Chronology of the universe

Expansion The current accepted model of the history of the universe is based on the concept of the Big Bang: the universe started hot and dense then expanded and cooled. Different particles interact during each major stage in the expansion; as the universe expands the density falls and some particle interactions cease to be important. The character of the universe changes. Moreover, the rate of the expansion itself depends upon the nature of the existing particles, creating an interplay between cosmology and particle physics. Time of extragalactic observations by their cosmological redshift up to z=20. In cosmology, time and space are connected: space expands as time increases. Time at each point in space (for example a galaxy) can be uniquely defined in terms of an imaginary clock at that point. These clocks move with the point in space as the universe expands; they are synchronized to a single point in the distant past. Light from distant galaxies is emitted in the past then travels at the speed of light: knowledge about a distant galaxy is limited to one point in time called the lookback time. During the journey from a distant point, the universe continues to expand, stretching the wavelength of the light along the way, an effect called cosmological redshift. Consequently, experimental knowledge about the chronology of the universe is derived by observing distant light. == Overview ==

The chronology of the universe can be divided into five parts: • Inflation, the first era supported by experimental evidence, a period of exponential expansion that ends with the conversion of energy into particles, • Quark soup, the initial particles cool and coalesce, dark matter forms, • Big bang nucleosynthesis, combining nucleons create the cores of the first atoms, • Gravity builds cosmic structure, reduced density allows matter to dominate over radiation for control of expansion, photons decouple to form the cosmic background radiation, and gravitational attraction builds stars, galaxies, and clusters of galaxies. • Cosmic acceleration, continued expansion allows dark energy to overcome gravitational force, inhibiting larger structures. With these large subsections are many events and transitions. Older models divided the chronology differently, using different terminology or emphasis. Tabular summary Modern cosmological chronologies begin with inflation, the earliest time period supported by solid observational evidence. Anything earlier is considered non-standard cosmology, the subject of a great deal of as-yet-unconfirmed research. == Inflation ==

: Before c. 10−32 seconds after the Big Bang At this point of the very early universe, the universe is thought to have expanded by at least a factor of in time on the order of . All of the mass-energy in all of the galaxies currently visible started in a sphere with a radius around , then grew to a sphere with a radius around 0.9m by the end of inflation. This phase of the cosmic expansion history is known as inflation or sometimes as the inflationary epoch. Inflation explains how today's universe has concentrations of matter, like galaxies and clusters of galaxies, rather than having matter spatially uniform through the universe. The mechanism that drove inflation remains unknown, although many models have been put forward. In several of the more prominent models, it is thought to have been triggered by the separation of the strong and electroweak interactions which ended the grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field. As this field settled into its lowest-energy state throughout the universe, it generated an enormous repulsive force that led to a rapid expansion of the universe. The rapid expansion meant that any potential particles (or other "unwanted" artifacts, such as topological defects) remaining from the time before inflation were now distributed very thinly across the universe. == Reheating ==

It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10−33 and 10−32 seconds after the Big Bang. The rapid expansion of space meant that any elementary particles remaining from the grand unification epoch were now distributed very thinly across the universe to the point where there is no physical temperature that can be associated with them. However, the large potential energy of the inflaton field was released at the end of the inflationary epoch, as the inflaton field decayed into other particles, known as reheating. This heating effect led to the universe being repopulated with a dense, hot mixture of Standard Model particles. After inflation ended, the universe continued to expand. A region the size of a melon at that time has since grown to be the entire observable universe. == Hot Big Bang ==

The physical model for the chronology of the universe with strong observational and theoretical support is called the hot Big Bang model. Within the standard model of cosmology the initial state is set by a process called inflation. The relative timeline for the earliest phenomena is unclear. Speculation on processes occurring before inflation involves physics considered outside of standard cosmology. == Baryogenesis ==

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, 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. == Electroweak phase transition ==

