Universe

For other uses, see Universe (disambiguation).

The Universe is all of time and space and itscontents.[8][9][10][11] The Universe includes planets, stars,galaxies, the contents of intergalactic space, the small-est subatomic particles, and all matter and energy. Theobservable universe is about 28 billion parsecs (91 billionlight-years) in diameter at the present time.[2] The size ofthe whole Universe is not known and may be infinite.[12]Observations and the development of physical theorieshave led to inferences about the composition and evolu-tion of the Universe.Throughout recorded history, cosmologies andcosmogonies, including scientific models, have beenproposed to explain observations of the Universe.The earliest quantitative geocentric models were de-veloped by ancient Greek philosophers and Indianphilosophers.[13][14] Over the centuries, more preciseastronomical observations led to Nicolaus Copernicus'sheliocentric model of the Solar System and JohannesKepler's improvement on that model with ellipticalorbits, which was eventually explained by Isaac Newton'stheory of gravity. Further observational improvementsled to the realization that the Solar System is located ina galaxy composed of billions of stars, the Milky Way.It was subsequently discovered that our galaxy is justone of many. Observations of the distribution of thesegalaxies and their spectral lines have led to many of thetheories of modern physical cosmology. The discoveryin the early 20th century that galaxies are systematicallyredshifted suggested that the Universe is expanding,and the discovery of the cosmic microwave backgroundradiation suggested that the Universe had a beginning.[15]Finally, observations in the late 1990s indicated therate of the expansion of the Universe is increasing[16]indicating that the majority of energy is most likely in anunknown form called dark energy.[17][18] The majority ofmass in the universe also appears to exist in an unknownform, called dark matter.[17][18]

The Big Bang theory is the prevailing cosmological modeldescribing the development of the Universe. Space andtime were created in the Big Bang, and these were im-bued with a fixed amount of energy and matter; as spaceexpands, the density of that matter and energy decreases.After the initial expansion, the Universe cooled suffi-ciently to allow the formation first of subatomic parti-cles and later of simple atoms. Giant clouds of theseprimordial elements later coalesced through gravity toform stars. Assuming that the prevailing model is correct,

the age of the Universe is measured to be 13.798±0.037billion years.[1][19]

There are many competing hypotheses about the ultimatefate of the Universe. Physicists and philosophers remainunsure about what, if anything, preceded the Big Bang.Many refuse to speculate, doubting that any informa-tion from any such prior state could ever be accessible.There are various multiverse hypotheses, in which somephysicists have suggested that the Universe might be oneamong many universes that likewise exist.[20][21]

1 Definition

TheUniverse is customarily defined as everything that ex-ists, everything that has existed, and everything that willexist.[22][23][24] According to our current understanding,the Universe consists of three constituents: spacetime,forms of energy (including electromagnetic radiation andmatter), and the physical laws that relate them. The Uni-verse also encompasses all of life, all of history, and somephilosophers and scientists even suggest that it encom-passes ideas such as mathematics.[25][26][27]

2 Etymology

The word universe derives from the Old French wordunivers, which in turn derives from the Latin word uni-versum.[28] The Latin word was used by Cicero and laterLatin authors in many of the same senses as the modernEnglish word is used.[29] The Latin word derives from thepoetic contraction unvorsum— first used by Lucretius inBook IV (line 262) of his De rerum natura (On the Na-ture of Things) — which connects un, uni (the combin-ing form of unus, or “one”) with vorsum, versum (a nounmade from the perfect passive participle of vertere, mean-ing “something rotated, rolled, changed”).[29]

An alternative interpretation of unvorsum is “everythingrotated as one” or “everything rotated by one”. In thissense, it may be considered a translation of an earlierGreek word for the Universe, περιφορά, (periforá, “cir-cumambulation”), originally used to describe a course ofa meal, the food being carried around the circle of dinnerguests.[30] This Greek word refers to celestial spheres, anearly Greek model of the Universe. Regarding Plato’sMetaphor of the Sun, Aristotle suggests that the rota-tion of the sphere of fixed stars inspired by the prime

1

2 4 PROPERTIES

mover,[31] motivates, in turn, terrestrial change via theSun. Astronomical and physical measurements, such asthe Foucault pendulum, demonstrate that the Earth ro-tates on its axis.

2.1 Synonyms

A term for the Universe in ancient Greece was τὸ πᾶν(tò pán, The All, Pan). Related terms were matter, (τὸὅλον, tò hólon, see also Hyle, lit. wood) and place (τὸκενόν, tò kenón).[32][33] Other synonyms for the Uni-verse among the ancient Greek philosophers included κό-σμος (cosmos) and φύσις (meaning nature, Greek physis,from which we derive the word physics).[34] The samesynonyms are found in Latin authors (totum, mundus,natura)[35] and survive in modern languages, e.g., theGerman words Das All, Weltall, and Natur for Universe.The same synonyms are found in English, such as every-thing (as in the theory of everything), the cosmos (as incosmology), the world (as in the many-worlds interpre-tation), and nature (as in natural laws or natural philoso-phy).[36]

3 Chronology and the Big Bang

Main articles: Big Bang and Chronology of the Universe

The prevailing model for the evolution of the Universe isthe Big Bang theory.[37][38] The Big Bang model statesthat the earliest state of the Universe was extremelyhot and dense and that it subsequently expanded. Themodel is based on general relativity and on simplifyingassumptions such as homogeneity and isotropy of space.A version of the model with a cosmological constant(Lambda) and cold dark matter, known as the Lambda-CDM model, is the simplest model that provides a rea-sonably good account of various observations about theUniverse. The Big Bang model accounts for observationssuch as the correlation of distance and redshift of galax-ies, the ratio of the number of hydrogen to helium atoms,and the microwave radiation background.The initial hot, dense state is called the Planck epoch, abrief period extending from time zero to one Planck timeunit of approximately 10−43 seconds. During the Planckepoch, all types of matter and all types of energy wereconcentrated into a dense state, where gravitation is be-lieved to have been as strong as the other fundamentalforces, and all the forces may have been unified. Sincethe Planck epoch, the Universe has been expanding toits present form, possibly with a very brief period ofcosmic inflation which caused the Universe to reach amuch larger size in less than 10−32 seconds.[39]

After the Planck epoch and inflation came the quark,hadron, and lepton epochs. Together, these epochs en-compassd less than 10 seconds of time following the Big

Bang. The observed abundance of the elements can beexplained by combining the overall expansion of spacewith nuclear and atomic physics. As the Universe ex-pands, the energy density of electromagnetic radiation de-creases more quickly than does that of matter because theenergy of a photon decreases with its wavelength. As theUniverse expanded and cooled, elementary particles as-sociated stably into ever larger combinations. Thus, inthe early part of the matter-dominated era, stable protonsand neutrons formed, which then formed atomic nu-clei through nuclear reactions. This process, known asBig Bang nucleosynthesis, led to the present abundancesof lighter nuclei, particularly hydrogen, deuterium, andhelium. Big Bang nucleosynthesis ended about 20 min-utes after the Big Bang, when the Universe had cooledenough so that nuclear fusion could no longer occur. Atthis stage, matter in the Universe was mainly a hot, denseplasma of negatively charged electrons, neutral neutrinosand positive nuclei. This era, called the photon epoch,lasted about 380 thousand years.Eventually, at time known as recombination, electronsand nuclei formed stable atoms, which are transparent tomost wavelengths of radiation. With photons decoupledfrom matter, the Universe entered the matter-dominatedera. Light from this era could now travel freely, and itcan still be seen in the Universe as the cosmic microwavebackground (CMB). After around 100 million years, thefirst stars formed; these were likely very massive, lumi-nous, and responsible for the reionization of the Universe.Having no elements heavier than lithium, these stars alsoproduced the first heavy elements through stellar nucle-osynthesis.[40] The Universe also contains a mysteriousenergy called dark energy; the energy density of dark en-ergy does not change over time. After about 9.8 billionyears, the Universe had expanded sufficiently so that thedensity of matter was less than the density of dark en-ergy, marking the beginning of the present dark-energy-dominated era.[41] In this era, the expansion of the Uni-verse is accelerating due to dark energy.

