Higgs field, the electroweak force and the strong force are all mediated through different kinds of bosons in the universe today. This epoch as a time when bosons are figuring themselves out. The two symmetry breaking processes that bind this epoch show us that the strong force, weak and electromagnetic boson particles are all related to each other - they are all pieces of a grand unified force.

As these new fundamental forces usher in, the universe explodes with super-dense gas-like quark-gluon plasma. This is the result of another phase transition called the QCD transition, which occurs at an energy of about 100 MeV. Quarks uncouple from the energy of the universe and behave like free particles. This is the first form of matter in the universe. It is thousands of times hotter than the inside of the Sun and denser than a neutron star. When the weak and electromagnetic forces break from the electroweak force, this epoch ends, about 10-12 seconds after the Big Bang. We will explore this quark explosion in the next article, Quark Epoch.

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