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=[[en:Bulbous corpuscle]]=

==Universo==

=[[Corpuscolo di Ruffini]]=

The '''Universe''' is commonly defined as the totality of everything that [[existence|exist]]s,<ref>

{{cite book

|url=http://www.yourdictionary.com/universe

|title=Webster's New World College Dictionary

|year=2010

|publisher=Wiley Publishing, Inc.}}

</ref> including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space<ref>

{{cite book

|url=http://www.yourdictionary.com/universe

|title=The American Heritage® Dictionary of the English Language

|edition=4th

|year=2010

|publisher=Houghton Mifflin Harcourt Publishing Company}}

</ref><ref>

{{cite book

|url=http://dictionary.cambridge.org/dictionary/british/universe

|title=Cambridge Advanced Learner's Dictionary}}

</ref>, although this usage may differ with the context (see definitions, below).

The term ''Universe'' may be used in slightly different contextual senses, denoting such concepts as the ''[[cosmos]]'', the ''[[world (philosophy)|world]]'', or ''[[nature]]''.

{{anatomia

Observations of earlier stages in the development of the universe, which can be seen at great distances, suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and history.

|Nome=Corpuscolo di Ruffini

|GraySubject=233

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|Immagine=Gray937.png

|Didascalia=Terminazioni nervose del corpuscolo di Ruffini

|Sviluppo embriologico=

|Sistema=

|Arteria=

|Vena=

|Nervo=

|Linfatici=

|MeshName=

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}}

Il '''corpuscolo di Ruffini''' è una classe di [[meccanorecettore|meccanorecettori]] ad adattamento lento. Si pensa che siano presenti solo nelle zone naturalmente glabre del [[derma]] (dorso della [[mano]], pianta del [[Piede (anatomia)|piede]], [[labbro|labbra]], [[piccole labbra]] e [[glande]]), e nell'[[ipoderma]] umano. Il nome deriva dal medico italiano [[Angelo Ruffini]].

==History==

Throughout recorded history, several [[cosmology|cosmologies]] and [[cosmogony|cosmogonies]] have been proposed to account for observations of the Universe. The earliest quantitative [[geocentric]] models were developed by the [[ancient Greece|ancient Greeks]], who proposed that the Universe possesses infinite space and has existed eternally, but contains a single set of concentric [[sphere]]s of finite size&nbsp;– corresponding to the fixed stars, the [[Sun]] and various [[planet]]s&nbsp;– rotating about a spherical but unmoving [[Earth]]. Over the centuries, more precise observations and improved theories of gravity led to [[Copernicus|Copernicus's]] [[heliocentrism|heliocentric model]] and the [[Isaac Newton|Newtonian]] model of the [[Solar System]], respectively. Further improvements in astronomy led to the realization that the Solar System is embedded in a [[galaxy]] composed of millions of stars, the [[Milky Way]], and that other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their [[spectral line]]s have led to much of [[physical cosmology|modern cosmology]]. Discovery of the [[red shift]] and cosmic [[microwave background radiation]] revealed that the Universe is expanding and apparently had a beginning.

==Function==

[[Image:HubbleUltraDeepFieldwithScaleComparison.jpg|thumb|right|290px|This high-resolution image of the [[Hubble ultra deep field]] shows a diverse range of [[Galaxy|galaxies]], each consisting of billions of [[star]]s. The equivalent area of sky that the picture occupies is shown in the lower left corner. The smallest, reddest galaxies, about 100, are some of the most distant galaxies to have been imaged by an optical telescope, existing at the time shortly after the Big Bang.]]

This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense of and control of finger position and movement.<ref>{{Cita libro|cognome=Mountcastle |nome=Vernon C. |anno=2005 |titolo=The Sensory Hand: Neural Mechanisms of Somatic Sensation |editore=Harvard University Press |p=34}}</ref> It is believed to be useful for monitoring slippage of objects along the surface of the skin, allowing modulation of grip on an object.

According to the prevailing scientific model of the Universe, known as the [[Big bang|Big Bang]], the Universe expanded from an extremely hot, dense phase called the [[Planck epoch]], in which all the matter and energy of the [[observable universe]] was concentrated. Since the Planck epoch, the Universe has been [[Cosmic expansion|expanding]] to its present form, possibly with a brief period (less than 10⁻³² seconds) of [[cosmic inflation]]. Several independent experimental measurements support this theoretical [[Metric expansion of space|expansion]] and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of [[dark energy]], and that most of the matter in the Universe may be in a form which cannot be detected by present instruments, and so is not accounted for in the present models of the universe; this has been named [[dark matter]]. The imprecision of current observations has hindered predictions of the [[ultimate fate of the Universe]].

Ruffinian endings are located in the deep layers of the skin, and register mechanical deformation within joints, more specifically angle change, with a specificity of up to 2 degrees, as well as continuous pressure states.They also act as a thermoreceptors that respond for a long time, so in case of deep burn there will be no pain as these receptors will be burned off.<ref>{{Cita libro|cognome=Hamilton |nome=Nancy |anno=2008 |titolo=Kinesiology: Scientific Basis of Human Motion |editore=McGraw-Hill |pp=76–7}}</ref>

Current interpretations of [[observable universe|astronomical observations]] indicate that the [[age of the Universe]] is 13.75 ±0.17 [[1000000000 (number)|billion]] years,<ref name="marshallaugerhilbertblandford">S. H. Suyu, P. J. Marshall, M. W. Auger, S. Hilbert, R. D. Blandford, L. V. E. Koopmans, C. D. Fassnacht and T. Treu. [http://www.iop.org/EJ/abstract/0004-637X/711/1/201/ Dissecting the Gravitational Lens B1608+656. II. Precision Measurements of the Hubble Constant, Spatial Curvature, and the Dark Energy Equation of State.] The Astrophysical Journal, 2010; 711 (1): 201 DOI: 10.1088/0004-637X/711/1/201</ref> and that the diameter of the [[observable universe]] is at least 93 billion [[light year]]s, or [[Scientific Notation|8.80 x 10²⁶]] [[metre]]s.<ref name=ly93>{{cite web | last = Lineweaver | first = Charles | coauthors = Tamara M. Davis | year = 2005 | url = http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03&page=5 | title = Misconceptions about the Big Bang | publisher = [[Scientific American]] | accessdate = 2008-11-06}}</ref> According to [[general relativity]], space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the Universe is finite or infinite.

==Etymology, synonyms and definitions==

{{See also|Cosmos|Nature|World (philosophy)|Celestial spheres}}

The word ''Universe'' derives from the [[Old French]] word ''Univers'', which in turn derives from the [[Latin]] word ''universum''.<ref>''The Compact Edition of the Oxford English Dictionary'', volume II, Oxford: Oxford University Press, 1971, p.3518.</ref> The Latin word was used by [[Cicero]] and later Latin authors in many of the same senses as the modern [[English language|English]] word is used.<ref name="lewis_short" /> The Latin word derives from the poetic contraction ''Unvorsum''&nbsp;— first used by [[Lucretius]] in Book IV (line 262) of his ''[[On the Nature of Things|De rerum natura]]'' (''On the Nature of Things'')&nbsp;— which connects ''un, uni'' (the combining form of ''unus', or "one") with ''vorsum, versum'' (a noun made from the perfect passive participle of ''vertere'', meaning "something rotated, rolled, changed").<ref name="lewis_short">Lewis and Short, ''A Latin Dictionary'', Oxford University Press, ISBN 0-19-864201-6, pp. 1933, 1977–1978.</ref> Lucretius used the word in the sense "everything rolled into one, everything combined into one".

[[Image:Foucault pendulum animated.gif|thumb|right|Artistic rendition (highly exaggerated) of a [[Foucault pendulum]] showing that the Earth is not stationary, but rotates.]]

An alternative interpretation of ''unvorsum'' is "everything rotated as one" or "everything rotated by one". In this sense, it may be considered a translation of an earlier Greek word for the Universe, περιφορα, "something transported in a circle", originally used to describe a course of a meal, the food being carried around the circle of dinner guests.<ref>Liddell and Scott, ''A Greek-English Lexicon'', Oxford University Press, ISBN 0-19-864214-8, p.1392.</ref> This Greek word refers to [[celestial spheres|an early Greek model of the Universe]], in which all matter was contained within rotating spheres centered on the Earth; according to [[Aristotle]], the rotation of [[Primum Mobile|the outermost sphere]] was responsible for the motion and change of everything within. It was natural for the Greeks to assume that the Earth was stationary and that the heavens rotated about the [[Earth]], because careful [[astronomy|astronomical]] and physical measurements (such as the [[Foucault pendulum]]) are required to prove otherwise.

The most common term for "Universe" among the ancient [[Greek philosophy|Greek philosophers]] from [[Pythagoras]] onwards was το παν (The All), defined as all matter (το ολον) and all space (το κενον).<ref>Liddell and Scott, pp.1345–1346.</ref><ref>{{cite book | author = Yonge, Charles Duke | year = 1870 | title = An English-Greek lexicon | publisher = American Bok Company | ___location = New York | pages = 567}}</ref> Other synonyms for the Universe among the ancient Greek philosophers included κοσμος (meaning the [[world (philosophy)|world]], the [[cosmos]]) and φυσις (meaning [[Nature]], from which we derive the word [[physics]]).<ref>Liddell and Scott, pp.985, 1964.</ref> The same synonyms are found in Latin authors (''totum'', ''mundus'', ''natura'')<ref>Lewis and Short, pp. 1881–1882, 1175, 1189–1190.</ref> and survive in modern languages, e.g., the German words ''Das All'', ''Weltall'', and ''Natur'' for Universe. The same synonyms are found in English, such as everything (as in the [[theory of everything]]), the cosmos (as in [[cosmology]]), the [[world (philosophy)|world]] (as in the [[many-worlds hypothesis]]), and [[Nature]] (as in [[natural law]]s or [[natural philosophy]]).<ref>OED, pp. 909, 569, 3821–3822, 1900.</ref>

===Broadest definition: reality and probability===

{{See also|Introduction to quantum mechanics|Interpretation of quantum mechanics|Many-worlds hypothesis}}