4 Properties

Main articles: Observable universe, Age of the Universeand Metric expansion of space

The spacetime of the Universe is usually interpreted froma Euclidean perspective, with space as consisting of threedimensions, and time as consisting of one dimension, the"fourth dimension".[42] By combining space and time intoa single manifold calledMinkowski space, physicists havesimplified a large number of physical theories, as well asdescribed in amore uniformway the workings of the Uni-verse at both the supergalactic and subatomic levels.Spacetime events are not absolutely defined spatially andtemporally but rather are known relative to the motion

4.2 Size and regions 3

of an observer. Minkowski space approximates the Uni-verse without gravity; the pseudo-Riemannian manifoldsof general relativity describe spacetime with matter andgravity. String theory postulates the existence of addi-tional dimensions.Of the four fundamental interactions, gravitation is domi-nant at cosmological length scales, including galaxies andlarger-scale structures. Gravity’s effects are cumulative;by contrast, the effects of positive and negative chargestend to cancel one another, making electromagnetism rel-atively insignificant on cosmological length scales. Theremaining two interactions, the weak and strong nuclearforces, decline very rapidly with distance; their effects areconfined mainly to sub-atomic length scales.The Universe appears to have much more matter thanantimatter, an asymmetry possibly related to the obser-vations of CP violation.[43] The Universe appears to haveno net electric charge, and therefore gravity appears to bethe dominant interaction on cosmological length scales.The Universe also appears to have neither net momentumnor angular momentum. The absence of net charge andmomentum would follow from accepted physical laws(Gauss’s law and the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the Universewere finite.[44]

4.1 Shape

The three possible options of the shape of the Universe.

Main article: Shape of the Universe

General relativity describes how spacetime is curved andbent by mass and energy. The topology or geometryof the Universe includes both local geometry in theobservable universe and global geometry. Cosmolo-gists often work with a given space-like slice of space-time called the comoving coordinates. The section ofspacetime which can be observed is the backward light

cone, which delimits the cosmological horizon. The cos-mological horizon (also called the particle horizon orthe light horizon) is the maximum distance from whichparticles can have traveled to the observer in the age ofthe Universe. This horizon represents the boundary be-tween the observable and the unobservable regions ofthe Universe.[45][46] The existence, properties, and signif-icance of a cosmological horizon depend on the particularcosmological model.Observations, including the Cosmic Background Ex-plorer (COBE), Wilkinson Microwave Anisotropy Probe(WMAP), and Planck maps of the CMB, suggest thatthe Universe is infinite in extent with a finite age, as de-scribed by the Friedmann–Lemaître–Robertson–Walker(FLRW) models.[47][48][49][50] These FLRW models thussupport inflationary models and the standard modelof cosmology, describing a flat, homogeneous uni-verse presently dominated by dark matter and dark en-ergy.[1][51]

4.2 Size and regions

See also: Observable universe and Observational cos-mology

According to a restrictive definition, the Universe is ev-erything within our connected spacetime that could havea chance to interact with us and vice versa.[52] Accord-ing to the general theory of relativity, some regions ofspace may never interact with ours even in the lifetime ofthe Universe due to the finite speed of light and the on-going expansion of space. For example, radio messagessent from Earth may never reach some regions of space,even if the Universe were to exist forever: space may ex-pand faster than light can traverse it.[53] Since we cannotobserve space beyond the limitations of light, or any elec-tromagnetic radiation, it is unknown whether the size ofthe Universe is finite or infinite.[12][54][55]

Distant regions of space are taken to exist and be partof reality as much as we are, yet we can never interactwith them. The spatial region within which we can affectand be affected is the observable universe. The observ-able Universe depends on the location of the observer.By traveling, an observer can come into contact with agreater region of spacetime than an observer who remainsstill. Nevertheless, even the most rapid traveler will notbe able to interact with all of space. Typically, the observ-able Universe is taken to mean the Universe observablefrom our vantage point in the Milky Way Galaxy.The proper distance – the distance as would be measuredat a specific time, including the present – between Earthand the edge of the observable universe is 46 billion light-years (14×109 pc), making the diameter of the observableuniverse about 91 billion light-years (28×109 pc). Thedistance the light from the edge of the observable uni-verse has travelled is very close to the age of the Universe

4 4 PROPERTIES

times the speed of light, 13.8 billion light-years (4.2×109pc), but this does not represent the distance at any giventime because the edge of the universe and the Earth havemoved since further apart.[56] For comparison, the diam-eter of a typical galaxy is 30,000 light-years, and the typi-cal distance between two neighboring galaxies is 3 millionlight-years.[57] As an example, the Milky Way Galaxyis roughly 100,000 light years in diameter,[58] and thenearest sister galaxy to the Milky Way, the AndromedaGalaxy, is located roughly 2.5 million light years away.[59]

4.3 Age and expansion

Main articles: Age of the universe and Metric expansionof space

Astronomers calculate the age of the Universe by assum-ing that the Lambda-CDM model accurately describesthe evolution of the Universe from a very uniform, hot,dense primordial state to its present state and measuringthe cosmological parameters which constitute the model.This model is well understood theoretically and supportedby recent high-precision astronomical observations suchas WMAP and Planck. Commonly, the set of observa-tions fitted includes the cosmic microwave backgroundanisotropy, the brightness/redshift relation for Type Iasupernovae, and large-scale galaxy clustering includingthe baryon acoustic oscillation feature. Other observa-tions, such as the Hubble constant, the abundance ofgalaxy clusters, weak gravitational lensing and globularcluster ages, are generally consistent with these, provid-ing a check of themodel, but are less accurately measuredat present. With the prior that the Lambda-CDM modelis correct and the values of the parameters measured byvarious experiments including Planck, the best value ofthe age of the Universe as of 2015 is 13.798 ± 0.037 bil-lion years.[1]

Over time, the Universe and its contents have evolved;for example, the relative population of quasars and galax-ies has changed[60] and space itself has expanded. Thisexpansion accounts for how it is that scientists on Earthcan observe the light from a galaxy 30 billion light yearsaway, even if that light has traveled for only 13 bil-lion years; the very space between them has expanded,and that is one of the tools used to calculate the age ofthe Universe. This expansion is consistent with the ob-servation that the light from distant galaxies has beenredshifted; the photons emitted have been stretched tolonger wavelengths and lower frequency during their jour-ney. Analyses of Type Ia supernovae indicate that thespatial expansion is accelerating.[61][62]

The more matter there is in the Universe, the strongerwill be the gravitational pull among the matter. If theUniverse were too dense then it would re-collapse intosingularity. However, if the Universe contained too littlematter then the expansion is accelerated greatly, thereby

leaving no time for planets and planetary systems to form.After the Big Bang, the universe is continuously expand-ing. The rate of expansion is affected by the gravityamong the matter present. Surprisingly, our universe hasjust the right mass density of about 5 protons per cubicmeter which has allowed it to expand for last 13.8 bil-lion years, giving time to form the universe as we see ittoday.[63]

There are dynamical forces acting on the particles in theUniverse which affect the expansion rate. It was earlierexpected that the Hubble Constant would be decreasingas time went on due to the influence of gravitational inter-actions in the Universe, and thus there is an additional ob-servable quantity in the Universe called the decelerationparameter which cosmologists expected to be directlyrelated to the matter density of the Universe. Surpris-ingly, the deceleration parameter was measured by twodifferent groups to be less than zero (actually, consis-tent with −1) which implied that today Hubble’s Con-stant is increasing as time goes on. Some cosmologistshave whimsically called the effect associated with the“accelerating universe” the “cosmic jerk".[64] The 2011Nobel Prize in Physics was given for the discovery of thisphenomenon.[65]

4.4 Spacetime

Main articles: Spacetime and World lineSee also: Lorentz transformation

Spacetimes are the arenas in which all physical eventstake place—an event is a point in spacetime specified byits time and place. For example, the motion of planetsaround the sun may be described in a particular type ofspacetime, or the motion of light around a rotating starmay be described in another type of spacetime. The basicelements of spacetime are events. In any given spacetime,an event is a unique position at a unique time. Becauseevents are spacetime points, an example of an event inclassical relativistic physics is (x, y, z, t) , the location ofan elementary (point-like) particle at a particular time. Aspacetime itself can be viewed as the union of all eventsin the same way that a line is the union of all of its points,formally organized into a manifold, a space which can bedescribed at small scales using coordinate systems.The Universe appears to be a smooth spacetime contin-uum consisting of three spatial dimensions and one tem-poral (time) dimension. On the average, space is ob-served to be very nearly flat (close to zero curvature),meaning that Euclidean geometry is empirically truewith high accuracy throughout most of the Universe.[66]Spacetime also appears to have a simply connectedtopology, in analogy with a sphere, at least on the length-scale of the observable Universe. However, presentobservations cannot exclude the possibilities that theUniverse has more dimensions and that its spacetime

5

may have a multiply connected global topology, in anal-ogy with the cylindrical or toroidal topologies of two-dimensional spaces.[48][67]

5 Contents

The formation of clusters and large-scale filaments in the ColdDark Matter model with dark energy. The frames show the evo-lution of structures in a 43 million parsecs (or 140 million lightyears) box from redshift of 30 to the present epoch (upper leftz=30 to lower right z=0).

