Universe

For other uses, see Universe (disambiguation).

The Universe is all of time and space.[8][9][10][11]This includes planets, stars, galaxies, the contents ofintergalactic space, the smallest subatomic particles, andall matter and energy, the majority of which are mostlikely in the form of dark matter and dark energy.[12][13]

The part of the Universe that we can see, referred to as theobservable universe, is about 28 billion parsecs (91 billionlight-years) in diameter at the present time.[1] The size ofthe whole universe is not known and may be infinite.[14]Scientific observation of the Universe has led to infer-ences about its evolution. These observations suggest thatthe Universe has been governed by the same physical lawsand constants throughout most of its extent and for alltime. The Big Bang theory is the prevailing cosmologicalmodel that describes the development of the Universe.Assuming that the prevailing model is correct, the age ofthe Universe is measured to be 13.798 ± 0.037 billionyears.[15][16] Space in the Universe is expanding, and therate of its expansion is increasing.[17]

There are many competing theories about the ultimatefate of the Universe. Physicists remain unsure aboutwhat, if anything, preceded the Big Bang. Many refuse tospeculate, doubting that any information from any suchprior state could ever be accessible. There are variousmultiverse hypotheses, in which some physicists havesuggested that the Universe might be one among manyuniverses that likewise exist.[14][18][19]

1 Etymology and historical obser-vation

See also: Cosmos, Nature, World (philosophy) andCelestial spheres

The word universe derives from the Old French wordunivers, which in turn derives from the Latin word uni-versum.[20] The Latin word was used by Cicero and laterLatin authors in many of the same senses as the modernEnglish word is used.[21] 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”).[21]

Throughout recorded history, cosmologies andcosmogonies have been proposed to account forobservations of the Universe. The earliest quantitativegeocentric models were developed by the ancient Greekphilosophers and Indian philosophers.[22][23] Over thecenturies, more precise observations led to Copernicus’sheliostatic model of the Solar System and Kepler’sheliocentric and elliptical model of the Solar System.The concept of gravity led to the Newtonian model ofthe Solar System. Further improvements in astronomicalobservations led to the realization that the Solar Systemis located in a galaxy composed of billions of stars,the Milky Way. It was subsequently discovered thatour galaxy is just one of many. Careful studies of thedistribution of these galaxies and their spectral lineshave led to much of modern physical cosmology. Thediscovery in the early 20th century that galaxies aresystematically redshifted suggested that the Universe isexpanding, and the discovery of the cosmic microwavebackground radiation suggested that the Universe had abeginning.[24]

2 Synonyms and definitions

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 din-ner guests.[25] This Greek word refers to celestial spheres,an early Greek model of the Universe. Regarding Plato’sMetaphor of the Sun, Aristotle suggests that the rotationof the sphere of fixed stars inspired by the prime mover,motivates, in turn, terrestrial change via the Sun. Care-ful astronomical and physical measurements (such as theFoucault pendulum) are required to prove the Earth ro-tates on its axis.A term for 'universe' in ancient Greece was τὸ πᾶν (tòpán, The All, Pan (mythology)). Related terms were mat-ter, (τὸ ὅλον, tò hólon, see also Hyle, lit. wood) and place(τὸ κενόν, tò kenón).[26][27] Other synonyms for the Uni-verse among the ancient Greek philosophers included κό-σμος (cosmos) and φύσις (meaning Nature, from whichwe derive the word physics).[28] The same synonyms arefound in Latin authors (totum, mundus, natura)[29] andsurvive in modern languages, e.g., the German wordsDas

1

2 3 HISTORICAL MODELS

All,Weltall, and Natur for Universe. The same synonymsare found in English, such as everything (as in the theoryof everything), the cosmos (as in cosmology), the world(as in the many-worlds interpretation), and Nature (as innatural laws or natural philosophy).[30]

2.1 Broadest definition: reality and prob-ability

See also: Essence–Energies distinction § Distinctionbetween created and uncreated

The broadest definition of the Universe is found inDe divisione naturae by the medieval philosopher andtheologian Johannes Scotus Eriugena, who defined it assimply everything: everything that is created and every-thing that is not created.

2.2 Definition as reality

See also: Reality and Physics

More customarily, the Universe is defined as every-thing that exists, from its beginning to end.[31] Accord-ing to our current understanding, the Universe consistsof three principles: spacetime, forms of energy, includingmomentum and matter, and the physical laws that relatethem.

2.3 Definition as connected spacetime

See also: Eternal inflation

It is possible to conceive of disconnected spacetimes,each existing but unable to interact with one another. Aneasily visualized metaphor is a group of separate soapbubbles, in which observers living on one soap bubblecannot interact with those on other soap bubbles, evenin principle.[32] According to one common terminology,each “soap bubble” of spacetime is denoted as a universe,whereas our particular spacetime is denoted as the Uni-verse,[18] just as we call our moon the Moon. The en-tire collection of these separate spacetimes is denoted asthe multiverse.[18] With this terminology, different uni-verses are not causally connected to each other.[18] Inprinciple, the other unconnected universes may have dif-ferent dimensionalities and topologies of spacetime, dif-ferent forms of matter and energy, and different physicallaws and physical constants, although such possibilitiesare purely speculative.[18] Others consider each of severalbubbles created as part of chaotic inflation to be separateuniverses, though in this model these universes all sharea causal origin.[18]

2.4 Definition as observable reality

See also: Observable universe and Observational cos-mology

According to a still more restrictive definition, the Uni-verse is everything within our connected spacetime thatcould have a chance to interact with us and vice versa.[33]According to the general theory of relativity, some re-gions of space may never interact with ours even in thelifetime of the Universe due to the finite speed of lightand the ongoing expansion of space. For example, radiomessages sent from Earth may never reach some regionsof space, even if the Universe would live forever: spacemay expand faster than light can traverse it.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.

3 Historical models

See also: Cosmology and Timeline of cosmology

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 theGreeks and Indians.[23]Over the cen-turies, improvements in astronomical observations andtheories of motion and gravitation led to ever more accu-rate descriptions of the Universe. The modern era of cos-mology began with Albert Einstein’s 1915 general theoryof relativity, which made it possible to quantitatively pre-dict the origin, evolution, and conclusion of the Universeas a whole. Most modern, accepted theories of cosmol-ogy are based on general relativity and, more specifically,the predicted Big Bang.

3.1 Creation

Main articles: Creation myth and Creator deity

Many cultures have stories describing the origin of theworld and universe. Cultures generally regard these sto-ries as having some truth.[34] There are however manydiffering beliefs in how these stories apply amongst thosebelieving in a supernatural origin, ranging from a god di-

3.2 Philosophical models 3

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 andPrakrti, the creation myth of the Serers,[35] or the yin andyang of the Tao.

3.2 Philosophical models

Further information: CosmologySee also: Pre-Socratic philosophy, Physics (Aristo-tle), Hindu cosmology, Islamic cosmology, Time andPhilosophy of space and time

The pre-Socratic Greek philosophers and Indian philoso-phers developed some of the earliest philosophical con-cepts of the Universe.[23][36] 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 atomismand proposed that light and heat were varieties of thesame substance.[37] In the 5th century AD, the Buddhistatomist 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.[38]

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 (Saa-dia ben Joseph); and the Muslim theologian, Al-Ghazali(Algazel). Borrowing from Aristotle’s Physics and Meta-physics, they employed two logical arguments against aninfinite past, the first being the “argument from the im-possibility of the existence of an actual infinite”, whichstates:[39]

“An actual infinite cannot exist.”“An infinite temporal regress of events is an ac-tual infinite.”" ∴ An infinite temporal regress of events can-not exist.”

The second argument, the “argument from the impossi-bility of completing an actual infinite by successive addi-tion”, states:[39]

“An actual infinite cannot be completed by suc-cessive addition.”“The temporal series of past events has beencompleted by successive addition.”

4 3 HISTORICAL MODELS

" ∴ The temporal series of past events cannotbe an actual infinite.”

Both arguments were adopted by Christian philosophersand theologians, and the second argument in particularbecame more famous after it was adopted by ImmanuelKant in his thesis of the first antinomy concerningtime.[39]

3.2.1 Fine tuning

Main article: Fine-tuned Universe

Many of the properties of the Universe have the appear-ance of having been tuned or selected so as to permit theemergence of intelligent life.[24][40][41] Not all scientistsagree that this fine-tuning exists.[42][43] [44] In particular,it is not known under what conditions intelligent life couldform and what form or shape that would take. A relevantobservation in this discussion is that for an observer toexist to observe fine-tuning, the Universe must be ableto support intelligent life. As such the conditional prob-ability of observing a universe that is fine-tuned to sup-port intelligent life is 1. This observation is known asthe anthropic principle and is particularly relevant if thecreation of the Universe was probabilistic or if multipleuniverses with a variety of properties exist (see below).However, the observation that the chemistry of life mayhave begun shortly after the Big Bang, 13.8 billion yearsago, during a habitable epoch when the Universe was only10–17 million years old, may differ, in part, with the an-thropic principle.[45][46][47]

3.3 Astronomical concepts

Main article: History of astronomyAstronomical 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 floating

in 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 physicalinterpretation of the model, celestial spheres eternallyrotate with uniform motion around a stationary Earth.Normal matter is entirely contained within the terrestrialsphere. This model was also refined by Callippus andafter concentric spheres were abandoned, it was broughtinto nearly perfect agreement with astronomical observa-tions by Ptolemy. The success of such a model is largelydue to the mathematical fact that any function (such asthe position of a planet) can be decomposed into a setof circular functions (the Fourier modes). Other Greekscientists, such as the Pythagorean philosopher Philolaus,postulated that at the center of the Universe was a “cen-tral fire” around which the Earth, Sun, Moon and Planetsrevolved in uniform circular motion.[48]

The Greek astronomer Aristarchus of Samos was thefirst known individual to propose a heliocentric modelof the Universe. Though the original text has been lost,a reference in Archimedes' book The Sand Reckonerdescribes Aristarchus’ heliocentric model. Archimedeswrote: (translated into English):

“You, King Gelon, are aware the 'Universe'is 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, which

3.3 Astronomical concepts 5

is 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 dutyof 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’ heliocentric modelwas Seleucus of Seleucia, a Hellenistic astronomer wholived a century after Aristarchus.[49][50][51] According toPlutarch, Seleucus was the first to prove the heliocentricsystem through reasoning, but it is not known what ar-guments he used. Seleucus’ arguments for a heliocen-tric cosmology were probably related to the phenomenonof tides.[52] 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.[53] 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.[54] During the Middle Ages,heliocentric models were also proposed by the Indian as-tronomer Aryabhata,[55] and by the Persian astronomersAlbumasar[56] and Al-Sijzi.[57]

