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| − | {{otheruses|Universe (disambiguation)}}
| + | The '''universe''' is (according to the Big Bang Theory) everything that came from the [[Big Bang]]. Many cosmologists now believe that our universe is one of many other universes in the [[multiverse]]. |
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| − | The '''Universe''' is most commonly defined as [[everything]] that physically exists: the entirety of [[space]] and [[time]], all forms of [[matter]], [[energy]] and [[momentum]], and the [[physical law]]s and [[physical constant|constant]]s that govern them. However, the term "universe" may be used in slightly different contextual senses, denoting such concepts as the ''[[cosmos]]'', the ''[[world (philosophy)|world]]'' or ''[[Nature]]''. | + | |
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| − | [[observable universe|Astronomical observations]] indicate that the universe is at least [[age of the universe|158.7 trillion years old]] and at least 180 billion [[light year]]s across. The event that started the universe is called the [[Big Bang]]. At this point in time all matter and energy of the [[observable universe]] was concentrated in one point of infinite density. After the Big Bang the universe started to expand to its present form. Since [[special relativity]] states that matter cannot exceed the [[speed of light]] in a fixed [[space-time]]. | + | [[Category:Cosmos]] |
| − | Experiments suggest that the universe has been governed by the same physical laws and constants throughout its extent and history. The dominant force at cosmological distances is [[gravity]], and [[general relativity]] is currently the most accurate theory of gravitation. The remaining three [[fundamental force]]s and the particles on which they act are described by the [[Standard Model]]. The universe has at least three [[dimension]]s of space and one of time, although [[compactification (physics)|extremely small]] additional dimensions cannot be ruled out experimentally. [[Spacetime]] appears to be [[differentiable manifold|smoothly]] and [[simply connected]], and [[3-space|space]] has very small mean [[Riemann curvature tensor|curvature]], so that [[Euclidean geometry]] is accurate ''on the average'' throughout the universe.
| + | [[Category:Cosmology]] |
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| − | According to some speculations, this universe may be one of many disconnected universes, which are collectively denoted as the [[multiverse]]. In [[bubble universe theory|one theory]], there is an infinite variety of universes, each with different [[physical constant]]s. In [[many-worlds hypothesis|another theory]], new universes are spawned with every [[quantum measurement]]. By definition, these speculations cannot currently be tested experimentally.
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| − | Throughout recorded history, several [[cosmology|cosmologies]] and [[cosmogony|cosmogonies]] have been proposed to account for observations of the universe. The earliest quantitative 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]]' [[heliocentrism|heliocentric model]] and the [[Isaac Newton|Newtonian]] model of the [[solar system]], respectively. Further improvements in astronomy led to the characterization of the [[Milky Way]], and the discovery of other galaxies and the microwave background radiation; careful studies of the distribution of these galaxies and their [[spectral line]]s have led to much of [[physical cosmology|modern cosmology]].
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| − | {{cosmology}}
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| − | ==Etymology, synonyms and definitions==
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| − | {{See also|Cosmos|Nature|World (philosophy)|Celestial spheres}}
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| − | 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".
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| − | [[Image:Foucault pendulum animated.gif|thumb|right|Artistic rendition of a [[Foucault pendulum]] showing that the Earth is not stationary, but rotates.]]
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| − | 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, πεÏιφοÏα, "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, since careful [[astronomy|astronomical]] and physical measurements (such as the [[Foucault pendulum]]) are required to prove otherwise.
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| − | The most common term for "universe" among the ancient 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 = p. 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>
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| − | ===Broadest definition: reality and probability===
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| − | {{See also|Introduction to quantum mechanics|Interpretation of quantum mechanics|Many-worlds hypothesis}}
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| − | The broadest definition of the universe is found in ''[[De divisione naturae]]'' by the [[Middle Ages|medieval]] [[philosopher]] [[Johannes Scotus Eriugena]], who defined it as simply everything: everything that exists and everything that does not exist. 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 it is relevant in [[quantum physics]], particularly 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}}</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 is loosely related to the [[Buddhism|Buddhist]] doctrines of [[shunyata]] and [[pratitya-samutpada|interdependent development of reality]], and to [[Gottfried Leibniz]]'s more modern concepts of [[contingency]] and the [[identity of indiscernibles]].
