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==Universo==
=[[Corpuscolo di Ruffini]]=
{{anatomia
===Philosophical models===
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{{See|Cosmology}}
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{{See also|Pre-Socratic philosophy|Physics (Aristotle)|Hindu cosmology|Islamic cosmology|Time}}
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Il '''corpuscolo di Ruffini''' è una classe di [[meccanorecettore|meccanorecettori]] ad adattamento lento. Si pensa che siano presenti solo nelle zone naturalmente glabre del [[derma]] (dorso della [[mano]], pianta del [[Piede (anatomia)|piede]], [[labbro|labbra]], [[piccole labbra]] e [[glande]]), e nell'[[ipoderma]] umano. Il nome deriva dal medico italiano [[Angelo Ruffini]].
From the 6th century BCE, the [[pre-Socratic philosophy|pre-Socratic Greek philosophers]] developed the earliest known philosophical models of the Universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the apparently different materials of the world (wood, metal, etc.) are all different forms of a single material, the [[arche]]. The first to do so was [[Thales]], who called this material [[Water (classical element)|Water]]. Following him, [[Anaximenes]] called it [[Air (classical element)|Air]], and posited that there must be attractive and repulsive [[force]]s that cause the arche to condense or dissociate into different forms. [[Empedocles]] proposed that multiple fundamental materials were necessary to explain the diversity of the universe, and proposed that all four classical elements (Earth, Air, Fire and Water) existed, albeit in different combinations and forms. This four-element theory was adopted by many of the subsequent philosophers. Some philosophers before Empedocles advocated less material things for the arche; [[Heraclitus]] argued for a [[Logos]], [[Pythagoras]] believed that all things were composed of [[number]]s, whereas Thales' student, [[Anaximander]], proposed that everything was composed of a chaotic substance known as [[Apeiron (cosmology)|apeiron]], roughly corresponding to the modern concept of a [[quantum foam]]. Various modifications of the apeiron theory were proposed, most notably that of [[Anaxagoras]], which proposed that the various matter in the world was spun off from a rapidly rotating apeiron, set in motion by the principle of [[Nous]] (Mind). Still other philosophers — 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]].
==Function==
Although Heraclitus argued for eternal change, his quasi-contemporary [[Parmenides]] made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides denoted this reality as το εν (The One). Parmenides' theory seemed implausible to many Greeks, but his student [[Zeno of Elea]] challenged them with several famous [[Zeno's paradoxes|paradoxes]]. Aristotle resolved these paradoxes by developing the notion of an infinitely divisible continuum, and applying it to [[space]] and [[time]].
This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense of and control of finger position and movement.<ref>{{Cita libro|cognome=Mountcastle |nome=Vernon C. |anno=2005 |titolo=The Sensory Hand: Neural Mechanisms of Somatic Sensation |editore=Harvard University Press |p=34}}</ref> It is believed to be useful for monitoring slippage of objects along the surface of the skin, allowing modulation of grip on an object.
The [[Indian philosophy|Indian philosopher]] [[Kanada]], founder of the [[Vaisheshika]] school, developed a theory of [[atomism]] and proposed that [[light]] and [[heat]] were varieties of the same substance.<ref>[[Will Durant]], ''Our Oriental Heritage'':
Ruffinian endings are located in the deep layers of the skin, and register mechanical deformation within joints, more specifically angle change, with a specificity of up to 2 degrees, as well as continuous pressure states.They also act as a thermoreceptors that respond for a long time, so in case of deep burn there will be no pain as these receptors will be burned off.<ref>{{Cita libro|cognome=Hamilton |nome=Nancy |anno=2008 |titolo=Kinesiology: Scientific Basis of Human Motion |editore=McGraw-Hill |pp=76–7}}</ref>
{{quote|"Two systems of Hindu thought propound physical theories suggestively similar to those of [[Ancient Greece|Greece]]. Kanada, founder of the Vaisheshika philosophy, held that the world was composed of atoms as many in kind as the various elements. The [[Jainism|Jains]] more nearly approximated to [[Democritus]] by teaching that all atoms were of the same kind, producing different effects by diverse modes of combinations. Kanada believed light and heat to be varieties of the same substance; [[Udayana]] taught that all heat comes from the sun; and [[Vācaspati Miśra|Vachaspati]], like Newton, interpreted light as composed of minute particles emitted by substances and striking the eye."}}</ref> In the 5th century AD, the [[Buddhist atomism|Buddhist atomist]] philosopher [[Dignāga]] proposed [[atom]]s to be point-sized, durationless, and made of energy. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.<ref>F. Th. Stcherbatsky (1930, 1962), ''Buddhist Logic'', Volume 1, p.19, Dover, New York:
{{quote|"The Buddhists denied the existence of substantial matter altogether. Movement consists for them of moments, it is a staccato movement, momentary flashes of a stream of energy... "Everything is evanescent“,... says the Buddhist, because there is no stuff... Both systems <nowiki>[</nowiki>[[Samkhya|Sānkhya]], and later Indian Buddhism] share in common a tendency to push the analysis of Existence up to its minutest, last elements which are imagined as absolute qualities, or things possessing only one unique quality. They are called “qualities” (guna-dharma) in both systems in the sense of absolute qualities, a kind of atomic, or intra-atomic, energies of which the empirical things are composed. Both systems, therefore, agree in denying the objective reality of the categories of Substance and Quality,... and of the relation of Inference uniting them. There is in Sānkhya philosophy no separate existence of qualities. What we call quality is but a particular manifestation of a subtle entity. To every new unit of quality corresponds a subtle quantum of matter which is called guna “quality”, but represents a subtle substantive entity. The same applies to early Buddhism where all qualities are substantive... or, more precisely, dynamic entities, although they are also called dharmas ('qualities')."}}</ref>
The theory of [[temporal finitism]] was inspired by the doctrine of creation shared by the three [[Abrahamic religions]]: [[Judaism]], [[Christianity]] and [[Islam]]. The [[Christian philosophy|Christian philosopher]], [[John Philoponus]], presented the philosophical arguments against the ancient Greek notion of an infinite past. Philoponus' arguments against an infinite past were used by the [[Early Islamic philosophy|early Muslim philosopher]], [[Al-Kindi]] (Alkindus); the [[Jewish philosophy|Jewish philosopher]], [[Saadia Gaon]] (Saadia ben Joseph); and the [[Kalam|Muslim theologian]], [[Al-Ghazali]] (Algazel). They employed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:<ref name=Craig>{{Cite journal|title=Whitrow and Popper on the Impossibility of an Infinite Past|first=William Lane|last=Craig|journal=The British Journal for the Philosophy of Science|volume=30|issue=2|date=June 1979|pages=165–170 [165–6]|doi=10.1093/bjps/30.2.165}}</ref>
:"An actual infinite cannot exist."
:"An infinite temporal regress of events is an actual infinite."
:"<math>\therefore</math> An infinite temporal regress of events cannot exist."
The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:<ref name=Craig/>
:"An actual infinite cannot be completed by successive addition."
:"The temporal series of past events has been completed by successive addition."
:"<math>\therefore</math> The temporal series of past events cannot be an actual infinite."
Both arguments were adopted by later Christian philosophers and theologians, and the second argument in particular became more famous after it was adopted by [[Immanuel Kant]] in his thesis of the first [[antinomy]] concerning [[time]].<ref name=Craig/>
===Astronomical models===
{{Main|History of astronomy}}
Astronomical models of the Universe were proposed soon after [[astronomy]] began with the [[Babylonian astronomy|Babylonian astronomers]], who viewed the Universe as a [[Flat Earth|flat disk]] floating in the ocean, and this forms the premise for early Greek maps like those of [[Anaximander]] and [[Hecataeus of Miletus]].
Later [[Ancient Greece|Greek]] philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the Universe based more profoundly on empirical evidence. The first coherent model was proposed by [[Eudoxus of Cnidos]]. According to this model, space and time are infinite and eternal, the Earth is spherical and stationary, and all other matter is confined to rotating concentric spheres. This model was refined by [[Callippus]] and [[Aristotle]], and brought into nearly perfect agreement with astronomical observations by [[Ptolemy]]. The success of this model is largely due to the mathematical fact that any function (such as the position of a planet) can be decomposed into a set of circular functions (the [[Fourier modes]]). However, not all Greek scientists accepted the geocentric model of the Universe. The [[Pythagoreans|Pythagorean]] philosopher [[Philolaus]] postulated that at the center of the Universe was a "central fire" around which the [[Earth]], [[Sun]], [[Moon]] and [[Planets]] revolved in uniform circular motion.<ref>Boyer, C. ''A History of Mathematics.'' Wiley, p. 54.</ref>
The [[Greek astronomy|Greek astronomer]] [[Aristarchus of Samos]] was the first known individual to propose a [[Heliocentrism|heliocentric]] model of the universe. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus' heliocentric theory. [[Archimedes]] wrote: (translated into English)
<blockquote>
You King Gelon are aware the 'Universe' is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'Universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.
</blockquote>
Aristarchus thus believed the stars to be very far away, and saw this as the reason why there was no visible parallax, that is, an observed movement of the stars relative to each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times, which is why stellar parallax is only detectable with telescopes. The geocentric model, consistent with planetary parallax, was assumed to be an explanation for the unobservability of the parallel phenomenon, stellar parallax. The rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests (On the Apparent Face in the Orb of the Moon):
<blockquote>
[[Cleanthes]] [a contemporary of Aristarchus and head of the Stoics] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe [i.e. the earth], . . . supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis. [1]
</blockquote>
The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was [[Seleucus of Seleucia]], a [[Hellenization|Hellenized]] [[Babylonia]]n astronomer who lived a century after Aristarchus.<ref>[[Otto E. Neugebauer]] (1945). "The History of Ancient Astronomy Problems and Methods", ''Journal of Near Eastern Studies'' '''4''' (1), p. 1–38.