See also: Galaxy formation and evolution, Galaxy clusterand Nebula

The Universe is composed almost completely of darkenergy, dark matter, and ordinary matter. Other con-tents are electromagnetic radiation(estimated to be from0.005% to close to 0.01%,) and antimatter.[68][69][70]The total amount of electromagnetic radiation generatedwithin the universe has decreased by 1/2 in the past2 billion years, indicating no electromagnetic radiationwill be generated in the distant future as the universeages.[71][72][73]

Ordinary baryonic matter, which includes atoms, stars,galaxies, and life, is only 4.6% of the contents. Thepresent overall density of this type of matter is very low,roughly 4.5 × 10−31 grams per cubic centimetre, corre-sponding to a density of the order of only one proton forevery four cubic meters of volume.[4] The nature of bothdark energy and dark matter is unknown. Dark matter,a mysterious form of matter that has not yet been identi-fied, is 26.8% of the contents. Dark energy, which is theenergy of empty space and that is causing the expansionof the Universe to accelerate, is the remaining 68.3% ofthe contents.[6][74][75]

Matter, dark matter, and dark energy are distributed ho-mogeneously throughout the Universe over length scaleslonger than 300 million light-years or so.[76] However, onsmaller length-scales, matter is observed to clump hierar-chically; many atoms are condensed into stars, most stars

A map of the Superclusters and voids nearest to Earth

into galaxies, most galaxies into clusters, superclustersand, finally, the largest-scale structures such as the Sloangreat wall. There are probably more than 100 billion(1011) galaxies in the observable Universe.[77] Typicalgalaxies range from dwarfs with as few as ten million[78](107) stars up to giants with one trillion[79] (1012) stars.A 2010 study by astronomers estimated that the observ-able Universe contains 300 sextillion (3×1023) stars.[80]Between the structures are voids, which are typically 10–150 Mpc (33 million–490 million ly) in diameter. TheMilky Way is in the Local Group of galaxies, which inturn is in the Laniakea Supercluster.[81] This superclusterspans over 500 million light years, while the Local Groupspans over 10 million light years.[82] In April 2015, as-tronomers announced the discovery of a supervoid, thelargest known structure in the universe. This big hole is1.8 billion ly (550 Mpc) across, characterized by its un-usual emptiness.[83]

Contents of the Universe updated with WMAP 9-YEARRESULTS[84][85] as of 04-08-2013

The observable matter of the Universe is also isotropic onlarge scales, meaning that no direction of observation ap-pears to be different from any other.[87] The Universe is

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Contents of the Universe comparison today to theearly Universe.[86] As of 04-08-2013 made with 5 year WMAP data.

WMAP data is accurate to two digits, the total of these numbers is not 100%. This reflects the current limits of WMAP’s ability to define Dark Matter and Dark Energy. Electromagnetic radiation, antimatter and other contents are not included.

also bathed in a highly isotropic microwave radiation thatcorresponds to a thermal equilibrium blackbody spec-trum of roughly 2.725 kelvin.[88] The hypothesis thatthe large-scale Universe is homogeneous and isotropicis known as the cosmological principle,[89] which issupported by astronomical observations.

5.1 Dark energy

Main article: Dark energy

An explanation for why the expansion of the Universeis accelerating remains elusive. It is often attributed to“dark energy”, an unknown form of energy that is hypoth-esized to permeate space.[74] On a mass–energy equiv-alence basis, the density of dark energy (6.91 × 10−27kg/m3) is much less than the density of ordinary matteror dark matter within galaxies. However, in the presentdark-energy era, it dominates the mass–energy of the uni-verse because it is uniform across space.[90]

Two proposed forms for dark energy are the cosmologicalconstant, a constant energy density filling spacehomogeneously,[91] and scalar fields such as quintessenceor moduli, dynamic quantities whose energy density canvary in time and space. Contributions from scalar fieldsthat are constant in space are usually also included in the

cosmological constant. The cosmological constant canbe formulated to be equivalent to vacuum energy. Scalarfields having only a slight amont of spatial inhomogeneitycan be difficult to distinguish from a cosmologicalconstant.

5.2 Dark matter

Main article: Dark matter

Dark matter is a hypothetical kind of matter that can-not be seen with telescopes, but which accounts for mostof the matter in the Universe. The existence and prop-erties of dark matter are inferred from its gravitationaleffects on visible matter, radiation, and the large-scalestructure of the Universe. Other than neutrinos, a formof hot dark matter, dark matter has not been detected di-rectly, making it one of the greatest mysteries in modernastrophysics. Dark matter neither emits nor absorbs lightor any other electromagnetic radiation at any significantlevel. Dark matter is estimated to constitute 26.8% ofthe total mass–energy and 84.5% of the total matter inthe Universe.[6][92]

5.3 Ordinary Matter

Main article: Matter

The remaining 4.9% of the mass–energy of the Universeis composed of matter made up of ordinary matter, thatis atoms, ions, electrons and the objects they form. Thismatter includes stars, which produce nearly all of the lightwe see from galaxies, as well as interstellar gas in theinterstellar and intergalactic media, planets, and all theobjects from everyday life that we can bump into, touchor squeeze.[93]

Ordinary matter commonly exists in four states (orphases): solid, liquid and gas, and plasma. However,advances in experimental techniques have revealed otherpreviously theoretical phases, such as Bose–Einstein con-densates and fermionic condensates.More fundamentally, matter is composed of two types ofelementary particles: quarks and leptons.[94] For exam-ple, the neutron is formed of two down quarks and one upquark, while the proton is formed of two up quarks andone down quark. Atoms are made up of several neutronsand protons (two types of baryons), which have severalelectrons (a type of lepton) orbiting them. Because mostof the mass of the atom is concentrated in the atomic nu-cleus, which is made up of baryons, astronomers oftenuse the term baryonic matter to describe ordinary mat-ter, although a small fraction of this baryonic matter isalso composed of electrons. A focus on an elementary-particle view of matter also leads to new phases of matter,such as the quark–gluon plasma.[95]

5.4 Particles 7

The primordial protons and neutrons themselves wereformed from the quark–gluon plasma during the Big Bangas it cooled below two trillion degrees. A few minutes af-terwards, starting with only protons and neutrons, nucleiup to lithium and beryllium were formed, but the abun-dances of other elements dropped sharply with growingatomic mass. Some boron may have been formed at thistime, but the process stopped before significant carboncould be formed. This process, known as Big Bang nu-cleosynthesis, essentially shut down after about 20 min-utes, due to drops in temperature and density as the uni-verse continued to expand. The subsequent nucleosyn-thesis of heavier elements required the extreme temper-atures and pressures found within stars and supernovas,in the processes of stellar nucleosynthesis and supernovanucleosynthesis.

5.4 Particles

The tree-level interactions between elementary particles de-scribed in the Standard Model.

Six of the particles in the Standard Model are quarks (shown inpurple). Each of the first three columns forms a generation ofmatter.

Main article: Particle physics

The ordinary matter in the Universe consists of parti-cles described by particle physics. An elementary particleor fundamental particle is a particle whose substructure(domain of the bigger structure which shares the simi-lar characteristics of the domain) is unknown, thus it isunknown whether it is composed of other particles.[96]Known elementary particles include the fundamentalfermions (quarks, leptons, antiquarks, and antileptons),which generally are “matter particles” and "antimatterparticles”, as well as the fundamental bosons (gaugebosons and Higgs boson), which generally are “force par-ticles” that mediate interactions among fermions.[96] Aparticle containing two or more elementary particles isa composite particle.The Standard Model of particle physics suggests a uni-versal set of physical laws and physical constants.[97] Itconcerns the electromagnetic, weak, and strong nuclearinteractions, as well as classifying all the subatomic par-ticles known. It was developed throughout the latter halfof the 20th century as a collaborative effort of scientistsaround the world.[98] The current formulation was final-ized in the mid-1970s upon experimental confirmation ofthe existence of quarks. Since then, discoveries of the topquark (1995), the tau neutrino (2000), and more recentlythe Higgs boson (2013), have given further credence tothe Standard Model. Because of its success in explain-ing a wide variety of experimental results, the StandardModel is sometimes regarded as a “theory of almost ev-erything”.Each particle generation is divided into two types ofleptons and two types of quarks, both of which arefermions. Bosons are the other fundamental class of el-ementary particles. Additionally, there are hypotheticalparticles.