The Aristotelian model was accepted in the Westernworld for roughly two millennia, until Copernicus revivedAristarchus’ perspective that the astronomical data could

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.

be 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 suggestion that theEarth rotates was very old, dating at least to Philolaus(c. 450 BC), Heraclides Ponticus (c. 350 BC) andEcphantus the Pythagorean. Roughly a century beforeCopernicus, the Christian scholar Nicholas of Cusa alsoproposed that the Earth rotates on its axis in his book,On Learned Ignorance (1440).[58] Aryabhata (476–550AD/CE), Brahmagupta (598–668), and Al-Sijzi,[59] alsoproposed that the Earth rotates on its axis. The firstempirical evidence for the Earth’s rotation on its axis, us-ing the phenomenon of comets, was given by Tusi (1201–1274) and Ali Qushji (1403–1474).This cosmology was accepted by Isaac Newton,Christiaan Huygens and later scientists.[60] Edmund Hal-ley (1720)[61] and Jean-Philippe de Chéseaux (1744)[62]noted independently that the assumption of an infinitespace filled uniformly with stars would lead to theprediction that the nighttime sky would be as bright asthe Sun itself; this became known as Olbers’ paradoxin the 19th century.[63] Newton believed that an infinitespace uniformly filled with matter would cause infiniteforces and instabilities causing the matter to be crushedinwards under its own gravity.[60] This instability wasclarified in 1902 by the Jeans instability criterion.[64]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.[65][66] A

6 5 PROPERTIES AND LAWS

significant astronomical advance of the 18th century wasthe realization by Thomas Wright, Immanuel Kant andothers of nebulae.[61]

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.[67]

4 Chronology

Main article: Chronology of the Universe

According to the prevailing scientific model of theUniverse, known as the Big Bang,[68][69] the Universeexpanded from an extremely hot, dense phase called thePlanck epoch, a brief period extending from time zero toapproximately 10−43 seconds (the Planck time). Duringthe Planck epoch, all types of matter, all types of energy,and all spacetime were concentrated into a dense state,where gravitation is believed to have been as strong asthe other fundamental forces, and all the forces may havebeen unified. Since the Planck epoch, the Universe hasbeen expanding to its present form, possibly with a verybrief period (less than 10−32 seconds) of cosmic inflationwhich caused the Universe to reach a much largersize almost instantaneously. Several independent exper-imental measurements support this theoretical expansion.

In the early Universe, after the Planck epoch and infla-tion, came the quark epoch, hadron epoch and the leptonepoch. All of these phases together lasted only up to 10seconds after the Big Bang. The photon epoch that fol-lowed lasted 380 thousand years. After that, hydrogenand helium atoms began to form as the density of theUniverse falls, allowing light to travel freely. That is theearliest light possible to see in the Universe and is knownas the cosmicmicrowave background (CMB), also knownas the afterglow of the Big Bang. The Universe continuesto expand to this day, studies have shown that this expan-sion is accelerating due to a mysterious force called darkenergy.Under general relativity, space can expand faster than thespeed of light, although we can view only a small portionof the Universe due to the limitation imposed by lightspeed. Since we cannot observe space beyond the lim-itations of light (or any electromagnetic radiation), it isuncertain whether the size of the Universe is finite or in-finite.

5 Properties and laws

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

5.1 Size

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 observ-able universe about 91 billion light-years (28×109 pc).This corresponds to a volume of 1.2×1013 Mpc3 (4×1083liters).[70] The distance the light from the edge of the ob-servable universe has travelled is very close to the age ofthe Universe times the speed of light, 13.8 billion light-years (4.2×109 pc), but this does not represent the dis-tance at any given time because the edge of the universeand the Earth have moved since further apart.[71] Forcomparison, the diameter of a typical galaxy is 30,000light-years, and the typical distance between two neigh-boring galaxies is 3million light-years.[65] As an example,the Milky Way Galaxy is roughly 100,000 light years indiameter,[72] and the nearest sister galaxy to the MilkyWay, the Andromeda Galaxy, is located roughly 2.5 mil-lion light years away.[73]

5.2 Contents

There are probably more than 100 billion (1011) galaxiesin the observable Universe.[74] Typical galaxies rangefrom dwarfs with as few as ten million[75] (107) stars upto giants with one trillion[76] (1012) stars, all orbiting thegalaxy’s center of mass. A 2010 study by astronomersestimated that the observable Universe contains 300 sex-tillion (3×1023) stars.[77]

The Universe is composed of ordinary baryonic mat-ter (only 4.9% of the contents), which includes atoms,stars, galaxies, and life. The present overall density ofthe this type of matter is very low, roughly 4.5 × 10−31grams per cubic centimetre, corresponding to a densityof the order of only one protons for every four cubic me-ters of volume.[4] The Universe also contains dark mat-ter (26.8%), a mysterious form of matter that has notyet been identified, and dark energy (68.3%), which isthe energy of empty space and that is causing the ex-pansion of the Universe to accelerate.[6][78] The commonuse of the “dark matter” and “dark energy” placeholdernames for the unknown entities (purported to account forabout 95% of the mass-energy density of the Universe)demonstrates the present observational and conceptualshortcomings and uncertainties concerning the nature andultimate fate of the Universe.[79]

Ordinary observable matter is spread homogeneously,that is, uniformly, throughout the Universe, when av-eraged over distances longer than 300 million light-years.[80] However, on smaller length-scales, matter is ob-served to form “clumps”, i.e., to cluster hierarchically;many atoms are condensed into stars, most stars intogalaxies, most galaxies into clusters, superclusters and,finally, the largest-scale structures such as the Sloan greatwall. The observable matter of the Universe is also spread

5.4 Laws 7

isotropically on large scales, meaning that no direction ofobservation seems different from any other; each regionof the sky has roughly the same content.[81] The Universeis also bathed in a highly isotropic microwave radiationthat corresponds to a thermal equilibrium blackbodyspectrum of roughly 2.725 kelvin.[82] The hypothesis thatthe large-scale Universe is homogeneous and isotropicis known as the cosmological principle,[83] which issupported by astronomical observations.

5.3 Age and expansion

The age of the Universe is measured to be 13.798 ±0.037 billion years with the prior that the prevailingmodel of the evolution of the Universe, a Big Bang dom-inated by a cosmological constant and cold dark matter,is correct.[16] Over its history, the Universe and its con-tents have evolved; for example, the relative populationof quasars and galaxies has changed and space itself hasexpanded. This expansion accounts for how it is that sci-entists on Earth can observe the light from a galaxy 30billion light years away, even if that light has traveled foronly 13 billion years; the very space between them has ex-panded, and that is one of the tools used to calculate theage of the Universe. This expansion is consistent withthe observation that the light from distant galaxies hasbeen redshifted; the photons emitted have been stretchedto longer wavelengths and lower frequency during theirjourney. The rate of this spatial expansion is accelerating,based on studies of Type Ia supernovae.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, therebyleaving 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 gently for last 13.8billion years, giving time to form the universe as we seeit today.[84]

5.4 Laws

The relative fractions of different chemical elements— particularly the lightest atoms such as hydrogen,deuterium and helium — seem to be identical through-out the Universe and during its observable history.[85]The Universe seems to have much more matter thanantimatter, an asymmetry possibly related to the obser-vations of CP violation.[86] 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 momentum

nor 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.[87]

Higgs Boson

Photon Weak Gluons

QuarksLeptons

Bosons

e µ τ ν ν νe µ τ q

gW Zγ

H

The elementary particles from which the Universe is constructed.Six leptons and six quarks comprise most of the matter; for ex-ample, the protons and neutrons of atomic nuclei are composedof quarks, and the ubiquitous electron is a lepton. These particlesinteract via the gauge bosons shown in the middle row, each cor-responding to a particular type of gauge symmetry. The Higgsboson is believed to confer mass on the particles with which it isconnected. The graviton, a supposed gauge boson for gravity, isnot shown.

The Universe appears to have 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 experimentally truewith high accuracy throughout most of the Universe.[88]Spacetime also appears to have a simply connectedtopology, at least on the length-scale of the observableUniverse. However, present observations cannot excludethe possibilities that the Universe has more dimensionsand that its spacetime may have a multiply connectedglobal topology, in analogy with the cylindrical or toroidaltopologies of two-dimensional spaces.[89][90]

Our Standard Model of physics seems to follow a univer-sal set of physical laws and physical constants.,[91] whereall matter is composed of three generations of leptons andquarks, both of which are fermions. These elementaryparticles interact via at most three fundamental in-teractions: the electroweak interaction which includeselectromagnetism and the weak nuclear force; the strongnuclear force described by quantum chromodynamics;and gravity, which is best described at present by generalrelativity. The first two interactions can be described byrenormalized quantum field theory, and are mediated bygauge bosons that correspond to a particular type of gaugesymmetry. A renormalized quantum field theory of gen-eral relativity has not yet been achieved. The theory ofspecial relativity is believed to hold throughout the Uni-verse, provided that the spatial and temporal length scales

8 7 THEORETICAL MODELS

are sufficiently short; otherwise, the more general the-ory of general relativity must be applied. There is noexplanation for the particular values that physical con-stants appear to have throughout our Universe, such asPlanck’s constant h or the gravitational constant G. Sev-eral conservation laws have been identified, such as theconservation of charge, momentum, angular momentumand energy; in many cases, these conservation laws canbe related to symmetries or mathematical identities.

6 Shape of the Universe

Main article: Shape of the UniverseThe curvature, topology shape or geometry of the Uni-

The three possible options of the shape of the Universe.

verse includes both local geometry in the observable uni-verse and global geometry, which is possibly measur-able. More formally, this practical subject investigateswhich 3-manifold corresponds to the spatial section incomoving coordinates of the four-dimensional spacetimeof the Universe. Cosmologists normally work with agiven space-like slice of spacetime called the comovingcoordinates. In terms of observation, the section ofspacetime that can be observed is the backward lightcone, being the time it takes to reach a given observerwithin the cosmic light horizon. On assumption that theobservable universe is smaller than the entire universe,which somemodels consider is many orders of magnitudesmaller, we cannot determine the true global structure byobservation alone, but are restricted only to localised re-gions.Observational data suggests the cosmological topo-logical model of the Universe is infinite with fi-nite age, supported by the so-called Friedmann–Lemaître–Robertson–Walker (FLRW) models,[92] in-cluding other FLRW models like the Poincaré dodeca-hedral space[90][93] and the Picard horn.[94] The data fitby these FLRW models of space especially include the

Wilkinson Microwave Anisotropy Probe (WMAP) andPlanck maps of cosmic background radiation. NASA re-leased the first WMAP cosmic background radiation datain February 2003, while a higher resolution map regard-ing Planck data was released by ESA in March 2013.Both probes have found almost perfect agreement withinflationary models and the standard model of cosmol-ogy, describing a flat, homogeneous universe dominatedby dark matter and dark energy.[16][95]

7 Theoretical models

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.