{{quote|"the [[Chaldaea]]n Seleucus from Seleucia"}}</ref><ref>[[George Sarton]] (1955). "Chaldaean Astronomy of the Last Three Centuries B. C.", ''Journal of the American Oriental Society'' '''75''' (3), pp. 166–173 [169]:
{{quote|"the heliocentrical astronomy invented by Aristarchos of Samos and still defended a century later by Seleucos the [[Babylonia]]n"}}</ref><ref>William P. D. Wightman (1951, 1953), ''The Growth of Scientific Ideas'', Yale University Press p.38, where Wightman calls him [[Seleukos]] the [[Chaldea]]n.</ref> According to [[Plutarch]], Seleucus was the first to prove the heliocentric system through [[reasoning]], but it is not known what arguments he used. Seleucus' arguments for a heliocentric theory were probably related to the phenomenon of [[tide]]s.<ref>[[Lucio Russo]], ''Flussi e riflussi'', Feltrinelli, Milano, 2003, ISBN 88-07-10349-4.</ref> According to [[Strabo]] (1.1.9), Seleucus was the first to state that the [[tide]]s are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.<ref>[[Bartel Leendert van der Waerden]] (1987), "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [527]</ref> Alternatively, he may have proved the heliocentric theory by determining the constants of a [[Geometry|geometric]] model for the heliocentric theory and by developing methods to compute planetary positions using this model, like what [[Nicolaus Copernicus]] later did in the 16th century.<ref>[[Bartel Leendert van der Waerden]] (1987), "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [527–9]</ref> During the [[Middle Ages]], heliocentric models may have also been proposed by the [[Indian astronomy|Indian astronomer]], [[Aryabhata]],<ref>[[Bartel Leendert van der Waerden]] (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [529–34]</ref> and by the [[Islamic astronomy|Persian astronomers]], [[Ja'far ibn Muhammad Abu Ma'shar al-Balkhi|Albumasar]]<ref>[[Bartel Leendert van der Waerden]] (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [534–7]</ref> and [[Al-Sijzi]].<ref name=Nasr>{{Cite book |last=Nasr |first=Seyyed H. |authorlink=Hossein Nasr |date=1st edition in 1964, 2nd edition in 1993 |title=An Introduction to Islamic Cosmological Doctrines |edition=2nd |publisher=1st edition by [[Harvard University Press]], 2nd edition by [[State University of New York Press]] |isbn=0791415155 |pages=135–6}}</ref>
[[Image:ThomasDiggesmap.JPG|thumb|left|Model of the [[Copernicus|Copernican]] universe by [[Thomas Digges]] in 1576, with the amendment that the stars are no longer confined to a sphere, but spread uniformly throughout the space surrounding the [[planet]]s.]]
The Aristotelian model was accepted in the [[Western world]] for roughly two millennia, until [[Copernicus]] revived Aristarchus' theory that the astronomical data could be explained more plausibly if the [[earth]] rotated on its axis and if the [[sun]] were placed at the center of the Universe.
{{cquote|In the center rests the sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can illuminate everything at the same time?|20px|20px|[[Copernicus]]| in Chapter 10, Book 1 of ''De Revolutionibus Orbium Coelestrum'' (1543)}}
As noted by Copernicus himself, the suggestion that the [[Earth's rotation|Earth rotates]] was very old, dating at least to [[Philolaus]] (c. 450 BC), [[Heraclides Ponticus]] (c. 350 BC) and [[Ecphantus the Pythagorean]]. Roughly a century before Copernicus, Christian scholar [[Nicholas of Cusa]] also proposed that the Earth rotates on its axis in his book, ''On Learned Ignorance'' (1440).<ref>Misner, Thorne and Wheeler (1973), p. 754.</ref> Aryabhata (476–550), [[Brahmagupta]] (598–668), [[Albumasar]] and [[Al-Sijzi]], also proposed that the Earth rotates on its axis.{{Citation needed|date=April 2010}} The first [[Empirical research|empirical evidence]] for the Earth's rotation on its axis, using the phenomenon of [[comet]]s, was given by [[Nasīr al-Dīn al-Tūsī|Tusi]] (1201–1274) and [[Ali Kuşçu]] (1403–1474).{{Citation needed|date=April 2010}} Tusi, however, continued to support the Aristotelian universe, thus Kuşçu was the first to refute the Aristotelian notion of a stationary Earth on an [[empirical]] basis, similar to how Copernicus later justified the Earth's rotation. [[Al-Birjandi]] (d. 1528) further developed a theory of "circular [[inertia]]" to explain the Earth's rotation, similar to how [[Galileo Galilei]] explained it.<ref>{{Cite journal |last=Ragep |first=F. Jamil |year=2001a |title=Tusi and Copernicus: The Earth's Motion in Context |journal=Science in Context |volume=14 |issue=1–2 |pages=145–63 |publisher=[[Cambridge University Press]] }}</ref><ref>{{Cite journal |last=Ragep |first=F. Jamil |year=2001b |title=Freeing Astronomy from Philosophy: An Aspect of Islamic Influence on Science |journal=Osiris, 2nd Series |volume=16 |issue=Science in Theistic Contexts: Cognitive Dimensions |pages=49–64 & 66–71}}</ref>
[[Image:Libr0309.jpg|thumb|[[Johannes Kepler]] published the ''[[Rudolphine Tables]]'' containing a star catalog and planetary tables using [[Tycho Brahe]]'s measurements.]]