: 10−12 seconds after the Big Bang As the universe's temperature continued to fall below , electroweak symmetry breaking happened. So far as is known, 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 became massive, having been massless at higher energy levels. • As a side-effect, the weak nuclear force and electromagnetic force, and their respective bosons (the W and Z bosons and photon) began to manifest differently in the present universe. Before electroweak symmetry breaking, these bosons were all massless particles and interacted over long distances, but at this point the W and Z bosons abruptly became massive particles only interacting over distances smaller than the size of an atom, while the photon remained massless and remained a long-distance interaction. After electroweak symmetry breaking, the fundamental interactions that are known—gravitation, electromagnetic, weak and strong interactions—all took their present forms, and fundamental particles had their expected masses, but the temperature of the universe was still too high to allow the stable formation of many of the particles observed in the universe, so there were no protons or neutrons, and therefore no atoms, atomic nuclei, or molecules. (More precisely, any composite particles that formed by chance almost immediately broke up again due to the extreme energies.) == Quantum chromodynamics phase transition ==

: Between 10−12 seconds and 10−5 seconds after the Big Bang After cosmic inflation ended, the universe was filled with a hot quark–gluon plasma, the remains of reheating. From this point onwards the physics of the early universe is much better understood, and the energies involved in the quark epoch are directly accessible in particle physics experiments and other detectors. 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. The quark epoch ended when the universe was about 10−5 seconds old; two non-equilibrium events must have occurred next, formation of baryons and of dark matter. == 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. == Electron-positron annihilation ==

: Between 1 second and 10 seconds after the Big Bang The majority of hadrons and anti-hadrons annihilate each other leaving leptons (such as the electron, muons and certain neutrinos) and antileptons, dominating the mass of the universe. 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. After most leptons and antileptons are annihilated, most of the mass–energy in the universe is left in the form of photons. == Nucleosynthesis of light elements ==

: Between 3 minutes and 20 minutes after the Big Bang About 25% of the protons, and all Small amounts of tritium (another hydrogen isotope) and beryllium-7 and -8 are formed, but these are unstable and quickly decay. A small amount of deuterium is left unfused. == Matter-radiation equality ==

: 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. As the universe cools, from around 47,000 years (redshift z = 3600), the universe's large-scale behavior becomes dominated by matter instead. 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. According to the Lambda-CDM model, by this stage, the matter in the universe is around 84.5% cold dark matter and 15.5% "ordinary" matter. There is overwhelming evidence that dark matter exists and dominates the universe, but since the exact nature of dark matter is still not understood, the Big Bang theory does not presently cover any stages in its formation. 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 were 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. 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) image of the cosmic microwave background radiation (2012). The radiation is isotropic to roughly one part in 100,000. 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. Just before recombination, the baryonic matter in the universe was at a temperature where it formed a hot ionized plasma. Most of the photons in the universe interacted with electrons and protons, and could not travel significant distances without interacting with ionized particles. As a result, the universe was opaque or "foggy". Although there was light, it was not possible to see, nor is that light observable through telescopes. Starting around 18,000 years, the universe has cooled to a point where free electrons can combine with helium nuclei to form atoms. After around 50,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. Much later, hydrogen and helium hydride react to form molecular hydrogen (H2), the fuel needed for the first stars. At about 370,000 years, neutral hydrogen atoms finish forming ("recombination" of hydrogen ions and electrons), greatly reducing the Thomson scattering of photons. 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, and they provide crucial evidence of the 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. == Gravity builds cosmic structure ==

