Electroweak epoch

Lore & Background

In physical cosmology, the electroweak epoch was the period in the early universe when the strong force had already separated from the combined electronuclear interaction, but the universe remained hot enough that electromagnetism and the weak interaction were still unified as a single electroweak force. This condition persisted above a critical temperature of approximately 159.5±1.5 GeV, as defined by the Standard Model of particle physics. The exact timing of this epoch is debated: some cosmologists place its start at the onset of the inflationary epoch, roughly 10⁻³⁶ seconds after the Big Bang, while others date it to around 10⁻³² seconds, when the inflaton field’s potential energy was released, filling the universe with a dense, hot quark–gluon plasma. During this phase, particle interactions were sufficiently energetic to produce large numbers of exotic particles, including W and Z bosons and Higgs bosons. As the universe expanded and cooled, these interactions weakened; by about 10⁻¹² seconds, W and Z bosons ceased to be created at observable rates, and the remaining ones decayed quickly, causing the weak interaction to become a short-range force in the subsequent quark epoch. The electroweak epoch ended with an electroweak phase transition, the nature of which remains uncertain. Speculation in the 1990s suggested it could be a first-order transition, potentially sourcing a gravitational wave background and baryogenesis if the Sakharov conditions were met and the Higgs boson energy was below 45 GeV. However, subsequent work with the Standard Model and a measured Higgs mass of 125 GeV showed the transition is not first- or second-order but a continuous crossover, with a width of about 5 GeV around the crossover temperature. In this scenario, no electroweak baryogenesis or observable gravitational wave background peaking in the mHz range occurs. Many extensions to the Standard Model, such as supersymmetry and the two-Higgs-doublet model, do predict a first-order transition, though they require additional CP violation.

Reader's Guide

The electroweak epoch began when the universe’s temperature dropped enough for the strong force to separate from the combined electronuclear interaction, yet remained above the critical temperature for electroweak symmetry breaking, which in the Standard Model is 159.5±1.5 GeV. Some cosmologists place this epoch at the start of the inflationary epoch, roughly 10⁻³⁶ seconds after the Big Bang, while others set it at about 10⁻³² seconds, when the inflaton field’s potential energy was released, creating a dense, hot quark–gluon plasma. During this phase, particle interactions were sufficiently energetic to produce large numbers of exotic particles, including W and Z bosons and Higgs bosons. As the universe expanded and cooled, interactions became less energetic; by the time the universe was about 10⁻¹² seconds old, W and Z bosons ceased forming at observable rates, and the remaining ones decayed quickly, making the weak interaction a short-range force in the subsequent quark epoch. The epoch ended with an electroweak phase transition, whose nature was initially unknown. Speculation in the 1990s suggested it could be a first-order transition, potentially sourcing a gravitational wave background and baryogenesis if the Higgs boson energy was below 45 GeV. However, subsequent work with the Standard Model and a measured Higgs mass over 114 GeV showed the transition is not first- or second-order but a continuous crossover; with the actual Higgs mass of 125 GeV, the crossover width is about 5 GeV around the crossover temperature. In this case, no electroweak baryogenesis or observable gravitational wave background peaking in the mHz range occurs. Many extensions to the Standard Model, such as supersymmetry and the two-Higgs-doublet model, still allow a first-order electroweak phase transition, though they require additional CP violation. The epoch’s significance is that it directly preceded the quark epoch and involved the last unification of electromagnetism and the weak interaction before their separation, constraining models of baryogenesis and early-universe physics.

Did You Know?

• The electroweak epoch ended when the universe was about 10⁻¹² seconds old, after which W and Z bosons ceased to be created at observable rates.
• Extensions to the Standard Model such as supersymmetry and the two-Higgs-doublet model can have a first-order electroweak phase transition but require additional CP violation.