Of the four fundamental interactions, gravitation is dom-inant at cosmological length scales; that is, the other threeforces play a negligible role in determining structures atthe level of planetary systems, galaxies and larger-scalestructures. Gravity’s effects are cumulative; by contrast,the effects of positive and negative charges tend to can-cel one another, making electromagnetism relatively in-significant on cosmological length scales. The remainingtwo interactions, the weak and strong nuclear forces, de-cline very rapidly with distance; their effects are confinedmainly to sub-atomic length scales.

7.2 Special relativity and spacetime 9

7.1 General theory of relativity

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

Given gravitation’s predominance in shaping cosmologi-cal structures, accurate predictions of the Universe’s pastand future require an accurate theory of gravitation. Thebest theory available is Albert Einstein's general theoryof relativity, which has passed all experimental tests todate. However, because rigorous experiments have notbeen carried out on cosmological length scales, generalrelativity could conceivably be inaccurate. Nevertheless,its cosmological predictions appear to be consistent withobservations, so there has been no compelling reason toadopt another theory.General relativity provides a set of ten nonlinear partialdifferential equations for the spacetime metric (Einstein’sfield equations) that must be solved for the distributionof mass-energy and momentum throughout the Universe.Because these are unknown in exact detail, cosmologi-cal models have been based on the cosmological princi-ple, which states that the Universe is homogeneous andisotropic. In effect, this principle asserts that the grav-itational effects of the various galaxies making up theUniverse are equivalent to those of a fine dust distributeduniformly throughout the Universe with the same averagedensity. The assumption of a uniform dust makes it easyto solve Einstein’s field equations and predict the past andfuture of the Universe on cosmological time scales.Einstein’s field equations include a cosmological constant(Λ),[67][96] that corresponds to an energy density of emptyspace.[97] Depending on its sign, the cosmological con-stant can either slow (negative Λ) or accelerate (posi-tive Λ) the expansion of the Universe. Although manyscientists, including Einstein, had speculated that Λ waszero,[98] recent astronomical observations of type Ia su-pernovae have detected a large amount of "dark energy"that is accelerating the Universe’s expansion.[99] Prelim-inary studies suggest that this dark energy correspondsto a positive Λ, although alternative theories cannot beruled out as yet.[100] Russian physicist Zel'dovich sug-gested that Λ is a measure of the zero-point energy as-sociated with virtual particles of quantum field theory, apervasive vacuum energy that exists everywhere, even inempty space.[101] Evidence for such zero-point energy isobserved in the Casimir effect.

7.2 Special relativity and spacetime

Main articles: Introduction to special relativity andSpecial relativityThe Universe has at least three spatial and one tempo-ral (time) dimension. It was long thought that the spatialand temporal dimensions were different in nature and in-dependent of one another. However, according to the

η

ξ

x

y

2 = Δ x 2 + Δ y 2

= Δ ξ 2 + Δ η 2

L

Δ x

Δ y

Δ ξ

Δ ηL

Only its length L is intrinsic to the rod (shown in black); coor-dinate differences between its endpoints (such as Δx, Δy or Δξ,Δη) depend on their frame of reference (depicted in blue andred, respectively).

special theory of relativity, spatial and temporal sepa-rations are interconvertible (within limits) by changingone’s motion.To understand this interconversion, it is helpful to con-sider the analogous interconversion of spatial separationsalong the three spatial dimensions. Consider the two end-points of a rod of length L. The length can be determinedfrom the differences in the three coordinates Δx, Δy andΔz of the two endpoints in a given reference frame

L2 = ∆x2 +∆y2 +∆z2

using the Pythagorean theorem. In a rotated referenceframe, the coordinate differences differ, but they give thesame length

L2 = ∆ξ2 +∆η2 +∆ζ2.

Thus, the coordinates differences (Δx, Δy, Δz) and (Δξ,Δη, Δζ) are not intrinsic to the rod, but merely reflectthe reference frame used to describe it; by contrast, thelength L is an intrinsic property of the rod. The coor-dinate differences can be changed without affecting therod, by rotating one’s reference frame.The analogy in spacetime is called the interval betweentwo events; an event is defined as a point in spacetime, aspecific position in space and a specific moment in time.The spacetime interval between two events is given by

s2 = L21 − c2∆t21 = L2

2 − c2∆t22

10 7 THEORETICAL MODELS

where c is the speed of light. According to special relativ-ity, one can change a spatial and time separation (L1, Δt1)into another (L2, Δt2) by changing one’s reference frame,as long as the change maintains the spacetime interval s.Such a change in reference frame corresponds to chang-ing one’smotion; in amoving frame, lengths and times aredifferent from their counterparts in a stationary referenceframe. The precise manner in which the coordinate andtime differences change with motion is described by theLorentz transformation.

7.3 Solving Einstein’s field equations

See also: Big Bang and Ultimate fate of the UniverseThe distances between the spinning galaxies increase

Animation illustrating the metric expansion of the Universe

with time, but the distances between the stars within eachgalaxy stay roughly the same, due to their gravitational in-teractions. This animation illustrates a closed FriedmannUniverse with zero cosmological constant Λ; such a Uni-verse oscillates between a Big Bang and a Big Crunch.In non-Cartesian (non-square) or curved coordinate sys-tems, the Pythagorean theorem holds only on infinites-imal length scales and must be augmented with a moregeneral metric tensor gμν, which can vary from placeto place and which describes the local geometry in theparticular coordinate system. However, assuming thecosmological principle that the Universe is homogeneousand isotropic everywhere, every point in space is like ev-ery other point; hence, the metric tensor must be the sameeverywhere. That leads to a single form for the met-ric 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 that can vary with time, and acurvature index k that can be only 0, 1 or −1, correspond-ing to flat Euclidean geometry, or spaces of positive ornegative curvature. In cosmology, solving for the historyof the Universe is done by calculating R as a function oftime, given k and the value of the cosmological constantΛ, which is a (small) parameter in Einstein’s field equa-tions. The equation describing how R varies with timeis known as the Friedmann equation, after its inventor,Alexander Friedmann.[102]

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. However, this equilibrium is unstable and be-cause the Universe is known to be inhomogeneous onsmaller scales, R must change, according to general rel-ativity. When R changes, all the spatial distances in theUniverse change in tandem; there is an overall expansionor contraction of space itself. This accounts for the ob-servation that galaxies appear to be flying apart; the spacebetween them is stretching. The stretching of space alsoaccounts for the apparent paradox that two galaxies canbe 40 billion light years apart, although they started fromthe same point 13.8 billion years ago[103] and nevermovedfaster than the speed of 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. Acommon misconception is that the Big Bang model pre-dicts that matter and energy exploded from a single pointin space and time; that is false. Rather, space itself wascreated in the Big Bang and imbued with a fixed amountof energy and matter distributed uniformly throughout;as space expands (i.e., as R(t) increases), the density ofthat matter and energy decreases.Third, the curvature index k determines the sign of themean spatial curvature of spacetime averaged over lengthscales greater than a billion light years. If k=1, thecurvature is positive and the Universe has a finite vol-ume. Such universes are often visualized as a three-dimensional sphere S3 embedded in a four-dimensionalspace. Conversely, if k is zero or negative, the Uni-

7.5 Multiverse hypothesis 11

verse may have infinite volume, depending on its over-all topology. It may seem counter-intuitive that an infi-nite and yet infinitely dense Universe could be created ina single instant at the Big Bang when R=0, but exactlythat is predicted mathematically when k does not equal 1.For comparison, an infinite plane has zero curvature butinfinite area, whereas an infinite cylinder is finite in onedirection and a torus is finite in both. A toroidal Universecould behave like a normal Universe with periodic bound-ary conditions, as seen in “wrap-around” video gamessuch as Asteroids; a traveler crossing an outer “bound-ary” of space going outwards would reappear instantly atanother point on the boundary moving inwards.The ultimate fate of the Universe is still unknown, be-cause it depends critically on the curvature index k and thecosmological constant Λ. If the Universe is sufficientlydense, k equals +1, meaning that its average curvaturethroughout is positive and the Universe will eventuallyrecollapse in a Big Crunch, possibly starting a new Uni-verse in a Big Bounce. Conversely, if the Universe is in-sufficiently dense, k equals 0 or −1 and the Universe willexpand forever, cooling off and eventually becoming in-hospitable for all life, as the stars die and all matter coa-lesces into black holes (the Big Freeze and the heat deathof the Universe). As noted above, recent data suggeststhat the expansion speed of the Universe is not decreasingas originally expected, but increasing; if this continues in-definitely, the Universe will eventually rip itself to shreds(the Big Rip). Experimentally, the Universe has an over-all density that is very close to the critical value betweenrecollapse and eternal expansion; more careful astronom-ical observations are needed to resolve the question.

7.4 Big Bang model

Main articles: Big Bang, Timeline of the Big Bang,Nucleosynthesis and Lambda-CDM model

The prevailing Big Bang model accounts for many ofthe experimental observations described above, such asthe correlation of distance and redshift of galaxies, theuniversal ratio of hydrogen:helium atoms, and the ubiq-uitous, isotropic microwave radiation background. Asnoted above, the redshift arises from the metric expan-sion of space. As space itself expands, the wavelengthof a photon traveling through space likewise increases,decreasing its energy. The longer a photon has beentraveling, the more expansion it has undergone; hence,older photons from more distant galaxies are the mostred-shifted. Determining the correlation between dis-tance and redshift is an important problem in experimen-tal physical cosmology.Other experimental observations can be explained bycombining the overall expansion of space with nuclearand atomic physics. As the Universe expands, the energydensity of the electromagnetic radiation decreases more

Chief nuclear reactions responsible for the relative abundancesof light atomic nuclei observed throughout the Universe.

quickly than does that of matter, because the energy ofa photon decreases with its wavelength. Thus, althoughthe energy density of the Universe is now dominated bymatter, it was once dominated by radiation; poeticallyspeaking, all was light. As the Universe expanded, itsenergy density decreased and it became cooler; as it didso, the elementary particles of matter could associate sta-bly into ever larger combinations. Thus, in the early partof the matter-dominated era, stable protons and neutronsformed, which then associated into atomic nuclei. At thisstage, the matter in the Universe was mainly a hot, denseplasma of negative electrons, neutral neutrinos and pos-itive nuclei. Nuclear reactions among the nuclei led tothe present abundances of the lighter nuclei, particularlyhydrogen, deuterium, and helium. Eventually, the elec-trons and nuclei combined to form stable atoms, whichare transparent to most wavelengths of radiation; at thispoint, the radiation decoupled from the matter, formingthe ubiquitous, isotropic background of microwave radi-ation observed today.Under the prevailing theory, a slight imbalance of matterover antimatter happened in the Universe shortly afterits creation, possibly caused by the CP violation that hasbeen observed by particle physicists. Although the matterand antimatter mostly annihilated one another, produc-ing photons, a small residue of matter survived, givingthe present matter-dominated Universe. Several lines ofevidence also suggest that a rapid cosmic inflation of theUniverse occurred very early in its history, lasting roughly10−35 seconds after its creation. Recent observations alsosuggest that the cosmological constant (Λ) is not zero andthat the net mass-energy content of the Universe is domi-nated by a dark energy and dark matter that have not beencharacterized scientifically. They differ in their gravita-tional effects. Dark matter gravitates as ordinary matterdoes, and thus slows the expansion of the Universe; bycontrast, dark energy serves to accelerate the Universe’sexpansion.