5.4.1 Hadrons

Main article: Hadron

A hadron is a composite particle made of quarks boundstate by the strong force (in a similar way as moleculesare held together by the electromagnetic force). Fromapproximately 10−6 seconds after the Big Bang, when thetemperature of the universe had fallen sufficiently to al-low the quarks from the preceding quark epoch to bindtogether into hadrons, the mass of the universe was dom-inated by hadrons. This period is known as the hadronepoch. Initially the temperature was high enough to al-low the formation of hadron/anti-hadron pairs, whichkept matter and antimatter in thermal equilibrium. How-ever, as the temperature of the Universe continued tofall, hadron/anti-hadron pairs were no longer produced.Most of the hadrons and anti-hadrons were then elimi-nated in annihilation reactions, leaving a small residual

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of hadrons. The elimination of anti-hadrons was com-pleted by one second after the Big Bang, when the fol-lowing lepton epoch began.Hadrons are categorized into two families: baryons (suchas protons and neutrons) made of three quarks andmesons (such as pions) made of one quark and oneantiquark. Of the hadrons, protons are stable, and neu-trons are bound within atomic nuclei are stable. Otherhadrons are unstable under ordinary conditions; free neu-trons decay with a half-life of about 611 seconds. Exper-imentally, hadron physics is studied by colliding protonsor nuclei of heavy elements such as lead, and detectingthe debris in the produced particle showers.

5.4.2 Leptons

Main article: Lepton

A lepton is an elementary, half-integer spin (spin 1⁄2) par-ticle that does not undergo strong interactions but is sub-ject to the Pauli exclusion principle; no two leptons ofthe same species can be in exactly the same state at thesame time.[99] Fermions differ from bosons, which obeyBose–Einstein statistics. The best known of all leptonsis the electron, which governs nearly all of chemistry asit is found in atoms and is directly tied to all chemicalproperties. Two main classes of leptons exist: chargedleptons (also known as the electron-like leptons), and neu-tral leptons (better known as neutrinos). The two lep-tons may be classified into one with electric charge −1(electron-like) and one neutral (neutrino). Thus elec-trons are stable and the most common charged leptonin the Universe, whereas muons and taus can only beproduced in high energy collisions (such as those in-volving cosmic rays and those carried out in particle ac-celerators).[100][101] Charged leptons can combine withother particles to form various composite particles suchas atoms and positronium, while neutrinos rarely inter-act with anything, and are consequently rarely observed.Neutrinos of all generations stream throughout the Uni-verse but rarely interact with normal matter.[102]

The lepton epoch was the period in the evolution ofthe early Universe in which the leptons dominated themass of the Universe. It started roughly 1 second af-ter the Big Bang, after the majority of hadrons andanti-hadrons annihilated each other at the end of thehadron epoch. During the lepton epoch the temper-ature of the Universe was still high enough to createlepton/anti-lepton pairs, so leptons and anti-leptons werein thermal equilibrium. Approximately 10 seconds af-ter the Big Bang, the temperature of the Universe hadfallen to the point where lepton/anti-lepton pairs were nolonger created.[103] Most leptons and anti-leptons werethen eliminated in annihilation reactions, leaving a smallresidue of leptons. The mass of the Universe was thendominated by photons as it entered the following photon

epoch.Leptons have various intrinsic properties, includingelectric charge, spin, and mass. Unlike quarks how-ever, leptons are not subject to the strong interaction, butthey are subject to the other three fundamental interac-tions: gravitation (best described at present by generalrelativity), electroweak (includes electromagnetism), andthe weak interaction. During the quark epoch, the elec-troweak force split into the electromagnetic and weakforces. Interactions can be described by renormalizedquantum field theory, and are mediated by gauge bosonsthat correspond to a particular type of gauge symmetry.A renormalized quantum field theory of general relativityhas not yet been achieved.

5.4.3 Bosons

See also: Higgs boson

Bosons may be either elementary, like photons, orcomposite, like mesons. Bosons are characterized byBose–Einstein statistics and all have integer spins. Whilemost bosons are composite particles, and these are im-portant in superfluidity and other applications of Bose–Einstein condensates, in the Standard Model there arefive bosons which are elementary: photons, which carrythe electromagnetic interaction; W and Z bosons, whichcarry the weak interaction; gluons, which carry the stronginteraction, and the scalar boson Higgs Boson.[104] Addi-tionally, the graviton is a hypothetical elementary parti-cle not incorporated in the Standard Model. If it exists,a graviton must be a boson, and could conceivably be agauge boson.The Higgs boson or Higgs particle is an elementaryparticle in the Standard Model. Observations ofthe particle allows scientists to explore the Higgsfield[105][106]—a fundamental field of crucial importanceto particle physics theory, that unlike the more familiarelectromagnetic field cannot be “turned off”, but insteadtakes a non-zero constant value almost everywhere.[106]The presence of this field, now believed to be con-firmed, explains why some fundamental particles havemass even though the symmetries controlling their inter-actions should require them to be massless, and also an-swers several other long-standing puzzles in physics, suchas the reason the weak force has a much shorter rangethan the electromagnetic force. The Higgs field can bedetected through its excitations (i.e. Higgs particles), butthese are extremely hard to produce and detect.

5.4.4 Photons

Main article: Photon epochSee also: Photino

9

A photon is an elementary particle, the quantum of lightand all other forms of electromagnetic radiation. It isthe force carrier for the electromagnetic force, even whenstatic via virtual photons. The effects of this force are eas-ily observable at the microscopic and at the macroscopiclevel because the photon has zero rest mass; this allowslong distance interactions. Like all elementary particles,photons are currently best explained by quantummechan-ics and exhibit wave–particle duality, exhibiting proper-ties of waves and of particles. For example, a single pho-tonmay be refracted by a lens or exhibit wave interferencewith itself but also act as a particle giving a definite resultwhen its position is measured.In the Standard Model, the photon is one of four gaugebosons in the electroweak interaction; the other threeare denoted W+, W− and Z0 and are responsible for theweak interaction. Unlike the photon, these gauge bosonshave mass, owing to a mechanism that breaks their SU(2)gauge symmetry.The photon epoch started after most leptons and anti-leptons were annihilated at the end of the lepton epoch,about 10 seconds after the Big Bang. Atomic nuclei werecreated in the process of nucleosynthesis which occurredduring the first few minutes of the photon epoch. For theremainder of the photon epoch the universe contained ahot dense plasma of nuclei, electrons and photons. About380,000 years after the Big Bang the temperature of theuniverse fell to the point where nuclei could combine withelectrons to create neutral atoms. As a result, photons nolonger interacted frequently with matter, the universe be-came transparent and the cosmic microwave backgroundradiation was created and then structure formation tookplace.In the Standard Model, photons and other elementaryparticles are described as a necessary consequence ofphysical laws having a certain symmetry at every pointin spacetime. The intrinsic properties of particles, suchas electric charge, mass and spin, are determined by theproperties of this gauge symmetry.

6 Geometry

See also: Physics beyond the Standard Model andElementary particle § Beyond the Standard Model

In modern physics, space and time are unified in a four-dimensional Minkowski continuum (a single interwovencontinuum) called Spacetime, with the goal to create aphase space (dynamical system in which all possible statesof a system are represented), whose metric treats the timedimension differently from the three spatial dimensions.The geometry of 4-dimensional space is muchmore com-plex than that of 3-dimensional space, due to the ex-tra degree of freedom (time, t). The set of points inEuclidean 4-space having the same distance R from a

High-precision test of general relativity by the Cassini space probe(artist’s impression): radio signals sent between the Earth and theprobe (green wave) are delayed by the warping of space and time(blue lines) due to the Sun's mass.

fixed point P0 forms a hypersurface known as a 3-sphere(see also Hypersphere). The hypervolume of the enclosedspace can be calculated with:

V = 12π

2R4

This is part of the Friedmann–Lemaître–Robertson–Walker metric in General relativity where R is substitutedby function R(t) with t meaning the cosmological age ofthe universe. Growing or shrinking R with time meansexpanding or collapsing universe, depending on the massdensity inside.[107] Thus, after the singularity, space be-gun to expand, andmodels try to recapture past and futureevolution of the Universe, and to understand the funda-mental principles at work.

