In particle physics , a baryon is a type of composite subatomic particle that contains an odd number of valence quarks , conventionally three. Protons and neutrons are examples of baryons; because baryons are composed of quarks , they belong to the hadron family of particles . Baryons are also classified as fermions because they have half-integer spin .
97-475: The Big Bang was, according to the prevailing cosmological theory of the universe's early development, the event that led to the formation of the universe. Big Bang may also refer to: Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature . The notion of an expanding universe was first scientifically originated by physicist Alexander Friedmann in 1922 with
194-441: A Belgian physicist and Roman Catholic priest , proposed that the recession of the nebulae was due to the expansion of the universe. He inferred the relation that Hubble would later observe, given the cosmological principle. In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in
291-624: A Russian cosmologist and mathematician , derived the Friedmann equations from the Einstein field equations, showing that the universe might be expanding in contrast to the static universe model advocated by Albert Einstein at that time. In 1924, American astronomer Edwin Hubble 's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed
388-484: A future horizon , which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the Friedmann–Lemaître–Robertson–Walker (FLRW) metric that describes the expansion of the universe. Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by
485-455: A proton is made of two up quarks and one down quark ; and its corresponding antiparticle, the antiproton , is made of two up antiquarks and one down antiquark. Baryons participate in the residual strong force , which is mediated by particles known as mesons . The most familiar baryons are protons and neutrons , both of which contain three quarks, and for this reason they are sometimes called triquarks . These particles make up most of
582-599: A combination of three u or d quarks. Under the isospin model, they were considered to be a single particle in different charged states. The mathematics of isospin was modeled after that of spin. Isospin projections varied in increments of 1 just like those of spin, and to each projection was associated a " charged state ". Since the " Delta particle " had four "charged states", it was said to be of isospin I = 3 / 2 . Its "charged states" Δ , Δ , Δ , and Δ , corresponded to
679-538: A more generic early hot, dense phase of the universe. In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the Standard Model of particle physics ) work. Based on measurements of
776-422: A particle is therefore the combination of intrinsic angular momentum (spin) and orbital angular momentum. It can take any value from J = | L − S | to J = | L + S | , in increments of 1. Particle physicists are most interested in baryons with no orbital angular momentum ( L = 0), as they correspond to ground states —states of minimal energy. Therefore, the two groups of baryons most studied are
873-500: A process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the Hubble parameter . The larger the ratio, the more time particles had to thermalize before they were too far away from each other. According to the Big Bang models, the universe at the beginning was very hot and very compact, and since then it has been expanding and cooling. In
970-467: A series of distance indicators, the forerunner of the cosmic distance ladder , using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory . This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recessional velocity —now known as Hubble's law. Independently deriving Friedmann's equations in 1927, Georges Lemaître ,
1067-515: A singularity in which space and time lose meaning (typically named "the Big Bang singularity"). Physics lacks a widely accepted theory of quantum gravity that can model the earliest conditions of the Big Bang. In 1964 the CMB was discovered, which convinced many cosmologists that the competing steady-state model of cosmic evolution was falsified , since the Big Bang models predict a uniform background radiation caused by high temperatures and densities in
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#17328580778121164-452: A surrounding space, the Big Bang only describes the intrinsic expansion of the contents of the universe. Another issue pointed out by Santhosh Mathew is that bang implies sound, which is not an important feature of the model. An attempt to find a more suitable alternative was not successful. The Big Bang models developed from observations of the structure of the universe and from theoretical considerations. In 1912, Vesto Slipher measured
1261-430: A temperature of approximately 10 degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a theory of quantum gravity . The Planck epoch was succeeded by the grand unification epoch beginning at 10 seconds, where gravitation separated from the other forces as the universe's temperature fell. At approximately 10 seconds into
1358-514: A vector of length S = 1 / 2 with two spin projections ( S z = + 1 / 2 , and S z = − 1 / 2 ). There is another quantity of angular momentum, called the orbital angular momentum ( azimuthal quantum number L ), that comes in increments of 1 ħ, which represent the angular moment due to quarks orbiting around each other. The total angular momentum ( total angular momentum quantum number J ) of
1455-492: Is 1 ħ), a single quark has a spin vector of length 1 / 2 , and has two spin projections ( S z = + 1 / 2 and S z = − 1 / 2 ). Two quarks can have their spins aligned, in which case the two spin vectors add to make a vector of length S = 1 and three spin projections ( S z = +1, S z = 0, and S z = −1). If two quarks have unaligned spins,