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| − | ===Definition as reality===
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| − | {{See also|Reality|Physics}}
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| − | More customarily, the universe is defined as everything that exists, has existed and will exist. 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 "universe" is everything that exists at a single moment of time, such as [[present (time)|the present]], as in the sentence "The universe is now bathed uniformly in [[cosmic microwave background radiation|microwave radiation]]".
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| − | 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; 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.
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| − | ===Definition as connected space-time===
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| − | [[Image:Hubble ultra deep field.jpg|thumb|right|350px|[[Hubble Ultra Deep Field]] image of a small region of the [[observable universe]], near the [[constellation]] [[Fornax]]. The light from the smallest, most [[redshift]]ed galaxies originated roughly 13 billion years ago.]]
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| − | {{See also|Bubble universe theory|Chaotic inflation}}
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| − | 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
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| − | | last = Ellis
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| − | | first = George F.R.
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| − | | authorlink = George Ellis
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| − | | coauthors = U. Kirchner, W.R. Stoeger
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| − | | title = Multiverses and physical cosmology
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| − | | journal = Monthly Notices of the Royal Astronomical Society
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| − | | volume = 347
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| − | | issue =
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| − | | pages = 921–936
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| − | | publisher =
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| − | | date = 2004
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| − | | url = http://arxiv.org/abs/astro-ph/0305292
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| − | | doi =
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| − | | accessdate = 2007-01-09}}</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 it is impossible to know for sure. These multiverses could also exist within other universes, in the same way that the interior of a black hole is discontinuous with our world; once something goes in it will never come out.
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| − | ===Definition as observable reality===
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| − | {{See also|Observable universe|Observational cosmology}}
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| − | According to a still more restrictive definition, the universe is everything within our connected [[space-time]] that could ever interact with us and vice versa. According to the theory of [[general 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 [[expansion of space]]. For example, radio messages sent from Earth may never reach some regions of space, even if the universe lives forever; space may expand faster than light can cover 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 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 a stationary observer on Earth.
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| − | {{clear}}
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| − | ==Size, age, contents, structure, and laws==<!-- [[Hubble's law]] links to this section -->
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| − | {{main|Observable universe|Age of the universe|Large-scale structure of the universe|Abundance of the chemical elements}}
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| − | The universe is very large and possibly infinite in volume; the observable matter is spread over a space at least 180 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&catID=2 | 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
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| − | | last = Christian
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| − | | first = Eric
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| − | | last2 = Samar
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| − | | first2 = Safi-Harb
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| − | | title = How large is the Milky Way?
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| − | | url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.html
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| − | | 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
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| − | | author=I. Ribas, C. Jordi, F. Vilardell, E.L. Fitzpatrick, R.W. Hilditch, F. Edward
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| − | | title=First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy
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| − | | journal=Astrophysical Journal
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| − | | year=2005
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| − | | volume=635
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| − | | pages=L37-L40
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| − | | url=http://adsabs.harvard.edu/abs/2005ApJ...635L..37R
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| − | | doi = 10.1086/499161 <!--Retrieved from CrossRef by DOI bot-->
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| − | }}<br />{{cite journal
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| − | | author=McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N.
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| − | | title=Distances and metallicities for 17 Local Group galaxies
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| − | | journal=Monthly Notices of the Royal Astronomical Society
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| − | | year=2005
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| − | | volume=356
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| − | | pages=979-997
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| − | | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005MNRAS.356..979M
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| − | | doi = 10.1111/j.1365-2966.2004.08514.x <!--Retrieved from CrossRef by DOI bot-->
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| − | [[Image:Cosmological composition.jpg|thumb|450px|right|The universe is believed to be mostly composed of [[dark energy]] and [[dark matter]], both of which are poorly understood at present. Only ≈4% of the universe is ordinary matter, a relatively small perturbation.]]
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| − | 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 [[galaxy|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]].
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| − | 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% 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]].