7 Theories of physics

See also: Supersymmetry

10 7 THEORIES OF PHYSICS

7.1 Special relativity

Main articles: Introduction to special relativity andSpecial relativity

Special relativity is the generally accepted physical the-ory regarding the relationship between space and time. Itis based on two postulates: (1) that the laws of physicsare invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference); and (2) that the speedof light in a vacuum is the same for all observers, regard-less of the motion of the light source. It was originallyproposed in 1905 by Albert Einstein in the paper "On theElectrodynamics of Moving Bodies".[108] The inconsis-tency of Newtonian mechanics with Maxwell’s equationsof electromagnetism and the inability to discover Earth’smotion through a luminiferous aether led to the develop-ment of special relativity, which corrects mechanics tohandle situations involving motions nearing the speed oflight. As of today, special relativity is the most accuratemodel of motion at any speed. Even so, Newtonian me-chanics is still useful (due to its simplicity and high accu-racy) as an approximation at small velocities relative tothe speed of light.Special relativity implies a wide range of consequences,which have been experimentally verified,[109] includinglength contraction, time dilation, relativistic mass, mass–energy equivalence, a universal speed limit, and relativityof simultaneity. It has replaced the conventional no-tion of an absolute universal time with the notion of atime that is dependent on reference frame and spatialposition. Rather than an invariant time interval be-tween two events, there is an invariant spacetime in-terval. Combined with other laws of physics, the twopostulates of special relativity predict the equivalence ofmass and energy, as expressed in the mass–energy equiv-alence formula E = mc2, where c is the speed of light invacuum.[110][111]

A defining feature of special relativity is the replacementof the Galilean transformations of Newtonian mechanicswith the Lorentz transformations. Time and space can-not be defined separately from each other. Rather spaceand time are interwoven into a single continuum knownas spacetime. Events that occur at the same time for oneobserver could occur at different times for another.

7.2 General relativity

Main articles: Introduction to general relativity, Generalrelativity and Einstein’s field equations

General relativity is the geometric theory of gravitationpublished by Albert Einstein in 1915[112] and the cur-rent description of gravitation in modern physics. It isthe basis of current cosmological models of a consistentlyexpanding universe. General relativity generalizes special

relativity and Newton’s law of universal gravitation, pro-viding a unified description of gravity as a geometricproperty of space and time, or spacetime. In particu-lar, the curvature of spacetime is directly related to theenergy and momentum of whatever matter and radiationare present. The relation is specified by the Einstein fieldequations, a system of partial differential equations.Some predictions of general relativity differ significantlyfrom those of classical physics, especially concerning thepassage of time, the geometry of space, the motion ofbodies in free fall, and the propagation of light. Exam-ples of such differences include gravitational time dila-tion, gravitational lensing, the gravitational redshift oflight, and the gravitational time delay. The predictionsof general relativity have been confirmed in all observa-tions and experiments to date. Although general relativityis not the only relativistic theory of gravity, it is the sim-plest theory that is consistent with experiments and ob-servations. However, unanswered questions remain, themost fundamental being how general relativity can be rec-onciled with the laws of quantum physics to produce acomplete and self-consistent theory of quantum gravity.

7.2.1 Model of the Universe based on general rela-tivity

Main article: Solutions of the Einstein field equationsSee also: Big Bang and Ultimate fate of the Universe

In general relativity, the distribution of matter and energydetermines the geometry of spacetime, which in turn de-scribes the acceleration of matter. Therefore, solutions ofthe Einstein field equations describe the evolution of theUniverse. Combined with measurements of the amount,type, and distribution of matter in the Universe, the equa-tions of general relativity describe the evolution of theUniverse over time.[113]

With the assumption of the cosmological principle thatthe Universe is homogeneous and isotropic everywhere,a specific solution of the field equations that describesthe Universe is the metric tensor called the Friedmann–Lemaître–Robertson–Walker metric,

ds2 = −c2dt2+R(t)2(

dr2

1− kr2+ r2dθ2 + r2 sin2 θ dϕ2

)where (r, θ, φ) correspond to a spherical coordinate sys-tem. This metric has only two undetermined parameters.An overall length scale R describes the size scale of theUniverse as a function of time; an increase in R is theexpansion of the Universe. A curvature index k describesthe geometry. The index k can be only 0, correspondingto flat Euclidean geometry, 1, corresponding to a spaceof positive curvature, or −1, a space of positive or nega-tive curvature. One can solve for the history of the Uni-verse by calculating R as a function of time t, given k and

7.3 Multiverse hypothesis 11

the value of the cosmological constant Λ.[113] The cos-mological constant represents the energy density of thevacuum of space and could be related to dark energy.[74]The equation describing how R varies with time is knownas the Friedmann equation after its inventor, AlexanderFriedmann.[114]

The solutions for R(t) depend on k and Λ, but somequalitative features of such solutions are general. Firstand most importantly, the length scale R of the Uni-verse can remain constant only if the Universe is perfectlyisotropic with positive curvature (k=1) and has one pre-cise value of density everywhere, as first noted by AlbertEinstein.[113] However, this equilibrium is unstable andbecause the Universe is known to be inhomogeneous onsmaller scales, so R must change. When R changes, allthe spatial distances in the Universe change in tandem;there is an overall expansion or contraction of space it-self. This accounts for the observation that galaxies ap-pear to be flying apart; the space between them is stretch-ing. The stretching of space also accounts for the appar-ent paradox that two galaxies can be 40 billion light yearsapart, although they started from the same point 13.8 bil-lion years ago[115] and never moved faster than the speedof light.Second, all solutions suggest that there was a gravitationalsingularity in the past, whenR goes to zero andmatter andenergy became infinitely dense. It may seem that this con-clusion is uncertain because it is based on the questionableassumptions of perfect homogeneity and isotropy (thecosmological principle) and that only the gravitational in-teraction is significant. However, the Penrose–Hawkingsingularity theorems show that a singularity should ex-ist for very general conditions. Hence, according to Ein-stein’s field equations, R grew rapidly from an unimagin-ably hot, dense state that existed immediately followingthis singularity (when R had a small, finite value); this isthe essence of the Big Bang model of the Universe.Third, the curvature index k determines the sign of themean spatial curvature of spacetime averaged over suf-ficiently large length scales (greater than about a billionlight years). If k=1, the curvature is positive and the Uni-verse has a finite volume. Such universes are often vi-sualized as a three-dimensional sphere 3-sphere embed-ded in a four-dimensional space. Conversely, if k is zeroor negative, the Universe may have infinite volume, de-pending on its overall topology. It may seem counter-intuitive that an infinite and yet infinitely dense Universecould be created in a single instant at the Big Bang whenR=0, but exactly that is predicted mathematically when kdoes not equal 1. For comparison, an infinite plane haszero curvature but infinite area, whereas an infinite cylin-der is finite in one direction and a torus is finite in both.A toroidal Universe could behave like a normal Universewith periodic boundary conditions.The ultimate fate of the Universe is still unknown, be-cause it depends critically on the curvature index k and the

cosmological constant Λ. If the Universe were sufficientlydense, k would equal +1, meaning that its average curva-ture throughout is positive and the Universe will eventu-ally recollapse in a Big Crunch, possibly starting a newUniverse in a Big Bounce. Conversely, if the Universewere insufficiently dense, k would equal 0 or −1 and theUniverse would expand forever, cooling off and eventu-ally reaching the Big Freeze and the heat death of the Uni-verse.[113] Modern data suggests that the expansion speedof the Universe is not decreasing as originally expected,but increasing; if this continues indefinitely, the Universemay eventually reach a Big Rip. Observationally, the Uni-verse appears to be flat (k = 0), with an overall density thatis very close to the critical value between recollapse andeternal expansion.

7.3 Multiverse hypothesis

Main articles: Multiverse, Many-worlds interpretation,Bubble universe theory and Parallel universe (fiction)See also: Eternal inflation

Depiction of a multiverse of seven “bubble” universes, whichare separate spacetime continua, each having different physicallaws, physical constants, and perhaps even different numbers ofdimensions or topologies.

