part 3

Electroweak symmetry breaking 10−12 seconds after the Big Bang As the universe's temperature continued to fall below 159.5±1.5 GeV, electroweak symmetry breaking happened.[29] So far as we know, it was the penultimate symmetry breaking event in the formation of the universe, the final one being chiral symmetry breaking in the quark sector. This has two related effects: Via the Higgs mechanism, all elementary particles interacting with the Higgs field become massive, having been massless at higher energy levels. After electroweak symmetry breaking, the fundamental interactions we know of—gravitation, electromagnetic, weak and strong interactions—have all taken their present forms, and fundamental particles have their expected masses, but the temperature of the universe is still too high to allow the stable formation of many particles we now see in the universe, so there are no protons or neutrons, and therefore no atoms, atomic nuclei, or molecules. (More exactly, any composite particles that form by chance, almost immediately break up again due to the extreme energies.) The quark epoch Between 10−12 seconds and 10−5 seconds after the Big Bang The quark epoch began approximately 10−12 seconds after the Big Bang. This was the period in the evolution of the early universe immediately after electroweak symmetry breaking, when the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction had taken their present forms, but the temperature of the universe was still too high to allow quarks to bind together to form hadrons. During the quark epoch the universe was filled with a dense, hot quark–gluon plasma, containing quarks, leptons and their antiparticles. Collisions between particles were too energetic to allow quarks to combine into mesons or baryons. The quark epoch ended when the universe was about 10−5 seconds old, when the average energy of particle interactions had fallen below the mass of the lightest hadron, the pion.[Sources:i) Petter 2013, p. 68,ii) Morison 2015, p. 298] Baryogenesis Perhaps by 10−11 seconds Baryons are subatomic particles such as protons and neutrons, that are composed of three quarks. It would be expected that both baryons, and particles known as antibaryons would have formed in equal numbers. However, this does not seem to be what happened—as far as we know, the universe was left with far more baryons than antibaryons. In fact, almost no antibaryons are observed in nature. It is not clear how this came about. Any explanation for this phenomenon must allow the Sakharov conditions related to baryogenesis to have been satisfied at some time after the end of cosmological inflation. Current particle physics suggests asymmetries under which these conditions would be met, but these asymmetries appear to be too small to account for the observed baryon-antibaryon asymmetry of the universe. Hadron epoch Between 10−5 second and 1 second after the Big Bang The quark–gluon plasma that composes the universe cools until hadrons, including baryons such as protons and neutrons, can form. Initially, hadron/anti-hadron pairs could form, so matter and antimatter were in thermal equilibrium. However, as the temperature of the universe continued to fall, new hadron/anti-hadron pairs were no longer produced, and most of the newly formed hadrons and anti-hadrons annihilated each other, giving rise to pairs of high-energy photons. A comparatively small residue of hadrons remained at about 1 second of cosmic time, when this epoch ended. Theory predicts that about 1 neutron remained for every 6 protons, with the ratio falling to 1:7 over time due to neutron decay. This is believed to be correct because, at a later stage, the neutrons and some of the protons fused, leaving hydrogen, a hydrogen isotope called deuterium, helium and other elements, which can be measured. A 1:7 ratio of hadrons would indeed produce the observed element ratios in the early and current universe.[ Source: Karki, Ravi (May 2010). "The Foreground of Big Bang Nucleosynthesis" (PDF). The Himalayan Physics. 1 (1): 79–82. doi:10.3126/hj.v1i0.5186] Neutrino decoupling and cosmic neutrino background (CνB) Around 1 second after the Big Bang At approximately 1 second after the Big Bang neutrinos decouple and begin travelling freely through space. As neutrinos rarely interact with matter, these neutrinos still exist today, analogous to the much later cosmic microwave background emitted during recombination, around 370,000 years after the Big Bang. The neutrinos from this event have a very low energy, around 10−10 times the amount of those observable with present-day direct detection. Even high-energy neutrinos are notoriously difficult to detect, so this cosmic neutrino background (CνB) may not be directly observed in detail for many years, if at all.[Siegel, Ethan (9 September 2016). "Cosmic Neutrinos Detected, Confirming The Big Bang's Last Great Prediction" (Blog). Science. Forbes. Jersey City, NJ. ISSN 0015-6914] In 2015, it was reported that such shifts had been detected in the CMB. Moreover, the fluctuations corresponded to neutrinos of almost exactly the temperature predicted by Big Bang theory (1.96 ± 0.02K compared to a prediction of 1.95K), and exactly three types of neutrino, the same number of neutrino flavors predicted by the Standard Model.