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| − | The universe is [[age of the universe|old]] and evolving. The [[Wilkinson Microwave Anisotropy Probe|most precise estimate]] of the universe's age is 158.7 trillion 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|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
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| − | | author =Britt RR
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| − | | title =Age of Universe Revised, Again
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| − | | publisher =[[space.com]]
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| − | | date = [[2003-01-03]]
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| − | | url = http://www.space.com/scienceastronomy/age_universe_030103.html
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| − | | accessdate = 2007-01-08}}</ref>
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| − | to 13–15 billion years.<ref>{{cite web
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| − | | author = Wright EL
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| − | | title =Age of the Universe
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| − | | publisher =[[UCLA]]
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| − | | date = 2005
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| − | | url = http://www.astro.ucla.edu/~wright/age.html
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| − | | accessdate = 2007-01-08}}<br />{{cite journal
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| − | | author = Krauss LM, Chaboyer B
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| − | | title =Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology
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| − | | journal =[[Science (journal)|Science]]
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| − | | volume = 299
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| − | | issue = 5603
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| − | | pages = 65–69
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| − | | publisher =[[American Association for the Advancement of Science]]
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| − | | date = [[3 January]] [[2003]]
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| − | | url = http://www.sciencemag.org/cgi/content/abstract/299/5603/65?ijkey=3D7y0Qonz=GO7ig.&keytype=3Dref&siteid=3Dsci
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| − | | accessdate = 2007-01-08 }}</ref> The universe has not been the same at all times in its history; for example, the relative populations of [[quasar]]s and [[galaxy|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.
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| − | The [[abundance of the chemical elements|relative fractions]] of different [[chemical element]]s — particularly the lightest [[atom]]s 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 <!--Retrieved from CrossRef by DOI bot-->}}<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 appears to have no net [[momentum]] and [[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>
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| − | [[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.]]
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| − | The universe appears to have a smooth [[spacetime]] [[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 [[cylinder|cylindrical]] or [[toroid]]al topologies of two-dimensional [[space]]s.<ref name="_spacetime_topology">{{cite conference
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| − | | coauthors = Boudewijn F. Roukema
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| − | | title = Topology of the Universe: Theory and Observations
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| − | | booktitle = Proceedings of Cosmology School held at Cargese, Corsica, August 1998
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| − | | authorlink =
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| − | | coauthors = J. Weeks, A. Riazuelo, R. Lehoucq, J.-P. Uzan
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| − | | title = Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background
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| − | | journal = [[Nature]]
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| − | | volume = 425
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| − | | issue =
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| − | | pages = 593
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| − | | publisher =
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| − | | date=2003
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| − | | url = http://arxiv.org/abs/astro-ph/0310253
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| − | | doi =
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| − | | id =
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| − | | accessdate = 2007-01-09}}</ref>
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| − | The universe appears to be governed throughout by the same [[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]].
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| − | ==Historical models==
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| − | {{main|Timeline of cosmology}}
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| − | Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the then available data and conceptions of the universe. Initially, cosmologies and cosmogonies were based on narratives of gods acting in various ways. The Greeks were the first to propose theories of an impersonal universe governed by physical laws. 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 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.
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| − | ===Creation myths===
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| − | {{main|Creation myth|Creator deity}}
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| − | [[Image:Song of Ur-Nammu AO5378 mp3h9129.jpg|thumb|200px|right|[[Sumer]]ian account of the creatrix goddess [[Nammu]], the precursor of the [[Assyrian]] goddess [[Tiamat]]; perhaps the earliest surviving creation myth.]]
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| − | Many cultures have [[creation myth|stories describing the creation 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]]. The Vedic religion beleives that the Universe is continually manifested and unmanifested. In the Vedic Religion Maha Vishnu breahtes out innumerble Universes. These universes expand for 311 trillion,40 Billion years and then they start to contract and enter Maha Vishnu once again. Once Maha Vishnu inhales and contracts the innnumerable Universes time once again comes to a singularity. At this time the Universe is unmainfest inside Maha Vishnu for 311 trillion, 40 billion years. The cycle repeats itself as the Universe expands for 311 trillion, 40 billion years and then contracts into a singularity. Currently the Universe is 158.7 billion years old, because Brahma is 158.7 billion years old, and he was created at the beginning of this current manifestation of the Universe. In related stories, the creation is caused by a single god emanating or producing something by themselves, as in [[Buddhism|Buddhist]] concept of [[Adi-Buddha]], the [[ancient Greece|ancient Greek]] story of [[Gaia]] (Mother Earth), the [[Aztec mythology|Aztec]] goddess [[Coatlicue]] or the [[ancient Egyptian religion|ancient Egyptian]] [[Ennead|god]] [[Atum]]. 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 another type of story, the world is created by the command of a [[god|divinity]], as in the [[ancient Egypt]]ian story of [[Ptah]] or the [[Bible|Biblical]] account in [[Creation according to Genesis|Genesis]], wherein some Christians believe that the universe was created 6,000-10,000 years ago, while other Christians believe that the Creation account is compatible with modern science. In other stories, the universe emanates from fundamental principles, such as [[Brahman]] and [[Prakrti]], or the [[yin]] and [[yang]] of the [[Tao]].