7.5 Multiverse hypothesis

Main articles: Multiverse, Many-worlds interpretation,Bubble universe theory and Parallel universe (fiction)Some speculative theories have proposed that our uni-verse is but one of a set of disconnected universes, collec-

12 9 NOTES AND REFERENCES

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.

tively denoted as the multiverse, challenging or enhancingmore limited definitions of the Universe.[18][104] 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.[105] 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-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).[106][107] 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.

8 See also• Religious cosmology

• Cosmic Calendar (scaled down timeline)

• Cosmic latte

• Dyson’s eternal intelligence

• Esoteric cosmology

• False vacuum

• Final anthropic principle

• Fine-tuned Universe

• Hindu cosmology

• Illustris project

• Jain cosmology

• Kardashev scale

• The Mysterious Universe (book)

• Nucleocosmochronology

• Non-standard cosmology

• Omega Point

• Rare Earth hypothesis

• Vacuum genesis

• World view

• Zero-energy Universe

9 Notes and references[1] Itzhak Bars; John Terning (November 2009). Extra Di-

mensions in Space and Time. Springer. pp. 27–. ISBN978-0-387-77637-8. Retrieved 2011-05-01.

[2] “volume universe - Wolfram-Alpha”. Retrieved 16 April2015.

[3] Paul Davies (2006). The Goldilocks Enigma. FirstMariner Books. p. 43–. ISBN 978-0-618-59226-5. Re-trieved 1 July 2013.

[4] “Universe 101: What is the Universe Made Of?". NASA:WMAP’s Universe. Jan 24, 2014. Retrieved 2015-02-17.

[5] Fixsen, D. J. (December 2009). “The Temperatureof the Cosmic Microwave Background”. The Astro-physical 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...”

13

[7] “WMAP- Shape of the Universe”. Retrieved 16 April2015.

[8] Universe. Webster’s New World College Dictionary, WileyPublishing, Inc. 2010.

[9] “Universe”. Dictionary.com. Retrieved 2012-09-21.

[10] “Universe”. Merriam-Webster Dictionary. Retrieved2012-09-21.

[11] Zeilik, Michael; Gregory, Stephen A. (1998). Introduc-tory Astronomy & Astrophysics (4th ed.). Saunders Col-lege Publishing. ISBN 0030062284. The totality of allspace and time; all that is, has been, and will be.

[12] The American Heritage Dictionary of the English Lan-guage (4th ed.). Houghton Mifflin Harcourt PublishingCompany. 2010.

[13] Cambridge Advanced Learner’s Dictionary.

[14] Brian Greene (2011). The Hidden Reality. Alfred A.Knopf.

[15] “Planck reveals an almost perfect universe”. Planck. ESA.2013-03-21. Retrieved 2013-03-21.

[16] Planck collaboration (2014). “Planck 2013 results. XVI.Cosmological parameters”. Astronomy & Astrophysics.arXiv:1303.5076. Bibcode:2014A&A...571A..16P.doi:10.1051/0004-6361/201321591.

[17] “The Nobel Prize in Physics 2011”. Retrieved 16 April2015.

[18] Ellis, George F.R.; U. Kirchner; W.R. Stoeger(2004). “Multiverses and physical cosmology”.Monthly Notices of the Royal Astronomical Soci-ety 347 (3): 921–936. arXiv:astro-ph/0305292.Bibcode:2004MNRAS.347..921E. doi:10.1111/j.1365-2966.2004.07261.x.

[19] Palmer, Jason. (2011-08-03) BBC News – 'Multiverse'theory suggested by microwave background. Retrieved2011-11-28.

[20] The Compact Edition of the Oxford English Dictionary,volume II, Oxford: Oxford University Press, 1971, p.3518.

[21] Lewis, C. T. and Short, S (1879) A Latin Dictionary, Ox-ford University Press, ISBN 0-19-864201-6, pp. 1933,1977–1978.

[22] Dold-Samplonius, Yvonne (2002). From China to Paris:2000 Years Transmission of Mathematical Ideas. FranzSteiner Verlag.

[23] Thomas F. Glick; Steven Livesey; Faith Wallis. Me-dieval Science Technology and Medicine: An Encyclope-dia. Routledge.

[24] Hawking, Stephen (1988). A Brief History of Time. Ban-tam Books. p. 125. ISBN 0-553-05340-X.

[25] Liddell & Scott (1968, p. 1392)

[26] Liddell & Scott (1968, pp. 1345–1346)

[27] Yonge, Charles Duke (1870). An English-Greek lexicon.New York: American Book Company. p. 567.

[28] Liddell & Scott (1968, pp. 985, 1964)

[29] Lewis, C. T.; Short, S (1879). A Latin Dictionary. Ox-ford University Press. pp. 1881–1882, 1175, 1189–1190.ISBN 0-19-864201-6.

[30] The Compact Edition of the Oxford English Dictionary II.Oxford: Oxford University Press. 1971. pp. 909, 569,3821–3822, 1900. ISBN 978-0198611172.

[31] Paul Copan; William Lane Craig (2004). Creation Out ofNothing: A Biblical, Philosophical, and Scientific Explo-ration. Baker Academic. p. 220. ISBN 9780801027338.

[32] Clara Moskowitz (August 12, 2011). “Weird! Our Uni-verse May Be a 'Multiverse,' Scientists Say”. livescience.

[33] McCall, Storrs. A Model of the Universe: Space-time,Probability, and Decision. Oxford University. p. 23.

[34] Leeming, David A. (2010). Creation Myths of the World.ABC-CLIO. p. xvii. ISBN 978-1-59884-174-9. In com-mon usage the word 'myth' refers to narratives or beliefsthat are untrue or merely fanciful; the stories that makeup national or ethnic mythologies describe characters andevents that common sense and experience tell us are im-possible. Nevertheless, all cultures celebrate such mythsand attribute to them various degrees of literal or symbolictruth.

[35] (Henry Gravrand, “La civilisation Sereer -Pangool”)[in] Universität Frankfurt am Main, Frobenius-Institut,Deutsche Gesellschaft für Kulturmorphologie, FrobeniusGesellschaft, “Paideuma: Mitteilungen zur Kulturkunde,Volumes 43–44”, F. Steiner (1997), pp. 144–5, ISBN3515028420

[36] B. Young, Louise. The Unfinished Universe. Oxford Uni-versity Press. p. 21.

[37] Will Durant, Our Oriental Heritage:

“Two systems of Hindu thought pro-pound physical theories suggestively similarto those of Greece. Kanada, founder of theVaisheshika philosophy, held that the worldis composed of atoms as many in kind as thevarious elements. The Jains more nearly ap-proximated to Democritus by teaching thatall atoms were of the same kind, producingdifferent effects by diverse modes of com-binations. Kanada believed light and heat tobe varieties of the same substance; Udayanataught that all heat comes from the Sun; andVachaspati, like Newton, interpreted light ascomposed ofminute particles emitted by sub-stances and striking the eye.”

[38] Stcherbatsky, F. Th. (1930, 1962), Buddhist Logic, Vol-ume 1, p. 19, Dover, New York:

“The Buddhists denied the existence ofsubstantial matter altogether. Movementconsists for them of moments, it is a staccato

14 9 NOTES AND REFERENCES

movement, momentary flashes of a streamof energy... “Everything is evanescent”,...says the Buddhist, because there is no stuff...Both systems [Sānkhya, and later Indian Bud-dhism] share in common a tendency to pushthe analysis of existence up to its minutest,last elements which are imagined as abso-lute qualities, or things possessing only oneunique quality. They are called “qualities”(guna-dharma) in both systems in the senseof absolute qualities, a kind of atomic, orintra-atomic, energies of which the empiricalthings are composed. Both systems, there-fore, agree in denying the objective realityof the categories of Substance and Quality,...and of the relation of Inference uniting them.There is in Sānkhya philosophy no separateexistence of qualities. What we call qualityis but a particular manifestation of a subtleentity. To every new unit of quality corre-sponds a subtle quantum of matter which iscalled guna, “quality”, but represents a sub-tle substantive entity. The same applies toearly Buddhism where all qualities are sub-stantive... or, more precisely, dynamic en-tities, although they are also called dharmas('qualities’).”

[39] Craig, William Lane (June 1979). “Whitrow and Popperon the Impossibility of an Infinite Past”. The British Jour-nal for the Philosophy of Science 30 (2): 165–170 (165–6). doi:10.1093/bjps/30.2.165.

[40] Alister E. McGrath (2009). A Fine-Tuned Universe: TheQuest for God in Science and Theology. Westminster JohnKnox Press. p. 288.

[41] Rees, Martin (1999). Just Six Numbers. HarperCollinsPublishers. ISBN 0-465-03672-4.

[42] Adams, F.C. (2008). “Stars in other universes: stellarstructure with different fundamental constants”. Jour-nal of Cosmology and Astroparticle Physics 2008 (8):010. arXiv:0807.3697. Bibcode:2008JCAP...08..010A.doi:10.1088/1475-7516/2008/08/010.

[43] Harnik, R.; Kribs, G.D. & Perez, G. (2006).“A Universe without weak interactions”. Phys-ical Review D 74 (3): 035006. arXiv:hep-ph/0604027. Bibcode:2006PhRvD..74c5006H.doi:10.1103/PhysRevD.74.035006.

[44] Victor J. Stenger (2011). The Fallacy of Fine-Tuning:Why the Universe Is Not Designed For Us. PrometheusBooks. p. 330.

