|
==Universo==
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]]''.
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.
==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 – corresponding to the fixed stars, the [[Sun]] and various [[planet]]s – 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.
[[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.]]
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<sup>−32</sup> 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]].
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 10<sup>26</sup>]] [[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'' — first used by [[Lucretius]] in Book IV (line 262) of his ''[[On the Nature of Things|De rerum natura]]'' (''On the Nature of Things'') — 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 (10<sup>11</sup>) [[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> (10<sup>7</sup>) [[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> (10<sup>12</sup>) stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around one [[sextillion]] (10<sup>21</sup>) stars in the observable universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion (7 x 10<sup>22</sup>)<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<sup>−30</sup> 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 — particularly the lightest atoms such as [[hydrogen]], [[deuterium]] and [[helium]] — 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 – 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 — as from [[Tiamat]] in the [[Babylon]]ian epic [[Enuma Elish]] or from the giant [[Ymir]] in [[Norse mythology]] – 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===
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