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| − | [[Image:Vessillo della Repubblica Ambrosiana (1447-1450).jpg|thumb|left|200px|[[Milan]]ese [[flag]] (''c.'' 1450) depicting the four [[classical element]]s in the outermost ring. [[Fire (classical element)|Fire]] and [[Air (classical element)|Air]] are above, holding red and white spheres, respectively; [[Water (classical element)|Water]] and [[Earth (classical element)|Earth]] are below, holding blue and green spheres, respectively.]]
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| − | ===Philosophical models===
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| − | {{main|Pre-Socratic philosophy|Physics (Aristotle)}}
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| − | The first philosophical models of the universe were developed by the [[pre-Socratic philosophy|pre-Socratic philosophers]]. 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 element]]s (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 — most notably [[Leucippus]] and [[Democritus]] — 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]].
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| − | Although Heraclitus argued for eternal change, his rough 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]].
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| − | ===Astronomical models===
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| − | [[Image:Universum.jpg|thumb|250px|right|Hand-colored version of the [[Flammarion woodcut]], depicting the [[Aristotle|Aristotelian]] conception of the universe that preceded the models of [[Copernicus]] and [[Thomas Digges]].]]
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| − | {{main|History of astronomy}}
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| − | More practical Greek philosophers were concerned with developing models of the universe that would account for the observed motion of the stars and planets. 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. [[Aristarchus of Samos]] was the first astronomer to propose a heliocentric model. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus' heliocentric theory. [[Archimedes]] wrote: (translated into English)
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| − | <blockquote>
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| − | 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.
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| − | </blockquote>
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| − | 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):
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| − | | + | |
| − | <blockquote>
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| − | [[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]
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| − | </blockquote>
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| − | The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was Seleucus of Seleucia, a Greek astronomer who lived a century after Aristarchus.
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| − | [[Image:ThomasDiggesmap.JPG|thumb|left|200px|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.]]
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| − | The Aristotelian model was accepted 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
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| − | {{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)}}
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| − | As noted by Copernicus himself, the suggestion that the 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, [[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> 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"/>
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| − | 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]] | date = 1744 | title = Traité de la Comète | publisher = Lausanne | pages = pp. 223ff}}. Reprinted as Appendix II in {{cite book | author = Dickson FP | date = 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]] | date = 1826 | title = Unknown title | journal = Bode's Jahrbuch | volume = 111}}. Reprinted as Appendix I in {{cite book | author = Dickson FP | date = 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]], [[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>
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| − | The modern era of [[physical cosmology]] began in [[1917]], when [[Albert Einstein]] first applied his [[general relativity|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.
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| − | ==Theoretical models==
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| − | [[Image:Cassini-science-br.jpg|thumb|right|225px|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.]]
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| − | 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.
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| − | ===General relativity===
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| − | {{main|Introduction to general relativity|General relativity|Einstein's field equations}}
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| − | 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 relativity|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.
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| − | 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.
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| − | 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 [[Albert Einstein|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}}</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]] | date = 1967 | title = Cosmological constant and elementary particles | journal = Zh. Eksp. & Teor. Fiz. Pis'ma | volume = 6 | pages = 883–884}} English translation in ''Sov. Phys. — JTEP Lett.'', '''6''', pp. 316–317 (1967).</ref> Evidence for such zero-point energy is observed in the [[Casimir effect]].
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| − | ===Special relativity and space-time===
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| − | {{main|Introduction to special relativity|Special relativity}}
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| − | [[Image:Only distance is real.svg|thumb|300px|left|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).]]
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| − | 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.
