9 LEPTON EPOCH

 

9.1 General

 

The Lepton Epoch is a period in the early history of the universe, occurring between roughly 1 second and 10 seconds after the Big Bang. During this time, the universe was dominated by leptons, a class of elementary particles that includes electrons, muons, taus, and their associated neutrinos. The Lepton Epoch is a crucial stage in the evolution of the universe, and several important processes took place during this time.

Here's a step-by-step description of the events that occurred during the Lepton Epoch:

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• Temperature and expansion: As the universe continued to expand and cool down from the earlier Quark Epoch (10¹⁰K → 10⁹K), it reached temperatures where quarks combined to form protons and neutrons. This transition marked the beginning of the Lepton Epoch.

 

• Electron-positron annihilation: At the start of the Lepton Epoch, the universe was filled with a dense plasma of electrons, positrons and photons. As the universe cooled further, electrons and positrons began to annihilate each other, producing photons. This process was initially in equilibrium, meaning that the rate of electron-positron creation was equal to the rate of annihilation. However, as the temperature continued to drop, the equilibrium shifted, and the rate of annihilation began to exceed the rate of creation.

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• Neutrino decoupling: Around 1 second after the Big Bang, neutrinos (very light, neutral particles that interact only weakly with other matter) decoupled from the primordial plasma. This occurred because the expansion of the universe caused the temperature to drop, reducing the interaction rate between neutrinos and other particles. Neutrinos began to travel freely through the universe, forming what we now call the Cosmic Neutrino Background (CNB).

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As neutrinos rarely interact with matter, these neutrinos still exist today. They have a very low energy, around 10−4 to 10−6 eV. Even high energy neutrinos are notoriously difficult to detect, and the CνB has energies around 1010 times smaller, so the CνB may not be directly observed in detail for many years, if at all. However, Big Bang cosmology makes many predictions about the CνB, and there is very strong indirect evidence that the CνB exists.  [5]  [6]

 

• Photon dominance: The annihilation of electrons and positrons resulted in a surplus of photons in the universe. As the Lepton Epoch progressed, the number of electrons and positrons decreased, while the number of photons increased. By the end of the Lepton Epoch, the universe was dominated by photons, and the remaining electrons were outnumbered by protons, which would later combine to form neutral hydrogen atoms during the Recombination Era.

 

• Baryogenesis: Although not yet fully understood, baryogenesis is believed to have taken place during the Lepton Epoch. This process resulted in the observed asymmetry between matter (baryons) and antimatter (antibaryons) in the universe. The exact mechanism behind baryogenesis is still a topic of ongoing research in the field of cosmology and particle physics.

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We do not yet have a unique theory of neutrino masses, but there are two possibilities. The neutrino might have a Majorana mass, in which case the mysterious right-handed particle in the above thought experiment would be an ordinary antineutrino. Since the antineutrino has already been included in the black-body formulas, they will not be changed. The other possibility is that the neutrino can have a Dirac mass, which would be the same type of mass that an electron has. In that case, the mysterious right-handed particle in the thought experiment would be a new spin state of the neutrino. The statement that neutrinos are always left-handed would be blatantly false. Nonetheless, our theories would allow us to calculate the strength of the interactions of these righthanded neutrinos, and they would be incredibly weak. They would be so weak that they would essentially never be produced in the early inverse, so again our black-body formulas would not require modification. [8]

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• Transition to the Photon Epoch: As the universe continued to expand and cool, it entered the Photon Epoch, which lasted from about 10 seconds to 380,000 years after the Big Bang. During this time, the universe was filled with a hot plasma of photons, protons, and electrons, which would eventually lead to the formation of the first atoms and the Cosmic Microwave Background (CMB).

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