Some speculative theories have proposed that our Uni-verse is but one of a set of disconnected universes, collec-tively denoted as the multiverse, challenging or enhancingmore limited definitions of the Universe.[20][116] Scien-tific multiverse models are distinct from concepts such asalternate planes of consciousness and simulated reality.Max Tegmark developed a four-part classificationscheme for the different types of multiverses that sci-entists have suggested in various problem domains. Anexample of such a model is the chaotic inflation modelof the early universe.[117] Another is the many-worlds in-terpretation of quantum mechanics. Parallel worlds aregenerated in a manner similar to quantum superpositionand decoherence, with all states of the wave function be-

12 8 HISTORICAL DEVELOPMENT

ing realized in separate worlds. Effectively, the multi-verse evolves as a universal wavefunction. If the Big Bangthat created our multiverse created an ensemble of mul-tiverses, the wave function of the ensemble would be en-tangled in this sense.The least controversial category of multiverse inTegmark’s scheme is Level I, which describes distantspacetime events “in our own universe”, but suggests thatstatistical analysis exploiting the anthropic principle pro-vides an opportunity to test multiverse theories in somecases. If space is infinite, or sufficiently large and uni-form, identical instances of the history of Earth’s entireHubble volume occur every so often, simply by chance.Tegmark calculated our nearest so-called doppelgänger,is 1010115 meters away from us (a double exponentialfunction larger than a googolplex).[118][119] In principle,it would be impossible to scientifically verify an identi-cal Hubble volume. However, it does follow as a fairlystraightforward consequence from otherwise unrelatedscientific observations and theories.It is possible to conceive of disconnected spacetimes,each existing but unable to interact with oneanother.[118][120] An easily visualized metaphor is agroup of separate soap bubbles, in which observers livingon one soap bubble cannot interact with those on othersoap bubbles, even in principle.[121] According to onecommon terminology, each “soap bubble” of spacetimeis denoted as a universe, whereas our particular spacetimeis denoted as the Universe,[20] just as we call our moon theMoon. The entire collection of these separate spacetimesis denoted as the multiverse.[20] With this terminology,different Universes are not causally connected to eachother.[20] In principle, the other unconnected Universesmay have different dimensionalities and topologies ofspacetime, different forms of matter and energy, anddifferent physical laws and physical constants, althoughsuch possibilities are purely speculative.[20] Othersconsider each of several bubbles created as part ofchaotic inflation to be separate Universes, though in thismodel these universes all share a causal origin.[20]

7.4 Fine-tuned Universe

Main article: Fine-tuned Universe

The fine-tuned Universe is the proposition that the con-ditions that allow life in the Universe can only oc-cur when certain universal fundamental physical con-stants lie within a very narrow range, so that if anyof several fundamental constants were only slightly dif-ferent, the Universe would be unlikely to be conduciveto the establishment and development of matter, as-tronomical structures, elemental diversity, or life as itis understood.[122] The proposition is discussed amongphilosophers, scientists, theologians, and proponents anddetractors of creationism.

8 Historical development

See also: Cosmology, Timeline of cosmology, NicolausCopernicus § Copernican system and PhilosophiæNaturalis Principia Mathematica § Beginnings of theScientific Revolution

Historically, there have been many ideas of the cosmos(cosmologies) and its origin (cosmogonies). Theories ofan impersonal Universe governed by physical laws werefirst proposed by the Greeks and Indians.[14] Ancient Chi-nese philosophy encompassed the notion of the Universeincluding both all of space and all of time.[123][124] Overthe centuries, improvements in astronomical observationsand theories of motion and gravitation led to ever moreaccurate descriptions of the Universe. The modern eraof cosmology began with Albert Einstein's 1915 generaltheory of relativity, which made it possible to quantita-tively predict the origin, evolution, and conclusion of theUniverse as a whole. Most modern, accepted theoriesof cosmology are based on general relativity and, morespecifically, the predicted Big Bang.[125]

8.1 Mythologies

Main articles: Creation myth, Creator deity and Religiouscosmology

Many cultures have stories describing the origin of theworld and universe. Cultures generally regard these sto-ries as having some truth.[126] There are however manydiffering beliefs in how these stories apply amongst thosebelieving in a supernatural origin, ranging from a god di-rectly creating the Universe as it is now to a god just set-ting the “wheels in motion” (for example via mechanismssuch as the big bang and evolution).Creation stories may be roughly grouped into commontypes. In one type of story, the world is born from a worldegg; such stories include the Finnish epic poem Kalevala,the Chinese story of Pangu or the Indian Brahmanda Pu-rana. In related stories, the Universe is created by a singleentity emanating or producing something by him- or her-self, as in the Tibetan Buddhism concept of Adi-Buddha,the ancient Greek story ofGaia (Mother Earth), theAztecgoddess Coatlicue myth, the ancient Egyptian god Atumstory, and the Judeo-Christian Genesis creation narrativein which the Abrahamic God created the Universe. Inanother type of story, the Universe is created from theunion of male and female deities, as in the Maori storyof Rangi and Papa. In other stories, the Universe is cre-ated by crafting it from pre-existing materials, such as thecorpse of a dead god— as from Tiamat in the BabylonianepicEnuma Elish or from the giant Ymir inNorsemythol-ogy – or from chaotic materials, as in Izanagi and Izanamiin Japanese mythology. In other stories, the Universe em-anates from fundamental principles, such as Brahman and

8.3 Astronomical concepts 13

Prakrti, the creationmyth of the Serers,[127] or the yin andyang of the Tao.

8.2 Philosophical models

Further information: CosmologySee also: Pre-Socratic philosophy, Physics (Aristotle),Hindu cosmology, Islamic cosmology and Philosophy ofspace and time

The pre-Socratic Greek philosophers and Indian philoso-phers developed some of the earliest philosophical con-cepts of the Universe.[14][128] The earliest Greek philoso-phers noted that appearances can be deceiving, andsought to understand the underlying reality behind the ap-pearances. In particular, they noted the ability of matterto change forms (e.g., ice to water to steam) and sev-eral philosophers proposed that all the physical materi-als in the world are different forms of a single primor-dial material, or arche. The first to do so was Thales,who proposed this material is water. Thales’ student,Anaximander, proposed that everything came from thelimitless apeiron. Anaximenes proposed air on accountof its perceived attractive and repulsive qualities thatcause the arche to condense or dissociate into differ-ent forms. Anaxagoras proposed the principle of Nous(Mind). Heraclitus proposed fire (and spoke of logos).Empedocles proposed the elements: earth, water, airand fire. His four-element model became very popu-lar. Like Pythagoras, Plato believed that all things werecomposed of number, with Empedocles’ elements tak-ing the form of the Platonic solids. Democritus, andlater philosophers—most notably Leucippus—proposedthat the Universe is composed of indivisible atoms mov-ing through void (vacuum). Aristotle did not believe thatwas feasible because air, like water, offers resistance tomotion. Air will immediately rush in to fill a void, andmoreover, without resistance, it would do so indefinitelyfast.Although Heraclitus argued for eternal change, his con-temporary Parmenides made the radical suggestion thatall change is an illusion, that the true underlying reality iseternally unchanging and of a single nature. Parmenidesdenoted this reality as τὸ ἐν (The One). Parmenides’ ideaseemed implausible to many Greeks, but his student Zenoof Elea challenged them with several famous paradoxes.Aristotle responded to these paradoxes by developing thenotion of a potential countable infinity, as well as the in-finitely divisible continuum. Unlike the eternal and un-changing cycles of time, he believed the world is boundedby the celestial spheres and that cumulative stellar mag-nitude is only finitely multiplicative.The Indian philosopher Kanada, founder of theVaisheshika school, developed a notion of atomism andproposed that light and heat were varieties of the samesubstance.[129] In the 5th century AD, the Buddhist

atomist philosopher Dignāga proposed atoms to bepoint-sized, durationless, and made of energy. Theydenied the existence of substantial matter and proposedthat movement consisted of momentary flashes of astream of energy.[130]

The notion of temporal finitism was inspired by the doc-trine of creation shared by the three Abrahamic religions:Judaism, Christianity and Islam. The Christian philoso-pher, John Philoponus, presented the philosophical ar-guments against the ancient Greek notion of an infinitepast and future. Philoponus’ arguments against an infinitepast were used by the earlyMuslim philosopher, Al-Kindi(Alkindus); the Jewish philosopher, Saadia Gaon (Saadiaben Joseph); and the Muslim theologian, Al-Ghazali (Al-gazel).