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| − | 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
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| − | :<math>
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| − | L^{2} = \Delta x^{2} + \Delta y^{2} + \Delta z^{2}
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| − | </math>
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| − | using the [[Pythagorean theorem]]. In a rotated reference frame, the coordinate differences differ, but they give the same length
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| − | :<math>
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| − | L^{2} = \Delta \xi^{2} + \Delta \eta^{2} + \Delta \zeta^{2}
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| − | </math>
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| − | 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.
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| − | 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
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| − | :<math>
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| − | s^{2} = L_{1}^{2} - c^{2} \Delta t_{1}^{2} = L_{2}^{2} - c^{2} \Delta t_{2}^{2}
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| − | </math>
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| − | where ''c'' is the speed of light. According to [[special relativity]], one can change a spatial and time separation (''L''<sub>1</sub>, Δ''t''<sub>1</sub>) into another (''L''<sub>2</sub>, Δ''t''<sub>2</sub>) 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]].
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| − | ===Solving Einstein's field equations===
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| − | {{See also|Big Bang|Ultimate fate of the universe}}
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| − | 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''<sub>μν</sub>, 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]]
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| − | :<math>
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| − | ds^2 = -c^{2} dt^2 +
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| − | 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)
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| − | </math>
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| − | 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 = Ãœber die Krümmung des Raumes | journal = Zeitschrift für Physik | volume = 10 | pages = 377–386}}</ref>
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| − | [[Image:CMB Timeline75.jpg|thumb|550px|Prevailing model of the creation and expansion of [[spacetime]] and all that it contains.]]
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| − | 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. The 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]].
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| − | 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.
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| − | {| 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"
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| − | Space has no boundary - that is empirically more certain than any external observation. However, that does not imply that space is infinite...(translated, original German)
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| − | |-
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| − | | style="text-align: left;" | [[Bernhard Riemann]] (Habilitationsvortrag, 1854)
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| − | |}
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| − | 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''<sup>3</sup> 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 "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''.
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| − | The [[ultimate fate of the universe]] is still unknown, since 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 [[Big Freeze]] and the [[heat death of the universe]]). As noted above, recent data suggests that the expansion of the universe is not decreasing as originally expected, but accelerating; 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 decide the question.
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| − | ===Big Bang model===
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| − | {{main|Big Bang|Timeline of the Big Bang|Nucleosynthesis|Lambda-CDM model}}
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| − | 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]].
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| − | [[Image:Primordial nucleosynthesis.JPG|thumb|left|450px|Chief nuclear reactions responsible for the [[abundance of the chemical elements|relative abundances]] of light [[atomic nucleus|atomic nuclei]] observed throughout the universe.]]
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| − | 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]] 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 [[atom]]s, 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.
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| − | 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<sup>-35</sup> 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.
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| − | ==Multiverse==
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| − | {{main|Multiverse|Many-worlds hypothesis|Bubble universe theory|Parallel universe (fiction)}}
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| − | [[Image:Multiverse - level II.GIF|thumb|250px|right|Artistic depiction of a [[multiverse]] of seven [[bubble universe theory|"bubble" universes]], which are separate [[spacetime]] [[continuum|continua]], each having different [[physical law]]s, [[physical constant]]s, and perhaps even different numbers of [[dimension]]s or [[topology|topologies]].]]
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| − | Some speculative theories have proposed that this universe is but one of a [[set]] of disconnected universes, collectively denoted as the [[multiverse]].<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}}</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. Thus such physical disconnected universes should be distinguished from the [[metaphysics|metaphysical]] conception of [[plane (metaphysics)|alternate planes of consciousness]], which are not thought to be physical places. 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>
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| − | There are two scientific senses in which multiple universes can occur. First, disconnected [[spacetime]] [[continuum|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. Authors have explored this concept in some fiction, most notably [[Jorge Borges]]' short story ''[[The Garden of Forking Paths]]''. However, both senses of the term "multiverse" are speculative and may be considered unscientific; the fact that universes cannot interact makes it impossible to test experimentally in this universe whether another universe exists.
