Chronology of the universe
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Graphical timeline of the Big Bang
This chronology of the universe describes the history and future of the universe according to Big Bang cosmology, the prevailing scientific model of how the universe came into being and developed over time, using the cosmological time parameter of comoving coordinates. The instant in which the universe is thought to have begun rapidly expanding from a singularity is known as the Big Bang. As of 2013, this expansion is estimated to have begun 13.798 ± 0.037 billion years ago. It is convenient to divide the evolution of the universe so far into three phases.
The very earliest universe was so hot, or energetic, that initially no particles existed or could exist (except perhaps in the most fleeting sense), and the forces we see around us today were believed to be merged into one unified force. Space itself expanded during an inflationary epoch due to the immensity of the energies involved. Gradually the immense energies cooled - still to a temperature inconceivably hot compared to any we see around us now, but sufficiently to allow forces to gradually undergo symmetry breaking, a kind of repeated condensation from one status quo to another, leading finally to the separation of the strong force from the electroweak force and the first particles.
In the second phase, this quark-gluon plasma universe then cooled further, the current fundamental forces we know take their present forms through further symmetry breaking - notably the breaking of electroweak symmetry - and the full range of complex and composite particles we see around us today became possible, leading to a matter dominated universe, the first neutral atoms (almost all of them hydrogen), and the cosmic microwave background radiation we can detect today. Modern high energy particle physics theories are satisfactory at these energy levels, and so physicists believe they have a good understanding of this and subsequent development of the fundamental universe around us. Because of these changes, space had also become largely transparent to light and other electromagnetic energy, rather than "foggy", by the end of this phase.
The third phase started with a universe whose fundamental particles and forces were as we know them, and witnessed the emergence of large scale stable structures, such as the earliest stars, quasars, galaxies, clusters of galaxies and superclusters, and the development of these to create the kind of universe we see today. Some researchers call the development of all this physical structure over billions of years "cosmic evolution." Other, more interdisciplinary, researchers refer to "cosmic evolution" as the entire scenario of growing complexity from big bang to humankind, thereby incorporating biology and culture into a grand unified view of all complex systems in the universe to date.
Beyond the present day, scientists anticipate that the Earth will cease to support life in about a billion years, and will be drawn into the Sun in about 5 billion years. On a far longer timescale, the Stelliferous Era will end as stars eventually die and fewer are born to replace them, leading to a darkening universe. Various theories suggest a number of subsequent possibilities. If particles such as protons are unstable then eventually matter may evaporate into low level energy in a kind of entropy related heat death. Alternatively the universe may collapse in a big crunch, although current data shows the rate of expansion is still increasing. If this is correct then it may end in a "big freeze" as matter and energy become very thinly spread and cool down. Alternative suggestions include a false vacuum catastrophe or a Big Rip as possible ends to the universe.
Very early universe 
All ideas concerning the very early universe (cosmogony) are speculative. No accelerator experiments have yet probed energies of sufficient magnitude to provide any experimental insight into the behavior of matter at the energy levels that prevailed during this period. Proposed scenarios differ radically. Some examples are the Hartle–Hawking initial state, string landscape, brane inflation, string gas cosmology, and the ekpyrotic universe. Some of these are mutually compatible, while others are not.
Planck epoch 
- Up to 10–43 seconds after the Big Bang
The Planck epoch is an era in traditional (non-inflationary) big bang cosmology in which the temperature is high enough that the four fundamental forces—electromagnetism, gravitation, weak nuclear interaction, and strong nuclear interaction—are all unified in one fundamental force. Little is understood about physics at this temperature, and different theories propose different scenarios. Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory is based on general relativity and is expected to break down due to quantum effects. Physicists hope that proposed theories of quantum gravitation, such as string theory, loop quantum gravity, and causal sets, will eventually lead to a better understanding of this epoch. In inflationary cosmology, times prior to the end of inflation (roughly 10−32 seconds after the Big Bang) do not follow the traditional big bang timeline. The universe before the end of inflation is a near-vacuum with a very low temperature, and persists for much longer than 10−32 second. Times from the end of inflation are based on the big bang time of the non-inflationary big bang model, not on the actual age of the universe at that time, which cannot be determined in inflationary cosmology. Thus, in inflationary cosmology there is no Planck epoch in the traditional sense, though similar conditions may have prevailed in a pre-inflationary era of the universe.
Grand unification epoch 
- Between 10–43 seconds and 10–36 seconds after the Big Bang
As the universe expands and cools, it crosses transition temperatures at which forces separate from each other. These are phase transitions much like condensation and freezing. The grand unification epoch begins when gravitation separates from the other forces of nature, which are collectively known as gauge forces. The non-gravitational physics in this epoch would be described by a so-called grand unified theory (GUT). The grand unification epoch ends when the GUT forces further separate into the strong and electroweak forces. This transition should produce magnetic monopoles in large quantities, which are not observed. The lack of magnetic monopoles was one problem solved by the introduction of inflation.