: 370 thousand to about 1 billion years after the Big Bang Even before recombination and decoupling, matter began to accumulate around clumps of dark matter. During the Dark Ages, the temperature of the universe cooled from some 4000 K to about 60 K (3727 °C to about −213 °C), and only two sources of photons existed: the photons released during recombination/decoupling (as neutral hydrogen atoms formed), which is still detectable today as the cosmic microwave background (CMB), and photons occasionally released by neutral hydrogen atoms, known as the 21 cm spin line of neutral hydrogen. The hydrogen spin line is in the microwave range of frequencies, and within 3 million years, the CMB photons had redshifted out of visible light to infrared; from that time until the first stars, there were no visible light photons. Other than perhaps some rare statistical anomalies, the universe was truly dark. The first generation of stars, known as Population III stars, formed within a few hundred million years after the Big Bang. These stars were the first source of visible light in the universe after recombination. Structures may have begun to emerge from around 150 million years, and early galaxies emerged from around 180 to 700 million years. As they emerged, the Dark Ages gradually ended. Because this process was gradual, the Dark Ages only ended fully at around 1 billion years, as the universe took on its present appearance. Oldest observations of stars and galaxies At present, the oldest observations of stars and galaxies are from shortly after the start of reionization, with galaxies such as GN-z11 (Hubble Space Telescope, 2016) at about z≈11.1 (about 400 million years cosmic time). Hubble's successor, the James Webb Space Telescope, launched December 2021, is designed to detect objects up to 100 times fainter than Hubble, and much earlier in the history of the universe, back to redshift z≈20 (about 180 million years cosmic time). This is believed to be earlier than the first galaxies, and around the era of the first stars. Quasars provide some additional evidence of early structure formation. Their light shows evidence of elements such as carbon, magnesium, iron and oxygen. This is evidence that by the time quasars formed, a massive phase of star formation had already taken place, including sufficient generations of Population III stars to give rise to these elements. Reionization As the first stars, dwarf galaxies and quasars gradually form, the intense radiation they emit reionizes much of the surrounding universe; splitting the neutral hydrogen atoms back into a plasma of free electrons and protons for the first time since recombination and decoupling. Reionization is evidenced from observations of quasars. Quasars are a form of active galaxy, and the most luminous objects observed in the universe. Electrons in neutral hydrogen have specific patterns of absorbing ultraviolet photons, related to electron energy levels and called the Lyman series. Ionized hydrogen does not have electron energy levels of this kind. Therefore, light travelling through ionized hydrogen and neutral hydrogen shows different absorption lines. Ionized hydrogen in the intergalactic medium (particularly electrons) can scatter light through Thomson scattering as it did before recombination, but the expansion of the universe and clumping of gas into galaxies resulted in a concentration too low to make the universe fully opaque by the time of reionization. Because of the immense distance travelled by light (billions of light years) to reach Earth from structures existing during reionization, any absorption by neutral hydrogen is redshifted by various amounts, rather than by one specific amount, indicating when the absorption of then-ultraviolet light happened. These features make it possible to study the state of ionization at many different times in the past. Reionization began as "bubbles" of ionized hydrogen which became larger over time until the entire intergalactic medium was ionized, when the absorption lines by neutral hydrogen become rare. The absorption was due to the general state of the universe (the intergalactic medium) and not due to passing through galaxies or other dense areas. With these constraints, it is expected that quasars and first generation stars and galaxies were the main sources of energy. The current leading candidates from most to least significant are currently believed to be Population III stars (the earliest stars; possibly 70%), dwarf galaxies (very early small high-energy galaxies; possibly 30%), and a contribution from quasars (a class of active galactic nuclei). However, by this time, matter had become far more spread out due to the ongoing expansion of the universe. Although the neutral hydrogen atoms were again ionized, the plasma was much more thin and diffuse, and photons were much less likely to be scattered. Despite being reionized, the universe remained largely transparent during reionization due how sparse the intergalactic medium was. Reionization gradually ended as the intergalactic medium became virtually completely ionized, although some regions of neutral hydrogen do exist, creating Lyman-alpha forests. In August 2023, images of black holes and related matter in the very early universe by the James Webb Space Telescope were reported and discussed. Galaxies, clusters and superclusters Matter continues to draw together under the influence of gravity, to form galaxies. The stars from this time period, known as Population II stars, are formed early on in this process, with more recent Population I stars formed later. Gravitational attraction also gradually pulls galaxies towards each other to form groups, clusters and superclusters. Hubble Ultra Deep Field observations has identified a number of small galaxies merging to form larger ones, at 800 million years of cosmic time (13 billion years ago). (This age estimate is now believed to be slightly overstated). == Present and future ==

