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76-450: THESEUS is a mission concept that would monitor transient events in the high-energy Universe across the whole sky and over the entirety of cosmic history . In particular, it expects to make a complete census of gamma-ray bursts (GRBs) from the Universe's first billion years, to help understand the life cycle of the first stars. THESEUS would provide real-time triggers and accurate locations of

152-401: A {\displaystyle a} is the scale parameter . The Hubble parameter, however, is proportional to a − 2 {\displaystyle a^{-2}} . Going back in time and higher in energy, and assuming no new physics at these energies, a careful estimate gives that thermalization was first possible when the temperature was: approximately 10 seconds after

228-498: A valence quark and a valence antiquark . Because mesons have integer spin (0 or 1) and are not themselves elementary particles, they are classified as "composite" bosons , although being made of elementary fermions . Examples of mesons include the pion , kaon , and the J/ψ . In quantum hadrodynamics , mesons mediate the residual strong force between nucleons. At one time or another, positive signatures have been reported for all of

304-568: A net positive charge, and anions which have a net negative charge. Quasiparticles are effective particles that exist in many particle systems. The field equations of condensed matter physics are remarkably similar to those of high energy particle physics. As a result, much of the theory of particle physics applies to condensed matter physics as well; in particular, there are a selection of field excitations, called quasi-particles , that can be created and explored. These include: The following categories are not unique or distinct: For example, either

380-468: A new selection process (2022) and a Phase-0 study (2023), THESEUS was selected by ESA for a new 2.5 year Phase-A study as one of the three candidates M7 missions (together with M-Matisse and Plasma Observatory). The space observatory would study GRBs and X-rays and their association with the explosive death of massive stars, supernova shock break-outs, black hole tidal disruption events , and magnetar flares. This can provide fundamental information on

456-421: A phase transition of this kind, when gravitation separated from the universal combined gauge force . This caused two forces to now exist: gravity , and an electrostrong interaction . There is no hard evidence yet that such a combined force existed, but many physicists believe it did. The physics of this electrostrong interaction would be described by a Grand Unified Theory. The grand unification epoch ended with

532-450: A phase transition. For example, in a later epoch, a side effect of one phase transition is that suddenly, many particles that had no mass at all acquire a mass (they begin to interact differently with the Higgs field ), and a single force begins to manifest as two separate forces. Assuming that nature is described by a so-called Grand Unified Theory (GUT), the grand unification epoch began with

608-432: A phenomenon of quantum fields called " symmetry breaking ". In everyday terms, as the universe cools, it becomes possible for the quantum fields that create the forces and particles around us, to settle at lower energy levels and with higher levels of stability. In doing so, they completely shift how they interact. Forces and interactions arise due to these fields, so the universe can behave very differently above and below

684-401: A redshift of z=13.2, from 13.4 billion years ago. The JWST was designed to observe as far as z≈20 (180 million years cosmic time). To derive the age of the universe from redshift, numeric integration or its closed-form solution involving the special Gaussian hypergeometric function 2 F 1 may be used: Lookback time is the age of the observation subtracted from the present age of

760-517: A result the universe also became transparent for the first time. The newly formed atoms—mainly hydrogen and helium with traces of lithium —quickly reach their lowest energy state ( ground state ) by releasing photons (" photon decoupling "), and these photons can still be detected today as the cosmic microwave background (CMB). This is the oldest direct observation we currently have of the universe. This period measures from 370,000 years until about 1 billion years. After recombination and decoupling ,

836-432: A second phase transition, as the electrostrong interaction in turn separated, and began to manifest as two separate interactions, called the strong and the electroweak interactions. Depending on how epochs are defined, and the model being followed, the electroweak epoch may be considered to start before or after the inflationary epoch. In some models, it is described as including the inflationary epoch. In other models,

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912-402: A small excess of matter in the universe. At about one second, neutrinos decouple ; these neutrinos form the cosmic neutrino background (CνB). If primordial black holes exist, they are also formed at about one second of cosmic time. Composite subatomic particles emerge—including protons and neutrons —and from about 2 minutes, conditions are suitable for nucleosynthesis : around 25% of

