hadron epoch

The hadron epoch is a period in the history of the universe that occurred from approximately 10⁻⁶ seconds after the Big Bang to about 1 second after the Big Bang. During this time, the universe was filled with a hot, dense plasma of particles known as hadrons, which include protons and neutrons. Most important steps in this epoch are :

The universe was incredibly hot and dense, with temperatures exceeding 10¹² Kelvin. At these extreme temperatures, particles were moving at very high speeds and colliding with each other frequently.
Initially, the universe was dominated by a sea of subatomic particles, including quarks and gluons, which are the building blocks of hadrons.
Cooling of the universe: As the universe expanded and cooled, the energy density and temperature decreased, allowing the quarks and gluons to interact with each other through the strong nuclear force. This led to the formation of clusters of quarks and gluons, which eventually combined to form hadrons.
Quark combination: The quarks in the plasma combined with each other to form different types of hadrons, including protons, neutrons, and mesons. The combination of quarks is governed by the strong nuclear force, which is mediated by gluons. The process of quark combination is a complex one, and it is not fully understood. However, it is thought that the quarks combine to form color-neutral hadrons through a process known as confinement.
Meson formation: In addition to the formation of baryons such as protons and neutrons, the quarks in the plasma can also combine to form mesons, which are hadrons composed of a quark-antiquark pair. Mesons are less stable than baryons, and they decay quickly into other particles.
Final state hadrons: Once the quarks have combined to form hadrons, these particles are no longer free to move around in the plasma. Instead, they become stable and interact with other particles in the plasma through the strong nuclear force.
The interactions between hadrons and photons, which are particles of light, were frequent and intense during the hadron epoch. As a result, the plasma was constantly absorbing and emitting radiation, leading to a process known as thermalization.
As the universe continued to expand and cool, the energy of the photons decreased, and they were eventually too low-energy to ionize the plasma. This marked the end of the hadron epoch and the beginning of the lepton epoch, during which the universe was dominated by leptons such as electrons and neutrinos.

Overall, the hadron epoch was a critical period in the history of the universe, as it marked the formation of stable hadrons from the initial quark-gluon plasma. This laid the groundwork for the formation of atomic nuclei and the eventual emergence of the first atoms, which occurred during the following era known as the photon epoch.

7.2 Recent Developments

Quark-gluon plasma studies: Recent experimental and theoretical studies have shed light on the properties and behavior of the quark-gluon plasma, which is thought to have existed during the hadron epoch. One example is the paper "Challenges in QCD matter physics --The scientific programme of the Compressed Baryonic Matter experiment at FAIR," by J. Stroth et al., published in The European Physical Journal A in 2020. The paper describes the goals and expected results of the Compressed Baryonic Matter (CBM) experiment at the Facility for Antiproton and Ion Research (FAIR), which aims to study the properties of the quark-gluon plasma and its transition to hadrons.
Improved simulations: Advances in computer simulations have allowed researchers to model the behavior of the early universe more accurately. One recent example is the paper "Improved cosmological simulations with the IllustrisTNG model: simulations of the EoR and reionization," by D. Vogelsberger et al., published in Monthly Notices of the Royal Astronomical Society in 2021. The paper describes a set of cosmological simulations that improve on previous models by including more detailed physics, such as radiative transfer and a more accurate treatment of baryonic physics.
Improved measurements of the cosmic microwave background: Recent measurements of the cosmic microwave background (CMB) radiation have provided important insights into the history and composition of the universe. One example is the paper "The Atacama Cosmology Telescope: DR4 maps and cosmological parameters," by A. Kusaka et al., published in The Astrophysical Journal Supplement Series in 2021. The paper describes the latest data release from the Atacama Cosmology Telescope (ACT), which includes new measurements of the CMB and improved estimates of cosmological parameters such as the Hubble constant.
Dark matter studies: The properties and behavior of dark matter, which is thought to have played a crucial role in the evolution of the universe during the hadron epoch, remain a subject of intense study. One recent example is the paper "Constraints on Light Dark Matter from Nuclear-Detector- Based Direct Detection Experiments," by J. Brod et al., published in Physical Review Letters in 2020. The paper presents new constraints on the properties of dark matter particles, based on data from nuclear-detector-based experiments such as XENON1T and CRESST-III.