The broadest definition of the Universe can be found in ''[[De divisione naturae]]'' by the [[Middle Ages|medieval]] [[philosopher]] and [[theology|theologian]] [[Johannes Scotus Eriugena]], who defined it as simply everything: everything that is created and everything that is not created. Time is not considered in Eriugena's definition; thus, his definition includes everything that exists, has existed and will exist, as well as everything that does not exist, has never existed and will never exist. This all-embracing definition was not adopted by most later philosophers, but something not entirely dissimilar reappears in [[quantum physics]], perhaps most obviously in the [[path integral formulation|path-integral formulation]] of [[Richard Feynman|Feynman]].<ref name="path_integral">{{cite book | author = Feynman RP, Hibbs AR | year = 1965 | title = Quantum Physics and Path Integrals | publisher = McGraw–Hill | ___location = New York | isbn = 0-07-020650-3}}<br />{{cite book | author = Zinn Justin J | year = 2004 | title = Path Integrals in Quantum Mechanics | publisher = Oxford University Press | isbn = 0-19-856674-3 | oclc = 212409192}}</ref> According to that formulation, the [[probability amplitude]]s for the various outcomes of an experiment given a perfectly defined initial state of the system are determined by summing over all possible paths by which the system could progress from the initial to final state. Naturally, an experiment can have only one outcome; in other words, only one possible outcome is made real in this Universe, via the mysterious process of [[measurement in quantum mechanics|quantum measurement]], also known as the [[wavefunction collapse|collapse of the wavefunction]] (but see the [[many-worlds hypothesis]] below in the [[Multiverse]] section). In this well-defined mathematical sense, even that which does not exist (all possible paths) can influence that which does finally exist (the experimental measurement). As a specific example, every [[electron]] is intrinsically identical to every other; therefore, probability amplitudes must be computed allowing for the possibility that they exchange positions, something known as [[exchange symmetry]]. This conception of the Universe embracing both the existent and the non-existent loosely parallels the [[Buddhism|Buddhist]] doctrines of [[shunyata]] and [[pratitya-samutpada|interdependent development of reality]], and [[Gottfried Leibniz]]'s more modern concepts of [[contingency]] and the [[identity of indiscernibles]].

===Definition as reality===

{{See also|Reality|Physics}}

More customarily, the Universe is defined as everything that exists, has existed, and will exist {{Citation needed|date=May 2010}}. According to this definition and our present understanding, the Universe consists of three elements: [[space]] and [[time]], collectively known as [[space-time]] or the [[vacuum]]; [[matter]] and various forms of [[energy]] and [[momentum]] occupying [[space-time]]; and the [[physical law]]s that govern the first two. These elements will be discussed in greater detail below. A related definition of the term ''Universe'' is everything that exists at a single moment of [[cosmological time]], such as the present, as in the sentence "The Universe is now bathed uniformly in [[cosmic microwave background radiation|microwave radiation]]".

The three elements of the Universe (spacetime, matter-energy, and physical law) correspond roughly to the ideas of [[Aristotle]]. In his book ''[[Physics (Aristotle)|The Physics]]'' (Φυσικης, from which we derive the word "physics"), Aristotle divided το παν (everything) into three roughly analogous elements: ''matter'' (the stuff of which the Universe is made), ''form'' (the arrangement of that matter in space) and ''change'' (how matter is created, destroyed or altered in its properties, and similarly, how form is altered). [[Physical law]]s were conceived as the rules governing the properties of matter, form and their changes. Later philosophers such as [[Lucretius]], [[Averroes]], [[Avicenna]] and [[Baruch Spinoza]] altered or refined these divisions{{Citation needed|date=May 2010}}; for example, Averroes and Spinoza discern ''[[natura naturans]]'' (the active principles governing the Universe) from ''[[natura naturata]]'', the passive elements upon which the former act.

===Definition as connected space-time===

{{See also|Chaotic Inflation theory}}

It is possible to conceive of disconnected [[space-time]]s, each existing but unable to interact with one another. An easily visualized metaphor is a group of separate [[soap bubble]]s, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle. According to one common terminology, each "soap bubble" of space-time is denoted as a universe, whereas our particular [[space-time]] is denoted as ''the Universe'', just as we call our moon ''the [[Moon]]''. The entire collection of these separate space-times is denoted as the [[multiverse]].<ref name="EllisKS03">{{cite journal

| last = Ellis

| first = George F.R.

| authorlink = George Ellis

| coauthors = U. Kirchner, W.R. Stoeger

| title = Multiverses and physical cosmology

| journal = Monthly Notices of the Royal Astronomical Society

| volume = 347

| issue =

| pages = 921–936

| publisher =

| year = 2004

| url = http://arxiv.org/abs/astro-ph/0305292

| doi =10.1111/j.1365-2966.2004.07261.x

| id =

| accessdate = 2007-01-09

| format = subscription required}}</ref> In principle, the other unconnected universes may have different [[dimension]]alities and [[topology|topologies]] of [[space-time]], different forms of [[matter]] and [[energy]], and different [[physical law]]s and [[physical constant]]s, although such possibilities are currently speculative.

===Definition as observable reality===

{{See also|Observable universe|Observational cosmology}}

According to a still-more-restrictive definition, the Universe is everything within our connected [[space-time]] that could have a chance to interact with us and vice versa.{{Citation needed|date=May 2010}} According to the [[general relativity|general theory of relativity]], some regions of [[space]] may never interact with ours even in the lifetime of the Universe, due to the finite [[speed of light]] and the ongoing [[expansion of space]]. For example, radio messages sent from Earth may never reach some regions of space, even if the Universe would live forever; space may expand faster than light can traverse it. It is worth emphasizing that those distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with them. The spatial region within which we can affect and be affected is denoted as the [[observable universe]]. Strictly speaking, the observable universe depends on the ___location of the observer. By traveling, an observer can come into contact with a greater region of space-time than an observer who remains still, so that the observable universe for the former is larger than for the latter. Nevertheless, even the most rapid traveler may not be able to interact with all of space. Typically, the observable universe is taken to mean the universe observable from our vantage point in the Milky Way Galaxy.

== Size, age, contents, structure, and laws ==<!-- [[Hubble's law]] links to this section -->

{{Main|Observable universe|Age of the Universe|Large-scale structure of the Universe|Abundance of the chemical elements}}

The Universe is very large and possibly infinite in volume; the observable matter is spread over a space at least 92 billion [[light years]] across.<ref>{{cite web | last = Lineweaver | first = Charles | coauthors = Tamara M. Davis | year = 2005 | url = http://www.sciam.com/article.cfm?articleID=0009F0CA-C523-1213-852383414B7F0147&pageNumber=5| title = Misconceptions about the Big Bang | publisher = [[Scientific American]] | accessdate = 2007-03-05}}</ref> For comparison, the diameter of a typical [[galaxy]] is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million [[light-years]].<ref>Rindler (1977), p.196.</ref> As an example, our [[Milky Way]] Galaxy is roughly 100,000 light years in diameter,<ref>{{cite web

| last = Christian

| first = Eric

| last2 = Samar

| first2 = Safi-Harb

| title = How large is the Milky Way?

| url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.html

| accessdate = 2007-11-28 }}</ref> and our nearest sister galaxy, the [[Andromeda Galaxy]], is located roughly 2.5 million light years away.<ref>{{cite journal

| author=I. Ribas, C. Jordi, F. Vilardell, E.L. Fitzpatrick, R.W. Hilditch, F. Edward

| title=First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy

| journal=Astrophysical Journal

| year=2005

|volume=635

| pages=L37–L40

| url=http://adsabs.harvard.edu/abs/2005ApJ...635L..37R

| doi = 10.1086/499161

}}<br />{{cite journal

| author=McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N.

| title=Distances and metallicities for 17 Local Group galaxies

| journal=Monthly Notices of the Royal Astronomical Society

| year=2005

|volume=356

|issue=4

| pages=979–997

| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005MNRAS.356..979M

| doi = 10.1111/j.1365-2966.2004.08514.x

}}</ref> There are probably more than 100 billion (10¹¹) [[Galaxy|galaxies]] in the [[observable universe]].<ref>{{cite web | last = Mackie | first = Glen |date= February 1, 2002 | url = http://astronomy.swin.edu.au/~gmackie/billions.html | title = To see the Universe in a Grain of Taranaki Sand | publisher = Swinburne University | accessdate = 2006-12-20 }}</ref> Typical galaxies range from [[dwarf galaxy|dwarfs]] with as few as ten million<ref>{{cite web | date=2000-05-03 | url = http://www.eso.org/outreach/press-rel/pr-2000/pr-12-00.html

| title = Unveiling the Secret of a Virgo Dwarf Galaxy

| publisher = ESO | accessdate = 2007-01-03 }}</ref> (10⁷) [[star]]s up to giants with one [[Orders of magnitude (numbers)#1012|trillion]]<ref name="M101">{{cite web | date=2006-02-28 | url = http://www.nasa.gov/mission_pages/hubble/science/hst_spiral_m10.html | title = Hubble's Largest Galaxy Portrait Offers a New High-Definition View

| publisher = NASA | accessdate = 2007-01-03 }}</ref> (10¹²) stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around one [[sextillion]] (10²¹) stars in the observable universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion (7 x 10²²)<ref>{{cite web | date=2003-07-17 | url = http://info.anu.edu.au/ovc/media/Media_Releases/2003/030717StarCount

| title = Star Count: ANU Astronomer makes best yet

| accessdate = 2010-02-19 }}</ref>.

[[File:Cosmological Composition - Pie Chart.png|thumb|450px|The universe is believed to be mostly composed of [[dark energy]] and [[dark matter]], both of which are poorly understood at present. Less than 5% of the universe is ordinary matter, a relatively small perturbation.]]

The observable matter is spread uniformly (''homogeneously'') throughout the universe, when averaged over distances longer than 300 million light-years.<ref>{{cite journal | author=N. Mandolesi, P. Calzolari, S. Cortiglioni, F. Delpino, G. Sironi | title=Large-scale homogeneity of the Universe measured by the microwave background | journal=Letters to Nature | year=1986 |volume=319 | pages=751–753 | doi= 10.1038/319751a0 }}</ref> However, on smaller length-scales, matter is observed to form "clumps", i.e., to cluster hierarchically; many [[atoms]] are condensed into [[star]]s, most stars into galaxies, most galaxies into [[galaxy groups and clusters|clusters, superclusters]] and, finally, the [[large-scale structure of the Universe|largest-scale structures]] such as the [[Great Wall (astronomy)|Great Wall of galaxies]]. The observable matter of the Universe is also spread ''isotropically'', meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content.<ref>{{cite web | last = Hinshaw | first = Gary |date= November 29, 2006 | url = http://map.gsfc.nasa.gov/m_mm.html | title = New Three Year Results on the Oldest Light in the Universe | publisher = NASA WMAP | accessdate = 2006-08-10 }}</ref> The Universe is also bathed in a highly isotropic [[microwave]] [[electromagnetic radiation|radiation]] that corresponds to a [[thermal equilibrium]] [[blackbody spectrum]] of roughly 2.725 [[kelvin]].<ref>{{cite web | last = Hinshaw | first = Gary |date= December 15, 2005 | url = http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html | title = Tests of the Big Bang: The CMB | publisher = NASA WMAP | accessdate = 2007-01-09 }}</ref> The hypothesis that the large-scale Universe is homogeneous and isotropic is known as the [[cosmological principle]],<ref>Rindler (1977), p. 202.</ref> which is [[End of Greatness|supported by astronomical observations]].