8.3 Astronomical concepts

Main articles: History of astronomy and Timeline of as-tronomyAstronomical models of the Universe were proposed

Aristarchus’s 3rd century BCE calculations on the relative sizesof from left the Sun, Earth and Moon, from a 10th-century ADGreek copy

soon after astronomy began with the Babylonian as-tronomers, who viewed the Universe as a flat disk floatingin the ocean, and this forms the premise for early Greekmaps like those of Anaximander and Hecataeus of Mile-tus.Later Greek philosophers, observing the motions of theheavenly bodies, were concerned with developing mod-els of the Universe-based more profoundly on empiricalevidence. The first coherent model was proposed byEudoxus of Cnidos. According to Aristotle’s physical in-terpretation of themodel, celestial spheres eternally rotatewith uniform motion around a stationary Earth. Normalmatter is entirely contained within the terrestrial sphere.De Mundo (composed before 250 BC or between 350and 200 BC), stated, Five elements, situated in spheres infive regions, the less being in each case surrounded by thegreater — namely, earth surrounded by water, water byair, air by fire, and fire by ether — make up the whole

14 8 HISTORICAL DEVELOPMENT

Universe.[131]

This model was also refined by Callippus and after con-centric spheres were abandoned, it was brought intonearly perfect agreement with astronomical observationsby Ptolemy. The success of such a model is largely dueto the mathematical fact that any function (such as theposition of a planet) can be decomposed into a set of cir-cular functions (the Fourier modes). Other Greek scien-tists, such as the Pythagorean philosopher Philolaus, pos-tulated (according to Stobaeus account) that at the cen-ter of the Universe was a “central fire” around which theEarth, Sun, Moon and Planets revolved in uniform circu-lar motion.[132]

The Greek astronomer Aristarchus of Samos was thefirst known individual to propose a heliocentric model ofthe Universe. Though the original text has been lost, areference in Archimedes' book The Sand Reckoner de-scribes Aristarchus’s heliocentric model. Archimedeswrote: (translated into English):

“You, King Gelon, are aware the Universeis the name given by most astronomers to thesphere the center of which is the center of theEarth, while its radius is equal to the straightline between the center of the Sun and thecenter of the Earth. This is the common ac-count as you have heard from astronomers. ButAristarchus has brought out a book consistingof certain hypotheses, wherein it appears, as aconsequence of the assumptions made, that theUniverse is many times greater than the Uni-verse just mentioned. His hypotheses are thatthe fixed stars and the Sun remain unmoved,that the Earth revolves about the Sun on thecircumference of a circle, the Sun lying in themiddle of the orbit, and that the sphere of fixedstars, situated about the same center as the Sun,is so great that the circle in which he supposesthe Earth to revolve bears such a proportion tothe distance of the fixed stars as the center ofthe sphere bears to its surface”

Aristarchus thus believed the stars to be very far away,and saw this as the reason why stellar parallax had notbeen observed, that is, the stars had not been observed tomove relative each other as the Earth moved around theSun. The stars are in fact much farther away than the dis-tance that was generally assumed in ancient times, whichis why stellar parallax is only detectable with precisioninstruments. The geocentric model, consistent with plan-etary parallax, was assumed to be an explanation for theunobservability of the parallel phenomenon, stellar paral-lax. The rejection of the heliocentric view was apparentlyquite strong, as the following passage from Plutarch sug-gests (On the Apparent Face in the Orb of the Moon):

"Cleanthes [a contemporary of Aristarchusand head of the Stoics ] thought it was the duty

of the Greeks to indict Aristarchus of Samoson the charge of impiety for putting in motionthe Hearth of the Universe [i.e. the Earth], .. . supposing the heaven to remain at rest andthe Earth to revolve in an oblique circle, whileit rotates, at the same time, about its own axis”

Flammarion engraving, Paris 1888

The only other astronomer from antiquity known byname who supported Aristarchus’s heliocentric modelwas Seleucus of Seleucia, a Hellenistic astronomer wholived a century after Aristarchus.[133][134][135] Accordingto Plutarch, Seleucus was the first to prove the heliocen-tric system through reasoning, but it is not known whatarguments he used. Seleucus’ arguments for a heliocen-tric cosmology were probably related to the phenomenonof tides.[136] According to Strabo (1.1.9), Seleucus wasthe first to state that the tides are due to the attractionof the Moon, and that the height of the tides dependson the Moon’s position relative to the Sun.[137] Alter-natively, he may have proved heliocentricity by deter-mining the constants of a geometric model for it, andby developing methods to compute planetary positionsusing this model, like what Nicolaus Copernicus laterdid in the 16th century.[138] During the Middle Ages,heliocentric models were also proposed by the Indian as-tronomer Aryabhata,[139] and by the Persian astronomersAlbumasar[140] and Al-Sijzi.[141]

The Aristotelian model was accepted in the Westernworld for roughly two millennia, until Copernicus revivedAristarchus’s perspective that the astronomical data couldbe explained more plausibly if the earth rotated on its axisand if the sun were placed at the center of the Universe.As noted by Copernicus himself, the notion that the Earthrotates is very old, dating at least to Philolaus (c. 450BC), Heraclides Ponticus (c. 350 BC) and Ecphantus thePythagorean. Roughly a century before Copernicus, theChristian scholar Nicholas of Cusa also proposed that theEarth rotates on its axis in his book, On Learned Igno-rance (1440).[142] Aryabhata (476–550 AD/CE)[143] and

15

Model of the Copernican Universe by Thomas Digges in 1576,with the amendment that the stars are no longer confined to asphere, but spread uniformly throughout the space surroundingthe planets.

Al-Sijzi[144] also proposed that the Earth rotates on itsaxis. Empirical evidence for the Earth’s rotation on itsaxis, using the phenomenon of comets, was given by Tusi(1201–1274) and Ali Qushji (1403–1474).[145]

This cosmology was accepted by Isaac Newton,Christiaan Huygens and later scientists.[146] EdmundHalley (1720)[147] and Jean-Philippe de Chéseaux(1744)[148] noted independently that the assumption ofan infinite space filled uniformly with stars would lead tothe prediction that the nighttime sky would be as brightas the Sun itself; this became known as Olbers’ paradoxin the 19th century.[149] Newton believed that an infinitespace uniformly filled with matter would cause infiniteforces and instabilities causing the matter to be crushedinwards under its own gravity.[146] This instability wasclarified in 1902 by the Jeans instability criterion.[150]One solution to these paradoxes is the Charlier Universe,in which the matter is arranged hierarchically (systemsof orbiting bodies that are themselves orbiting in alarger system, ad infinitum) in a fractal way such thatthe Universe has a negligibly small overall density;such a cosmological model had also been proposedearlier in 1761 by Johann Heinrich Lambert.[57][151] Asignificant astronomical advance of the 18th century wasthe realization by Thomas Wright, Immanuel Kant andothers of nebulae.[147]

The modern era of physical cosmology began in 1917,when Albert Einstein first applied his general theory ofrelativity to model the structure and dynamics of theUniverse.[152]

9 See also• Cosmic Calendar (scaled down timeline)

• Cosmic latte

• Dyson’s eternal intelligence

• Esoteric cosmology

• False vacuum

• Fine-tuned Universe

• Hindu cosmology

• Illustris project

• Jain cosmology

• Kardashev scale

• Galaxy And Mass Assembly survey

• Nucleocosmochronology

• Non-standard cosmology

• Rare Earth hypothesis

• Vacuum genesis

• World view

• Zero-energy Universe

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[2] Itzhak Bars; John Terning (2009). Extra Dimensions inSpace and Time. Springer. pp. 27ff. ISBN 978-0-387-77637-8. Retrieved 2011-05-01.

[3] Paul Davies (2006). The Goldilocks Enigma. FirstMariner Books. p. 43ff. ISBN 978-0-618-59226-5. Re-trieved 2013-07-01.

[4] NASA/WMAP Science Team (24 January 2014).“Universe 101: What is the Universe Made Of?". NASA.Retrieved 2015-02-17.

[5] Fixsen, D. J. (2009). “The Temperature of theCosmic Microwave Background”. The Astrophys-ical Journal 707 (2): 916–920. arXiv:0911.1955.Bibcode:2009ApJ...707..916F. doi:10.1088/0004-637X/707/2/916.

[6] Sean Carroll, Ph.D., Cal Tech, 2007, The Teaching Com-pany, Dark Matter, Dark Energy: The Dark Side of theUniverse, Guidebook Part 1 pages 1 and 3, Accessed Oct.7, 2013, "...only 5% of the Universe is made of ordinarymatter, with 25 percent being some kind of unseen darkmatter and a full 70% being a smoothly distributed darkenergy...”