Copernicus' [[Heliocentrism|heliocentric model]] allowed the stars to be placed uniformly through the (infinite) space surrounding the planets, as first proposed by [[Thomas Digges]] in his ''Perfit Description of the Caelestiall Orbes according to the most aunciente doctrine of the Pythagoreans, latelye revived by Copernicus and by Geometricall Demonstrations approved'' (1576).<ref name = "Misner-p755">Misner, Thorne, and Wheeler (1973), p.755.</ref> [[Giordano Bruno]] accepted the idea that space was infinite and filled with solar systems similar to our own; for the publication of this view, he was [[execution by burning|burned at the stake]] in the [[Campo de' Fiori|Campo dei Fiori]] in Rome on 17 February 1600.<ref name = "Misner-p755"/>
This cosmology was accepted provisionally by [[Isaac Newton]], [[Christiaan Huygens]] and later scientists,<ref name = "Misner-p755">Misner, Thorne, and Wheeler (1973), p. 755–756.</ref> although it had several paradoxes that were resolved only with the development of [[general relativity]]. The first of these was that it assumed that space and time were infinite, and that the stars in the universe had been burning forever; however, since stars are constantly radiating [[energy]], a finite star seems inconsistent with the radiation of infinite energy. Secondly, Edmund Halley (1720)<ref>Misner, Thorne, and Wheeler (1973), p. 756.</ref> and [[Jean-Philippe de Cheseaux]] (1744)<ref>{{cite book | author = [[Jean-Philippe de Cheseaux|de Cheseaux JPL]] | year = 1744 | title = Traité de la Comète | publisher = Lausanne | pages = 223ff}}. Reprinted as Appendix II in {{cite book | author = Dickson FP | year = 1969 | title = The Bowl of Night: The Physical Universe and Scientific Thought | publisher = M.I.T. Press | ___location = Cambridge, MA | isbn = 978-0262540032}}</ref> noted independently that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the sun itself; this became known as [[Olbers' paradox]] in the 19th century.<ref>{{cite journal | author = [[Heinrich Wilhelm Matthäus Olbers|Olbers HWM]] | year = 1826 | title = Unknown title | journal = Bode's Jahrbuch | volume = 111}}. Reprinted as Appendix I in {{cite book | author = Dickson FP | year = 1969 | title = The Bowl of Night: The Physical Universe and Scientific Thought | publisher = M.I.T. Press | ___location = Cambridge, MA | isbn = 978-0262540032}}</ref> Third, Newton himself showed that an infinite space uniformly filled with matter would cause infinite forces and instabilities causing the matter to be crushed inwards under its own gravity.<ref name = "Misner-p755"/> This instability was clarified in 1902 by the [[Jeans instability]] criterion.<ref>Jeans, J. H. (1902) ''Philosophical Transactions Royal Society of London, Series A'', '''199''', 1.</ref> One solution to these latter two paradoxes is the [[Carl Charlier|Charlier universe]], in which the matter is arranged hierarchically (systems of orbiting bodies that are themselves orbiting in a larger system, ''ad infinitum'') in a [[fractal]] way such that the universe has a negligibly small overall density; such a cosmological model had also been proposed earlier in 1761 by [[Johann Heinrich Lambert]].<ref>Rindler, p. 196; Misner, Thorne, and Wheeler (1973), p. 757.</ref> A significant astronomical advance of the 18th century was the realization by [[Thomas Wright (astronomer)|Thomas Wright]], [[Immanuel Kant]] and others that stars are not distributed uniformly throughout space; rather, they are grouped into [[galaxy|galaxies]].<ref>Misner, Thorne and Wheeler, p.756.</ref>
The modern era of [[physical cosmology]] began in 1917, when [[Albert Einstein]] first applied his general theory of relativity to model the structure and dynamics of the universe.<ref name="einstein_1917">{{cite journal | last = Einstein | first = A | authorlink = Albert Einstein | year = 1917 | title = Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie | journal = Preussische Akademie der Wissenschaften, Sitzungsberichte | volume = 1917 (part 1) | pages = 142–152}}</ref> This theory and its implications will be discussed in more detail in the following section.
==Theoretical models==
[[Image:Cassini-science-br.jpg|thumb|High-precision test of general relativity by the [[Cassini-Huygens|Cassini]] space probe (artist's impression): [[radio]] signals sent between the Earth and the probe (green wave) are [[Shapiro effect|delayed]] by the warping of [[space and time]] (blue lines) due to the [[Sun]]'s mass.]]
Of the four [[fundamental interaction]]s, [[gravitation]] is dominant at cosmological length scales; that is, the other three forces are believed to play a negligible role in determining structures at the level of planets, stars, galaxies and larger-scale structures. Since all matter and energy gravitate, gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on cosmological length scales. The remaining two interactions, the [[weak nuclear force|weak]] and [[strong nuclear force]]s, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.
===General theory of relativity===
{{Main|Introduction to general relativity|General relativity|Einstein's field equations}}
Given gravitation's predominance in shaping cosmological structures, accurate predictions of the universe's past and future require an accurate theory of gravitation. The best theory available is [[Albert Einstein]]'s general theory of relativity, which has passed all experimental tests hitherto. However, since rigorous experiments have not been carried out on cosmological length scales, general relativity could conceivably be inaccurate. Nevertheless, its cosmological predictions appear to be consistent with observations, so there is no compelling reason to adopt another theory.