The present overall [[density]] of the Universe is very low, roughly 9.9 × 10⁻³⁰ grams per cubic centimetre. This mass-energy appears to consist of 73% [[dark energy]], 23% [[cold dark matter]] and 4% [[baryonic matter|ordinary matter]]. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume.<ref>{{cite web | last = Hinshaw | first = Gary |date= February 10, 2006 | url = http://map.gsfc.nasa.gov/m_uni/uni_101matter.html | title = What is the Universe Made Of? | publisher = NASA WMAP | accessdate = 2007-01-04}}</ref> The properties of dark energy and dark matter are largely unknown. Dark matter [[gravity|gravitates]] as ordinary matter, and thus works to slow the [[metric expansion of space|expansion of the Universe]]; by contrast, dark energy [[accelerating Universe|accelerates its expansion]].

The Universe is [[age of the universe|old]] and evolving. The [[Wilkinson Microwave Anisotropy Probe|most precise estimate]] of the Universe's age is 13.73±0.12 billion years old, based on observations of the [[cosmic microwave background radiation]].<ref name="NASA_age">{{cite web | title = Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results | url=http://lambda.gsfc.nasa.gov/product/map/dr3/pub_papers/fiveyear/basic_results/wmap5basic.pdf|format=PDF|publisher=nasa.gov|accessdate=2008-03-06}}</ref> Independent estimates (based on measurements such as [[radioactive dating]]) agree, although they are less precise, ranging from 11–20 billion years<ref>{{cite web

| author =Britt RR

| title =Age of Universe Revised, Again

| publisher =[[space.com]]

| date = 2003-01-03

| url = http://www.space.com/scienceastronomy/age_universe_030103.html

| accessdate = 2007-01-08}}</ref>

to 13–15 billion years.<ref>{{cite web

| author = Wright EL

| title =Age of the Universe

| publisher =[[UCLA]]

| year = 2005

| url = http://www.astro.ucla.edu/~wright/age.html

| accessdate = 2007-01-08

}}<br />{{cite journal

| author = Krauss LM, Chaboyer B

| title =Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology

| journal =[[Science (journal)|Science]]

| volume = 299

| issue = 5603

| pages = 65–69

| publisher =[[American Association for the Advancement of Science]]

| date = 3 January 2003

| url = http://www.sciencemag.org/cgi/content/abstract/299/5603/65?ijkey=3D7y0Qonz=GO7ig.&keytype=3Dref&siteid=3Dsci

| accessdate = 2007-01-08

| doi =10.1126/science.1075631

| pmid =12511641}}</ref> The universe has not been the same at all times in its history; for example, the relative populations of [[quasar]]s and galaxies have changed and [[space]] itself appears to have [[metric expansion of space|expanded]]. This expansion accounts for how Earth-bound scientists can observe the light from a galaxy 30 billion light years away, even if that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been [[redshift]]ed; the [[photon]]s emitted have been stretched to longer [[wavelength]]s and lower [[frequency]] during their journey. The rate of this spatial expansion is [[accelerating universe|accelerating]], based on studies of [[Type Ia supernova]]e and corroborated by other data.

The [[abundance of the chemical elements|relative fractions]] of different [[chemical element]]s&nbsp;— particularly the lightest atoms such as [[hydrogen]], [[deuterium]] and [[helium]]&nbsp;— seem to be identical throughout the universe and throughout its observable history.<ref>{{cite web | last = Wright | first = Edward L. |date= September 12, 2004 | url = http://www.astro.ucla.edu/~wright/BBNS.html | title = Big Bang Nucleosynthesis | publisher = UCLA | accessdate = 2007-01-05 }}<br />{{cite journal | author=M. Harwit, M. Spaans | title=Chemical Composition of the Early Universe | journal=The Astrophysical Journal | year=2003 |volume=589 |issue=1 | pages=53–57 | url=http://adsabs.harvard.edu/abs/2003ApJ...589...53H | doi = 10.1086/374415}}<br />{{cite journal | author=C. Kobulnicky, E. D. Skillman | title=Chemical Composition of the Early Universe | journal=Bulletin of the American Astronomical Society | year=1997 |volume=29 | pages=1329 | url=http://adsabs.harvard.edu/abs/1997AAS...191.7603K }}</ref> The universe seems to have much more [[matter]] than [[antimatter]], an asymmetry possibly related to the observations of [[CP violation]].<ref>{{cite web |date= October 28, 2003 | url = http://www.pparc.ac.uk/ps/bbs/bbs_antimatter.asp | title = Antimatter | publisher = Particle Physics and Astronomy Research Council | accessdate = 2006-08-10 }}</ref> The Universe appears to have no net [[electric charge]], and therefore [[gravity]] appears to be the dominant interaction on cosmological length scales. The Universe also appears to have neither net [[momentum]] nor [[angular momentum]]. The absence of net charge and momentum would follow from accepted physical laws ([[Gauss's law]] and the non-divergence of the [[stress-energy-momentum pseudotensor]], respectively), if the universe were finite.<ref>Landau and Lifshitz (1975), p.361.</ref>

[[Image:Elementary particle interactions.svg|thumb|left|300px|The [[elementary particle]]s from which the Universe is constructed. Six [[lepton]]s and six [[quark]]s comprise most of the [[matter]]; for example, the [[proton]]s and [[neutron]]s of [[atomic nucleus|atomic nuclei]] are composed of quarks, and the ubiquitous [[electron]] is a lepton. These particles interact via the [[gauge boson]]s shown in the middle row, each corresponding to a particular type of [[gauge symmetry]]. The [[Higgs boson]] (as yet unobserved) is believed to confer [[mass]] on the particles with which it is connected. The [[graviton]], a supposed gauge boson for [[gravity]], is not shown.]]

The Universe appears to have a smooth [[space-time continuum]] consisting of three [[space|spatial]] [[dimension]]s and one temporal ([[time]]) dimension. On the average, [[3-space|space]] is observed to be very nearly flat (close to zero [[curvature]]), meaning that [[Euclidean geometry]] is experimentally true with high accuracy throughout most of the Universe.<ref name="Shape">[http://map.gsfc.nasa.gov/m_mm/mr_content.html WMAP Mission: Results&nbsp;– Age of the Universe<!-- Bot generated title -->]</ref> Spacetime also appears to have a [[simply connected space|simply connected]] [[topology]], at least on the length-scale of the observable universe. However, present observations cannot exclude the possibilities that the universe has more dimensions and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or [[toroid]]al topologies of two-dimensional [[space]]s.<ref name="_spacetime_topology">{{cite conference

| first = Jean-Pierre

| last = Luminet

| authorlink =

| coauthors = Boudewijn F. Roukema

| title = Topology of the Universe: Theory and Observations

| booktitle = Proceedings of Cosmology School held at Cargese, Corsica, August 1998

| pages =

| publisher =

| year = 1999

| ___location =

| url = http://arxiv.org/abs/astro-ph/9901364

| doi =

| id =

| accessdate = 2007-01-05

}}<br />{{cite journal

| last = Luminet

| first = Jean-Pierre

| authorlink =

| coauthors = J. Weeks, A. Riazuelo, R. Lehoucq, J.-P. Uzan

| title = Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background

| journal = [[Nature]]

| volume = 425

| issue =

6958| pages = 593

| publisher =

| year=2003

| pmid = 14534579

| url = http://arxiv.org/abs/astro-ph/0310253

| doi =10.1038/nature01944

| id =

| accessdate = 2007-01-09

| format = subscription required}}</ref>

The Universe appears to behave in a manner that regularly follows a set of [[physical law]]s and [[physical constant]]s.<ref>{{cite web | last = Strobel | first = Nick |date= May 23, 2001 | url = http://www.astronomynotes.com/starprop/s7.htm | title = The Composition of Stars | publisher = Astronomy Notes | accessdate = 2007-01-04 }}<br />{{cite web | url=http://www.faqs.org/faqs/astronomy/faq/part4/section-4.html | title = Have physical constants changed with time? | publisher = Astrophysics (Astronomy Frequently Asked Questions) | accessdate = 2007-01-04 }}</ref> According to the prevailing [[Standard Model]] of physics, all matter is composed of three generations of [[lepton]]s and [[quark]]s, both of which are [[fermion]]s. These [[elementary particle]]s interact via at most three [[fundamental interaction]]s: the [[electroweak]] interaction which includes [[electromagnetism]] and the [[weak nuclear force]]; the [[strong nuclear force]] described by [[quantum chromodynamics]]; and [[gravity]], which is best described at present by [[general relativity]]. The first two interactions can be described by [[renormalization|renormalized]] [[quantum field theory]], and are mediated by [[gauge boson]]s that correspond to a particular type of [[gauge symmetry]]. A renormalized quantum field theory of general relativity has not yet been achieved, although various forms of [[string theory]] seem promising. The theory of [[special relativity]] is believed to hold throughout the universe, provided that the spatial and temporal length scales are sufficiently short; otherwise, the more general theory of general relativity must be applied. There is no explanation for the particular values that [[physical constant]]s appear to have throughout our Universe, such as [[Planck's constant]] ''h'' or the [[gravitational constant]] ''G''. Several [[conservation law]]s have been identified, such as the [[conservation of charge]], [[conservation of momentum|momentum]], [[conservation of angular momentum|angular momentum]] and [[conservation of energy|energy]]; in many cases, these conservation laws can be related to [[symmetry|symmetries]] or [[Bianchi identity|mathematical identities]].