1552-572: Is a vector quantity that represents the "intrinsic" angular momentum of a particle. It comes in increments of 1 / 2 ħ (pronounced "h-bar"). The ħ is often dropped because it is the "fundamental" unit of spin, and it is implied that "spin 1" means "spin 1 ħ". In some systems of natural units , ħ is chosen to be 1, and therefore does not appear anywhere. Quarks are fermionic particles of spin 1 / 2 ( S = 1 / 2 ). Because spin projections vary in increments of 1 (that
1649-492: Is accelerating , an observation attributed to an unexplained phenomenon known as dark energy . The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the light elements , the CMB , large-scale structure , and Hubble's law . The models depend on two major assumptions: the universality of physical laws and the cosmological principle . The universality of physical laws
1746-427: Is assumed that the Big Bang produced a state with equal amounts of baryons and antibaryons. The process by which baryons came to outnumber their antiparticles is called baryogenesis . Experiments are consistent with the number of quarks in the universe being conserved alongside the total baryon number , with antibaryons being counted as negative quantities. Within the prevailing Standard Model of particle physics,
1843-422: Is assumed to be cold. (Warm dark matter is ruled out by early reionization .) This CDM is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. In an "extended model" which includes hot dark matter in the form of neutrinos, then the "physical baryon density" Ω b h 2 {\displaystyle \Omega _{\text{b}}h^{2}}
1940-421: Is baryonic matter , which includes atoms of any sort, and provides them with the property of mass. Non-baryonic matter, as implied by the name, is any sort of matter that is not composed primarily of baryons. This might include neutrinos and free electrons , dark matter , supersymmetric particles , axions , and black holes . The very existence of baryons is also a significant issue in cosmology because it
2037-417: Is called " intrinsic parity " or simply "parity" ( P ). Gravity , the electromagnetic force , and the strong interaction all behave in the same way regardless of whether or not the universe is reflected in a mirror, and thus are said to conserve parity (P-symmetry). However, the weak interaction does distinguish "left" from "right", a phenomenon called parity violation (P-violation). Based on this, if
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#17328580778122134-403: Is estimated at 0.023. (This is different from the 'baryon density' Ω b {\displaystyle \Omega _{\text{b}}} expressed as a fraction of the total matter/energy density, which is about 0.046.) The corresponding cold dark matter density Ω c h 2 {\displaystyle \Omega _{\text{c}}h^{2}} is about 0.11, and
2231-425: Is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder . When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed: v = H 0 D {\displaystyle v=H_{0}D} where Hubble's law implies that
2328-536: Is modeled by a cosmological constant term in Einstein field equations of general relativity, but its composition and mechanism are unknown. More generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theory. All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by
2425-427: Is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10 via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound on the order of 10% inhomogeneity, as of 1995. An important feature of the Big Bang spacetime is the presence of particle horizons . Since
2522-700: Is not generally accepted. The particle physics community as a whole did not view their existence as likely in 2006, and in 2008, considered evidence to be overwhelmingly against the existence of the reported pentaquarks. However, in July 2015, the LHCb experiment observed two resonances consistent with pentaquark states in the Λ b → J/ψK p decay, with a combined statistical significance of 15σ. In theory, heptaquarks (5 quarks, 2 antiquarks), nonaquarks (6 quarks, 3 antiquarks), etc. could also exist. Nearly all matter that may be encountered or experienced in everyday life
2619-410: Is one of the underlying principles of the theory of relativity . The cosmological principle states that on large scales the universe is homogeneous and isotropic —appearing the same in all directions regardless of location. These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that
2716-432: Is the proper distance, v {\displaystyle v} is the recessional velocity, and v {\displaystyle v} , H {\displaystyle H} , and D {\displaystyle D} vary as the universe expands (hence we write H 0 {\displaystyle H_{0}} to denote the present-day Hubble "constant"). For distances much smaller than
2813-472: Is thought to be due to non- conservation of baryon number in the very early universe, though this is not well understood. The concept of isospin was first proposed by Werner Heisenberg in 1932 to explain the similarities between protons and neutrons under the strong interaction . Although they had different electric charges, their masses were so similar that physicists believed they were the same particle. The different electric charges were explained as being
2910-555: The Gell-Mann–Nishijima formula : where S , C , B ′, and T represent the strangeness , charm , bottomness and topness flavour quantum numbers, respectively. They are related to the number of strange, charm, bottom, and top quarks and antiquark according to the relations: meaning that the Gell-Mann–Nishijima formula is equivalent to the expression of charge in terms of quark content: Spin (quantum number S )