[45] Loeb, Abraham (October 2014). “The Habit-able Epoch of the Early Universe”. InternationalJournal of Astrobiology 13 (04): 337–339.arXiv:1312.0613. Bibcode:2014IJAsB..13..337L.doi:10.1017/S1473550414000196. Retrieved 15December 2014.

[46] Loeb, Abraham (2 December 2013). “The HabitableEpoch of the Early Universe”. arXiv:1312.0613v3.

[47] Dreifus, Claudia (2 December 2014). “Much-DiscussedViews That Go Way Back - Avi Loeb Ponders the EarlyUniverse, Nature and Life”. New York Times. Retrieved3 December 2014.

[48] Boyer, C. (1968) A History of Mathematics. Wiley, p. 54.

[49] Neugebauer, Otto E. (1945). “The History of Ancient As-tronomy Problems and Methods”. Journal of Near East-ern Studies 4 (1): 1–38. doi:10.1086/370729. JSTOR595168. the Chaldaean Seleucus from Seleucia

[50] Sarton, George (1955). “Chaldaean Astronomy ofthe Last Three Centuries B. C”. Journal of theAmerican Oriental Society 75 (3): 166–173 (169).doi:10.2307/595168. JSTOR 595168. the heliocentricalastronomy invented by Aristarchos of Samos and still de-fended a century later by Seleucos the Babylonian

[51] William P. D. Wightman (1951, 1953), The Growth ofScientific Ideas, Yale University Press p. 38, whereWight-man calls him Seleukos the Chaldean.

[52] Lucio Russo, Flussi e riflussi, Feltrinelli, Milano, 2003,ISBN 88-07-10349-4.

[53] Bartel (1987, p. 527)

[54] Bartel (1987, pp. 527–9)

[55] Bartel (1987, pp. 529–34)

[56] Bartel (1987, pp. 534–7)

[57] Nasr, Seyyed H. (1993) [1964]. An Introduction to IslamicCosmological Doctrines (2nd ed.). 1st edition by HarvardUniversity Press, 2nd edition by State University of NewYork Press. pp. 135–6. ISBN 0-7914-1515-5.

[58] Misner, Thorne and Wheeler, p. 754.

[59] Ālī, Ema Ākabara. Science in the Quran 1. Malik Library.p. 218.

[60] Misner, Thorne and Wheeler, p. 755–756.

[61] Misner, Thorne and Wheeler, p. 756.

[62] de Cheseaux JPL (1744). Traité de la Comète. Lausanne.pp. 223ff.. Reprinted as Appendix II in Dickson FP(1969). The Bowl of Night: The Physical Universe andScientific Thought. Cambridge, MA: M.I.T. Press. ISBN978-0-262-54003-2.

[63] Olbers HWM (1826). “Unknown title”. Bode’s Jahrbuch111.. Reprinted as Appendix I in Dickson FP (1969).The Bowl of Night: The Physical Universe and ScientificThought. Cambridge, MA: M.I.T. Press. ISBN 978-0-262-54003-2.

[64] Jeans, J. H. (1902). “The Stability of a Spherical Nebula”(PDF). Philosophical Transactions of the Royal Society A199 (312–320): 1–53. Bibcode:1902RSPTA.199....1J.doi:10.1098/rsta.1902.0012. JSTOR 90845. Retrieved2011-03-17.

[65] Rindler, p. 196.

[66] Misner, Thorne and Wheeler, p. 757.

15

[67] Einstein, A (1917). “Kosmologische Betrachtungen zurallgemeinen Relativitätstheorie”. Preussische Akademieder Wissenschaften, Sitzungsberichte. 1917. (part 1):142–152.

[68] Joseph Silk (2009). Horizons of Cosmology. TempletonPressr. p. 208.

[69] Simon Singh (2005). Big Bang: The Origin of the Uni-verse. Harper Perennial. p. 560.

[70] “Wolfram alpha”.

[71] Christopher Crockett (February 20, 2013). “What is alight-year?". EarthSky.

[72] Christian, Eric; Samar, Safi-Harb. “How large is theMilky Way?". Retrieved 2007-11-28.

[73] I. Ribas; C. Jordi; F. Vilardell; E.L. Fitzpatrick; R.W.Hilditch; F. Edward Guinan (2005). “First Determina-tion of the Distance and Fundamental Properties of anEclipsing Binary in the Andromeda Galaxy”. Astrophysi-cal Journal 635 (1): L37–L40. arXiv:astro-ph/0511045.Bibcode:2005ApJ...635L..37R. doi:10.1086/499161.McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M.N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N. (2005).“Distances and metallicities for 17 Local Group galax-ies”. Monthly Notices of the Royal Astronomical So-ciety 356 (4): 979–997. arXiv:astro-ph/0410489.Bibcode:2005MNRAS.356..979M. doi:10.1111/j.1365-2966.2004.08514.x.

[74] Mackie, Glen (February 1, 2002). “To see the Universein a Grain of Taranaki Sand”. Swinburne University. Re-trieved 2006-12-20.

[75] “Unveiling the Secret of a Virgo Dwarf Galaxy”. ESO.2000-05-03. Retrieved 2007-01-03.

[76] “Hubble’s Largest Galaxy Portrait Offers a New High-Definition View”. NASA. 2006-02-28. Retrieved 2007-01-03.

[77] Vergano, Dan (1 December 2010). “Universe holds bil-lions more stars than previously thought”. USA Today.Retrieved 2010-12-14.

[78] Peebles, P. J. E. & Ratra, Bharat (2003). “Thecosmological constant and dark energy”. Reviewsof Modern Physics 75 (2): 559–606. arXiv:astro-ph/0207347. Bibcode:2003RvMP...75..559P.doi:10.1103/RevModPhys.75.559.

[79] Universe, ed. Martin Rees, pp. 54–55, Dorling Kinders-ley Publishing, New York 2005, ISBN 978-0-7566-1364-8

[80] Mandolesi, N.; Calzolari, P.; Cortiglioni, S.; Delpino,F.; Sironi, G.; Inzani, P.; Deamici, G.; Solheim, J. -E.;Berger, L.; Partridge, R. B.; Martenis, P. L.; Sangree, C.H.; Harvey, R. C. (1986). “Large-scale homogeneity ofthe Universe measured by the microwave background”.Nature 319 (6056): 751. doi:10.1038/319751a0.

[81] Hinshaw, Gary (November 29, 2006). “New Three YearResults on the Oldest Light in the Universe”. NASAWMAP. Retrieved 2006-08-10.

[82] Hinshaw, Gary (December 15, 2005). “Tests of the BigBang: The CMB”. NASA WMAP. Retrieved 2007-01-09.

[83] Rindler, p. 202.

[84] Sean Carroll and Michio Kaku (2014). How the UniverseWorks 3. End of the Universe. Discovery Channel.

[85] Wright, Edward L. (September 12, 2004). “Big Bang Nu-cleosynthesis”. UCLA. Retrieved 2007-01-05.M. Harwit; M. Spaans (2003). “Chemical Com-position of the Early Universe”. The AstrophysicalJournal 589 (1): 53–57. arXiv:astro-ph/0302259.Bibcode:2003ApJ...589...53H. doi:10.1086/374415.C. Kobulnicky; E. D. Skillman (1997). “Chemi-cal Composition of the Early Universe”. Bulletinof the American Astronomical Society 29: 1329.Bibcode:1997AAS...191.7603K.

[86] “Antimatter”. Particle Physics and Astronomy ResearchCouncil. October 28, 2003. Retrieved 2006-08-10.

[87] Landau and Lifshitz, p. 361.

[88] WMAP Mission: Results – Age of the Universe.Map.gsfc.nasa.gov. Retrieved 2011-11-28.

[89] Luminet, Jean-Pierre; Boudewijn F. Roukema (1999).“Topology of the Universe: Theory and Observations”.Proceedings of Cosmology School held at Cargese, Corsica,August 1998. arXiv:astro-ph/9901364.

[90] Luminet, Jean-Pierre; Weeks, Jeffrey R.; Riazuelo,Alain; Lehoucq, Roland; Uzan, Jean-Philippe(2003-10-09). “Dodecahedral space topologyas an explanation for weak wide-angle tempera-ture correlations in the cosmic microwave back-ground”. Nature 425 (6958): 593–5. arXiv:astro-ph/0310253. Bibcode:2003Natur.425..593L.doi:10.1038/nature01944. PMID 14534579.

[91] Strobel, Nick (May 23, 2001). “The Composition ofStars”. Astronomy Notes. Retrieved 2007-01-04.“Have physical constants changed with time?". Astro-physics (Astronomy Frequently Asked Questions). Re-trieved 2007-01-04.

[92] Will the Universe expand forever?, WMAP website atNASA.

[93] Roukema, Boudewijn; Zbigniew Buliński; AgnieszkaSzaniewska; Nicolas E. Gaudin (2008). “A test of thePoincare dodecahedral space topology hypothesis with theWMAPCMB data”. Astronomy and Astrophysics 482 (3):747. arXiv:0801.0006. Bibcode:2008A&A...482..747L.doi:10.1051/0004-6361:20078777.

[94] Aurich, Ralf; Lustig, S.; Steiner, F.; Then, H. (2004).“Hyperbolic Universes with a Horned Topology andthe CMB Anisotropy”. Classical and Quantum Grav-ity 21 (21): 4901–4926. arXiv:astro-ph/0403597.Bibcode:2004CQGra..21.4901A. doi:10.1088/0264-9381/21/21/010.

[95] “Planck reveals 'almost perfect' universe”. Michael Banks.Physics World. 2013-03-21. Retrieved 2013-03-21.

16 12 EXTERNAL LINKS

[96] Rindler, pp. 226–229.

[97] Landau and Lifshitz, pp. 358–359.

[98] Einstein, A (1931). “Zum kosmologischen Problemder allgemeinen Relativitätstheorie”. Sitzungsberichte derPreussischen Akademie der Wissenschaften, Physikalisch-mathematische Klasse 1931: 235–237.Einstein A., de Sitter W. (1932). “On the relation be-tween the expansion and the mean density of the Uni-verse”. Proceedings of the National Academy of Sci-ences 18 (3): 213–214. Bibcode:1932PNAS...18..213E.doi:10.1073/pnas.18.3.213. PMC 1076193. PMID16587663.

[99] Hubble Telescope news release. Hubblesite.org (2004-02-20). Retrieved 2011-11-28.

[100] “Mysterious force’s long presence”. BBC News. 2006-11-16.

[101] Zel'dovich YB (1967). “Cosmological constant andelementary particles”. JETP Letters 6: 316–317.Bibcode:1967JETPL...6..316Z.