===Fine tuning===

{{main|fine-tuned universe}}

It appears that many of properties of the universe have special values in the sense that a universe where these properties only differ slightly would not be able to support intelligent life.<ref>{{cite book|author=[[Stephen Hawking]]|year=1988|title=A Brief History of Time|publisher=Bantam Books|isbn=0-553-05340-X|page=125}}</ref><ref>{{cite book|year=1999|title=Just Six Numbers|publisher=HarperCollins Publishers|isbn=0-465-03672-4|author=[[Martin Rees]]}}</ref> Not all scientists agree that this [[fine-tuned universe|fine-tuning]] exists.<ref name="adams">{{cite journal | last=Adams | first=F.C. | year=2008 | title=Stars in other universes: stellar structure with different fundamental constants | journal= Journal of Cosmology and Astroparticle Physics | issue=08 | doi=10.1088/1475-7516/2008/08/010 | url=http://arxiv.org/abs/0807.3697 | volume=2008 | pages=010}}</ref><ref>{{cite journal | last=Harnik | first=R. | coauthors=Kribs, G.D. and Perez, G. | year=2006 | title=A universe without weak interactions | journal=Physical Review D | volume=74 | doi=10.1103/PhysRevD.74.035006 | url=http://arxiv.org/abs/hep-ph/0604027 | pages=035006 }}</ref> In particular, it is not known under what conditions intelligent life could form and what form or shape that would take. A relevant observation in this discussion is that existence of an observer to observe fine-tuning, requires that the universe supports intelligent life. As such the [[conditional probability]] of observing a universe that is fine-tuned to support intelligent life is 1. This observation is known as the [[anthropic principle]] and is particularly relevant if the creation of the universe was probabilistic or if multiple universes with a variety of properties exist (see [[#multiverse theory|below]]).

==Historical models==

{{See also|Cosmology|Timeline of cosmology}}

Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the then-available data and conceptions of the Universe. Historically, cosmologies and cosmogonies were based on narratives of gods acting in various ways. Theories of an impersonal Universe governed by physical laws were first proposed by the Greeks and Indians. Over the centuries, improvements in astronomical observations and theories of motion and gravitation led to ever more accurate descriptions of the Universe. The modern era of cosmology began with [[Albert Einstein|Albert Einstein's]] 1915 [[general relativity|general theory of relativity]], which made it possible to quantitatively predict the origin, evolution, and conclusion of the Universe as a whole. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted [[Big Bang]]; however, still more careful measurements are required to determine which theory is correct.

===Creation===

{{Main|Creation myth|Creator deity}}

[[Image:Song of Ur-Nammu AO5378 mp3h9129.jpg|thumb|[[Sumer]]ian account of the creatrix goddess [[Nammu]], the precursor of the [[Assyria]]n goddess [[Tiamat]]; perhaps the earliest surviving creation myth.]]

Many cultures have [[creation myth|stories describing the origin of the world]], which may be roughly grouped into common types. In one type of story, the world is born from a [[world egg]]; such stories include the [[Finnish people|Finnish]] [[epic poetry|epic poem]] ''[[Kalevala]]'', the [[China|Chinese]] story of [[Pangu]] or the [[History of India|Indian]] [[Brahmanda Purana]]. In related stories, the creation idea is caused by a single entity emanating or producing something by his or herself, as in the [[Tibetan Buddhism]] concept of [[Adi-Buddha]], the [[ancient Greece|ancient Greek]] story of [[Gaia (mythology)|Gaia]] (Mother Earth), the [[Aztec mythology|Aztec]] goddess [[Coatlicue]] myth, the [[ancient Egyptian religion|ancient Egyptian]] [[Ennead|god]] [[Atum]] story, or the [[Genesis creation narrative]]. In another type of story, the world is created from the union of male and female deities, as in the [[Maori mythology|Maori story]] of [[Rangi and Papa]]. In other stories, the Universe is created by crafting it from pre-existing materials, such as the corpse of a dead god&nbsp;— as from [[Tiamat]] in the [[Babylon]]ian epic [[Enuma Elish]] or from the giant [[Ymir]] in [[Norse mythology]]&nbsp;– or from chaotic materials, as in [[Izanagi]] and [[Izanami]] in [[Japanese mythology]]. In other stories, the universe emanates from fundamental principles, such as [[Brahman]] and [[Prakrti]], or the [[yin and yang]] of the [[Tao]].

===Philosophical models===

{{See|Cosmology}}

{{See also|Pre-Socratic philosophy|Physics (Aristotle)|Hindu cosmology|Islamic cosmology|Time}}

From the 6th century BCE, the [[pre-Socratic philosophy|pre-Socratic Greek philosophers]] developed the earliest known philosophical models of the Universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the apparently different materials of the world (wood, metal, etc.) are all different forms of a single material, the [[arche]]. The first to do so was [[Thales]], who called this material [[Water (classical element)|Water]]. Following him, [[Anaximenes]] called it [[Air (classical element)|Air]], and posited that there must be attractive and repulsive [[force]]s that cause the arche to condense or dissociate into different forms. [[Empedocles]] proposed that multiple fundamental materials were necessary to explain the diversity of the universe, and proposed that all four classical elements (Earth, Air, Fire and Water) existed, albeit in different combinations and forms. This four-element theory was adopted by many of the subsequent philosophers. Some philosophers before Empedocles advocated less material things for the arche; [[Heraclitus]] argued for a [[Logos]], [[Pythagoras]] believed that all things were composed of [[number]]s, whereas Thales' student, [[Anaximander]], proposed that everything was composed of a chaotic substance known as [[Apeiron (cosmology)|apeiron]], roughly corresponding to the modern concept of a [[quantum foam]]. Various modifications of the apeiron theory were proposed, most notably that of [[Anaxagoras]], which proposed that the various matter in the world was spun off from a rapidly rotating apeiron, set in motion by the principle of [[Nous]] (Mind). Still other philosophers&nbsp;— most notably [[Leucippus]] and Democritus&nbsp;— proposed that the Universe was composed of indivisible [[atom]]s moving through empty space, a [[vacuum]]; [[Aristotle]] opposed this view ("Nature abhors a vacuum") on the grounds that [[Drag (physics)|resistance to motion]] increases with [[density]]; hence, empty space should offer no resistance to motion, leading to the possibility of infinite [[speed]].

Although Heraclitus argued for eternal change, his quasi-contemporary [[Parmenides]] made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides denoted this reality as το εν (The One). Parmenides' theory seemed implausible to many Greeks, but his student [[Zeno of Elea]] challenged them with several famous [[Zeno's paradoxes|paradoxes]]. Aristotle resolved these paradoxes by developing the notion of an infinitely divisible continuum, and applying it to [[space]] and [[time]].

The [[Indian philosophy|Indian philosopher]] [[Kanada]], founder of the [[Vaisheshika]] school, developed a theory of [[atomism]] and proposed that [[light]] and [[heat]] were varieties of the same substance.<ref>[[Will Durant]], ''Our Oriental Heritage'':

{{quote|"Two systems of Hindu thought propound physical theories suggestively similar to those of [[Ancient Greece|Greece]]. Kanada, founder of the Vaisheshika philosophy, held that the world was composed of atoms as many in kind as the various elements. The [[Jainism|Jains]] more nearly approximated to [[Democritus]] by teaching that all atoms were of the same kind, producing different effects by diverse modes of combinations. Kanada believed light and heat to be varieties of the same substance; [[Udayana]] taught that all heat comes from the sun; and [[Vācaspati Miśra|Vachaspati]], like Newton, interpreted light as composed of minute particles emitted by substances and striking the eye."}}</ref> In the 5th century AD, the [[Buddhist atomism|Buddhist atomist]] philosopher [[Dignāga]] proposed [[atom]]s to be point-sized, durationless, and made of energy. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.<ref>F. Th. Stcherbatsky (1930, 1962), ''Buddhist Logic'', Volume 1, p.19, Dover, New York:

{{quote|"The Buddhists denied the existence of substantial matter altogether. Movement consists for them of moments, it is a staccato movement, momentary flashes of a stream of energy... "Everything is evanescent“,... says the Buddhist, because there is no stuff... Both systems <nowiki>[</nowiki>[[Samkhya|Sānkhya]], and later Indian Buddhism] share in common a tendency to push the analysis of Existence up to its minutest, last elements which are imagined as absolute qualities, or things possessing only one unique quality. They are called “qualities” (guna-dharma) in both systems in the sense of absolute qualities, a kind of atomic, or intra-atomic, energies of which the empirical things are composed. Both systems, therefore, agree in denying the objective reality of the categories of Substance and Quality,... and of the relation of Inference uniting them. There is in Sānkhya philosophy no separate existence of qualities. What we call quality is but a particular manifestation of a subtle entity. To every new unit of quality corresponds a subtle quantum of matter which is called guna “quality”, but represents a subtle substantive entity. The same applies to early Buddhism where all qualities are substantive... or, more precisely, dynamic entities, although they are also called dharmas ('qualities')."}}</ref>

The theory of [[temporal finitism]] was inspired by the doctrine of creation shared by the three [[Abrahamic religions]]: [[Judaism]], [[Christianity]] and [[Islam]]. The [[Christian philosophy|Christian philosopher]], [[John Philoponus]], presented the philosophical arguments against the ancient Greek notion of an infinite past. Philoponus' arguments against an infinite past were used by the [[Early Islamic philosophy|early Muslim philosopher]], [[Al-Kindi]] (Alkindus); the [[Jewish philosophy|Jewish philosopher]], [[Saadia Gaon]] (Saadia ben Joseph); and the [[Kalam|Muslim theologian]], [[Al-Ghazali]] (Algazel). They employed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:<ref name=Craig>{{Cite journal|title=Whitrow and Popper on the Impossibility of an Infinite Past|first=William Lane|last=Craig|journal=The British Journal for the Philosophy of Science|volume=30|issue=2|date=June 1979|pages=165–170 [165–6]|doi=10.1093/bjps/30.2.165}}</ref>

:"An actual infinite cannot exist."

:"An infinite temporal regress of events is an actual infinite."

:"<math>\therefore</math> An infinite temporal regress of events cannot exist."

The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:<ref name=Craig/>

:"An actual infinite cannot be completed by successive addition."

:"The temporal series of past events has been completed by successive addition."

:"<math>\therefore</math> The temporal series of past events cannot be an actual infinite."

Both arguments were adopted by later Christian philosophers and theologians, and the second argument in particular became more famous after it was adopted by [[Immanuel Kant]] in his thesis of the first [[antinomy]] concerning [[time]].<ref name=Craig/>

===Astronomical models===

{{Main|History of astronomy}}

Astronomical models of the Universe were proposed soon after [[astronomy]] began with the [[Babylonian astronomy|Babylonian astronomers]], who viewed the Universe as a [[Flat Earth|flat disk]] floating in the ocean, and this forms the premise for early Greek maps like those of [[Anaximander]] and [[Hecataeus of Miletus]].