3007-523: The Hubble Space Telescope and WMAP. Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating. "[The] big bang picture is too firmly grounded in data from every area to be proved invalid in its general features." — Lawrence Krauss The earliest and most direct observational evidence of
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3104-487: The Milne model , the oscillatory universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard C. Tolman ) and Fritz Zwicky 's tired light hypothesis. After World War II , two distinct possibilities emerged. One was Fred Hoyle's steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time. The other
3201-510: The Particle Data Group . These rules consider the up ( u ), down ( d ) and strange ( s ) quarks to be light and the charm ( c ), bottom ( b ), and top ( t ) quarks to be heavy . The rules cover all the particles that can be made from three of each of the six quarks, even though baryons made of top quarks are not expected to exist because of
3298-591: The S = 1 / 2 ; L = 0 and S = 3 / 2 ; L = 0, which corresponds to J = 1 / 2 and J = 3 / 2 , respectively, although they are not the only ones. It is also possible to obtain J = 3 / 2 particles from S = 1 / 2 and L = 2, as well as S = 3 / 2 and L = 2. This phenomenon of having multiple particles in
3395-1301: The circumgalactic medium , and the remaining 30 to 40% could be located in the warm–hot intergalactic medium (WHIM). Baryons are strongly interacting fermions ; that is, they are acted on by the strong nuclear force and are described by Fermi–Dirac statistics , which apply to all particles obeying the Pauli exclusion principle . This is in contrast to the bosons , which do not obey the exclusion principle. Baryons, alongside mesons , are hadrons , composite particles composed of quarks . Quarks have baryon numbers of B = 1 / 3 and antiquarks have baryon numbers of B = − 1 / 3 . The term "baryon" usually refers to triquarks —baryons made of three quarks ( B = 1 / 3 + 1 / 3 + 1 / 3 = 1). Other exotic baryons have been proposed, such as pentaquarks —baryons made of four quarks and one antiquark ( B = 1 / 3 + 1 / 3 + 1 / 3 + 1 / 3 − 1 / 3 = 1), but their existence
3492-417: The cosmic microwave background (CMB) radiation , and large-scale structure . The uniformity of the universe, known as the flatness problem , is explained through cosmic inflation : a sudden and very rapid expansion of space during the earliest moments. Extrapolating this cosmic expansion backward in time using the known laws of physics , the models describe an increasingly concentrated cosmos preceded by
3589-488: The dwarf galaxy problem of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible. Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are unsolved problems in physics. Observations of distant galaxies and quasars show that these objects are redshifted:
3686-412: The top quark 's short lifetime. The rules do not cover pentaquarks. It is also a widespread (but not universal) practice to follow some additional rules when distinguishing between some states that would otherwise have the same symbol. Quarks carry a charge, so knowing the charge of a particle indirectly gives the quark content. For example, the rules above say that a Λ c contains
3783-570: The wavefunction for each particle (in more precise terms, the quantum field for each particle type) were simultaneously mirror-reversed, then the new set of wavefunctions would perfectly satisfy the laws of physics (apart from the weak interaction). It turns out that this is not quite true: for the equations to be satisfied, the wavefunctions of certain types of particles have to be multiplied by −1, in addition to being mirror-reversed. Such particle types are said to have negative or odd parity ( P = −1, or alternatively P = –), while
3880-410: The "four pillars" of the Big Bang models. Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations. Remaining issues include the cuspy halo problem and
3977-497: The 1978 Nobel Prize in Physics . Baryonic matter The name "baryon", introduced by Abraham Pais , comes from the Greek word for "heavy" (βαρύς, barýs ), because, at the time of their naming, most known elementary particles had lower masses than the baryons. Each baryon has a corresponding antiparticle (antibaryon) where their corresponding antiquarks replace quarks. For example,
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4074-451: The Big Bang models. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles , and later atoms . The unequal abundances of matter and antimatter that allowed this to occur is an unexplained effect known as baryon asymmetry . These primordial elements—mostly hydrogen , with some helium and lithium —later coalesced through gravity , forming early stars and galaxies. Astronomers observe
4171-533: The Big Bang. Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, Alan Guth made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang models with the introduction of an epoch of rapid expansion in the early universe he called "inflation". Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over
4268-462: The Big Bang. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible density gradient . The earliest phases of the Big Bang are subject to much speculation, given the lack of available data. In the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures , and
4365-464: The absence of a perfect cosmological principle , extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This irregular behavior, known as the gravitational singularity , indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone cannot fully extrapolate toward
