Hadron era

The hadron era describes a specific period in an obsolete model of the early universe. In physical cosmology, this era represents a stage in how scientists once thought the universe evolved. This model is no longer used because our understanding of subatomic particles has changed. However, studying this era helps us understand how our current models developed. The era was defined by the presence of hadrons and anti-hadrons. These are fundamental building blocks of matter and antimatter.

In this older model, the hadron era had specific starting and ending points. Some versions of the theory say it began at 10^-44 seconds. Other versions suggest it started later, at 10^-8 seconds. The era ended when the universe reached 10^-4 seconds of age. During this time, the universe was extremely hot. This intense heat allowed for the formation of hadron and anti-hadron pairs. These pairs stayed in thermal equilibrium. This means matter and antimatter existed in a steady balance.

To understand the mechanism, we must look at the temperature and particle interactions. High temperatures provided the energy needed to create hadron and anti-hadron pairs. As long as the heat remained high, these particles stayed in equilibrium. However, this balance was temporary. The universe was also undergoing a process called the QCD phase transition. This transition occurred between 5x10^-5 seconds and 7x10^-5 seconds. During this tiny window, hadron and anti-hadron pairs were abundant.

There were distinct stages that preceded and followed this era. Before the hadron era, there was the quark epoch. During the quark epoch, the universe was too hot for hadrons to exist. Quarks were the smaller particles that make up hadrons. Because of the extreme heat, quarks did not combine to form hadrons yet. Once the temperature dropped, the QCD phase transition allowed these particles to form. This transition marked a major change in the state of the universe.

Our scientific understanding changed significantly during the 1970s. During this decade, researchers discovered quarks and gluons. Quarks are the particles that form hadrons. Gluons are the particles that hold them together. This discovery proved that the old hadron-based model no longer made sense. We realized that the era of quarks must come before the era of hadrons. This shifted how cosmologists view the timeline of the early universe.

The end of the era was caused by a drop in temperature. At 7x10^-5 seconds, the temperature fell below the pion mass. A pion is a type of particle related to the structure of hadrons. When this temperature threshold was crossed, a major event occurred. Most of the hadrons and anti-hadrons underwent annihilation reactions. In an annihilation reaction, matter and antimatter destroy each other. This process eliminated most of the hadrons and anti-hadrons from the universe.

This annihilation led to a new period called the lepton era. The universe changed from being dominated by hadrons to being dominated by other particles. After the annihilation, the universe was filled with photons. Photons are particles of light. It was also filled with neutrinos, which are very light particles. Additionally, the universe contained electron-positron pairs. These are electrons and their antimatter counterparts. This transition shows how temperature dictates which particles can exist in the cosmos.