General relativity provides of a set of ten nonlinear partial differential equations for the [[metric tensor (general relativity)|spacetime metric]] ([[Einstein field equations|Einstein's field equations]]) that must be solved from the distribution of [[mass-energy]] and [[momentum]] throughout the universe. Since these are unknown in exact detail, cosmological models have been based on the [[cosmological principle]], which states that the universe is homogeneous and isotropic. In effect, this principle asserts that the gravitational effects of the various galaxies making up the universe are equivalent to those of a fine dust distributed uniformly throughout the universe with the same average density. The assumption of a uniform dust makes it easy to solve Einstein's field equations and predict the past and future of the universe on cosmological time scales.
Einstein's field equations include a [[cosmological constant]] (''Λ''),<ref name="einstein_1917" /><ref>Rindler (1977), pp. 226–229.</ref> that corresponds to an energy density of empty space.<ref>Landau and Lifshitz (1975), pp. 358–359.</ref> Depending on its sign, the cosmological constant can either slow (negative ''Λ'') or accelerate (positive ''Λ'') the [[metric expansion of space|expansion of the universe]]. Although many scientists, including Einstein, had speculated that ''Λ'' was zero,<ref>{{cite journal | last = Einstein | first = A | authorlink = Albert Einstein | year = 1931 | title = Zum kosmologischen Problem der allgemeinen Relativitätstheorie | journal = Sitzungsberichte der Preussischen Akademie der Wissenschaften, Physikalisch-mathematische Klasse | volume = 1931 | pages = 235–237}}<br />{{cite journal | author = [[Albert Einstein|Einstein A.]], [[Willem de Sitter|de Sitter W.]] | year = 1932 | title = On the relation between the expansion and the mean density of the universe | journal = Proceedings of the National Academy of Sciences | volume = 18 | pages = 213–214 | doi = 10.1073/pnas.18.3.213 | pmid = 16587663 | issue = 3 | pmc = 1076193}}</ref> recent astronomical observations of [[type Ia supernova]]e have detected a large amount of "[[dark energy]]" that is accelerating the universe's expansion.<ref>[http://hubblesite.org/newscenter/archive/releases/2004/12/text/ Hubble Telescope news release]</ref> Preliminary studies suggest that this dark energy corresponds to a positive ''Λ'', although alternative theories cannot be ruled out as yet.<ref>[http://news.bbc.co.uk/1/hi/sci/tech/6156110.stm BBC News story: Evidence that dark energy is the cosmological constant]</ref> Russian [[physics|physicist]] [[Yakov Borisovich Zel'dovich|Zel'dovich]] suggested that ''Λ'' is a measure of the [[zero-point energy]] associated with [[virtual particle]]s of [[quantum field theory]], a pervasive [[vacuum energy]] that exists everywhere, even in empty space.<ref>{{cite journal | author = [[Yakov Borisovich Zel'dovich|Zel'dovich YB]] | year = 1967 | title = Cosmological constant and elementary particles | journal = Zh. Eksp. & Teor. Fiz. Pis'ma | volume = 6 | pages = 883–884}} English translation in ''Sov. Phys. — JTEP Lett.'', '''6''', pp. 316–317 (1967).</ref> Evidence for such zero-point energy is observed in the [[Casimir effect]].
===Special relativity and space-time===
{{Main|Introduction to special relativity|Special relativity}}
[[Image:Only distance is real.svg|thumb|300px|Only its length ''L'' is intrinsic to the rod (shown in black); coordinate differences between its endpoints (such as Δx, Δy or Δξ, Δη) depend on their frame of reference (depicted in blue and red, respectively).]]
The universe has at least three [[space|spatial]] and one temporal ([[time]]) dimension. It was long thought that the spatial and temporal dimensions were different in nature and independent of one another. However, according to the [[special relativity|special theory of relativity]], spatial and temporal separations are interconvertible (within limits) by changing one's motion.
To understand this interconversion, it is helpful to consider the analogous interconversion of spatial separations along the three spatial dimensions. Consider the two endpoints of a rod of length ''L''. The length can be determined from the differences in the three coordinates Δx, Δy and Δz of the two endpoints in a given reference frame
:<math>
L^{2} = \Delta x^{2} + \Delta y^{2} + \Delta z^{2}
</math>
using the [[Pythagorean theorem]]. In a rotated reference frame, the coordinate differences differ, but they give the same length
:<math>
L^{2} = \Delta \xi^{2} + \Delta \eta^{2} + \Delta \zeta^{2}.
</math>
Thus, the coordinates differences (Δx, Δy, Δz) and (Δξ, Δη, Δζ) are not intrinsic to the rod, but merely reflect the reference frame used to describe it; by contrast, the length ''L'' is an intrinsic property of the rod. The coordinate differences can be changed without affecting the rod, by rotating one's reference frame.
