The Cosmogenesis
Credit: CERN/Georges Boxaider
It is now almost generally agreed that the evolution of the Universe must have been determined in the first few moments of the Big Bang. A most convincing theory of the origin and evolution of the Universe is the standard model, which is based on recent theoretical and experimental advances in elementary particle physics.
The early Universe was extremely dense and extremely hot, with average particle energies many orders of magnitude beyond anything that exists in the present Universe. The temperature of the very early Universe ranged from 10¹⁰ K to 10¹³ K. In the standard model, at time t = 10⁻⁴³ s, known as the Planck time, the temperature of the Universe was about 10³² K, and the average energy per particle was approximately E = kT = (10⁻¹³ GeV/K)(10³² K) = 10¹⁹ GeV.
Prior to 10⁻⁴³ s, it is imagined that the four fundamental forces in nature were unified into one force. At t = 10⁻⁴³ s, a phase transition occurred, during which the gravitational force condensed out as a separate force. This symmetry breaking left the strong, weak, and electromagnetic forces unified, marking the beginning of the grand unified era. During this period, there was no distinction between quarks and leptons, and baryon and lepton numbers were not conserved.
By t = 10⁻³⁵ s, the Universe had expanded considerably, cooling to about 10²⁷ K and reducing the average energy to about 10¹⁴ GeV. At this energy, the strong force condensed out, and baryon and lepton numbers began to be separately conserved. At t = 10⁻³² s, the Universe was a mixture of quarks, leptons, and mediating bosons (gluons, photons, and the weak bosons W and Z°).
By t = 10⁻¹² s, the Universe had expanded further, with the temperature dropping to about 10²⁰ K. The four fundamental forces became distinct, and leptons separated into electrons, neutrinos, and antiparticles. Gravity began to control the expansion of the Universe.
At t = 10⁻⁶ s, the Universe expanded and cooled to about 10¹³ K, with typical energies around 1 GeV. Quarks began to bind together to form nucleons and antinucleons. There were still enough high-energy photons to produce nucleon-antinucleon pairs, balancing the process of nucleon-antinucleon annihilation. This led to the formation of protons, neutrons, and their antiparticles, with a slight excess of protons and neutrons remaining along with many photons.
By t = 1 s, the Universe had cooled to about 10 billion degrees (10¹⁰ K), and the average kinetic energy was about 1 MeV. Electrons and positrons were still being created, but within a few more seconds, the temperature dropped sufficiently to prevent their formation. However, annihilation-free photons continued to exist, making photons and neutrinos the major constituents of the Universe. As neutrinos rarely interacted, the Universe became radiation-dominated, with much more energy in radiation than in matter.
Around t = 2 to 3 minutes, nuclear fusion began (nucleosynthesis reactions started), as the temperature had dropped to about 10⁹ K, corresponding to kinetic energy of 100 keV. Deuterium, helium, and very small amounts of lithium were formed. The rapid cooling of the Universe prevented the formation of larger nuclei.
By 1 hour, the temperature had dropped further, halting nucleosynthesis. For millions of years, nucleosynthesis would not resume, except in stars. Thus, after the first hour, matter in the Universe mainly consisted of bare nuclei of hydrogen (about 75%) and helium (about 25%), along with electrons. Radiation continued to dominate.
At approximately 300,000 years, the Universe had expanded to about 1/1000 of its present size, with the temperature cooling to around 3000 K. During this era, atoms began to form. As the Universe continued to expand, the radiation cooled further (to 2.7 K today, forming cosmic microwave background radiation) and lost energy. However, the mass of material particles did not decrease, leading to the Universe becoming increasingly dominated by matter rather than radiation, a state it remains in today.
By 1 billion years, stars and galaxies had formed due to self-gravitation. The Universe continued to evolve until today, 10 to 20 billion years later.
One of the many questions regarding the history of the Universe is: when did time start? The Universe has not existed forever. Rather, the Universe, and time itself, had a beginning in the Big Bang, about 20 billion years ago. The no-boundary hypothesis of the Universe predicts that the Universe will eventually collapse again. However, the contracting phase will not reverse the arrow of time seen in the expanding phase. Therefore, time will not go backward, and we will not return to our youth.
Given that time is not going to reverse, I think it's better to stop now.