Later [[Ancient Greece|Greek]] philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the Universe based more profoundly on empirical evidence. The first coherent model was proposed by [[Eudoxus of Cnidos]]. According to this model, space and time are infinite and eternal, the Earth is spherical and stationary, and all other matter is confined to rotating concentric spheres. This model was refined by [[Callippus]] and [[Aristotle]], and brought into nearly perfect agreement with astronomical observations by [[Ptolemy]]. The success of this model is largely due to the mathematical fact that any function (such as the position of a planet) can be decomposed into a set of circular functions (the [[Fourier modes]]). However, not all Greek scientists accepted the geocentric model of the Universe. The [[Pythagoreans|Pythagorean]] philosopher [[Philolaus]] postulated that at the center of the Universe was a "central fire" around which the [[Earth]], [[Sun]], [[Moon]] and [[Planets]] revolved in uniform circular motion.<ref>Boyer, C. ''A History of Mathematics.'' Wiley, p. 54.</ref>

The [[Greek astronomy|Greek astronomer]] [[Aristarchus of Samos]] was the first known individual to propose a [[Heliocentrism|heliocentric]] model of the universe. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus' heliocentric theory. [[Archimedes]] wrote: (translated into English)

<blockquote>

You King Gelon are aware the 'Universe' is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'Universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.

</blockquote>

Aristarchus thus believed the stars to be very far away, and saw this as the reason why there was no visible parallax, that is, an observed movement of the stars relative to each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times, which is why stellar parallax is only detectable with telescopes. The geocentric model, consistent with planetary parallax, was assumed to be an explanation for the unobservability of the parallel phenomenon, stellar parallax. The rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests (On the Apparent Face in the Orb of the Moon):

<blockquote>

[[Cleanthes]] [a contemporary of Aristarchus and head of the Stoics] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe [i.e. the earth], . . . supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis. [1]

</blockquote>

The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was [[Seleucus of Seleucia]], a [[Hellenization|Hellenized]] [[Babylonia]]n astronomer who lived a century after Aristarchus.<ref>[[Otto E. Neugebauer]] (1945). "The History of Ancient Astronomy Problems and Methods", ''Journal of Near Eastern Studies'' '''4''' (1), p. 1–38.

{{quote|"the [[Chaldaea]]n Seleucus from Seleucia"}}</ref><ref>[[George Sarton]] (1955). "Chaldaean Astronomy of the Last Three Centuries B. C.", ''Journal of the American Oriental Society'' '''75''' (3), pp. 166–173 [169]:

{{quote|"the heliocentrical astronomy invented by Aristarchos of Samos and still defended a century later by Seleucos the [[Babylonia]]n"}}</ref><ref>William P. D. Wightman (1951, 1953), ''The Growth of Scientific Ideas'', Yale University Press p.38, where Wightman calls him [[Seleukos]] the [[Chaldea]]n.</ref> According to [[Plutarch]], Seleucus was the first to prove the heliocentric system through [[reasoning]], but it is not known what arguments he used. Seleucus' arguments for a heliocentric theory were probably related to the phenomenon of [[tide]]s.<ref>[[Lucio Russo]], ''Flussi e riflussi'', Feltrinelli, Milano, 2003, ISBN 88-07-10349-4.</ref> According to [[Strabo]] (1.1.9), Seleucus was the first to state that the [[tide]]s are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.<ref>[[Bartel Leendert van der Waerden]] (1987), "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [527]</ref> Alternatively, he may have proved the heliocentric theory by determining the constants of a [[Geometry|geometric]] model for the heliocentric theory and by developing methods to compute planetary positions using this model, like what [[Nicolaus Copernicus]] later did in the 16th century.<ref>[[Bartel Leendert van der Waerden]] (1987), "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [527–9]</ref> During the [[Middle Ages]], heliocentric models may have also been proposed by the [[Indian astronomy|Indian astronomer]], [[Aryabhata]],<ref>[[Bartel Leendert van der Waerden]] (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [529–34]</ref> and by the [[Islamic astronomy|Persian astronomers]], [[Ja'far ibn Muhammad Abu Ma'shar al-Balkhi|Albumasar]]<ref>[[Bartel Leendert van der Waerden]] (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [534–7]</ref> and [[Al-Sijzi]].<ref name=Nasr>{{Cite book |last=Nasr |first=Seyyed H. |authorlink=Hossein Nasr |date=1st edition in 1964, 2nd edition in 1993 |title=An Introduction to Islamic Cosmological Doctrines |edition=2nd |publisher=1st edition by [[Harvard University Press]], 2nd edition by [[State University of New York Press]] |isbn=0791415155 |pages=135–6}}</ref>

[[Image:ThomasDiggesmap.JPG|thumb|left|Model of the [[Copernicus|Copernican]] universe by [[Thomas Digges]] in 1576, with the amendment that the stars are no longer confined to a sphere, but spread uniformly throughout the space surrounding the [[planet]]s.]]

The Aristotelian model was accepted in the [[Western world]] for roughly two millennia, until [[Copernicus]] revived Aristarchus' theory that the astronomical data could be explained more plausibly if the [[earth]] rotated on its axis and if the [[sun]] were placed at the center of the Universe.

{{cquote|In the center rests the sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can illuminate everything at the same time?|20px|20px|[[Copernicus]]| in Chapter 10, Book 1 of ''De Revolutionibus Orbium Coelestrum'' (1543)}}

As noted by Copernicus himself, the suggestion that the [[Earth's rotation|Earth rotates]] was very old, dating at least to [[Philolaus]] (c. 450 BC), [[Heraclides Ponticus]] (c. 350 BC) and [[Ecphantus the Pythagorean]]. Roughly a century before Copernicus, Christian scholar [[Nicholas of Cusa]] also proposed that the Earth rotates on its axis in his book, ''On Learned Ignorance'' (1440).<ref>Misner, Thorne and Wheeler (1973), p. 754.</ref> Aryabhata (476–550), [[Brahmagupta]] (598–668), [[Albumasar]] and [[Al-Sijzi]], also proposed that the Earth rotates on its axis.{{Citation needed|date=April 2010}} The first [[Empirical research|empirical evidence]] for the Earth's rotation on its axis, using the phenomenon of [[comet]]s, was given by [[Nasīr al-Dīn al-Tūsī|Tusi]] (1201–1274) and [[Ali Kuşçu]] (1403–1474).{{Citation needed|date=April 2010}} Tusi, however, continued to support the Aristotelian universe, thus Kuşçu was the first to refute the Aristotelian notion of a stationary Earth on an [[empirical]] basis, similar to how Copernicus later justified the Earth's rotation. [[Al-Birjandi]] (d. 1528) further developed a theory of "circular [[inertia]]" to explain the Earth's rotation, similar to how [[Galileo Galilei]] explained it.<ref>{{Cite journal |last=Ragep |first=F. Jamil |year=2001a |title=Tusi and Copernicus: The Earth's Motion in Context |journal=Science in Context |volume=14 |issue=1–2 |pages=145–63 |publisher=[[Cambridge University Press]] }}</ref><ref>{{Cite journal |last=Ragep |first=F. Jamil |year=2001b |title=Freeing Astronomy from Philosophy: An Aspect of Islamic Influence on Science |journal=Osiris, 2nd Series |volume=16 |issue=Science in Theistic Contexts: Cognitive Dimensions |pages=49–64 & 66–71}}</ref>

[[Image:Libr0309.jpg|thumb|[[Johannes Kepler]] published the ''[[Rudolphine Tables]]'' containing a star catalog and planetary tables using [[Tycho Brahe]]'s measurements.]]

Copernicus' [[Heliocentrism|heliocentric model]] allowed the stars to be placed uniformly through the (infinite) space surrounding the planets, as first proposed by [[Thomas Digges]] in his ''Perfit Description of the Caelestiall Orbes according to the most aunciente doctrine of the Pythagoreans, latelye revived by Copernicus and by Geometricall Demonstrations approved'' (1576).<ref name = "Misner-p755">Misner, Thorne, and Wheeler (1973), p.755.</ref> [[Giordano Bruno]] accepted the idea that space was infinite and filled with solar systems similar to our own; for the publication of this view, he was [[execution by burning|burned at the stake]] in the [[Campo de' Fiori|Campo dei Fiori]] in Rome on 17 February 1600.<ref name = "Misner-p755"/>

This cosmology was accepted provisionally by [[Isaac Newton]], [[Christiaan Huygens]] and later scientists,<ref name = "Misner-p755">Misner, Thorne, and Wheeler (1973), p. 755–756.</ref> although it had several paradoxes that were resolved only with the development of [[general relativity]]. The first of these was that it assumed that space and time were infinite, and that the stars in the universe had been burning forever; however, since stars are constantly radiating [[energy]], a finite star seems inconsistent with the radiation of infinite energy. Secondly, Edmund Halley (1720)<ref>Misner, Thorne, and Wheeler (1973), p. 756.</ref> and [[Jean-Philippe de Cheseaux]] (1744)<ref>{{cite book | author = [[Jean-Philippe de Cheseaux|de Cheseaux JPL]] | year = 1744 | title = Traité de la Comète | publisher = Lausanne | pages = 223ff}}. Reprinted as Appendix II in {{cite book | author = Dickson FP | year = 1969 | title = The Bowl of Night: The Physical Universe and Scientific Thought | publisher = M.I.T. Press | ___location = Cambridge, MA | isbn = 978-0262540032}}</ref> noted independently that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the sun itself; this became known as [[Olbers' paradox]] in the 19th century.<ref>{{cite journal | author = [[Heinrich Wilhelm Matthäus Olbers|Olbers HWM]] | year = 1826 | title = Unknown title | journal = Bode's Jahrbuch | volume = 111}}. Reprinted as Appendix I in {{cite book | author = Dickson FP | year = 1969 | title = The Bowl of Night: The Physical Universe and Scientific Thought | publisher = M.I.T. Press | ___location = Cambridge, MA | isbn = 978-0262540032}}</ref> Third, Newton himself showed that an infinite space uniformly filled with matter would cause infinite forces and instabilities causing the matter to be crushed inwards under its own gravity.<ref name = "Misner-p755"/> This instability was clarified in 1902 by the [[Jeans instability]] criterion.<ref>Jeans, J. H. (1902) ''Philosophical Transactions Royal Society of London, Series A'', '''199''', 1.</ref> One solution to these latter two paradoxes is the [[Carl Charlier|Charlier universe]], in which the matter is arranged hierarchically (systems of orbiting bodies that are themselves orbiting in a larger system, ''ad infinitum'') in a [[fractal]] way such that the universe has a negligibly small overall density; such a cosmological model had also been proposed earlier in 1761 by [[Johann Heinrich Lambert]].<ref>Rindler, p. 196; Misner, Thorne, and Wheeler (1973), p. 757.</ref> A significant astronomical advance of the 18th century was the realization by [[Thomas Wright (astronomer)|Thomas Wright]], [[Immanuel Kant]] and others that stars are not distributed uniformly throughout space; rather, they are grouped into [[galaxy|galaxies]].<ref>Misner, Thorne and Wheeler, p.756.</ref>

The modern era of [[physical cosmology]] began in 1917, when [[Albert Einstein]] first applied his general theory of relativity to model the structure and dynamics of the universe.<ref name="einstein_1917">{{cite journal | last = Einstein | first = A | authorlink = Albert Einstein | year = 1917 | title = Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie | journal = Preussische Akademie der Wissenschaften, Sitzungsberichte | volume = 1917 (part 1) | pages = 142–152}}</ref> This theory and its implications will be discussed in more detail in the following section.