988-402: A specific chemical element . To date, 118 elements have been discovered or created. Exotic atoms may be composed of particles in addition to or in place of protons, neutrons, and electrons, such as hyperons or muons. Examples include pionium ( π   π ) and quarkonium atoms. Leptonic atoms, named using - onium , are exotic atoms constituted by

1064-444: A superposition of mass eigenstates . The hypothetical heavy right-handed neutrino, called a " sterile neutrino ", has been omitted. Bosons are one of the two fundamental particles having integral spinclasses of particles, the other being fermions . Bosons are characterized by Bose–Einstein statistics and all have integer spins. Bosons may be either elementary, like photons and gluons , or composite, like mesons . According to

1140-563: A superposition of them together with the Higgsinos. Other theories predict the existence of additional elementary bosons and fermions, with some theories also postulating additional superpartners for these particles: Composite particles are bound states of elementary particles. Hadrons are defined as strongly interacting composite particles . Hadrons are either: Quark models , first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks "aces"), describe

1216-486: Is a hypothetical particle that has been included in some extensions to the standard model to mediate the gravitational force. It is in a peculiar category between known and hypothetical particles: As an unobserved particle that is not predicted by, nor required for the Standard Model , it belongs in the table of hypothetical particles, below. But gravitational force itself is a certainty, and expressing that known force in

1292-420: Is defined by the specific number of each type of nucleon. Atoms are the smallest neutral particles into which matter can be divided by chemical reactions . An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. An atomic nucleus consists of 1 or more protons and 0 or more neutrons. Protons and neutrons are, in turn, made of quarks. Each type of atom corresponds to

1368-408: Is known about the details of these processes. The number density of each particle species was, by a similar analysis to Stefan–Boltzmann law : which is roughly just ( k B T / ℏ c ) 3 {\displaystyle (k_{B}T/\hbar c)^{3}} . Since the interaction was strong, the cross-section σ {\displaystyle \sigma }

1444-880: Is known as the cosmic Dark Ages . At some point around 200 to 500 million years, the earliest generations of stars and galaxies form (exact timings are still being researched), and early large structures gradually emerge, drawn to the foam-like dark matter filaments which have already begun to draw together throughout the universe. The earliest generations of stars have not yet been observed astronomically. They may have been very massive (100–300 solar masses ) and non-metallic , with very short lifetimes compared to most stars we see today , so they commonly finish burning their hydrogen fuel and explode as highly energetic pair-instability supernovae after mere millions of years. Other theories suggest that they may have included small stars, some perhaps still burning today. In either case, these early generations of supernovae created most of

1520-505: Is much better understood, and the energies involved in the Quark epoch are directly accessible in particle physics experiments and other detectors. Sometime after inflation, the created particles went through thermalization , where mutual interactions lead to thermal equilibrium . The earliest stage that we are confident about is some time before the electroweak symmetry breaking , at a temperature of around 10 K, approximately 10 seconds after

1596-401: Is no lower than 1 TeV , the electroweak scale. The masses of particles and their superpartners would then no longer be equal. This very high energy could explain why no superpartners of known particles have ever been observed. After cosmic inflation ends, the universe is filled with a hot quark–gluon plasma , the remains of reheating. From this point onwards the physics of the early universe

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1672-459: Is not known whether the neutrino is a Dirac fermion or a Majorana fermion . Fermions are the basic building blocks of all matter . They are classified according to whether they interact via the strong interaction or not. In the Standard Model, there are 12 types of elementary fermions: six quarks and six leptons . Quarks are the fundamental constituents of hadrons and interact via

1748-459: Is often considered to mark the start of the electroweak epoch, and some theories, such as warm inflation , avoid a reheating phase entirely. In non-traditional versions of Big Bang theory (known as "inflationary" models), inflation ended at a temperature corresponding to roughly 10 seconds after the Big Bang, but this does not imply that the inflationary era lasted less than 10 seconds. To explain

1824-514: Is one of the possibilities.) This provides a model of the universe which matches all current physical observations extremely closely. This initial period of the universe's chronology is called the " Big Bang ". The Standard Model of cosmology attempts to explain how the universe physically developed once that moment happened. The singularity from the FLRW metric is interpreted to mean that current theories are inadequate to describe what actually happened at