In modern inflationary cosmology, the traditional grand unification epoch, like the Planck epoch, does not exist, though similar conditions likely would have existed in the universe prior to inflation.[further explanation needed]
Electroweak epoch 
- Between 10–36 seconds (or the end of inflation) and 10–12 seconds after the Big Bang
In traditional big bang cosmology, the Electroweak epoch begins 10−36 seconds after the Big Bang, when the temperature of the universe is low enough (1028 K) to separate the strong force from the electroweak force (the name for the unified forces of electromagnetism and the weak interaction). In inflationary cosmology, the electroweak epoch begins when the inflationary epoch ends, at roughly 10−32 seconds.
Inflationary epoch 
- Unknown duration, ending 10–32(?) seconds after the Big Bang
Cosmic inflation is an era of accelerating expansion produced by a hypothesized field called the inflaton, which would have properties similar to the Higgs field and dark energy. While decelerating expansion magnifies deviations from homogeneity, making the universe more chaotic, accelerating expansion makes the universe more homogeneous. A sufficiently long period of inflationary expansion in our past could explain the high degree of homogeneity that is observed in the universe today at large scales, even if the state of the universe before inflation was highly disordered.
Inflation ends when the inflaton field decays into ordinary particles in a process called "reheating", at which point ordinary Big Bang expansion begins. The time of reheating is usually quoted as a time "after the Big Bang". This refers to the time that would have passed in traditional (non-inflationary) cosmology between the Big Bang singularity and the universe dropping to the same temperature that was produced by reheating, even though, in inflationary cosmology, the traditional Big Bang did not occur.
According to the simplest inflationary models, inflation ended at a temperature corresponding to roughly 10−32 seconds after the Big Bang. As explained above, this does not imply that the inflationary era lasted less than 10−32 seconds. In fact, in order to explain the observed homogeneity of the universe, the duration must be longer than 10−32 seconds. In inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation.
There is currently insufficient observational evidence to explain why the universe contains far more baryons than antibaryons. A candidate explanation for this phenomenon must allow the Sakharov conditions to be satisfied at some time after the end of cosmological inflation. While particle physics suggests asymmetries under which these conditions are met, these asymmetries are too small empirically to account for the observed baryon-antibaryon asymmetry of the universe.
Early universe 
After cosmic inflation ends, the universe is filled with a quark–gluon plasma. From this point onwards the physics of the early universe is better understood, and less speculative.
Supersymmetry breaking (speculative) 
If supersymmetry is a property of our universe, then it must be broken at an energy that is no lower than 1 TeV, the electroweak symmetry scale. The masses of particles and their superpartners would then no longer be equal, which could explain why no superpartners of known particles have ever been observed.
Electroweak symmetry breaking and the quark epoch 
- Between 10–12 seconds and 10–6 seconds after the Big Bang
As the universe's temperature falls below a certain very high energy level, it is believed that the Higgs field spontaneously acquires a vacuum expectation value, which breaks electroweak gauge symmetry. (If the Higgs field does not exist then a similar effect must occur, but with some other cause). This has two related effects:
- The weak force and electromagnetic force, and their respective bosons (the W and Z bosons and photon) manifest differently in the present universe, with different ranges;
- Via the Higgs mechanism, all elementary particles interacting with the Higgs field become massive, having been massless at higher energy levels.
At the end of this epoch, the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction have now taken their present forms, and fundamental particles have mass, but the temperature of the universe is still too high to allow quarks to bind together to form hadrons.
Hadron epoch 
- Between 10–6 seconds 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. At approximately 1 second after the Big Bang neutrinos decouple and begin traveling freely through space. This cosmic neutrino background, while unlikely to ever be observed in detail since the neutrino energies are very low, is analogous to the cosmic microwave background that was emitted much later. (See above regarding the quark-gluon plasma, under the String Theory epoch). However there is strong indirect evidence that the cosmic neutrino background exists, both from Big Bang nucleosynthesis predictions of the Helium abundance, and from anisotropies in the cosmic microwave background
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 and anti-leptons dominating the mass of the universe. Approximately 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton/anti-lepton pairs are no longer created and most leptons and anti-leptons are eliminated in annihilation reactions, leaving a small residue of leptons.
Photon epoch 
- Between 10 seconds and 380,000 years after the Big Bang
After most leptons and anti-leptons are annihilated at the end of the lepton epoch the energy of the universe is dominated by photons. These photons are still interacting frequently with charged protons, electrons and (eventually) nuclei, and continue to do so for the next 380,000 years.