==Theoretical models==

[[Image:Cassini-science-br.jpg|thumb|High-precision test of general relativity by the [[Cassini-Huygens|Cassini]] space probe (artist's impression): [[radio]] signals sent between the Earth and the probe (green wave) are [[Shapiro effect|delayed]] by the warping of [[space and time]] (blue lines) due to the [[Sun]]'s mass.]]

Of the four [[fundamental interaction]]s, [[gravitation]] is dominant at cosmological length scales; that is, the other three forces are believed to play a negligible role in determining structures at the level of planets, stars, galaxies and larger-scale structures. Since all matter and energy gravitate, gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on cosmological length scales. The remaining two interactions, the [[weak nuclear force|weak]] and [[strong nuclear force]]s, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.

===General theory of relativity===

{{Main|Introduction to general relativity|General relativity|Einstein's field equations}}

Given gravitation's predominance in shaping cosmological structures, accurate predictions of the universe's past and future require an accurate theory of gravitation. The best theory available is [[Albert Einstein]]'s general theory of relativity, which has passed all experimental tests hitherto. However, since rigorous experiments have not been carried out on cosmological length scales, general relativity could conceivably be inaccurate. Nevertheless, its cosmological predictions appear to be consistent with observations, so there is no compelling reason to adopt another theory.

General relativity provides of a set of ten nonlinear partial differential equations for the [[metric tensor (general relativity)|spacetime metric]] ([[Einstein field equations|Einstein's field equations]]) that must be solved from the distribution of [[mass-energy]] and [[momentum]] throughout the universe. Since these are unknown in exact detail, cosmological models have been based on the [[cosmological principle]], which states that the universe is homogeneous and isotropic. In effect, this principle asserts that the gravitational effects of the various galaxies making up the universe are equivalent to those of a fine dust distributed uniformly throughout the universe with the same average density. The assumption of a uniform dust makes it easy to solve Einstein's field equations and predict the past and future of the universe on cosmological time scales.

Einstein's field equations include a [[cosmological constant]] (''Λ''),<ref name="einstein_1917" /><ref>Rindler (1977), pp. 226–229.</ref> that corresponds to an energy density of empty space.<ref>Landau and Lifshitz (1975), pp. 358–359.</ref> Depending on its sign, the cosmological constant can either slow (negative ''Λ'') or accelerate (positive ''Λ'') the [[metric expansion of space|expansion of the universe]]. Although many scientists, including Einstein, had speculated that ''Λ'' was zero,<ref>{{cite journal | last = Einstein | first = A | authorlink = Albert Einstein | year = 1931 | title = Zum kosmologischen Problem der allgemeinen Relativitätstheorie | journal = Sitzungsberichte der Preussischen Akademie der Wissenschaften, Physikalisch-mathematische Klasse | volume = 1931 | pages = 235–237}}<br />{{cite journal | author = [[Albert Einstein|Einstein A.]], [[Willem de Sitter|de Sitter W.]] | year = 1932 | title = On the relation between the expansion and the mean density of the universe | journal = Proceedings of the National Academy of Sciences | volume = 18 | pages = 213–214 | doi = 10.1073/pnas.18.3.213 | pmid = 16587663 | issue = 3 | pmc = 1076193}}</ref> recent astronomical observations of [[type Ia supernova]]e have detected a large amount of "[[dark energy]]" that is accelerating the universe's expansion.<ref>[http://hubblesite.org/newscenter/archive/releases/2004/12/text/ Hubble Telescope news release]</ref> Preliminary studies suggest that this dark energy corresponds to a positive ''Λ'', although alternative theories cannot be ruled out as yet.<ref>[http://news.bbc.co.uk/1/hi/sci/tech/6156110.stm BBC News story: Evidence that dark energy is the cosmological constant]</ref> Russian [[physics|physicist]] [[Yakov Borisovich Zel'dovich|Zel'dovich]] suggested that ''Λ'' is a measure of the [[zero-point energy]] associated with [[virtual particle]]s of [[quantum field theory]], a pervasive [[vacuum energy]] that exists everywhere, even in empty space.<ref>{{cite journal | author = [[Yakov Borisovich Zel'dovich|Zel'dovich YB]] | year = 1967 | title = Cosmological constant and elementary particles | journal = Zh. Eksp. & Teor. Fiz. Pis'ma | volume = 6 | pages = 883–884}} English translation in ''Sov. Phys.&nbsp;— JTEP Lett.'', '''6''', pp. 316–317 (1967).</ref> Evidence for such zero-point energy is observed in the [[Casimir effect]].

===Special relativity and space-time===

{{Main|Introduction to special relativity|Special relativity}}

[[Image:Only distance is real.svg|thumb|300px|Only its length ''L'' is intrinsic to the rod (shown in black); coordinate differences between its endpoints (such as Δx, Δy or Δξ, Δη) depend on their frame of reference (depicted in blue and red, respectively).]]

The universe has at least three [[space|spatial]] and one temporal ([[time]]) dimension. It was long thought that the spatial and temporal dimensions were different in nature and independent of one another. However, according to the [[special relativity|special theory of relativity]], spatial and temporal separations are interconvertible (within limits) by changing one's motion.

To understand this interconversion, it is helpful to consider the analogous interconversion of spatial separations along the three spatial dimensions. Consider the two endpoints of a rod of length ''L''. The length can be determined from the differences in the three coordinates Δx, Δy and Δz of the two endpoints in a given reference frame

:<math>

L^{2} = \Delta x^{2} + \Delta y^{2} + \Delta z^{2}

</math>

using the [[Pythagorean theorem]]. In a rotated reference frame, the coordinate differences differ, but they give the same length

:<math>

L^{2} = \Delta \xi^{2} + \Delta \eta^{2} + \Delta \zeta^{2}.

</math>

Thus, the coordinates differences (Δx, Δy, Δz) and (Δξ, Δη, Δζ) are not intrinsic to the rod, but merely reflect the reference frame used to describe it; by contrast, the length ''L'' is an intrinsic property of the rod. The coordinate differences can be changed without affecting the rod, by rotating one's reference frame.

The analogy in [[spacetime]] is called the interval between two events; an event is defined as a point in spacetime, a specific position in space and a specific moment in time. The spacetime interval between two events is given by

:<math>

s^{2} = L_{1}^{2} - c^{2} \Delta t_{1}^{2} = L_{2}^{2} - c^{2} \Delta t_{2}^{2}

</math>

where ''c'' is the speed of light. According to [[special relativity]], one can change a spatial and time separation (''L''₁, Δ''t''₁) into another (''L''₂, Δ''t''₂) by changing one's reference frame, as long as the change maintains the spacetime interval ''s''. Such a change in reference frame corresponds to changing one's motion; in a moving frame, lengths and times are different from their counterparts in a stationary reference frame. The precise manner in which the coordinate and time differences change with motion is described by the [[Lorentz transformation]].

===Solving Einstein's field equations===

{{See also|Big Bang|Ultimate fate of the universe}}

The distances between the spinning galaxies increase with time, but the distances between the stars within each galaxy stay roughly the same, due to their gravitational interactions. This animation illustrates a closed Friedmann universe with zero [[cosmological constant]] Λ; such a universe oscillates between a [[Big Bang]] and a [[Big Crunch]].

[[File:Closed Friedmann universe zero Lambda.ogg|thumb|right|Animation illustrating the [[metric expansion of the universe]]]]

In non-Cartesian (non-square) or curved coordinate systems, the Pythagorean theorem holds only on infinitesimal length scales and must be augmented with a more general [[metric tensor]] ''g''_μν, which can vary from place to place and which describes the local geometry in the particular coordinate system. However, assuming the [[cosmological principle]] that the universe is homogeneous and isotropic everywhere, every point in space is like every other point; hence, the metric tensor must be the same everywhere. That leads to a single form for the metric tensor, called the [[Friedmann-Lemaître-Robertson-Walker metric]]

:<math>

ds^2 = -c^{2} dt^2 +

R(t)^2 \left( \frac{dr^2}{1-k r^2} + r^2 d\theta^2 + r^2 \sin^2 \theta \, d\phi^2 \right)

</math>

where (''r'', θ, φ) correspond to a [[spherical coordinate system]]. This [[metric (mathematics)|metric]] has only two undetermined parameters: an overall length scale ''R'' that can vary with time, and a curvature index ''k'' that can be only 0, 1 or −1, corresponding to flat [[Euclidean geometry]], or spaces of positive or negative [[curvature]]. In cosmology, solving for the history of the universe is done by calculating ''R'' as a function of time, given ''k'' and the value of the [[cosmological constant]] ''Λ'', which is a (small) parameter in Einstein's field equations. The equation describing how ''R'' varies with time is known as the [[Friedmann equation]], after its inventor, [[Alexander Friedmann]].<ref>{{cite journal | author = [[Alexander Friedmann|Friedmann A.]] | year = 1922 | title = Über die Krümmung des Raumes | journal = Zeitschrift für Physik | volume = 10 | pages = 377–386 | doi = 10.1007/BF01332580}}</ref>

The solutions for ''R(t)'' depend on ''k'' and ''Λ'', but some qualitative features of such solutions are general. First and most importantly, the length scale ''R'' of the universe can remain constant ''only'' if the universe is perfectly isotropic with positive curvature (''k''=1) and has one precise value of density everywhere, as first noted by [[Albert Einstein]]. However, this equilibrium is unstable and since the universe is known to be inhomogeneous on smaller scales, ''R'' must change, according to [[general relativity]]. When ''R'' changes, all the spatial distances in the universe change in tandem; there is an overall expansion or contraction of space itself. This accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point 13.7 billion years ago and never moved faster than the [[speed of light]].