The analogy in [[spacetime]] is called the interval between two events; an event is defined as a point in spacetime, a specific position in space and a specific moment in time. The spacetime interval between two events is given by
:<math>
s^{2} = L_{1}^{2} - c^{2} \Delta t_{1}^{2} = L_{2}^{2} - c^{2} \Delta t_{2}^{2}
</math>
where ''c'' is the speed of light. According to [[special relativity]], one can change a spatial and time separation (''L''<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]].
===Solving Einstein's field equations===
{{See also|Big Bang|Ultimate fate of the universe}}
The distances between the spinning galaxies increase with time, but the distances between the stars within each galaxy stay roughly the same, due to their gravitational interactions. This animation illustrates a closed Friedmann universe with zero [[cosmological constant]] Λ; such a universe oscillates between a [[Big Bang]] and a [[Big Crunch]].
[[File:Closed Friedmann universe zero Lambda.ogg|thumb|right|Animation illustrating the [[metric expansion of the universe]]]]
In non-Cartesian (non-square) or curved coordinate systems, the Pythagorean theorem holds only on infinitesimal length scales and must be augmented with a more general [[metric tensor]] ''g''<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]]
:<math>
ds^2 = -c^{2} dt^2 +
R(t)^2 \left( \frac{dr^2}{1-k r^2} + r^2 d\theta^2 + r^2 \sin^2 \theta \, d\phi^2 \right)
</math>
where (''r'', θ, φ) correspond to a [[spherical coordinate system]]. This [[metric (mathematics)|metric]] has only two undetermined parameters: an overall length scale ''R'' that can vary with time, and a curvature index ''k'' that can be only 0, 1 or −1, corresponding to flat [[Euclidean geometry]], or spaces of positive or negative [[curvature]]. In cosmology, solving for the history of the universe is done by calculating ''R'' as a function of time, given ''k'' and the value of the [[cosmological constant]] ''Λ'', which is a (small) parameter in Einstein's field equations. The equation describing how ''R'' varies with time is known as the [[Friedmann equation]], after its inventor, [[Alexander Friedmann]].<ref>{{cite journal | author = [[Alexander Friedmann|Friedmann A.]] | year = 1922 | title = Über die Krümmung des Raumes | journal = Zeitschrift für Physik | volume = 10 | pages = 377–386 | doi = 10.1007/BF01332580}}</ref>
The solutions for ''R(t)'' depend on ''k'' and ''Λ'', but some qualitative features of such solutions are general. First and most importantly, the length scale ''R'' of the universe can remain constant ''only'' if the universe is perfectly isotropic with positive curvature (''k''=1) and has one precise value of density everywhere, as first noted by [[Albert Einstein]]. However, this equilibrium is unstable and since the universe is known to be inhomogeneous on smaller scales, ''R'' must change, according to [[general relativity]]. When ''R'' changes, all the spatial distances in the universe change in tandem; there is an overall expansion or contraction of space itself. This accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point 13.7 billion years ago and never moved faster than the [[speed of light]].
Second, all solutions suggest that there was a [[gravitational singularity]] in the past, when ''R'' goes to zero and matter and energy became infinitely dense. It may seem that this conclusion is uncertain since it is based on the questionable assumptions of perfect homogeneity and isotropy (the cosmological principle) and that only the gravitational interaction is significant. However, the [[Penrose-Hawking singularity theorems]] show that a singularity should exist for very general conditions. Hence, according to Einstein's field equations, ''R'' grew rapidly from an unimaginably hot, dense state that existed immediately following this singularity (when ''R'' had a small, finite value); this is the essence of the [[Big Bang]] model of the universe. A common misconception is that the Big Bang model predicts that matter and energy exploded from a single point in space and time; that is false. Rather, space itself was created in the Big Bang and imbued with a fixed amount of energy and matter distributed uniformly throughout; as space expands (i.e., as ''R(t)'' increases), the density of that matter and energy decreases.
{| class="toccolours" style="float: left; margin-left: 1em; margin-right: 2em; font-size: 85%; background:#FFFDD0; color:black; width:30em; max-width: 35%;" cellspacing="5"
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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)
|-
| style="text-align: left;" | [[Bernhard Riemann]] (Habilitationsvortrag, 1854)
|}
Third, the curvature index ''k'' determines the sign of the mean spatial curvature of [[spacetime]] averaged over length scales greater than a billion [[light year]]s. If ''k''=1, the curvature is positive and the universe has a finite volume. Such universes are often visualized as a [[3-sphere|three-dimensional sphere ''S''<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 [[wraparound (video games)|"wrap-around" video games]] such as ''[[Asteroids (arcade game)|Asteroids]]''; a traveler crossing an outer "boundary" of space going ''outwards'' would reappear instantly at another point on the boundary moving ''inwards''.
[[Image:CMB Timeline75.jpg|thumb|600px|center|Prevailing model of the origin and expansion of [[spacetime]] and all that it contains.]]