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21

11 Text and image sources, contributors, and licenses

11.1 Text• Universe Source: https://en.wikipedia.org/wiki/Universe?oldid=676707660 Contributors: AxelBoldt, Lee Daniel Crocker, CYD, Bryan

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Brown, Honeycake, Alansohn, Atlant, Mr Adequate, Jeltz,WTGDMan1986, Andrewpmk, Plumbago, Bblackmoor, AzaToth, Lightdarkness, Fritzpoll, Kel-nage, Malo, Snowolf, Magnoliasucks,Wtmitchell, Velella, ProhibitOnions, Knowledge Seeker, RaiderRobert, Vcelloho, Eddie Dealtry, Bsadowski1, Reaverdrop, SteinbDJ,MIT Trekkie, Johntex, HenryLi, Kazvorpal, Oleg Alexandrov, Quirkie, WilliamKF, Jeffrey O. Gustafson, Woohookitty, Mindmatrix,FeanorStar7, Shreevatsa, Daniel Case, Uncle G, Savantnavas, Ruud Koot, WadeSimMiser, JeremyA, Chochopk, MONGO, Jok2000, Uris,Trevor Andersen, Jleon, Bbatsell, Sengkang, GregorB, Andromeda321, SDC, CharlesC, TheAlphaWolf, Joke137, Christopher Thomas,Palica, Dysepsion, GSlicer, Wulfila, Rnt20, Graham87, Deltabeignet, Magister Mathematicae, BD2412, Zeroparallax, FreplySpang, Zoz,Canderson7, Drbogdan, Jorunn, Rjwilmsi, Mayumashu, Joe Decker, P3Pp3r, Nightscream, Koavf, Zbxgscqf, Jake Wartenberg, Rillian,SMC, Mike Peel, The wub, DoubleBlue, Yamamoto Ichiro, Dionyseus, FayssalF, Old Moonraker, Chanting Fox, RexNL, Gurch, TheDJ,Gakon5, Thewolrab, TeaDrinker, Dsewell, Butros, King of Hearts, Chobot, Sharkface217, DVdm, Citizen Premier, Scoo, Napate, Gw-ernol, Wjfox2005, The Rambling Man, Siddhant, Siddharth Prabhu, YurikBot, Wavelength, Spacepotato, Err0neous, Vedranf, Splinter-cellguy, Sceptre, Blightsoot, Nipponese, Jimp, Retodon8, StuffOfInterest, RussBot, Petiatil, Hyad, Anonymous editor, Bhny, SpuriousQ,Stephenb, Chaos, Bullzeye, NawlinWiki, SEWilcoBot, Neural, Grafen, Erielhonan, Jaxl, RazorICE, Nsmith 84, Irishguy, Randolf Richard-son, Chrisbrl88, Matticus78, Rmky87, Haoie, Saggipie, Iamnotanorange~enwiki, Epipelagic, SFC9394, Roy Brumback, DeadEyeArrow,Psy guy, Martinwilke1980, Nlu, Dna-webmaster, Dv82matt, Jpmccord, 2over0, Lt-wiki-bot, Ageekgal, Breakfastchief, Theda, Closed-mouth, Arthur Rubin, KGasso, Nemu, Th1rt3en, Reyk, Exodio, GraemeL, JoanneB, LeonardoRob0t, Leeannedy, ArielGold, Caco devidro, Aeosynth, RG2, JuniorMuruin, Serendipodous, DVD R W, Eog1916, Dupz, Kicking222, Sardanaphalus, SmackBot, Roger Davies,Mehranwahid, Ashill, Kurochka, Zazaban, KnowledgeOfSelf, Olorin28, McGeddon, Unyoyega, Jacek Kendysz, Jagged 85, Davewild,WookieInHeat, Delldot, Hardyplants, Shai-kun, DreamOfMirrors, Gaff, Onsly, JFHJr, Gilliam, Ohnoitsjamie, Wlmg, Skizzik, Saros136,Amatulic, Rrscott, Persian Poet Gal, Telempe, Exploreuniverse, Miquonranger03, MalafayaBot, Silly rabbit, SchfiftyThree, Hibernian,Hurdygurdyman1234, Octahedron80, EdgeOfEpsilon, Patriarch, DHN-bot~enwiki, Sbharris, Sahsan~enwiki, Darth Panda, Firetrap9254,Bangarangmanchester, Diyako, Scwlong, Tsca.bot, Shalom Yechiel, Hve, Vanished User 0001, Vere scatman, Yidisheryid, Xiner, Rrburke,Addshore, Lobner, SundarBot, UU, Madman2001, Aldaron, Krich, Tvaughn05, Cybercobra, Kntrabssi, John D. 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Campbell, Q2op, Barts1a, Katalaveno, Ncmvocalist, McSly, Mikael Häggström, Gurchzilla, Bilbobee, Pyrospirit,Qazwsx197966, Spig a digs, Vanished User 4517, TomasBat, NewEnglandYankee, Djambalawa, Rebel700, Trilobitealive, SJP, Bobianite,LeighvsOptimvsMaximvs, Jorfer, Zojj, Mufka, Student7, Rickmeister~enwiki, Terik brunson, MetsFan76, KylieTastic, Mattu00, Remem-ber the dot, Gwen Gale, Sinep2, Vanished user 39948282, DavyJonesGSB, Robbiemasters89, HiEv, Cuckooman4, Bonadea, Rickmeister6,Dude00311, JavierMC, Martial75, Xiahou, Squids and Chips, Steel1943, CardinalDan, Idioma-bot, Funandtrvl, Thedjatclubrock, ABF,Jeff G., Fences and windows, Soriano9, Abhiag, Philip Trueman, Jhon montes24, TXiKiBoT, Dang3210, Cosmic Latte, Kip the Dip, Vipin-hari, Canuckle, Hqb, GDonato, DarrynJ, Ridernyc, Anonymous Dissident, GcSwRhIc, Vishal144, Qxz, Someguy1221, Trahern1994, AnnaLincoln, Lradrama, Clarince63, John haley, Patssle, JhsBot, Bob Andolusorn, Abdullais4u, Fbs. 13, LeaveSleaves, Manchurian candidate,UnitedStatesian, Dantheman2008, Geometry guy, Saturn star, Nighthawk380, Knightshield, SheffieldSteel, Billinghurst, Lamro, Maethor-

22 11 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

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11.2 Images 23

rerrrrrrfjgfufgvgjfjfjjfjfggfu, Catman3659, KasparBot and Anonymous: 1854

11.2 Images• File:080998_Universe_Content_240.jpg Source: https://upload.wikimedia.org/wikipedia/commons/a/a5/080998_Universe_Content_

240.jpg License: Public domain Contributors: http://map.gsfc.nasa.gov/media/080998/index.html Original artist: Credit: NASA / WMAPScience Team

• File:121236_NewPieChart320.png Source: https://upload.wikimedia.org/wikipedia/commons/3/32/121236_NewPieChart320.png Li-cense: Public domain Contributors: http://wmap.gsfc.nasa.gov/news/index.html Original artist: NASA/WMAP Science Team

• File:Aristarchus_working.jpg Source: https://upload.wikimedia.org/wikipedia/commons/2/2b/Aristarchus_working.jpg License: Pub-lic domain Contributors: ? Original artist: ?

• File:CMB_Timeline300_no_WMAP.jpg Source: https://upload.wikimedia.org/wikipedia/commons/6/6f/CMB_Timeline300_no_WMAP.jpg License: Public domain Contributors: Original version: NASA; modified by Ryan Kaldari Original artist: NASA/WMAPScience Team

• File:Cassini-science-br.jpg Source: https://upload.wikimedia.org/wikipedia/commons/2/24/Cassini-science-br.jpg License: Public do-main Contributors: ? Original artist: ?

• File:Commons-logo.svg Source: https://upload.wikimedia.org/wikipedia/en/4/4a/Commons-logo.svg License: ? Contributors: ? Originalartist: ?

• File:Crab_Nebula.jpg Source: https://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg License: Public domain Con-tributors: HubbleSite: gallery, release. Original artist: NASA, ESA, J. Hester and A. Loll (Arizona State University)

• File:Earth’{}s_Location_in_the_Universe_SMALLER_(JPEG).jpg Source: https://upload.wikimedia.org/wikipedia/commons/0/0f/Earth%27s_Location_in_the_Universe_SMALLER_%28JPEG%29.jpg License: CC BY-SA 3.0 Contributors: Own work Original artist:Andrew Z. Colvin

• File:Earth-moon.jpg Source: https://upload.wikimedia.org/wikipedia/commons/5/5c/Earth-moon.jpg License: Public domain Contribu-tors: NASA [1] Original artist: Apollo 8 crewmember Bill Anders

• File:Elementary_particle_interactions_in_the_Standard_Model.png Source: https://upload.wikimedia.org/wikipedia/commons/a/a7/Elementary_particle_interactions_in_the_Standard_Model.png License: CC0 Contributors: Own work Original artist: Eric Drexler

• File:End_of_universe.jpg Source: https://upload.wikimedia.org/wikipedia/commons/9/98/End_of_universe.jpg License: Public domainContributors: ? Original artist: ?

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