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| − | ==See also==
| + | |
| − | <div style="-moz-column-count:4; column-count:4;">
| + | |
| − | * [[Anthropic principle]]
| + | |
| − | * [[Big Bang]]
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| − | * [[Big Crunch]]
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| − | * [[Big Freeze]]
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| − | * [[Cosmic Latte]]
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| − | * [[Cosmology]]
| + | |
| − | * [[Dyson's eternal intelligence]]
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| − | * [[Esoteric cosmology]]
| + | |
| − | * [[False vacuum]]
| + | |
| − | * [[Final anthropic principle]]
| + | |
| − | * [[Fine-tuned universe]]
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| − | * [[Gaia hypothesis]]
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| − | * [[Heat death of the universe]]
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| − | * [[Hindu Cycle Of The Universe]]
| + | |
| − | * [[Kardashev scale]]
| + | |
| − | * [[Multiverse (science)|Multiverse]]
| + | |
| − | * [[Multiverse (religion)]]
| + | |
| − | * [[Nucleocosmochronology]]
| + | |
| − | * [[Non-standard cosmology]]
| + | |
| − | * [[Omega point]]
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| − | * [[Omniverse]]
| + | |
| − | * [[Origin of life]]
| + | |
| − | * [[Rare Earth hypothesis]]
| + | |
| − | * [[Reality]]
| + | |
| − | * [[Shape of the universe]]
| + | |
| − | * [[Ultimate fate of the universe]]
| + | |
| − | * [[World view]]
| + | |
| − | * [[World development]]
| + | |
| − | </div>
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| − | | + | |
| − | ==Notes and references==
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| − | {{reflist|2}}
| + | |
| − | | + | |
| − | ==Further reading==
| + | |
| − | * {{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 edition | publisher = Basic Books | location = New York | isbn = 978-0465024377}}
| + | |
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| − | * {{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 = pp. 193–244}}
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| − | * {{cite book | author = [[Lev Landau|Landau LD]], [[Evgeny Lifshitz|Lifshitz EM]] | year = 1975 | title = The Classical Theory of Fields (Course of Theoretical Physics, Vol. 2) | edition = revised 4th English ed. | publisher = Pergamon Press | location = New York | isbn = 978-0-08-018176-9 |pages = pp. 358–397}}
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| − | | + | |
| − | * {{cite book | author = [[Charles W. Misner|Misner CW]],[[Kip Thorne|Thorne K]], [[John Archibald Wheeler|Wheeler JA]] | title = Gravitation | location = San Francisco | publisher = W. H. Freeman | year = 1973 | isbn = 978-0-7167-0344-0 | pages = pp. 703–816 }} (See [[Gravitation (book)]].)
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| − | | + | |
| − | * {{cite book | author = [[Steven Weinberg|Weinberg S]] | year = 1972 | title = Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity | publisher = John Wiley and Sons | location = New York | isbn = 0-471-92567-5 | pages = pp. 407–633}}
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| − | ==External links==
| + | |
| − | {{Spoken Wikipedia|En-Universe.ogg|2007-07-07}}
| + | |
| − | {{Commonscat|Space}}
| + | |
| − | {{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.astro.ucla.edu/~wright/cosmology_faq.html Cosmology FAQ]
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| − | * [http://www.shekpvar.net/~dna/Publications/Cosmos/cosmos.html Cosmos - an "illustrated dimensional journey from microcosmos to macrocosmos"]
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| − | * [http://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets, the sun, and other stars]
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| − | * [http://www.astro.princeton.edu/~mjuric/universe/ Logarithmic Maps of the Universe]
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| − | * [http://www.slate.com/id/2087206/nav/navoa/ My So-Called Universe] arguments for and against an infinite and parallel universes
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| − | * [http://www.hep.upenn.edu/~max/multiverse1.html Parallel Universes] by Max Tegmark
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| − | * [http://cosmology.lbl.gov/talks/Ho_07.pdf The Dark Side and the Bright Side of the Universe] Princeton University, Shirley Ho
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| − | * [http://www.atlasoftheuniverse.com/ Richard Powell: ''An Atlas of the Universe''] - images at various scales, with explanations
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| − | * [http://www.space.com/scienceastronomy/mystery_monday_040524.html Size of the Universe] at Space.Com
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| − | * [http://www.pbs.org/wnet/hawking/html/home.html ''Stephen Hawking's Universe''] - why is the universe the way it is?
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| − | * [http://www.exploreuniverse.com/ic/ Universe - Space Information Centre] by Exploreuniverse.com
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