{{Clear}}
The [[ultimate fate of the universe]] is still unknown, because it depends critically on the curvature index ''k'' and the cosmological constant ''Λ''. If the universe is sufficiently dense, ''k'' equals +1, meaning that its average curvature throughout is positive and the universe will eventually recollapse in a [[Big Crunch]], possibly starting a new universe in a [[Big Bounce]]. Conversely, if the universe is insufficiently dense, ''k'' equals 0 or −1 and the universe will expand forever, cooling off and eventually becoming inhospitable for all life, as the stars die and all matter coalesces into black holes (the [[Future of an expanding universe|Big Freeze]] and the [[heat death of the universe]]). As noted above, recent data suggests that the expansion speed of the universe is not decreasing as originally expected, but increasing; if this continues indefinitely, the universe will eventually rip itself to shreds (the [[Big Rip]]). Experimentally, the universe has an overall density that is very close to the critical value between recollapse and eternal expansion; more careful astronomical observations are needed to resolve the question.
===Big Bang model===
{{Main|Big Bang|Timeline of the Big Bang|Nucleosynthesis|Lambda-CDM model}}
The prevailing Big Bang model accounts for many of the experimental observations described above, such as the correlation of distance and [[redshift]] of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous, isotropic microwave radiation background. As noted above, the redshift arises from the [[metric expansion of space]]; as the space itself expands, the wavelength of a [[photon]] traveling through space likewise increases, decreasing its energy. The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an important problem in experimental [[physical cosmology]].
[[Image:Primordial nucleosynthesis.svg|thumb|400px|Chief nuclear reactions responsible for the [[abundance of the chemical elements|relative abundances]] of light [[atomic nucleus|atomic nuclei]] observed throughout the universe.]]
Other experimental observations can be explained by combining the overall expansion of space with [[nuclear physics|nuclear]] and [[atomic physics]]. As the universe expands, the energy density of the [[electromagnetic radiation]] decreases more quickly than does that of [[matter]], since the energy of a photon decreases with its wavelength. Thus, although the energy density of the universe is now dominated by matter, it was once dominated by radiation; poetically speaking, all was [[light]]. As the universe expanded, its energy density decreased and it became cooler; as it did so, the [[elementary particle]]s of matter could associate stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable [[proton]]s and [[neutron]]s formed, which then associated into [[atomic nuclei]]. At this stage, the matter in the universe was mainly a hot, dense [[Plasma (physics)|plasma]] of negative [[electron]]s, neutral [[neutrino]]s and positive nuclei. [[Nuclear reaction]]s among the nuclei led to the present abundances of the lighter nuclei, particularly [[hydrogen]], [[deuterium]], and [[helium]]. Eventually, the electrons and nuclei combined to form stable atoms, which are transparent to most wavelengths of radiation; at this point, the radiation decoupled from the matter, forming the ubiquitous, isotropic background of microwave radiation observed today.
Other observations are not answered definitively by known physics. According to the prevailing theory, a slight imbalance of [[matter]] over [[antimatter]] was present in the universe's creation, or developed very shortly thereafter, possibly due to the [[CP violation]] that has been observed by [[particle physics|particle physicists]]. Although the matter and antimatter mostly annihilated one another, producing [[photon]]s, a small residue of matter survived, giving the present matter-dominated universe. Several lines of evidence also suggest that a rapid [[cosmic inflation]] of the universe occurred very early in its history (roughly 10<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.
==Untestable proposals==
===Multiverse theory===
{{Main|Multiverse|Many-worlds hypothesis|Bubble universe theory|Parallel universe (fiction)}}
[[File:Multiverse - level II.svg|thumb|Depiction of a [[multiverse]] of seven [[bubble universe theory|"bubble" universes]], which are separate [[spacetime]] continua, each having different [[physical law]]s, [[physical constant]]s, and perhaps even different numbers of [[dimension]]s or [[topology|topologies]].]]
Some speculative theories have proposed that this universe is but one of a [[set (mathematics)|set]] of disconnected universes, collectively denoted as the [[multiverse]], altering the concept that the universe encompasses everything.<ref name="EllisKS03" /><ref>{{cite journal | author = Munitz MK | year = 1959 | title = One Universe or Many? | journal = Journal of the History of Ideas | volume = 12 | pages = 231–255 | url = http://links.jstor.org/sici?sici=0022-5037(195104)12%3A2%3C231%3AOUOM%3E2.0.CO%3B2-F | doi = 10.2307/2707516 | issue = 2}}</ref> By definition, there is no possible way for anything in one universe to affect another; if two "universes" could affect one another, they would be part of a single universe. Thus, although some fictional characters travel between [[parallel universe (fiction)|parallel fictional "universes"]], this is, strictly speaking, an incorrect usage of the term ''universe''. The disconnected universes are conceived as being physical, in the sense that each should have its own space and time, its own matter and energy, and its own physical laws — that also challenges the definition of parallelity as these universes don't exist synchronously (since they have their own time) or in a geometrically parallel way (since there's no interpretable relation between spatial positions of the different universes). Such physically disconnected universes should be distinguished from the [[metaphysics|metaphysical]] conception of [[plane (esotericism)|alternate planes of consciousness]], which are not thought to be physical places and are connected through the flow of information. The concept of a multiverse of disconnected universes is very old; for example, Bishop [[Étienne Tempier]] of Paris ruled in 1277 that God could create as many universes as he saw fit, a question that was being hotly debated by the French theologians.<ref>Misner, Thorne and Wheeler (1973), p.753.</ref>
There are two scientific senses in which multiple universes are discussed. First, disconnected spacetime continua may exist; presumably, all forms of matter and energy are confined to one universe and cannot "tunnel" between them. An example of such a theory is the [[bubble universe theory|chaotic inflation]] model of the early universe.<ref name="chaotic_inflation">{{cite journal | author = [[Andrei Linde|Linde A.]] | year = 1986 | title = Eternal chaotic inflation | journal = Mod. Phys. Lett. | volume = A1 | pages = 81}}<br />{{cite journal | author = [[Andrei Linde|Linde A.]] | year = 1986 | title = Eternally existing self-reproducing chaotic inflationary universe | journal = Phys. Lett. | volume = B175 | pages = 395–400}}</ref> Second, according to the [[many-worlds hypothesis]], a parallel universe is born with every [[quantum measurement]]; the universe "forks" into parallel copies, each one corresponding to a different outcome of the quantum measurement. However, both senses of the term "multiverse" are speculative and may be considered [[Falsifiability|unscientific]]; no known experimental test in one universe could reveal the existence or properties of another non-interacting universe.