[102] Friedmann A. (1922). "Über die Krüm-mung des Raumes”. Zeitschrift für Physik 10(1): 377–386. Bibcode:1922ZPhy...10..377F.doi:10.1007/BF01332580.

[103] “Cosmic Detectives”. The European Space Agency(ESA). 2013-04-02. Retrieved 2013-04-15.

[104] Munitz MK (1959). “One Universe or Many?".Journal of the History of Ideas 12 (2): 231–255.doi:10.2307/2707516. JSTOR 2707516.

[105] Linde A. (1986). “Eternal chaotic inflation”. Mod. Phys.Lett. A1 (2): 81–85. Bibcode:1986MPLA....1...81L.doi:10.1142/S0217732386000129.Linde A. (1986). “Eternally existing self-reproducingchaotic inflationary Universe” (PDF). Phys. Lett.B175 (4): 395–400. Bibcode:1986PhLB..175..395L.doi:10.1016/0370-2693(86)90611-8. Retrieved 2011-03-17.

[106] Tegmark M. (2003). “Parallel universes. Not just a sta-ple of science fiction, other universes are a direct impli-cation of cosmological observations”. Scientific American288 (5): 40–51. doi:10.1038/scientificamerican0503-40.PMID 12701329.

[107] Tegmark, Max (2003). J. D. Barrow; P.C.W.Davies; C.L. Harper, eds. “Parallel Universes”.Scientific American: “Science and Ultimate Reality:from Quantum to Cosmos”, honoring John Wheeler’s90th birthday (Cambridge University Press): 2131.arXiv:astro-ph/0302131. Bibcode:2003astro.ph..2131T.doi:10.1038/scientificamerican0503-40.

10 Bibliography• Bartel, Leendert van der Waerden (1987).“The Heliocentric System in Greek, Per-sian and Hindu Astronomy”. Annals of the

New York Academy of Sciences 500 (1):525–545. Bibcode:1987NYASA.500..525V.doi:10.1111/j.1749-6632.1987.tb37224.x.

• Landau, Lev, Lifshitz, E.M. (1975). The ClassicalTheory of Fields (Course of Theoretical Physics) 2(revised 4th English ed.). New York: PergamonPress. pp. 358–397. ISBN 978-0-08-018176-9.

• Liddell, H. G. & Scott, R. (1968). A Greek-EnglishLexicon. Oxford University Press. ISBN 0-19-864214-8.

• Misner, C.W., Thorne, Kip, Wheeler, J.A. (1973).Gravitation. San Francisco: W. H. Freeman. pp.703–816. ISBN 978-0-7167-0344-0.

• Rindler, W. (1977). Essential Relativity: Special,General, and Cosmological. New York: SpringerVerlag. pp. 193–244. ISBN 0-387-10090-3.

11 Further reading• Weinberg, S. (1993). The First Three Minutes: AModern View of the Origin of the Universe (2nd up-dated ed.). New York: Basic Books. ISBN 978-0-465-02437-7. OCLC 28746057. For lay readers.

• Nussbaumer, Harry; Bieri, Lydia; Sandage, Allan(2009). Discovering the Expanding Universe. Cam-bridge University Press. ISBN 978-0-521-51484-2.

12 External links• Is there a hole in the Universe? at HowStuffWorks

• Stephen Hawking’s Universe – Why is the Universethe way it is?

• Cosmology FAQ

• Cosmos – An “illustrated dimensional journey frommicrocosmos to macrocosmos”

• Illustration comparing the sizes of the planets, thesun, and other stars

• My So-Called Universe – Arguments for and againstan infinite and parallel universes

• The Dark Side and the Bright Side of the UniversePrinceton University, Shirley Ho

• Richard Powell: An Atlas of the Universe – Imagesat various scales, with explanations

• Multiple Big Bangs

• Universe – Space Information Centre

• Cosmic Evolution – the Universe on a timeline

12.1 Videos 17

12.1 Videos

• Cosmography of the Local Universe at irfu.cea.fr(17:35) (arXiv)

• The Known Universe created by the American Mu-seum of Natural History

• Understand The Size Of The Universe – by Powersof Ten

• 3-D Video (01:46) – Over a Million Galaxies of Bil-lions of Stars each – BerkeleyLab/animated

18 13 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

13 Text and image sources, contributors, and licenses

13.1 Text• Universe Source: http://en.wikipedia.org/wiki/Universe?oldid=663693589 Contributors: AxelBoldt, Lee Daniel Crocker, CYD, Bryan

Derksen, Ed Poor, Wayne Hardman, XJaM, Aldie, William Avery, Montrealais, Stevertigo, Lir, Nealmcb, Patrick, Boud, MichaelHardy, Nixdorf, Liftarn, MartinHarper, Ixfd64, Minesweeper, Looxix~enwiki, Ahoerstemeier, Mac, CatherineMunro, Suisui, Angela,JWSchmidt, Setu, Glenn, AugPi, Andres, Evercat, Hectorthebat, Samuel~enwiki, Pizza Puzzle, Timwi, Janko, Hydnjo, DJ Clayworth, Tp-bradbury, DW40, Maximus Rex, Furrykef, Saltine, Xevi~enwiki, Joseaperez, Topbanana, Fvw, Gakrivas, Pakaran, BenRG, Frazzydee,Jni, Phil Boswell, Rossnixon, Paranoid, Astronautics~enwiki, Fredrik, Altenmann, Peak, Yelyos, Nurg, Romanm, Gandalf61, Mirv,Academic Challenger, DHN, Borislav, Johnstone, Aetheling, Guy Peters, Mattflaschen, Cordell, SpellBott, Giftlite, Dbenbenn, GraemeBartlett, Awolf002, Barbara Shack, Herbee, Mark.murphy, Peruvianllama, Everyking, No Guru, Michael Devore, Bensaccount, Jcobb,Pascal666, Eequor, Matt Crypto, Python eggs, Jackol, Bobblewik, Golbez, Chowbok, Gadfium, Utcursch, Andycjp, R. fiend, Gdm,Zeimusu, Quadell, Antandrus, Beland, OverlordQ, Lesgles, Kaldari, Yafujifide, ShakataGaNai, Karol Langner, JimWae, Latitude0116,Mike Storm, Kevin B12, Jawed, Icairns, CesarFelipe, Zfr, Karl-Henner, Neutrality, TJSwoboda, Trevor MacInnis, Randwicked, Canter-bury Tail, RevRagnarok, Mike Rosoft, Shahab, Freakofnurture, A-giau, Naryathegreat, Discospinster, Eb.hoop, Rich Farmbrough, Vsmith,Jpk, Florian Blaschke, StephanKetz, YUL89YYZ, Jordancpeterson, Zamfi, Dbachmann, Nchaimov, Martpol, Aardark, SpookyMulder,ESkog, Brian0918, RJHall, El C, Rgdboer, Art LaPella, Bjoern~enwiki, Orlady, Sajt, Adambro, Guettarda, Bobo192, Army1987, An-droo, Smalljim, K0hlrabi, Maurreen, ParticleMan, I9Q79oL78KiL0QTFHgyc, Juzeris, Sawadeekrap, Joe Jarvis, Acjelen, Nk, VBGF-scJUn3, Ardric47, MPerel, Sam Korn, Thialfi, Krellis, Storm Rider, Stephen G. 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, Splin-tercellguy, Sceptre, Blightsoot, Nipponese, Jimp, Retodon8, StuffOfInterest, RussBot, Petiatil, Hyad, Anonymous editor, Bhny, Spuri-ousQ, Stephenb, Chaos, Bullzeye, NawlinWiki, SEWilcoBot, Neural, Grafen, Erielhonan, Jaxl, RazorICE, Nsmith 84, Irishguy, RandolfRichardson, Chrisbrl88, Matticus78, Rmky87, Haoie, Saggipie, Iamnotanorange~enwiki, Epipelagic, SFC9394, Roy Brumback, Dead-EyeArrow, Psy guy, Martinwilke1980, Nlu, Dna-webmaster, Dv82matt, Jpmccord, 2over0, Lt-wiki-bot, Ageekgal, Breakfastchief, Theda,Closedmouth, Arthur Rubin, KGasso, Nemu, Th1rt3en, Reyk, Exodio, GraemeL, JoanneB, LeonardoRob0t, Leeannedy, ArielGold, Cacode vidro, Aeosynth, RG2, JuniorMuruin, Serendipodous, DVD R W, Eog1916, Dupz, Kicking222, Sardanaphalus, SmackBot, RogerDavies, Mehranwahid, Ashill, Kurochka, Zazaban, KnowledgeOfSelf, Olorin28, McGeddon, Unyoyega, Jacek Kendysz, Jagged 85, Dav-ewild, 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. Croft,Craner Murdock, Dreadstar, RandomP, Lcarscad, Alasdair Routh, BullRangifer, Drooling Sheep, Orczar, Kotjze, Iamorlando, Evlekis,Bejnar, Kukini, Ollj, Ged UK, DorJ, Weatherman1126, SashatoBot, Lambiam, Danielrcote, Dr. Sunglasses, 007david, Abob6, Kuru,John, T g7, MagnaMopus, N3bulous, Buchanan-Hermit, Kipala, Erdelyiek, Sir Nicholas de Mimsy-Porpington, JorisvS, Mgiganteus1,IronGargoyle, Ekrub-ntyh, Ckatz, Ian Dalziel, The Bread, Smith609, Beetstra, Hypnosifl, Waggers, Doczilla, Dr.K., EEPROM Eagle,Jose77, Yresh, MarkThomas, Autonova, Hu12, ThuranX, Nehrams2020, Iridescent, K, Hurricanefloyd, Shoeofdeath, Newone, NativeFor-eigner, J Di, Aeternus, AGK, Tawkerbot2, JRSpriggs, Firewall62, Chetvorno, Xammer, Uq, MarkTB, JForget, Friendly Neighbour, Alejrb, Insanephantom, Dycedarg, Cytocon, Scohoust, Albert.white, Woudloper, JohnCD, Dub8lad1, Mr plant420, Runningonbrains, Lawn-chair On Jupiter, CuriousEric, MarsRover, Geniustwin, Joelholdsworth, WeggeBot, Awesome streak, Lokal Profil, Karenjc, Myasuda,Phase Theory, Gregbard, Icarus of old, Cydebot, Ryan, Treybien, WillowW, Grahamec, Perfect Proposal, Steel, Peterdjones, UncleBubba,Gogo Dodo, Travelbird, FellowWikipedian, Frosty0814snowman, Llort, ST47, Scroggie, Eu.stefan, Wildnox, Tawkerbot4, Dougweller,Moingv, Dchristle, DumbBOT, Hontogaichiban, Kozuch, Omicronpersei8, RotaryAce, Satori Son, Mattisse, Malleus Fatuorum, Joernder-schlaue, Thijs!bot, Epbr123, Barticus88, Mbell, 271828182, Ramananrv123, Hazmat2, Keraunos, Mojo Hand, The Dark Side, Headbomb,Marek69, West Brom 4ever, Tapir Terrific, Kathovo, Peter Gulutzan, Picus viridis, Tellyaddict, Cool Blue, Dfrg.msc, AgentPeppermint,Pure maple sugar, Elert, Futurebird, Escarbot, Stannered, Mentifisto, WikiSlasher, AntiVandalBot, Majorly, Yonatan, Kba, Seaphoto,Quintote, Voortle, Nseidm1, Mal4mac, Jj137, Scepia, Geogeogeo, Dylan Lake, Danger, Spencer, Larry Lawrence, Legare, Myanw, PresN,Canadian-Bacon, JAnDbot, Jimothytrotter, Vorpal blade, Davewho2, Barek, MER-C, Epeefleche, The Transhumanist, DarkLouis, Fetch-comms, Andonic, Hut 8.5, Rdht, Snibbe, Badacmw90, Schmackity, ILSS, Murgh, Bongwarrior, VoABot II, MartinDK, Sushant gupta,AuburnPilot, JNW, Mclay1, Jéské Couriano, Think outside the box, Rivertorch, Depolarizer, Nyttend, Sruk77, Aka042, SparrowsWing,Bubba hotep, BrianGV, Fabrictramp, Catgut, Animum, Spacegoat, Bloodredrover, JJ Harrison, Mlhooten, Just James, DerHexer, FloriaL, Dirtyharry2, Patstuart, Jdorwin, Sjtarr, NatureA16, B9 hummingbird hovering, Blacksqr, Sonikkua, Jackson Peebles, Hdt83, Mart-inBot, NAHID, Meduban, GomeonaFinnigan, Rettetast, Keith D, CommonsDelinker, AlexiusHoratius, PrestonH, WelshMatt, Ssolbergj,AlphaEta, Watch37264, J.delanoy, Pharaoh of the Wizards, JEREMYBB, Tom Kitt, Ali, MikeBaharmast, Uncle Dick, Ciotog, MauriceCarbonaro, Brest, All Is One, G. Campbell, Q2op, Barts1a, Katalaveno, Ncmvocalist, McSly, Mikael Häggström, Gurchzilla, Bilbobee, Py-rospirit, Qazwsx197966, Spig a digs, Vanished User 4517, TomasBat, NewEnglandYankee, Djambalawa, Rebel700, Trilobitealive, SJP, Bo-bianite, LeighvsOptimvsMaximvs, Jorfer, Zojj, Mufka, Student7, Rickmeister~enwiki, Terik brunson, MetsFan76, KylieTastic, Mattu00,Remember the dot, Gwen Gale, Sinep2, Vanished user 39948282, DavyJonesGSB, Robbiemasters89, HiEv, Cuckooman4, Bonadea, Rick-meister6, 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,Vipinhari, Canuckle, Hqb, GDonato, DarrynJ, Ridernyc, Anonymous Dissident, GcSwRhIc, Vishal144, Qxz, Someguy1221, Trahern1994,Anna Lincoln, Lradrama, Clarince63, John haley, Patssle, JhsBot, Bob Andolusorn, Abdullais4u, Fbs. 13, LeaveSleaves, Manchurian can-didate, UnitedStatesian, Dantheman2008, Geometry guy, Saturn star, Nighthawk380, Knightshield, SheffieldSteel, Billinghurst, Maethor-