- Between 3 minutes and 20 minutes after the Big Bang
During the photon epoch the temperature of the universe falls to the point where atomic nuclei can begin to form. Protons (hydrogen ions) and neutrons begin to combine into atomic nuclei in the process of nuclear fusion. Free neutrons combine with protons to form deuterium. Deuterium rapidly fuses into helium-4. Nucleosynthesis only lasts for about seventeen minutes, since the temperature and density of the universe has fallen to the point where nuclear fusion cannot continue. By this time, all neutrons have been incorporated into helium nuclei. This leaves about three times more hydrogen than helium-4 (by mass) and only trace quantities of other nuclei.
Matter domination 
- 70,000 years after the Big Bang
At this time, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) are 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 ΛCDM, at this stage, cold dark matter dominates, paving the way for gravitational collapse to amplify the tiny inhomogeneities left by cosmic inflation, making dense regions denser and rarefied regions more rarefied. However, because present theories as to the nature of dark matter are inconclusive, there is as yet no consensus as to its origin at earlier times, as currently exist for baryonic matter.
- ca. 377,000 years after the Big Bang
Hydrogen and helium atoms begin to form as the density of the universe falls. This is thought to have occurred about 377,000 years after the Big Bang. Hydrogen and helium are at the beginning ionized, i.e., no electrons are bound to the nuclei, which (containing positively charged protons) are therefore electrically charged (+1 and +2 respectively). As the universe cools down, the electrons get captured by the ions, forming electrically neutral atoms. This process is relatively fast (actually faster for the helium than for the hydrogen) and is known as recombination. At the end of recombination, most of the protons in the universe are bound up in neutral atoms. Therefore, the photons' mean free path becomes effectively infinite and the photons can now travel freely (see Thomson scattering): the universe has become transparent. This cosmic event is usually referred to as decoupling.
The photons present at the time of decoupling are the same photons that we see in the cosmic microwave background (CMB) radiation, after being greatly cooled by the expansion of the Universe. Around the same time, 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 diagram), 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.
Dark Ages 
Before decoupling occurs, most of the photons in the universe are interacting with electrons and protons in the photon–baryon fluid. The universe is opaque or "foggy" as a result. There is light but not light we could observe through telescopes. The baryonic matter in the universe consisted of ionized plasma, and it only became neutral when it gained free electrons during "recombination," thereby releasing the photons creating the CMB. When the photons were released (or decoupled) the universe became transparent. At this point the only radiation emitted is the 21 cm spin line of neutral hydrogen. There is currently an observational effort underway to detect this faint radiation, as it is in principle an even more powerful tool than the cosmic microwave background for studying the early universe. The Dark Ages are currently thought to have lasted between 150 million to 800 million years after the Big Bang. The October 2010 discovery of UDFy-38135539, the first observed galaxy to have existed during the following reionization epoch, gives us a window into these times. The galaxy earliest in this period observed and thus also the most distant galaxy ever observed is currently on the record of Leiden University's Richard J. Bouwens and Garth D. Illingsworth from UC Observatories/Lick Observatory. They found the galaxy UDFj-39546284 to be at a time some 480 million years after the Big Bang or about halfway through the Cosmic Dark Ages at a distance of about 13.2 billion light-years. More recently, the UDFj-39546284 galaxy was found to be around "380 million years" after the Big Bang and at a distance of 13.37 billion light-years.
Structure formation 
Structure formation in the big bang model proceeds hierarchically, with smaller structures forming before larger ones. The first structures to form are quasars, which are thought to be bright, early active galaxies, and population III stars. Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulations with billions of particles.
- 150 million to 1 billion years after the Big Bang
The first stars and quasars form from gravitational collapse. The intense radiation they emit reionizes the surrounding universe. From this point on, most of the universe is composed of plasma.
Formation of stars 
The first stars, most likely Population III stars, form and start the process of turning the light elements that were formed in the Big Bang (hydrogen, helium and lithium) into heavier elements. However, as yet there have been no observed Population III stars, and understanding of them is currently based on computational models of their formation and evolution.
Formation of galaxies 
Johannes Schedler's project has identified a quasar CFHQS 1641+3755 at 12.7 billion light-years away, when the Universe was just 7% of its present age.
On July 11, 2007, using the 10-metre Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasadena and his team found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old. Only about 10 of these extremely early objects are currently known.
Formation of groups, clusters and superclusters 
Gravitational attraction pulls galaxies towards each other to form groups, clusters and superclusters.
Formation of the Solar System 
- 9 billion years after the Big Bang
The Solar System began forming about 4.6 billion years ago, or about 9 billion years after the Big Bang. A molecular cloud made mostly of hydrogen and traces of other elements began to collapse, forming a large sphere in the center which would become the Sun, as well as a surrounding disk. The surrounding accretion disk would coalesce into a multitude of smaller objects that would become planets, asteroids, and comets. The Sun is a late-generation star, and the Solar System incorporates matter created by previous generations of stars.