1900-421: Is said that the Big Bang "happened everywhere". During the earliest moments of cosmic time, the energies and conditions were so extreme that current knowledge can only suggest possibilities, which may turn out to be incorrect. To give one example, eternal inflation theories propose that inflation lasts forever throughout most of the universe, making the notion of "N seconds since Big Bang" ill-defined. Therefore,

1976-484: The Pauli exclusion principle . They include the quarks and leptons , as well as any composite particles consisting of an odd number of these, such as all baryons and many atoms and nuclei. Fermions have half-integer spin; for all known elementary fermions this is ⁠ 1 / 2 ⁠ . All known fermions except neutrinos , are also Dirac fermions ; that is, each known fermion has its own distinct antiparticle . It

2052-454: The Planck epoch , during which currently established laws of physics may not have applied; the emergence in stages of the four known fundamental interactions or forces —first gravitation , and later the electromagnetic , weak and strong interactions; and the accelerated expansion of the universe due to cosmic inflation . Tiny ripples in the universe at this stage are believed to be

2128-547: The Standard Model , the elementary bosons are: The Higgs boson is postulated by the electroweak theory primarily to explain the origin of particle masses . In a process known as the " Higgs mechanism ", the Higgs boson and the other gauge bosons in the Standard Model acquire mass via spontaneous symmetry breaking of the SU(2) gauge symmetry. The Minimal Supersymmetric Standard Model (MSSM) predicts several Higgs bosons. On 4 July 2012,

2204-457: The electromagnetic and weak interactions.) The exact point where electrostrong symmetry was broken is not certain, owing to speculative and as yet incomplete theoretical knowledge. At this point of the very early universe, the universe is thought to have expanded by a factor of at least 10 in volume. This is equivalent to a linear increase of at least 10 times in every spatial dimension—equivalent to an object 1 nanometre (10 m , about half

2280-413: The fields which define the universe's fundamental forces and particles also completely change their behaviors and structures when the temperature/energy falls below a certain point. This is not apparent in everyday life, because it only happens at far higher temperatures than usually seen in the present-day universe. These phase transitions in the universe's fundamental forces are believed to be caused by

2356-419: 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. Inflation explains several observed properties of

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2432-442: The strong force . Quarks are the only known carriers of fractional charge , but because they combine in groups of three quarks (baryons) or in pairs of one quark and one antiquark (mesons), only integer charge is observed in nature. Their respective antiparticles are the antiquarks , which are identical except that they carry the opposite electric charge (for example the up quark carries charge + ⁠ 2 / 3 ⁠ , while

2508-517: The Big Bang. List of particles#Composite particles This is a list of known and hypothesized particles. Elementary particles are particles with no measurable internal structure; that is, it is unknown whether they are composed of other particles. They are the fundamental objects of quantum field theory . Many families and sub-families of elementary particles exist. Elementary particles are classified according to their spin . Fermions have half-integer spin while bosons have integer spin. All

2584-597: The Big Bang. The electromagnetic and weak interaction have not yet separated , and the gauge bosons and fermions have not yet gained mass through the Higgs mechanism . However exotic massive particle-like entities, sphalerons , are thought to have existed. 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 anti-matter (though in extensions to this model, this may have happened earlier). Little

2660-404: The ability of gravity to decelerate the expansion of the universe; in contrast, dark energy (believed to be a constant scalar field throughout the visible universe) 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

2736-562: The basis of large-scale structures that formed much later. Different stages of the very early universe are understood to different extents. The earlier parts are beyond the grasp of practical experiments in particle physics but can be explored through the extrapolation of known physical laws to extremely high temperatures. This period lasted around 370,000 years. Initially, various kinds of subatomic particles are formed in stages. These particles include almost equal amounts of matter and antimatter , so most of it quickly annihilates, leaving

2812-414: The bound state of a lepton and an antilepton. Examples of such atoms include positronium ( e   e ), muonium ( e   μ ), and " true muonium " ( μ   μ ). Of these positronium and muonium have been experimentally observed, while "true muonium" remains only theoretical. Molecules are