4462-497: The age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds , indicated a much younger age for globular clusters. Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as the Cosmic Background Explorer (COBE),
4559-411: The big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded
4656-432: The corresponding neutrino density Ω v h 2 {\displaystyle \Omega _{\text{v}}h^{2}} is estimated to be less than 0.0062. Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy , which appears to homogeneously permeate all of space. Observations suggest that 73% of
4753-452: The determination of the Hubble constant is known as Hubble tension . Techniques based on observation of the CMB suggest a lower value of this constant compared to the quantity derived from measurements based on the cosmic distance ladder. In 1964, Arno Penzias and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band. Their discovery provided substantial confirmation of
4850-426: The distant past. A wide range of empirical evidence strongly favors the Big Bang event, which is now essentially universally accepted. Detailed measurements of the expansion rate of the universe place the Big Bang singularity at an estimated 13.787 ± 0.020 billion years ago, which is considered the age of the universe . There remain aspects of the observed universe that are not yet adequately explained by
4947-591: The expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event—known as the " age of the universe "—is 13.8 billion years. Despite being extremely dense at this time—far denser than is usually required to form a black hole —the universe did not re-collapse into a singularity. Commonly used calculations and limits for explaining gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as
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#17328580778125044-400: The expansion, a phase transition caused a cosmic inflation , during which the universe grew exponentially , unconstrained by the light speed invariance , and temperatures dropped by a factor of 100,000. This concept is motivated by the flatness problem , where the density of matter and energy is very close to the critical density needed to produce a flat universe . That is, the shape of
5141-415: The first Doppler shift of a " spiral nebula " (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way . Ten years later, Alexander Friedmann ,
5238-457: The four Deltas and the two nucleons were thought to be the different states of two particles. However, in the quark model, Deltas are different states of nucleons (the N or N are forbidden by Pauli's exclusion principle ). Isospin, although conveying an inaccurate picture of things, is still used to classify baryons, leading to unnatural and often confusing nomenclature. The strangeness flavour quantum number S (not to be confused with spin)
5335-404: The gravitational effects of an unknown dark matter surrounding galaxies. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang models and various observations indicate that this excess gravitational potential is not created by baryonic matter , such as normal atoms. Measurements of the redshifts of supernovae indicate that the expansion of the universe
5432-444: The isospin projections I 3 = + 3 / 2 , I 3 = + 1 / 2 , I 3 = − 1 / 2 , and I 3 = − 3 / 2 , respectively. Another example is the "nucleon particle". As there were two nucleon "charged states", it was said to be of isospin 1 / 2 . The positive nucleon N (proton)
5529-411: The lambda-CDM model of cosmology, which uses the independent frameworks of quantum mechanics and general relativity. There are no easily testable models that would describe the situation prior to approximately 10 seconds. Understanding this earliest of eras in the history of the universe is one of the greatest unsolved problems in physics . English astronomer Fred Hoyle is credited with coining
5626-487: The largest possible deviation of the fine-structure constant over much of the age of the universe is of order 10 . Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars . The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler Copernican principle , which states that there
5723-405: The light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift
5820-519: The mass of the visible matter in the universe and compose the nucleus of every atom ( electrons , the other major component of the atom, are members of a different family of particles called leptons ; leptons do not interact via the strong force). Exotic baryons containing five quarks, called pentaquarks , have also been discovered and studied. A census of the Universe's baryons indicates that 10% of them could be found inside galaxies, 50 to 60% in
5917-452: The mathematical derivation of the Friedmann equations . The earliest empirical observation of the notion of an expanding universe is known as Hubble's Law , published in work by physicist Edwin Hubble in 1929, which discerned that galaxies are moving away from Earth at a rate that accelerates proportionally with distance. Independent of Friedmann's work, and independent of Hubble's observations, physicist Georges Lemaître proposed that
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#17328580778126014-453: The notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time. During the 1930s, other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including
6111-411: The number of baryons may change in multiples of three due to the action of sphalerons , although this is rare and has not been observed under experiment. Some grand unified theories of particle physics also predict that a single proton can decay , changing the baryon number by one; however, this has not yet been observed under experiment. The excess of baryons over antibaryons in the present universe