- 13.8 billion years after the Big Bang
The Big Bang is estimated to have occurred about 13.8 billion years ago. Since the expansion of the universe appears to be accelerating, the cosmic web is likely to be the largest structure that will ever form in the universe. The present accelerated expansion prevents any more inflationary structures entering the horizon and prevents new gravitationally bound structures from forming.
Ultimate fate of the universe 
As with interpretations of what happened in the very early universe, advances in fundamental physics are required before it will be possible to know the ultimate fate of the universe with any certainty. Below are some of the main possibilities.
Fate of the Solar system: 1 to 5 billion years 
Over a timescale of a billion years or more, the Earth and Solar System are unstable. Earth's existing biosphere is expected to vanish in about a billion years, as the Sun's heat production gradually increases to the point that liquid water and life are unlikely; the Earth's magnetic fields, axial tilt and atmosphere are subject to long term change; and the Solar System itself is chaotic over million- and billion-year timescales; Eventually in around 5.4 billion years from now, the core of the Sun will become hot enough to trigger hydrogen fusion in its surrounding shell. This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a red giant. Within 7.5 billion years, the Sun will have expanded to a radius of 1.2 AU—256 times its current size, and studies announced in 2008 show that due to tidal interaction between Sun and Earth, Earth would actually fall back into a lower orbit, and get engulfed and incorporated inside the Sun before the Sun reaches its largest size, despite the Sun losing about 38% of its mass. The Sun itself will continue to exist for many billions of years, passing through a number of phases, and eventually (if nothing else changes) ending up as a long-lived white dwarf. Eventually, after billions more years, the Sun will finally cease to shine altogether, becoming a black dwarf.
Big freeze: 1014 years and beyond 
This scenario is generally considered to be the most likely, as it occurs if the universe continues expanding as it has been. Over a time scale on the order of 1014 years or less, existing stars burn out, stars cease to be created, and the universe goes dark., §IID. Over a much longer time scale in the eras following this, the galaxy evaporates as the stellar remnants comprising it escape into space, and black holes evaporate via Hawking radiation., §III, §IVG. In some grand unified theories, proton decay after at least 1034 years will convert the remaining interstellar gas and stellar remnants into leptons (such as positrons and electrons) and photons. Some positrons and electrons will then recombine into photons., §IV, §VF. In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known however whether it eventually achieves thermodynamic equilibrium., §VIB, VID.
Big Crunch: 100+ billion years from now 
If the energy density of dark energy were negative or the universe were closed, then it would be possible that the expansion of the universe would reverse and the universe would contract towards a hot, dense state. This is a required element of oscillatory universe scenarios, such as the cyclic model, although a Big Crunch does not necessarily imply an oscillatory Universe. Current observations suggest that this model of the universe is unlikely to be correct, and the expansion will continue or even accelerate.
Big Rip: 20+ billion years from now 
This scenario is possible only if the energy density of dark energy actually increases without limit over time. Such dark energy is called phantom energy and is unlike any known kind of energy. In this case, the expansion rate of the universe will increase without limit. Gravitationally bound systems, such as clusters of galaxies, galaxies, and ultimately the Solar System will be torn apart. Eventually the expansion will be so rapid as to overcome the electromagnetic forces holding molecules and atoms together. Finally even atomic nuclei will be torn apart and the universe as we know it will end in an unusual kind of gravitational singularity. At the time of this singularity, the expansion rate of the universe will reach infinity, so that any and all forces (no matter how strong) that hold composite objects together (no matter how closely) will be overcome by this expansion, literally tearing everything apart.
Vacuum metastability event 
If our universe is in a very long-lived false vacuum, it is possible that a small region of the universe will tunnel into a lower energy state. If this happens, all structures within will be destroyed instantaneously and the region will expand at near light speed, bringing destruction without any forewarning.
Heat death: 10150+ years from now 
The heat death is a possible final state of the universe, estimated at after 10150 years, in which it has "run down" to a state of no thermodynamic free energy to sustain motion or life. In physical terms, it has reached maximum entropy (because of this, the term "entropy" has often been confused with Heat Death, to the point of entropy being labelled as the "force killing the universe"). The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson (Lord Kelvin) who extrapolated the theory of heat views of mechanical energy loss in nature, as embodied in the first two laws of thermodynamics, to universal operation.
See also 
- Radiation-dominated era, matter-dominated era and Dark-energy-dominated era, another way that the universe's chronology is categorized.
- Graphical timeline of the Stelliferous Era
- Graphical timeline from Big Bang to Heat Death
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