Second, all solutions suggest that there was a [[gravitational singularity]] in the past, when ''R'' goes to zero and matter and energy became infinitely dense. It may seem that this conclusion is uncertain since it is based on the questionable assumptions of perfect homogeneity and isotropy (the cosmological principle) and that only the gravitational interaction is significant. However, the [[Penrose-Hawking singularity theorems]] show that a singularity should exist for very general conditions. Hence, according to Einstein's field equations, ''R'' grew rapidly from an unimaginably hot, dense state that existed immediately following this singularity (when ''R'' had a small, finite value); this is the essence of the [[Big Bang]] model of the universe. A common misconception is that the Big Bang model predicts that matter and energy exploded from a single point in space and time; that is false. Rather, space itself was created in the Big Bang and imbued with a fixed amount of energy and matter distributed uniformly throughout; as space expands (i.e., as ''R(t)'' increases), the density of that matter and energy decreases.

{| class="toccolours" style="float: left; margin-left: 1em; margin-right: 2em; font-size: 85%; background:#FFFDD0; color:black; width:30em; max-width: 35%;" cellspacing="5"

| style="text-align: left;"|

Space has no boundary&nbsp;– that is empirically more certain than any external observation. However, that does not imply that space is infinite...(translated, original German)

|-

| style="text-align: left;" | [[Bernhard Riemann]] (Habilitationsvortrag, 1854)

|}

Third, the curvature index ''k'' determines the sign of the mean spatial curvature of [[spacetime]] averaged over length scales greater than a billion [[light year]]s. If ''k''=1, the curvature is positive and the universe has a finite volume. Such universes are often visualized as a [[3-sphere|three-dimensional sphere ''S''³ embedded in a four-dimensional space]]. Conversely, if ''k'' is zero or negative, the universe ''may'' have infinite volume, depending on its overall [[topology]]. It may seem counter-intuitive that an infinite and yet infinitely dense universe could be created in a single instant at the Big Bang when ''R''=0, but exactly that is predicted mathematically when ''k'' does not equal 1. For comparison, an infinite plane has zero curvature but infinite area, whereas an infinite cylinder is finite in one direction and a [[torus]] is finite in both. A toroidal universe could behave like a normal universe with [[periodic boundary conditions]], as seen in [[wraparound (video games)|"wrap-around" video games]] such as ''[[Asteroids (arcade game)|Asteroids]]''; a traveler crossing an outer "boundary" of space going ''outwards'' would reappear instantly at another point on the boundary moving ''inwards''.

[[Image:CMB Timeline75.jpg|thumb|600px|center|Prevailing model of the origin and expansion of [[spacetime]] and all that it contains.]]

{{Clear}}

The [[ultimate fate of the universe]] is still unknown, because it depends critically on the curvature index ''k'' and the cosmological constant ''Λ''. If the universe is sufficiently dense, ''k'' equals +1, meaning that its average curvature throughout is positive and the universe will eventually recollapse in a [[Big Crunch]], possibly starting a new universe in a [[Big Bounce]]. Conversely, if the universe is insufficiently dense, ''k'' equals 0 or −1 and the universe will expand forever, cooling off and eventually becoming inhospitable for all life, as the stars die and all matter coalesces into black holes (the [[Future of an expanding universe|Big Freeze]] and the [[heat death of the universe]]). As noted above, recent data suggests that the expansion speed of the universe is not decreasing as originally expected, but increasing; if this continues indefinitely, the universe will eventually rip itself to shreds (the [[Big Rip]]). Experimentally, the universe has an overall density that is very close to the critical value between recollapse and eternal expansion; more careful astronomical observations are needed to resolve the question.

===Big Bang model===

{{Main|Big Bang|Timeline of the Big Bang|Nucleosynthesis|Lambda-CDM model}}

The prevailing Big Bang model accounts for many of the experimental observations described above, such as the correlation of distance and [[redshift]] of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous, isotropic microwave radiation background. As noted above, the redshift arises from the [[metric expansion of space]]; as the space itself expands, the wavelength of a [[photon]] traveling through space likewise increases, decreasing its energy. The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an important problem in experimental [[physical cosmology]].

[[Image:Primordial nucleosynthesis.svg|thumb|400px|Chief nuclear reactions responsible for the [[abundance of the chemical elements|relative abundances]] of light [[atomic nucleus|atomic nuclei]] observed throughout the universe.]]

Other experimental observations can be explained by combining the overall expansion of space with [[nuclear physics|nuclear]] and [[atomic physics]]. As the universe expands, the energy density of the [[electromagnetic radiation]] decreases more quickly than does that of [[matter]], since the energy of a photon decreases with its wavelength. Thus, although the energy density of the universe is now dominated by matter, it was once dominated by radiation; poetically speaking, all was [[light]]. As the universe expanded, its energy density decreased and it became cooler; as it did so, the [[elementary particle]]s of matter could associate stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable [[proton]]s and [[neutron]]s formed, which then associated into [[atomic nuclei]]. At this stage, the matter in the universe was mainly a hot, dense [[Plasma (physics)|plasma]] of negative [[electron]]s, neutral [[neutrino]]s and positive nuclei. [[Nuclear reaction]]s among the nuclei led to the present abundances of the lighter nuclei, particularly [[hydrogen]], [[deuterium]], and [[helium]]. Eventually, the electrons and nuclei combined to form stable atoms, which are transparent to most wavelengths of radiation; at this point, the radiation decoupled from the matter, forming the ubiquitous, isotropic background of microwave radiation observed today.

Other observations are not answered definitively by known physics. According to the prevailing theory, a slight imbalance of [[matter]] over [[antimatter]] was present in the universe's creation, or developed very shortly thereafter, possibly due to the [[CP violation]] that has been observed by [[particle physics|particle physicists]]. Although the matter and antimatter mostly annihilated one another, producing [[photon]]s, a small residue of matter survived, giving the present matter-dominated universe. Several lines of evidence also suggest that a rapid [[cosmic inflation]] of the universe occurred very early in its history (roughly 10⁻³⁵ seconds after its creation). Recent observations also suggest that the [[cosmological constant]] (''Λ'') is not zero and that the net [[mass-energy]] content of the universe is dominated by a [[dark energy]] and [[dark matter]] that have not been characterized scientifically. They differ in their gravitational effects. Dark matter gravitates as ordinary matter does, and thus slows the expansion of the universe; by contrast, dark energy serves to accelerate the universe's expansion.

==Untestable proposals==

===Multiverse theory===

{{Main|Multiverse|Many-worlds hypothesis|Bubble universe theory|Parallel universe (fiction)}}

[[File:Multiverse - level II.svg|thumb|Depiction of a [[multiverse]] of seven [[bubble universe theory|"bubble" universes]], which are separate [[spacetime]] continua, each having different [[physical law]]s, [[physical constant]]s, and perhaps even different numbers of [[dimension]]s or [[topology|topologies]].]]

Some speculative theories have proposed that this universe is but one of a [[set (mathematics)|set]] of disconnected universes, collectively denoted as the [[multiverse]], altering the concept that the universe encompasses everything.<ref name="EllisKS03" /><ref>{{cite journal | author = Munitz MK | year = 1959 | title = One Universe or Many? | journal = Journal of the History of Ideas | volume = 12 | pages = 231–255 | url = http://links.jstor.org/sici?sici=0022-5037(195104)12%3A2%3C231%3AOUOM%3E2.0.CO%3B2-F | doi = 10.2307/2707516 | issue = 2}}</ref> By definition, there is no possible way for anything in one universe to affect another; if two "universes" could affect one another, they would be part of a single universe. Thus, although some fictional characters travel between [[parallel universe (fiction)|parallel fictional "universes"]], this is, strictly speaking, an incorrect usage of the term ''universe''. The disconnected universes are conceived as being physical, in the sense that each should have its own space and time, its own matter and energy, and its own physical laws&nbsp;— that also challenges the definition of parallelity as these universes don't exist synchronously (since they have their own time) or in a geometrically parallel way (since there's no interpretable relation between spatial positions of the different universes). Such physically disconnected universes should be distinguished from the [[metaphysics|metaphysical]] conception of [[plane (esotericism)|alternate planes of consciousness]], which are not thought to be physical places and are connected through the flow of information. The concept of a multiverse of disconnected universes is very old; for example, Bishop [[Étienne Tempier]] of Paris ruled in 1277 that God could create as many universes as he saw fit, a question that was being hotly debated by the French theologians.<ref>Misner, Thorne and Wheeler (1973), p.753.</ref>

There are two scientific senses in which multiple universes are discussed. First, disconnected spacetime continua may exist; presumably, all forms of matter and energy are confined to one universe and cannot "tunnel" between them. An example of such a theory is the [[bubble universe theory|chaotic inflation]] model of the early universe.<ref name="chaotic_inflation">{{cite journal | author = [[Andrei Linde|Linde A.]] | year = 1986 | title = Eternal chaotic inflation | journal = Mod. Phys. Lett. | volume = A1 | pages = 81}}<br />{{cite journal | author = [[Andrei Linde|Linde A.]] | year = 1986 | title = Eternally existing self-reproducing chaotic inflationary universe | journal = Phys. Lett. | volume = B175 | pages = 395–400}}</ref> Second, according to the [[many-worlds hypothesis]], a parallel universe is born with every [[quantum measurement]]; the universe "forks" into parallel copies, each one corresponding to a different outcome of the quantum measurement. However, both senses of the term "multiverse" are speculative and may be considered [[Falsifiability|unscientific]]; no known experimental test in one universe could reveal the existence or properties of another non-interacting universe.