6208-460: The observational evidence, most notably from radio source counts , began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe. In 1968 and 1970, Roger Penrose , Stephen Hawking , and George F. R. Ellis published papers where they showed that mathematical singularities were an inevitable initial condition of relativistic models of
6305-482: The opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well. Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate thermodynamic equilibrium . Others were fast enough to reach thermalization . The parameter usually used to find out whether
6402-404: The original matter particles and none of their antiparticles . A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos ). A few minutes into the expansion, when
6499-515: The other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter (CDM), warm dark matter , hot dark matter , and baryonic matter . The best measurements available, from the Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter
6596-469: The other forces, with only the electromagnetic force and weak nuclear force remaining unified. Inflation stopped locally at around 10 to 10 seconds, with the observable universe's volume having increased by a factor of at least 10 . Reheating followed as the inflaton field decayed, until the universe obtained the temperatures required for the production of a quark–gluon plasma as well as all other elementary particles . Temperatures were so high that
6693-720: The other particles are said to have positive or even parity ( P = +1, or alternatively P = +). For baryons, the parity is related to the orbital angular momentum by the relation: As a consequence, baryons with no orbital angular momentum ( L = 0) all have even parity ( P = +). Baryons are classified into groups according to their isospin ( I ) values and quark ( q ) content. There are six groups of baryons: nucleon ( N ), Delta ( Δ ), Lambda ( Λ ), Sigma ( Σ ), Xi ( Ξ ), and Omega ( Ω ). The rules for classification are defined by
6790-427: The past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence. In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of
6887-512: The photon radiation . The recombination epoch began after about 379,000 years, when the electrons and nuclei combined into atoms (mostly hydrogen ), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background. After the recombination epoch, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and
6984-506: The picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators . At about 10 seconds, quarks and gluons combined to form baryons such as protons and neutrons . The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass annihilation immediately followed, leaving just one in 10 of
7081-481: The precise values of the Hubble Constant and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe ). In the mid-1990s, observations of certain globular clusters appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with
7178-461: The predominance of matter over antimatter in the present universe. The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10 seconds. After about 10 seconds,
7275-427: The random motions of particles were at relativistic speeds , and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point, an unknown reaction called baryogenesis violated the conservation of baryon number , leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in
7372-438: The result of some unknown excitation similar to spin. This unknown excitation was later dubbed isospin by Eugene Wigner in 1937. This belief lasted until Murray Gell-Mann proposed the quark model in 1964 (containing originally only the u, d, and s quarks). The success of the isospin model is now understood to be the result of the similar masses of u and d quarks. Since u and d quarks have similar masses, particles made of
7469-482: The same number then also have similar masses. The exact specific u and d quark composition determines the charge, as u quarks carry charge + 2 / 3 while d quarks carry charge − 1 / 3 . For example, the four Deltas all have different charges ( Δ (uuu), Δ (uud), Δ (udd), Δ (ddd)), but have similar masses (~1,232 MeV/c ) as they are each made of
7566-401: The same total angular momentum configuration is called degeneracy . How to distinguish between these degenerate baryons is an active area of research in baryon spectroscopy . If the universe were reflected in a mirror, most of the laws of physics would be identical—things would behave the same way regardless of what we call "left" and what we call "right". This concept of mirror reflection
7663-463: The same way to the strong interaction. Since quarks do not have the same mass, they do not interact in the same way (exactly like an electron placed in an electric field will accelerate more than a proton placed in the same field because of its lighter mass), and the symmetry is said to be broken . It was noted that charge ( Q ) was related to the isospin projection ( I 3 ), the baryon number ( B ) and flavour quantum numbers ( S , C , B ′, T ) by
7760-410: The singularity. In some proposals, such as the emergent Universe models, the singularity is replaced by another cosmological epoch. A different approach identifies the initial singularity as a singularity predicted by some models of the Big Bang theory to have existed before the Big Bang event. This primordial singularity is itself sometimes called "the Big Bang", but the term can also refer to
7857-490: The size of the observable universe , the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity v {\displaystyle v} . For distances comparable to the size of the observable universe, the attribution of the cosmological redshift becomes more ambiguous, although its interpretation as a kinematic Doppler shift remains the most natural one. An unexplained discrepancy with