13.1 Text 19

daer, Perníček~enwiki, Falcon8765, Enviroboy, FKmailliW, Seresin, Someguyonthestreet, Agüeybaná, Brianga, Flyingostrich, Thealltruth,Wavehunter, AlleborgoBot, Baaleos, Logan, Domi33, Neilk9393, Scottywong, NHRHS2010, Hcagri, EmxBot, Deconstructhis, Futurist-corporation, Blah987654, TimProof, Theoneintraining, HarryMcDeath, Shroitman, Brooktree, SieBot, Dusti, Ttony21, K. Annoyomous,Renil~enwiki, Gerakibot, Claus Ableiter, Caltas, Muraabi6, ConfuciusOrnis, Yintan, Kiefer100, Srushe, Mookiefurr, Keilana, Who3,TitanOne, The Evil Spartan, MinorContributor, Xingzeng, Oda Mari, ShinobiX200, Terper, Jojalozzo, Pontoots9, Jamiepgs, JetLover,Myotis, Teles.ME, Aperseghin, Wmpearl, Oxymoron83, RobertMel, Faradayplank, Nuttycoconut, Zharradan.angelfire, Vijinjain, KibaK-ibbles, Lightmouse, Beej175560, Jhacob, Ks0stm, RyanParis, Fratrep, Sunrise, Pediainsight, Silvergoat, LonelyMarble, Reneeholle, Stat-icGull, Mygerardromance, X31forest, Ascidian, Nathan1991nathan1991, LAS1180, Neo., Brave warrior, Moomoomoostwos, Pinkadelica,Pyrophotographer, Supraboy001, Denisarona, Escape Orbit, Benjamin Nicholas Johnston, Marmenta, Athenean, WikipedianMarlith, Jim-myjkjr, Loren.wilton, Martarius, Sfan00 IMG, FlamingSilmaril, Elassint, ClueBot, Compdude47, Rumping, GorillaWarfare, The ThingThat Should Not Be, Topher208, Eusticeconway, IceUnshattered, Plastikspork, EMC125, Wysprgr2005, Synthiac, Johnny4netglimse,Gopher65, Alecsdaniel, Aj767, Drmies, VQuakr, Ceris2, Uncle Milty, J8079s, Shadowdemon936, CounterVandalismBot, Wirelesspp,03ctodd, Blanchardb, Agge1000, Kbev, Neverquick, ChandlerMapBot, Deadman3215, Phobiaphobia, Awickert, Excirial, Pumpmeup,Alan268, -Midorihana-, Alexbot, Jusdafax, M4gnum0n, Sinteractive, Robbie098, Erebus Morgaine, McLovin123459, Alejandrocaro35,SeeYouNextTuesday999, Adric Kearney, Parresh, Prancibaldfpants, NuclearWarfare, Cenarium, Promethean, Vendeka, Hans Adler, Kent-gen1, Lilbrew369, Razorflame, Noosentaal, Olliegodwin, La Pianista, Panos84, Thingg, Lindberg, Friedpotatoeparty, Darren23, Porthos0,Aitias, Galor612, M karthikkannan, Versus22, Mancini141, SoxBot III, Apparition11, Goodvac, B15nes7, Vanished user uih38riiw4hjlsd,Vanished User 1004, DumZiBoT, Jolekweatin, TimothyRias, InternetMeme, Williamrlinden, Anubad95, Skunkboy74, BarretB, Paidoan-tonio, Arianewiki1, XLinkBot, DrOxacropheles, Popol0707, Jovianeye, Rror, Gwark, Grubtatorship, Purnajitphukon, Kasyap.d, Coldplay-rock08, Ilikepie2221, Denton22, Coreylook, NellieBly, Mifter, Maruthi Achsara, Noctibus, Aunt Entropy, Whizmd, ZooFari, Abomas-now, MaizeAndBlue86, Truthnlove, Dbannie07, Lemmey, HexaChord, Bigfatmonkeyturd, Houdabouda123, King Pickle, Addbot, Willk-ing1979, Tonezilla88, Moosester, Some jerk on the Internet, DOI bot, Tcncv, Shitonmydick1234, Fyrael, Landon1980, Vedjain, Captain-tucker, Rashaani, PaterMcFly, GSMR, Amirazemi, SpellingBot, Reidlophile, Ronhjones, TutterMouse, NiallJones, Njaelkies Lea, Ajp4,Xlec, Vishnava, CanadianLinuxUser, Leszek Jańczuk, Fluffernutter, NjardarBot, Ka Faraq Gatri, Looie496, Skyezx, Xxuberzang, Down-load, SoSaysChappy, Jeffo223344, CarsracBot, Gifðas, Mjr162006, PFSLAKES1, Muffin123456789, Punkrockpiper, Debresser, Scopes-monkeys, Favonian, Azazeel, Sappho'd, CuteHappyBrute, Xoxoxoxoxxx3, Cranberry5553, Brainmachine, Dath Dath Binks, Numbo3-bot,Debashish Mahapatra., Ljay9206, Tide rolls, Lightbot, OlEnglish, Libertype, Sky83, Teles, Lrrasd, Smallman21, Gail, SeniorInt, LuK3,Alfie66, Joshua098, Ben Ben, Luckas-bot, Yobot, 2D, Felixmonk1, Ptbotgourou, Oscardove, Legobot II, Justintan88, SashaTheAwesome,Deadly Matty, II MusLiM HyBRiD II, Adi, Aldebaran66, WikiworldJ, THENWHOWAS PHONE?, Zkczkc, Godzilla 2002, PoizonMyst,AnakngAraw, Thulasidharan2k8, Donthegon101, Echtner, Azcolvin429, Darkowlf8592, Madan.sreddy, Tempodivalse, Kookyunii, Syn-chronism, AnomieBOT, AndrooUK, Jaimeescobar, Hermanschrader, Killiondude, Jim1138, AdjustShift, Bobsexual, Cheese12345cheese,P123567890p, Georgiocj, Kingpin13, Sz-iwbot, Dr. Günter Bechly, Ulric1313, GWPFBE, Mann jess, Materialscientist, Hellopunish3r,RobertEves92, The High Fin SpermWhale, Citation bot, Srinivas, OllieFury, Mr.Kassner, Stevemanjones, Xeonxeon12, Berkeley626, Neu-rolysis, ArthurBot, Phlembowper99, Obersachsebot, Xqbot, Samoboow, Jstana, Typuifre, Animonster, S h i v a (Visnu), Intelati, JimVC3,Capricorn42, Oraculo miraculoso, Wperdue, Renaissancee, Jeffrey Mall, Nothingisayisreal, Smk65536, Nickkid5, Pwnage09, Jsharp-minor, Imstillhereyoh, Light-assasin, Grim23, Jzhuo, Superbrainr, Tomwsulcer, Br77rino, Rootswailer, Mlpearc, Gap9551, Logan6362,J04n, GrouchoBot, Nayvik, Solphusion~enwiki, Call me Bubba, Mario777Zelda, False vacuum, Omnipaedista, Shirik, Wooitscaroline,Akshat2, Prunesqualer, RibotBOT, SassoBot, Amaury, Permafry42, The Wiki ghost, GhalyBot, Someone0707, Jrossr, DarkElrad, LORD-GOD7777, Shadowjams, AlimanRuna, Peter470, Babebait227, BoomerAB, Captain-n00dle, Joel grover, Nagualdesign, George585, Fres-coBot, AtomsOrSystems, Jbvjkhgvkjukjhvkj, Tobby72, Bluto7, Dantesparda271191, Dannyat43, Endofskull, Tylergriswold, Machine Elf1735, Ruben3186, Airborne84, Amicaveritas, Wireless Keyboard, Dalekian, HamburgerRadio, Citation bot 1, Familyguylover64, Rom-mopaula, Redrose64, Cloroplast horse, Breadhead23, Pekayer11, Pinethicket, Jinsubpretzels, Tom.Reding, بيك ,قربانعلي CalmerWaters, StrawberryPink, Dazedbythebell, Procatcher31, Jschnur, RedBot, KnowYourCosmos, SpaceFlight89, VinnyXY, Vitaliy skryn-nik, Trec'hlid mitonet, Tamsier, Newgrounder, Infinitesolid, December21st2012Freak, Utility Monster, IVAN3MAN, Meier99, TobeBot,Trappist the monk, Bibleboy14, Jkveton08, Jules93, Hahaho~enwiki, AHeneen, Gafferuk, Jonkerz, Nickyus, Vrenator, ویکی ,علی Be-goon, Sidoburn, Canuck100, Dcs002, Zachareth, Diannaa, Michael.goulais, Earthandmoon, Tbhotch, Reach Out to the Truth, Hmmwhat-sthisdo, Minimac, Jpabian01, DARTH SIDIOUS 2, JedediahBaugh, Jeglikerdeg, Mean as custard, Nate5713, Ripchip Bot, Citation-eeded, Bhawani Gautam, Beyond My Ken, LucChickenfingers1873, Wintonian, Crazyj922, Slon02, Dumbdarwin, EmausBot, Mzi-likazi1939, WikitanvirBot, Eekerz, Akhil.aggarwal2, Katherine, Ihatejustinbeiber, GoingBatty, RA0808, JacobParker100, Strane1991,Kamakazii101, Bt8257, Miladragon3, Pcorty, Slightsmile, Phil2324, Tommy2010, Leemadd92, Alicam8, Wikipelli, Aynan678, Tele-ComNasSprVen, Ryuzakihateskira, Italia2006, Hhhippo, Yddam, Susfele, Tnvkumar, WelcomeBackWinter, Mattsarahcat, Xunknownxx,HyperSonic X, Jhum~enwiki, Aeonx, Bilbo571, H3llBot, Git2010, Confession0791, Kirstylovesatl, Kjgiant, David J Johnson, Weirdo1990,Tolly4bolly, Openstrings, Butterfly 25007, Tiiliskivi, IGeMiNix, Coasterlover1994, Purduecit, L Kensington, Quantumor, ,35יניב Inswoon,Farheen1973, Wikiloop, Bulwersator, Harmi.banik, Vanfug, Orange Suede Sofa, ChuispastonBot, Matthewrbowker, Bubble queen, Iketsi,LikeLakers2, 28bot, H1tchh1ker4, Mjbmrbot, Jesuschristlover, Petrzak, ClueBot NG, Snoopdoug22, Dr A Thompson, VLDG123, The-matrix007, JustKiddingPro2, Ulflund, 8732Spacefish, ALovelyOne1, Gilderien, EricWaltonBall, Sdht, Deepvalley, Joefromrandb, Threegeeks, Hermes the Wise, Zenithfel, Braincricket, Rezabot, Sudhir.routray, Widr, Karl 334, Squirtle1994, JackizCool69, Gunfists, Ale Ron-zani, Pluma, North Atlanticist Usonian, Mightymights, Helpful Pixie Bot, Art and Muscle, Rryswny, Faus, Jumpingjacksparrow, NikhilSasi, Winegum74, I LIVE 2 TROLL, UT200100, Bibcode Bot, Olso12345, Lowercase sigmabot, BG19bot, Island Monkey, Sahuanimesh,Gaithdroby500, Furkhaocean, Apeconmyth, Quarkgluonsoup, MongolWiki, Xzuiko, AvocatoBot, Inkpot80, Davidiad, J991, Abhinan-dan27, Dan653, Tracy49, Mariraja2007, Cadiomals, Joydeep, Omeed Was Here, Chretienorthodoxe, Maharding, Hotturbos, Stimuliecon-omy, Hamish59, Eguinto, Theone70, Glacialfox, Andi2011, Mozerella8, Oleg-ch, Ghostillnses, Boogawooga02, Anbu121, Amphibio,Adamlewis157, BattyBot, Tutelary, Hihihihibye, Priyamd, SFaddict42, Th4n3r, Lawlessballer23, Cyberbot II, Atularunpandey, Craigc29,Khazar2, Dexbot, Webclient101, Lugia2453, Frosty, Hair, Graphium, Philipandrew, Rmoole, Paryinmahpantz, LemonsWillWin, Keefyj,Alexfrench70, Guruparanjothijason, Bccmac14, Reatlas, 123yokomo, Heskey eats soup with a fork, Ihatefindingusernamestheyneverwork,USEFULMENACE, Faizan, Supercool900, Godot13, Ianreisterariola, Vcfahrenbruck, Redshiftimprove, AWolfgang, Christofkopera, Ex-tremind, Alihosseinisg, MarioVSYoshi, KermitTheFrogOwns, , Tentinator, Sakhail, Boofle1, Infamous Castle, Jayminsiple, Leave61,Seps123, Dorrphilip, Ventripotent13, Alaudduin, CuirassierX, Pmacclain, Ugog Nizdast, FrogySK, Eagle3399, Jwratner1, Kogge, DamiánA. Fernández Beanato, OccultZone, Jackmerius, Anrnusna, Asterixasterix, Mikey Camarista, Beenybobby, Aleksa is cool, Maderthaner,Goodkushandalcohol, BubbaLAquatics10, Concord hioz, Monkbot, Apipia, Sofia Koutsouveli, Pinkpills, Thatoneguy08, Astronomnom,PrivateNoPrivates, SkyFlubbler, Mannerheimcross, SarahTehCat, Analfistingjanick, SeaMosse, Corneel1, BenVinnie Tennieson, SquidzyMcgavin, Jojoisawsome, EoRdE6, Satki, Atomic bacon, Snabbkaffe, I'm your Grandma., Tetra quark, Isambard Kingdom, Anand2202,Mahad Asif khan, Zxzxzxzx29, Warrerrrrrrfjgfufgvgjfjfjjfjfggfu, Catman3659, KasparBot and Anonymous: 1854