2888-432: The cosmic star formation rate, the number density and properties of low-mass galaxies, the neutral hydrogen fraction, and the escape fraction of ultraviolet photons from galaxies. The conceptual payload of THESEUS includes: Chronology of the universe The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. Research published in 2015 estimates

2964-471: The current universe that are otherwise difficult to account for, including explaining how today's universe has ended up so exceedingly homogeneous (spatially uniform) on a very large scale, even though it was highly disordered in its earliest stages. It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10 and 10 seconds after the Big Bang. The rapid expansion of space meant that elementary particles remaining from

3040-400: The discovery of a new particle with a mass between 125 and 127 GeV/ c was announced; physicists suspected that it was the Higgs boson. Since then, the particle has been shown to behave, interact, and decay in many of the ways predicted for Higgs particles by the Standard Model, as well as having even parity and zero spin, two fundamental attributes of a Higgs boson. This also means it is

3116-417: The earliest stages are an active area of research and based on ideas that are still speculative and subject to modification as scientific knowledge improves. Although a specific "inflationary epoch" is highlighted at around 10 seconds, observations and theories both suggest that distances between objects in space have been increasing at all times since the moment of the Big Bang, and are still increasing (with

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3192-469: The earliest stages of the universe's existence as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level. For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated , into five parts. It is generally considered meaningless or unclear whether time existed before this chronology: The first picosecond  (10 seconds) of cosmic time includes

3268-461: The electroweak epoch is said to begin after the inflationary epoch ended, at roughly 10 seconds. According to traditional Big Bang cosmology, the electroweak epoch began 10 seconds after the Big Bang, when the temperature of the universe was low enough (10 K) for the electronuclear force to begin to manifest as two separate interactions, the strong and the electroweak interactions. (The electroweak interaction will also separate later, dividing into

3344-577: The end of inflation (roughly 10 seconds after the Big Bang) do not follow the same timeline as in traditional Big Bang cosmology. Models that aim to describe the universe and physics during the Planck epoch are generally speculative and fall under the umbrella of " New Physics ". Examples include the Hartle–Hawking initial state , string theory landscape , string gas cosmology , and the ekpyrotic universe . As

3420-482: The every day elements we see around us today, and seeded the universe with them. Galaxy clusters and superclusters emerge over time. At some point, high-energy photons from the earliest stars, dwarf galaxies and perhaps quasars lead to a period of reionization that commences gradually between about 250–500 million years and finishes by about 1 billion years (exact timings still being researched). The Dark Ages only fully came to an end at about 1 billion years as

3496-407: The exception of gravitationally bound objects such as galaxies and most clusters , once the rate of expansion had greatly slowed). The inflationary period marks a specific period when a very rapid change in scale occurred, but does not mean that it stayed the same at other times. More precisely, during inflation, the expansion accelerated. After inflation, and for about 9.8 billion years, the expansion

3572-430: The existence of more particles, none of which have been confirmed experimentally. Just as the photon, Z boson and W  bosons are superpositions of the B , W , W , and W fields, the photino , zino, and wino are superpositions of the bino , wino , wino , and wino . No matter if one uses the original gauginos or this superpositions as a basis, the only predicted physical particles are neutralinos and charginos as

3648-664: The far future and ultimate fate of the universe . More exact knowledge of the present-day universe may allow these to be better understood. The Standard Model of cosmology is based on a model of spacetime called the Friedmann–Lemaître–Robertson–Walker (FLRW) metric . A metric provides a measure of distance between objects, and the FLRW metric is the exact solution of Einstein field equations (EFE) if some key properties of space such as homogeneity and isotropy are assumed to be true. The FLRW metric very closely matches overwhelming other evidence, showing that

3724-402: The first elementary scalar particle discovered in nature. Elementary bosons responsible for the four fundamental forces of nature are called force particles ( gauge bosons ). The strong interaction is mediated by the gluon , the weak interaction is mediated by the W and Z bosons, electromagnetism by the photon, and gravity by the graviton, which is still hypothetical. The graviton