Universe as it appears today From 1 billion years, and for about 12.8 billion years, the universe has looked much as it does today and it will continue to appear very similar for many billions of years into the future. The thin disk of the Milky Way began to form when the universe was about 5 billion years old or Gya. The Solar System formed at about 9.2 billion years (4.6 Gya); The thinning of matter over time reduces the ability of the matter to gravitationally decelerate the expansion of the universe; in contrast, dark energy is a constant factor tending to accelerate the expansion of the universe. The universe's expansion passed an inflection point about five or six billion years ago when the universe entered the modern "dark-energy-dominated era" where the universe's expansion is now accelerating rather than decelerating. The present-day universe is quite well understood, but beyond about 100 billion years of cosmic time (about 86 billion years in the future), scientists are less sure which path the universe will take. Dark energy-dominated era : From about 9.8 billion years after the Big Bang From about 9.8 billion years of cosmic time, "Dark" in this context means that it is not directly observed, but its existence can be deduced by examining the gravitational effect it has on the universe. Research is ongoing to understand this dark energy. Dark energy is now believed to be the single largest component of the universe, as it constitutes about 68.3% of the entire mass–energy of the physical universe. Dark energy is believed to act like a cosmological constant—a scalar field that exists throughout space. Unlike gravity, the effects of such a field do not diminish (or only diminish slowly) as the universe grows. While matter and gravity have a greater effect initially, their effect quickly diminishes as the universe continues to expand. Objects in the universe, which are initially seen to be moving apart as the universe expands, continue to move apart, but their outward motion gradually slows down. This slowing effect becomes smaller as the universe becomes more spread out. Eventually, the outward and repulsive effect of dark energy begins to dominate over the inward pull of gravity. Instead of slowing down and perhaps beginning to move inward under the influence of gravity, from about 9.8 billion years of cosmic time, the expansion of space starts to slowly accelerate outward at a gradually increasing rate. == Beyond standard cosmology ==

Cosmogenesis Cosmological models extrapolated back to 10−43 seconds combined with particle physics models both with and beyond the Standard Model allow well-informed speculation on the character and properties of the early universe. Grand unification epoch : Between 10−43 seconds and 10−36 seconds after the Big Bang Before the GUT epoch, the temperature of the universe exceeded 1015 GeV. As the universe expanded and cooled, it may have crossed a cosmological phase transition, which may have resulted in the large ratio of matter to antimatter observed in the present era. This phase transition is a thermodynamic effect similar to condensation of a gas or freezing of a liquid. While the transition in the GUT epoch is speculative, electroweak and quark-hadron transitions which happen later are supported by theoretical models with some successful predictions. Electroweak epoch : Starting anywhere between 10−22 and 10−15 seconds after the Big Bang, until 10−12 seconds after the Big Bang Sometime after inflation, the created particles went through thermalization, where mutual interactions lead to thermal equilibrium. Before the electroweak symmetry breaking, at a temperature of around , approximately 10−15 seconds after the Big Bang, the electromagnetic and weak interaction had not yet separated, and the gauge bosons and fermions had not yet gained mass through the Higgs mechanism. This epoch ended with electroweak symmetry breaking, potentially through a phase transition. In some extensions of the Standard Model of particle physics, baryogenesis also happened at this stage, creating an imbalance between matter and antimatter (though in extensions to this model, this may have happened earlier). Little is known about the details of these processes. Far future and ultimate fate There are several competing scenarios for the long-term evolution of the universe. Which of them will happen, if any, depends on the precise values of physical constants such as the cosmological constant, the possibility of proton decay, the energy of the vacuum (meaning, the energy of "empty" space itself), and the natural laws beyond the Standard Model. If the expansion of the universe continues and it stays in its present form, eventually all but the nearest galaxies will be carried away from the Earth by the expansion of space at such a velocity that the observable universe will be limited to the solar system's gravitationally bound local galaxy cluster, the Laniakea Supercluster. In the very long term (after many trillions—thousands of billions—of years, cosmic time), the Stelliferous Era will end, as stars cease to be born and even the longest-lived stars gradually die. Beyond this, all objects in the universe will cool and (with the possible exception of protons) gradually decompose back to their constituent particles and then into subatomic particles and very low-level photons and other fundamental particles, by a variety of possible processes. The following scenarios have been proposed for the ultimate fate of the universe: In this kind of protracted timescale, extremely rare quantum phenomena may also occur that are unlikely to be seen on a timescale smaller than trillions of years. These may also lead to unpredictable changes to the state of the universe which would not be likely to be significant on any smaller timescale. For example, on a timescale of millions of trillions of years, black holes might appear to evaporate almost instantly, uncommon quantum tunnelling phenomena would appear to be common, and quantum (or other) phenomena so unlikely that they might occur just once in a trillion years may occur many times. == See also ==