20 13 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

13.2 Images• File:Aristarchus_working.jpg Source: http://upload.wikimedia.org/wikipedia/commons/2/2b/Aristarchus_working.jpg License: Public

domain Contributors: ? Original artist: ?• File:CMB_Timeline300_no_WMAP.jpg Source: http://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: http://upload.wikimedia.org/wikipedia/commons/2/24/Cassini-science-br.jpg License: Public do-main Contributors: ? Original artist: ?

• File:Closed_Friedmann_universe_zero_Lambda.ogg Source: http://upload.wikimedia.org/wikipedia/commons/2/2c/Closed_Friedmann_universe_zero_Lambda.ogg License: CC BY-SA 3.0 Contributors: Own work Original artist: WillowW

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

• File:Crab_Nebula.jpg Source: http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg License: Public domain Contrib-utors: 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: http://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: http://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.svg Source: http://upload.wikimedia.org/wikipedia/commons/4/4c/Elementary_particle_interactions.svg License: Public domain Contributors: en:Image:Interactions.png Original artist: en:User:TriTertButoxy, User:Stannered

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

• File:Flammarion.jpg Source: http://upload.wikimedia.org/wikipedia/commons/8/87/Flammarion.jpg License: Public domain Contribu-tors: Camille Flammarion, L'Atmosphere: Météorologie Populaire (Paris, 1888), pp. 163 Original artist: Anonymous

• File:Folder_Hexagonal_Icon.svg Source: http://upload.wikimedia.org/wikipedia/en/4/48/Folder_Hexagonal_Icon.svg License: Cc-by-sa-3.0 Contributors: ? Original artist: ?

• File:Ilc_9yr_moll4096.png Source: http://upload.wikimedia.org/wikipedia/commons/3/3c/Ilc_9yr_moll4096.png License: Public do-main Contributors: http://map.gsfc.nasa.gov/media/121238/ilc_9yr_moll4096.png Original artist: NASA / WMAP Science Team

• File:Multiverse_-_level_II.svg Source: http://upload.wikimedia.org/wikipedia/commons/3/34/Multiverse_-_level_II.svg License: Pub-lic domain Contributors: Vectorisation of Multiverse - level II.GIF (by K1234567890y), by Lokal_Profil Original artist:

• Original by K1234567890y• File:Only_distance_is_real.svg Source: http://upload.wikimedia.org/wikipedia/commons/3/31/Only_distance_is_real.svg License: CC

BY 3.0 Contributors: Own work Original artist: WillowW• File:Portal-puzzle.svg Source: http://upload.wikimedia.org/wikipedia/en/f/fd/Portal-puzzle.svg License: Public domain Contributors: ?Original artist: ?

• File:Primordial_nucleosynthesis.svg Source: http://upload.wikimedia.org/wikipedia/commons/8/85/Primordial_nucleosynthesis.svgLicense: Public domain Contributors: Latex code (see talk page for source) converted to svg using the TexText extension for Inkscape.Minor alignment issues solved by hand. The original set of equations come from Primordial nucleosynthesis.JPG by Szdori Original artist:Lokal_Profil

• File:Sound-icon.svg Source: http://upload.wikimedia.org/wikipedia/commons/4/47/Sound-icon.svg License: LGPL Contributors:Derivative work from Silsor's versio Original artist: Crystal SVG icon set

• File:ThomasDiggesmap.JPG Source: http://upload.wikimedia.org/wikipedia/commons/3/3e/ThomasDiggesmap.JPG License: Publicdomain Contributors: Thomas Digges map: public domain, copied from <a data-x-rel='nofollow' class='external text' href='http://www.astrosociety.org/pubs/mercury/30_05/copernicus.html'>here</a> Original artist: w:Thomas Digges (1546?−1595)

• File:Wikiquote-logo.svg Source: http://upload.wikimedia.org/wikipedia/commons/f/fa/Wikiquote-logo.svg License: Public domainContributors: ? Original artist: ?

13.3 Content license• Creative Commons Attribution-Share Alike 3.0