3800-401: The following exotic mesons but their existences have yet to be confirmed. Atomic nuclei typically consist of protons and neutrons, although exotic nuclei may consist of other baryons, such as hypertriton which contains a hyperon . These baryons (protons, neutrons, hyperons, etc.) which comprise the nucleus are called nucleons. Each type of nucleus is called a " nuclide ", and each nuclide

3876-416: The framework of a quantum field theory requires a boson to mediate it. If it exists, the graviton is expected to be massless because the gravitational force has a very long range, and appears to propagate at the speed of light. The graviton must be a spin -2 boson because the source of gravitation is the stress–energy tensor , a second-order tensor (compared with electromagnetism 's spin-1 photon ,

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3952-410: The grand unification epoch was now distributed very thinly across the universe. However, the huge 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 quarks, anti-quarks and gluons . In other models, reheating

4028-450: The known hadrons as composed of valence quarks and/or antiquarks, tightly bound by the color force , which is mediated by gluons . (The interaction between quarks and gluons is described by the theory of quantum chromodynamics .) A "sea" of virtual quark-antiquark pairs is also present in each hadron. Ordinary baryons (composite fermions ) contain three valence quarks or three valence antiquarks each. Ordinary mesons are made up of

4104-455: The observed homogeneity of the universe, the duration in these models must be longer than 10 seconds. Therefore, in inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation. After inflation ended, the universe continued to expand, but at a decelerating rate. About 4 billion years ago the expansion gradually began to speed up again. This is believed to be due to dark energy becoming dominant in

4180-441: The opposite electric charge and lepton number. The antiparticle of an electron is an antielectron, which is almost always called a " positron " for historical reasons. There are six leptons in total; the three charged leptons are called "electron-like leptons", while the neutral leptons are called " neutrinos ". Neutrinos are known to oscillate , so that neutrinos of definite flavor do not have definite mass: Instead, they exist in

4256-426: The particles of the Standard Model have been experimentally observed, including the Higgs boson in 2012. Many other hypothetical elementary particles, such as the graviton , have been proposed, but not observed experimentally. Fermions are one of the two fundamental classes of particles, the other being bosons . Fermion particles are described by Fermi–Dirac statistics and have quantum numbers described by

4332-481: The protons and all the neutrons fuse into heavier elements , initially deuterium which itself quickly fuses into mainly helium-4 . By 20 minutes, the universe is no longer hot enough for nuclear fusion , but far too hot for neutral atoms to exist or photons to travel far. It is therefore an opaque plasma . The recombination epoch begins at around 18,000 years, as electrons are combining with helium nuclei to form He . At around 47,000 years, as

4408-462: The smallest particles into which a substance can be divided while maintaining the chemical properties of the substance. Each type of molecule corresponds to a specific chemical substance . A molecule is a composite of two or more atoms. Atoms are combined in a fixed proportion to form a molecule. Molecule is one of the most basic units of matter. Ions are charged atoms ( monatomic ions ) or molecules ( polyatomic ions ). They include cations which have

4484-448: The source of which is the four-current , a first-order tensor). Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way that gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton. Supersymmetric theories predict

4560-662: The sources, which could also be followed up by other space- or ground-based telescopes operating at complementary wavelengths. The concept was selected in May 2018 as a finalist to become the fifth Medium-class mission (M5) of the Cosmic Vision programme by the European Space Agency (ESA). The other finalist was EnVision , a Venus orbiter. The winner, EnVision , was selected in June 2021 for launch in 2031. In November 2023, following

4636-420: The start of the Big Bang itself. It is widely believed that a correct theory of quantum gravity may allow a more correct description of that event, but no such theory has yet been developed. After that moment, all distances throughout the universe began to increase from (perhaps) zero because the FLRW metric itself changed over time, affecting distances between all non-bound objects everywhere. For this reason, it

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4712-407: The universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule . Much later, hydrogen and helium hydride react to form molecular hydrogen (H 2 ) the fuel needed for the first stars . At about 370,000 years, neutral hydrogen atoms finish forming ("recombination"), and as