==Shape of the universe==

{{Details|Shape of the Universe}}

The shape or [[geometry]] of the universe includes both [[Shape of the Universe#Local geometry (spatial curvature)|local geometry]] in the [[observable universe]] and [[Shape of the Universe#Global geometry|global geometry]], which we may or may not be able to measure. Shape can refer to curvature and [[topology]]. More formally, the subject in practice investigates which [[3-manifold]] corresponds to the spatial section in [[comoving coordinates]] of the four-dimensional [[spacetime|space-time]] of the Universe. Analysis of data from [[Wilkinson Microwave Anisotropy Probe|WMAP]] implies that the universe is [[Euclidean geometry|spatially flat]] with only a 2% margin of error.<ref>[http://map.gsfc.nasa.gov/universe/uni_shape.html Shape of the Universe], WMAP website at NASA.</ref>

Cosmologists normally work with a given [[space-like]] slice of spacetime called the [[Comoving distance|comoving coordinates]]. In terms of observation, the section of spacetime that can be observed is the backward [[light cone]] (points within the [[cosmic light horizon]], given time to reach a given observer). If the observable universe is smaller than the entire universe (in some models it is many orders of magnitude smaller), one cannot determine the global structure by observation: one is limited to a small patch.

In October 2001, NASA began collecting data with the [[Wilkinson Microwave Anisotropy Probe]] (WMAP) on cosmic background radiation. Like visible light from distant stars and galaxies, cosmic background radiation allows scientists to peer into the past to the time when the universe was in its infancy. Density fluctuations in this radiation can also tell scientists much about the physical nature of space.<ref>http://en.wikipedia.org/wiki/Homology_sphere#Cosmology</ref> NASA released the first WMAP cosmic background radiation data in February 2003. In 2009 the [[Planck (spacecraft)|Planck observatory]] was launched which will be able to analyze the microwave background at higher resolution, providing more information on the shape of the early universe. The preliminary data will be released in December 2010.

==See also==

{{Portal box|Astronomy|Space}}

<div style="-moz-column-count:4; column-count:4;">

* [[Anthropic principle]]

* [[Big Bang]]

* [[Big Crunch]]

* [[Cosmic latte]]

* [[Cosmology]]

* [[Dyson's eternal intelligence]]

* [[Esoteric cosmology]]

* [[False vacuum]]

* [[Final anthropic principle]]

* [[Fine-tuned Universe]]

* [[Heat death of the universe]]

* [[Hindu Cycle Of The Universe]]

* [[Kardashev scale]]

* [[Multiverse]]

* [[Nucleocosmochronology]]

* [[Non-standard cosmology]]

* [[Omega point]]

* [[Omniverse]]

* [[Rare Earth hypothesis]]

* [[Reality]]

* [[Shape of the Universe]]

* [[Ultimate fate of the universe]]

* [[Vacuum genesis]]

* [[World view]]

* [[Zero-energy Universe]]

</div>

==Notes and references==

{{Reflist|2}}

==Further reading==

* {{cite book|author = [[Lev Landau|Landau, Lev]], [[Evgeny Lifshitz|Lifshitz, E.M.]] | year = 1975 | title= The Classical Theory of Fields ([[Course of Theoretical Physics]], Vol. 2) | edition = revised 4th English|publisher=Pergamon Press|___location=New York|isbn=9780080181769|pages=358–397}}

* [[Edward Robert Harrison]] (2000) ''Cosmology'' 2nd ed. Cambridge University Press. Gentle.

* {{cite book | author = [[Charles W. Misner|Misner, C.W.]], [[Kip Thorne|Thorne, Kip]], [[John Archibald Wheeler|Wheeler, J.A.]] | title = [[Gravitation (book)|Gravitation]] | ___location = San Francisco | publisher = W. H. Freeman | year = 1973 | isbn = 978-0-7167-0344-0 | pages = 703–816 }} The classic text for a generation.

* {{cite book | author = [[Wolfgang Rindler|Rindler, W.]] | year = 1977 | title = Essential Relativity: Special, General, and Cosmological | publisher = Springer Verlag | ___location = New York | isbn = 0-387-10090-3 | pages = 193–244}}

* {{cite book | author = [[Steven Weinberg|Weinberg, S.]] | year = 1993 | title = The First Three Minutes: A Modern View of the Origin of the Universe | edition = 2nd updated | publisher = Basic Books | ___location = New York | isbn = 978-0465024377 | oclc = 28746057}} For lay readers.

* -------- (2008) ''Cosmology''. Oxford University Press. Challenging.

* Oscar Monchito (1987) ''Universe. What a concept''. Colton, 23rd edition. For advanced readers.

==Footnotes and references==

<references />

==External links==

* {{Cita pubblicazione|autore= Paré M, Behets C, Cornu O |titolo= Paucity of presumptive ruffini corpuscles in the index finger pad of humans. |rivista= J Comp Neurol |volume= 456 |numero= 3 |pp= 260–6 |anno= 2003 | pmid = 12528190 | doi = 10.1002/cne.10519}}

{{Spoken Wikipedia|En-Universe.ogg|2007-07-07}}

<nowiki>

{{Commons category|Universe}}

[[Category:Sensory receptors]]

{{wikiquote}}

* {{HSW|hole-in-universe|Is there a hole in the universe?}}

* [http://www.space.com/scienceastronomy/age_universe_030103.html Age of the Universe] at Space.Com

* [http://www.pbs.org/wnet/hawking/html/home.html ''Stephen Hawking's Universe'']&nbsp;– Why is the universe the way it is?

* [http://www.astro.ucla.edu/~wright/cosmology_faq.html Cosmology FAQ]

* [http://www.shekpvar.net/~dna/Publications/Cosmos/cosmos.html Cosmos&nbsp;– An "illustrated dimensional journey from microcosmos to macrocosmos"]

* [http://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets, the sun, and other stars]

* [http://www.astro.princeton.edu/~mjuric/universe/ Logarithmic Maps of the Universe]

* [http://www.slate.com/id/2087206/nav/navoa/ My So-Called Universe]&nbsp;– Arguments for and against an infinite and parallel universes

* [http://www.hep.upenn.edu/~max/multiverse1.html Parallel Universes] by Max Tegmark

* [http://cosmology.lbl.gov/talks/Ho_07.pdf The Dark Side and the Bright Side of the Universe] Princeton University, Shirley Ho

* [http://www.atlasoftheuniverse.com/ Richard Powell: ''An Atlas of the Universe'']&nbsp;– Images at various scales, with explanations

* [http://www.npr.org/templates/story/story.php?storyId=1142346 Multiple Big Bangs]

* [http://www.exploreuniverse.com/ic/ Universe&nbsp;– Space Information Centre]

* [http://www.nasa.gov/topics/universe/index.html Exploring the Universe] at Nasa.gov

* [http://www.zideo.nl/index.php?option=com_podfeed&zideo=6c4947596d673d3d&playzideo=6c3461566f56593d The Size Of The Universe, understand the size of the universe by starting with humans and going up by powers of ten ]

===Videos===

* [http://www.youtube.com/watch?v=17jymDn0W6U The Known Universe] created by the [[American Museum of Natural History]]

{{Earth's ___location}}

{{Nature nav}}

<nowiki>[[Category:Environments]]

[[Category:Universe]]

[[cs:Ruffiniho tělísko]]

[[af:Heelal]]

[[de:Ruffini-Körperchen]]

[[ar:فضاء كوني]]

[[es:Corpúsculos de Ruffini]]

[[an:Universo]]

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[[ast:Universu]]

[[gl:Corpúsculo de Ruffini]]

[[az:Kainat]]

[[he:גופיף רפיני]]

[[bn:মহাবিশ্ব]]

[[pl:Ciałka Ruffiniego]]

[[zh-min-nan:Ú-tiū]]

[[pt:Corpúsculo de Ruffini]]</nowiki>

[[be:Сусвет]]

[[be-x-old:Сусьвет]]

[[bs:Svemir]]

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[[cv:Çут Тĕнче]]

[[cs:Vesmír]]

[[cy:Bydysawd (seryddiaeth)]]

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[[de:Universum]]

[[nv:Yágháhookáán]]

[[dsb:Uniwersum]]

[[et:Universum]]

[[el:Σύμπαν]]

[[es:Universo]]

[[eo:Universo]]

[[eu:Unibertso]]

[[fa:گیتی]]

[[fr:Univers]]

[[fy:Hielal]]

[[gl:Universo]]

[[gu:બ્રહ્માંડ]]

[[hak:Yî-chhiu]]

[[ko:우주]]

[[hi:ब्रह्माण्ड]]

[[hr:Svemir]]

[[io:Universo]]

[[id:Alam semesta]]

[[ia:Universo]]

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[[it:Universo]]

[[he:היקום]]

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[[kn:ಬ್ರಹ್ಮಾಂಡ]]

[[pam:Sikluban]]

[[ka:სამყარო]]

[[csb:Swiatnica]]

[[sw:Ulimwengu]]

[[ku:Gerdûn]]

[[la:Universum]]

[[lv:Visums]]

[[lt:Visata]]

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[[lmo:Ünivers]]

[[hu:Világegyetem]]

[[mk:Вселена]]

[[ml:പ്രപഞ്ചം]]

[[mr:विश्व]]

[[arz:كون]]

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[[mwl:Ouniberso]]

[[mn:Ертөнц]]

[[my:စကြာဝဠာ]]

[[nah:Cemānāhuac]]

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[[ne:ब्रह्माण्ड]]

[[ja:宇宙]]

[[nap:Annevierzo]]

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[[oc:Univèrs]]

[[uz:Olam]]

[[pap:Universo]]

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[[pl:Wszechświat]]

[[pt:Universo]]

[[ksh:Weltall]]

[[ro:Univers]]

[[qu:Ch'askancha]]

[[ru:Вселенная]]

[[stq:Al]]

[[sq:Gjithësia]]

[[scn:Universu]]

[[simple:Universe]]

[[sk:Vesmír]]

[[sl:Vesolje]]

[[ckb:گەردوون]]

[[sr:Свемир]]

[[sh:Svemir]]

[[su:Jagat]]

[[fi:Maailmankaikkeus]]

[[sv:Universum]]

[[tl:Sanlibutan]]

[[ta:அண்டம்]]

[[tt:Космос]]

[[te:విశ్వం]]

[[th:เอกภพ]]

[[tg:Коинот]]

[[tr:Evren]]

[[uk:Всесвіт]]

[[ur:کائنات]]

[[vi:Vũ trụ]]

[[zh-classical:宇宙]]

[[war:Sangkalibutan]]

[[yi:אוניווערס]]

[[zh-yue:宇宙]]

[[bat-smg:Vėsata]]

[[zh:宇宙]]

</nowiki>