[aforesaid]. Possible formation of primordial black holes May have occurred within about 1 second after the Big Bang Primordial black holes are a hypothetical type of black hole proposed in 1966,[34] that may have formed during the so-called radiation-dominated era, due to the high densities and inhomogeneous conditions within the first second of cosmic time. Random fluctuations could lead to some regions becoming dense enough to undergo gravitational collapse, forming black holes. Current understandings and theories place tight limits on the abundance and mass of these objects. Lepton epoch Between 1 second and 10 seconds after the Big Bang The majority of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving leptons (such as the electron, muons and certain neutrinos) and antileptons, dominating the mass of the universe. The lepton epoch follows a similar path to the earlier hadron epoch. Initially leptons and antileptons are produced in pairs. About 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton–antilepton pairs are no longer created and most remaining leptons and antileptons quickly annihilated each other, giving rise to pairs of high-energy photons, and leaving a small residue of non-annihilated leptons. [3Sources: Kauffmann, Guinevere. "Thermal history of the universe and early growth of density fluctuations" (PDF) (Lecture). Garching: Max Planck Institute for Astrophysics. Chaisson, Eric J. (2013). "First Few Minutes". Cosmic Evolution. Cambridge, MA: Harvard–Smithsonian Center for Astrophysics. "Timeline of the Big Bang". The Physics of the Universe] Photon epoch Between 10 seconds and 370,000 years after the Big Bang After most leptons and antileptons are annihilated at the end of the lepton epoch, most of the mass–energy in the universe is left in the form of photons. Therefore, the energy of the universe, and its overall behavior, is dominated by its photons. These photons continue to interact frequently with charged particles, i.e., electrons, protons and (eventually) nuclei. They continue to do so for about the next 370,000 years. Nucleosynthesis of light elements Between 2 minutes and 20 minutes after the Big Bang Between about 2 and 20 minutes after the Big Bang, the temperature and pressure of the universe allowed nuclear fusion to occur, giving rise to nuclei of a few light elements beyond hydrogen ("Big Bang nucleosynthesis"). About 25% of the protons, and all the neutrons fuse to form deuterium, a hydrogen isotope, and most of the deuterium quickly fuses to form helium-4. Atomic nuclei will easily unbind (break apart) above a certain temperature, related to their binding energy. From about 2 minutes, the falling temperature means that deuterium no longer unbinds, and is stable, and starting from about 3 minutes, helium and other elements formed by the fusion of deuterium also no longer unbind and are stable.[Source: Ryden, Barbara Sue (12 March 2003). "Astronomy 162 – Lecture 44: The First Three Minutes". Barbara S. Ryden's Home Page. Columbus, OH: Department of Astronomy, Ohio State University. Ryden, Barbara Sue (12 March 2003). "Astronomy 162 – Lecture 44: The First Three Minutes". Barbara S. Ryden's Home Page. Columbus, OH: Department of Astronomy, Ohio State University. Ryden, Barbara Sue (12 March 2003). "Astronomy 162 – Lecture 44: The First Three Minutes". Barbara S. Ryden's Home Page. Columbus, OH: Department of Astronomy, Ohio State University]. The amounts of each light element in the early universe can be estimated from old galaxies, and is strong evidence for the Big Bang. The Big Bang should produce about 1 neutron for every 7 protons, allowing for 25% of all nucleons to be fused into helium-4 (2 protons and 2 neutrons out of every 16 nucleons), and this is the amount we find today, and far more than can be easily explained by other processes.Similarly, deuterium fuses extremely easily; any alternative explanation must also explain how conditions existed for deuterium to form, but also left some of that deuterium unfused and not immediately fused again into helium.[32] Any alternative must also explain the proportions of the various light elements and their isotopes. A few isotopes, such as lithium-7, were found to be present in amounts that differed from theory, but over time, these differences have been resolved by better observations. Matter domination 47,000 years after the Big Bang Until now, the universe's large-scale dynamics and behavior have been determined mainly by radiation—meaning, those constituents that move relativistically (at or near the speed of light), such as photons and neutrinos.[Ryden 2006] As the universe cools, from around 47,000 years (redshift z = 3600),[2] the universe's large-scale behavior becomes dominated by matter instead. This occurs because the energy density of matter begins to exceed both the energy density of radiation and the vacuum energy density.