4788-413: The universe expanded and cooled, it crossed transition temperatures at which forces separated from each other. These cosmological phase transitions can be visualized as similar to condensation and freezing phase transitions of ordinary matter. At certain temperatures/energies, water molecules change their behavior and structure, and they will behave completely differently. Like steam turning to water,

4864-505: The universe gradually transitioned into the universe we see around us today, but denser, hotter, more intense in star formation, and richer in smaller (particularly unbarred) spiral and irregular galaxies, as opposed to giant elliptical galaxies. While early stars have not been observed, galaxies have been observed from 329 million years since the Big Bang, with JADES-GS-z13-0 which the James Webb Space Telescope observed with

4940-409: The universe has expanded since the Big Bang. If the FLRW metric equations are assumed to be valid all the way back to the beginning of the universe, they can be followed back in time, to a point where the equations suggest all distances between objects in the universe were zero or infinitesimally small. (This does not necessarily mean that the universe was physically small at the Big Bang, although that

5016-469: The universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies , so there were few sources of light and the emission from these sources was immediately absorbed by hydrogen atoms. The only photons (electromagnetic radiation, or "light") in the universe were those released during decoupling (visible today as the cosmic microwave background) and 21 cm radio emissions occasionally emitted by hydrogen atoms. This period

5092-548: 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), we are less sure which path the universe will take. At some time, the Stelliferous Era will end as stars are no longer being born, and the expansion of the universe will mean that the observable universe becomes limited to local galaxies. There are various scenarios for

5168-521: The universe's large-scale behavior. It is still expanding (and, accelerating), today. On 17 March 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-modes power spectrum which was interpreted as clear experimental evidence for the theory of inflation. However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings

5244-548: The universe: 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 our galaxy began to form at about 5 billion years (8.8 Gya ), and the Solar System formed at about 9.2 billion years (4.6 Gya), with the earliest evidence of life on Earth emerging by about 10 billion years (3.8 Gya). The thinning of matter over time reduces

5320-515: The universe— gravitation , electromagnetism , the weak nuclear force , and the strong nuclear force —comprised a single fundamental force. Little is understood about physics in this environment. Traditional big bang cosmology predicts a gravitational singularity —a condition in which spacetime breaks down—before this time, but the theory relies on the theory of general relativity , which is thought to break down for this epoch due to quantum effects . In inflationary models of cosmology, times before

5396-426: The up antiquark carries charge − ⁠ 2 / 3 ⁠ ), color charge, and baryon number. There are six flavors of quarks; the three positively charged quarks are called "up-type quarks" while the three negatively charged quarks are called "down-type quarks". Leptons do not interact via the strong interaction . Their respective antiparticles are the antileptons , which are identical, except that they carry

5472-409: The width of a molecule of DNA ) in length, expanding to one approximately 10.6 light-years (100 trillion kilometres) long in a tiny fraction of a second. This phase of the cosmic expansion history is known as inflation . 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

5548-452: Was approximately the particle wavelength squared, which is roughly n − 2 / 3 {\displaystyle n^{-2/3}} . The rate of collisions per particle species can thus be calculated from the mean free path , giving approximately: For comparison, since the cosmological constant was negligible at this stage, the Hubble parameter was: where x ~ 10

5624-453: Was much slower and became slower yet over time (although it never reversed). About 4 billion years ago, it began slightly speeding up again. The Planck epoch is an era in traditional (non-inflationary) Big Bang cosmology immediately after the event that began the known universe. During this epoch, the temperature and average energies within the universe were so high that subatomic particles could not form. The four fundamental forces that shape

5700-501: Was reported and finally, on 2 February 2015, a joint analysis of data from BICEP2/Keck and the European Space Agency 's Planck microwave space telescope concluded that the statistical "significance [of the data] is too low to be interpreted as a detection of primordial B-modes" and can be attributed mainly to polarized dust in the Milky Way. If supersymmetry is a property of the universe, then it must be broken at an energy that

5776-395: Was the number of available particle species. Thus H is orders of magnitude lower than the rate of collisions per particle species. This means there was plenty of time for thermalization at this stage. At this epoch, the collision rate is proportional to the third root of the number density, and thus to a − 1 {\displaystyle a^{-1}} , where

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