7954-521: The spin vectors add up to make a vector of length S = 0 and has only one spin projection ( S z = 0), etc. Since baryons are made of three quarks, their spin vectors can add to make a vector of length S = 3 / 2 , which has four spin projections ( S z = + 3 / 2 , S z = + 1 / 2 , S z = − 1 / 2 , and S z = − 3 / 2 ), or
8051-406: The steady-state theory. This perception was enhanced by the fact that the originator of the Big Bang concept, Lemaître, was a Roman Catholic priest. Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz ., that matter is eternal . A beginning in time was "repugnant" to him. Lemaître, however, disagreed: If the world has begun with a single quantum ,
8148-447: The temperature was about a billion kelvin and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis (BBN). Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest energy density of matter came to gravitationally dominate that of
8245-465: The term "Big Bang" during a talk for a March 1949 BBC Radio broadcast, saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past." However, it did not catch on until the 1970s. It is popularly reported that Hoyle, who favored an alternative " steady-state " cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it
8342-452: The total energy density of the present day universe is in this form. When the universe was very young it was likely infused with dark energy, but with everything closer together, gravity predominated, braking the expansion. Eventually, after billions of years of expansion, the declining density of matter relative to the density of dark energy allowed the expansion of the universe to begin to accelerate. Dark energy in its simplest formulation
8439-404: The u and d mass are similar, this description of particle mass and charge in terms of isospin and flavour quantum numbers works well only for octet and decuplet made of one u, one d, and one other quark, and breaks down for the other octets and decuplets (for example, ucb octet and decuplet). If the quarks all had the same mass, their behaviour would be called symmetric , as they would all behave in
8536-416: The universe has no overall geometric curvature due to gravitational influence. Microscopic quantum fluctuations that occurred because of Heisenberg's uncertainty principle were "frozen in" by inflation, becoming amplified into the seeds that would later form the large-scale structure of the universe. At a time around 10 seconds, the electroweak epoch begins when the strong nuclear force separates from
8633-411: The universe emerged from a "primeval atom " in 1931, introducing the modern notion of the Big Bang. Various cosmological models of the Big Bang explain the evolution of the observable universe from the earliest known periods through its subsequent large-scale form. These models offer a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements ,
8730-422: The universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach earth. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines
8827-490: The universe is uniformly expanding everywhere. This cosmic expansion was predicted from general relativity by Friedmann in 1922 and Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang model as developed by Friedmann, Lemaître, Robertson, and Walker. The theory requires the relation v = H D {\displaystyle v=HD} to hold at all times, where D {\displaystyle D}
8924-449: The validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang nucleosynthesis (BBN). More recent evidence includes observations of galaxy formation and evolution , and the distribution of large-scale cosmic structures . These are sometimes called
9021-473: Was Lemaître's Big Bang theory, advocated and developed by George Gamow , who introduced BBN and whose associates, Ralph Alpher and Robert Herman , predicted the CMB. Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949. For a while, support was split between these two theories. Eventually,
9118-404: Was identified with I 3 = + 1 / 2 and the neutral nucleon N (neutron) with I 3 = − 1 / 2 . It was later noted that the isospin projections were related to the up and down quark content of particles by the relation: where the n' s are the number of up and down quarks and antiquarks. In the "isospin picture",
9215-411: Was just a striking image meant to highlight the difference between the two models. Helge Kragh writes that the evidence for the claim that it was meant as a pejorative is "unconvincing", and mentions a number of indications that it was not a pejorative. The term itself has been argued to be a misnomer because it evokes an explosion. The argument is that whereas an explosion suggests expansion into
9312-418: Was noticed to go up and down along with particle mass. The higher the mass, the lower the strangeness (the more s quarks). Particles could be described with isospin projections (related to charge) and strangeness (mass) (see the uds octet and decuplet figures on the right). As other quarks were discovered, new quantum numbers were made to have similar description of udc and udb octets and decuplets. Since only
9409-474: Was very rapidly expanding and cooling. The period up to 10 seconds into the expansion, the Planck epoch , was a phase in which the four fundamental forces —the electromagnetic force , the strong nuclear force , the weak nuclear force , and the gravitational force , were unified as one. In this stage, the characteristic scale length of the universe was the Planck length , 1.6 × 10 m , and consequently had
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