[Zeilik & Gregory 1998, p. 497] Around or shortly after 47,000 years, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) become equal, the Jeans length, which determines the smallest structures that can form (due to competition between gravitational attraction and pressure effects), begins to fall and perturbations, instead of being wiped out by free streaming radiation, can begin to grow in amplitude. From this point on, and for several billion years to come, the presence of dark matter accelerates the formation of structure in the universe. In the early universe, dark matter gradually gathers in huge filaments under the effects of gravity, collapsing faster than ordinary (baryonic) matter because its collapse is not slowed by radiation pressure. This amplifies the tiny inhomogeneities (irregularities) in the density of the universe which was left by cosmic inflation. Over time, slightly denser regions become denser and slightly rarefied (emptier) regions become more rarefied. Ordinary matter eventually gathers together faster than it would otherwise do, because of the presence of these concentrations of dark matter. The properties of dark matter that allow it to collapse quickly without radiation pressure, also mean that it cannot lose energy by radiation either. Losing energy is necessary for particles to collapse into dense structures beyond a certain point. Therefore, dark matter collapses into huge but diffuse filaments and haloes, and not into stars or planets. Ordinary matter, which can lose energy by radiation, forms dense objects and also gas clouds when it collapses. Recombination, photon decoupling, and the cosmic microwave background (CMB) 9-year WMAP image of the cosmic microwave background radiation (2012). The radiation is isotropic to roughly one part in 100,000.[ Wright 2004, p. 291] About 370,000 years after the Big Bang, two connected events occurred: the ending of recombination and photon decoupling. Recombination describes the ionized particles combining to form the first neutral atoms, and decoupling refers to the photons released ("decoupled") as the newly formed atoms settle into more stable energy states. Starting around 18,000 years, the universe has cooled to a point where free electrons can combine with helium nuclei to form He+ atoms. Neutral helium nuclei then start to form at around 100,000 years, with neutral hydrogen formation peaking around 260,000 years.[5 Sunyaev, R. A.; Chluba, J. (August 2009). "Signals From the Epoch of Cosmological Recombination". Astronomical Notes. 330 (7): 657–674. arXiv:0908.0435. doi:10.1002/asna.200911237] This process is known as recombination.[ Mukhanov 2005, p. 120] The name is slightly inaccurate and is given for historical reasons: in fact the electrons and atomic nuclei were combining for the first time. At around 100,000 years, the universe had cooled enough for helium hydride, the first molecule, to form.[52] In April 2019, this molecule was first announced to have been observed in interstellar space, in NGC 7027, a planetary nebula within this galaxy. (Much later, atomic hydrogen reacted with helium hydride to create molecular hydrogen, the fuel required for star formation.[ Mathewson, Samantha (18 April 2019). "Astronomers Finally Spot Universe's First Molecule in Distant Nebula". Space.com. New York: Future plc] Directly combining in a low energy state (ground state) is less efficient, so these hydrogen atoms generally form with the electrons still in a high-energy state, and once combined, the electrons quickly release energy in the form of one or more photons as they transition to a low energy state. This release of photons is known as photon decoupling. Some of these decoupled photons are captured by other hydrogen atoms, the remainder remain free. By the end of recombination, most of the protons in the universe have formed neutral atoms. This change from charged to neutral particles means that the mean free path photons can travel before capture in effect becomes infinite, so any decoupled photons that have not been captured can travel freely over long distances (see Thomson scattering). The universe has become transparent to visible light, radio waves and other electromagnetic radiation for the first time in its history. The background of this box approximates the original 4000 K color of the photons released during decoupling, before they became redshifted to form the cosmic microwave background. The entire universe would have appeared as a brilliantly glowing fog of a color similar to this and a temperature of 4000 K, at the time. The photons released by these newly formed hydrogen atoms initially had a temperature/energy of around ~ 4000 K. This would have been visible to the eye as a pale yellow/orange tinted, or "soft", white color.[ Mukhanov 2005, p. 120] Over billions of years since decoupling, as the universe has expanded, the photons have been red-shifted from visible light to radio waves (microwave radiation corresponding to a temperature of about 2.7 K). Red shifting describes the photons acquiring longer wavelengths and lower frequencies as the universe expanded over billions of years, so that they gradually changed from visible light to radio waves. These same photons can still be detected as radio waves today. They form the cosmic microwave background. early universe and how it developed. Around the same time as recombination, existing pressure waves within the electron-baryon plasma—known as baryon acoustic oscillations—became embedded in the distribution of matter as it condensed, giving rise to a very slight preference in distribution of large-scale objects. Therefore, the cosmic microwave background is a picture of the universe at the end of this epoch including the tiny fluctuations generated during inflation (see 9-year WMAP image), and the spread of objects such as galaxies in the universe is an indication of the scale and size of the universe as it developed over time.[i) Amos, Jonathan (13 November 2012). "Quasars illustrate dark energy's roller coaster ride". Science & Environment. BBC News. London: BBC].