Electroweak epoch

The electroweak epoch is a vital period in the history of our universe. It occurred during the very early stages of cosmic evolution. During this time, the universe was incredibly hot and dense. This heat kept certain fundamental forces of nature joined together. Specifically, electromagnetism and the weak interaction were merged into one. Scientists call this single force the electroweak interaction. This era is important because it marks how forces began to separate. Understanding this epoch helps us learn how the modern universe formed.

The mechanism of this epoch relied on extreme temperatures. In the very beginning, the universe was too hot for forces to stay separate. As the universe expanded, it began to cool down. First, the strong force separated from the electronuclear interaction. However, the electroweak interaction stayed merged for a longer period. This lasted until the temperature fell below a critical threshold. In the Standard Model of particle physics, this temperature is 159.5±1.5 GeV. This value represents the point of electroweak symmetry breaking.

Cosmologists debate exactly when this epoch began. Some researchers place its start at the beginning of the inflationary epoch. This would be roughly 10⁻³⁶ seconds after the Big Bang. Other scientists suggest it started later, at about 10⁻³² seconds. This later timing coincides with the release of potential energy. This energy came from the inflaton field that drove cosmic inflation. This release filled the universe with a hot, dense quark–gluon plasma.

Particle activity during this epoch was extremely intense. The high energy levels allowed for the creation of exotic particles. These included Higgs bosons and the W and Z bosons. These particles are essential to our understanding of physics. As the universe continued to expand and cool, energy levels dropped. By the time the universe was 10⁻¹² seconds old, things changed. W and Z bosons were no longer created at observable rates. The remaining bosons decayed very quickly into other things.

The end of this era is known as the electroweak phase transition. For many years, scientists speculated about the nature of this transition. In the 1990s, some thought it might be a first-order transition. A first-order transition is a sudden, sharp change. If this were true, it could have caused baryogenesis. It might also have created a background of gravitational waves. These ideas depended on the Higgs boson energy being below 45 GeV.

Recent measurements have changed our understanding of this transition. The Standard Model and new measurements show a different reality. The Higgs boson mass was found to be 125 GeV. This mass is much higher than the 45 GeV limit. Because of this, the transition was not first-order or second-order. Instead, it was a continuous crossover. This means the change was smooth rather than sudden. The crossover width is approximately 5 GeV around the transition temperature.

This smooth crossover has specific scientific consequences. Because it was a crossover, there was no electroweak baryogenesis. It also means we do not see a gravitational wave background in the mHz range. However, many scientific models still explore different possibilities. Some extensions to the Standard Model suggest different outcomes. These include supersymmetry and the two-Higgs-doublet model. These models can allow for a first-order electroweak phase transition. They simply require additional CP violation to work.