==Shape of the universe==
{{Details|Shape of the Universe}}
The shape or [[geometry]] of the universe includes both [[Shape of the Universe#Local geometry (spatial curvature)|local geometry]] in the [[observable universe]] and [[Shape of the Universe#Global geometry|global geometry]], which we may or may not be able to measure. Shape can refer to curvature and [[topology]]. More formally, the subject in practice investigates which [[3-manifold]] corresponds to the spatial section in [[comoving coordinates]] of the four-dimensional [[spacetime|space-time]] of the Universe. Analysis of data from [[Wilkinson Microwave Anisotropy Probe|WMAP]] implies that the universe is [[Euclidean geometry|spatially flat]] with only a 2% margin of error.<ref>[http://map.gsfc.nasa.gov/universe/uni_shape.html Shape of the Universe], WMAP website at NASA.</ref>
Cosmologists normally work with a given [[space-like]] slice of spacetime called the [[Comoving distance|comoving coordinates]]. In terms of observation, the section of spacetime that can be observed is the backward [[light cone]] (points within the [[cosmic light horizon]], given time to reach a given observer). If the observable universe is smaller than the entire universe (in some models it is many orders of magnitude smaller), one cannot determine the global structure by observation: one is limited to a small patch.
In October 2001, NASA began collecting data with the [[Wilkinson Microwave Anisotropy Probe]] (WMAP) on cosmic background radiation. Like visible light from distant stars and galaxies, cosmic background radiation allows scientists to peer into the past to the time when the universe was in its infancy. Density fluctuations in this radiation can also tell scientists much about the physical nature of space.<ref>http://en.wikipedia.org/wiki/Homology_sphere#Cosmology</ref> NASA released the first WMAP cosmic background radiation data in February 2003. In 2009 the [[Planck (spacecraft)|Planck observatory]] was launched which will be able to analyze the microwave background at higher resolution, providing more information on the shape of the early universe. The preliminary data will be released in December 2010.
==See also==
{{Portal box|Astronomy|Space}}
<div style="-moz-column-count:4; column-count:4;">
* [[Anthropic principle]]
* [[Big Bang]]
* [[Big Crunch]]
* [[Cosmic latte]]
* [[Cosmology]]
* [[Dyson's eternal intelligence]]
* [[Esoteric cosmology]]
* [[False vacuum]]
* [[Final anthropic principle]]
* [[Fine-tuned Universe]]
* [[Heat death of the universe]]
* [[Hindu Cycle Of The Universe]]
* [[Kardashev scale]]
* [[Multiverse]]
* [[Nucleocosmochronology]]
* [[Non-standard cosmology]]
* [[Omega point]]
* [[Omniverse]]
* [[Rare Earth hypothesis]]
* [[Reality]]
* [[Shape of the Universe]]
* [[Ultimate fate of the universe]]
* [[Vacuum genesis]]
* [[World view]]
* [[Zero-energy Universe]]
</div>
==Notes and references==
{{Reflist|2}}
==Further reading==
* {{cite book|author = [[Lev Landau|Landau, Lev]], [[Evgeny Lifshitz|Lifshitz, E.M.]] | year = 1975 | title= The Classical Theory of Fields ([[Course of Theoretical Physics]], Vol. 2) | edition = revised 4th English|publisher=Pergamon Press|___location=New York|isbn=9780080181769|pages=358–397}}
* [[Edward Robert Harrison]] (2000) ''Cosmology'' 2nd ed. Cambridge University Press. Gentle.
* {{cite book | author = [[Charles W. Misner|Misner, C.W.]], [[Kip Thorne|Thorne, Kip]], [[John Archibald Wheeler|Wheeler, J.A.]] | title = [[Gravitation (book)|Gravitation]] | ___location = San Francisco | publisher = W. H. Freeman | year = 1973 | isbn = 978-0-7167-0344-0 | pages = 703–816 }} The classic text for a generation.
* {{cite book | author = [[Wolfgang Rindler|Rindler, W.]] | year = 1977 | title = Essential Relativity: Special, General, and Cosmological | publisher = Springer Verlag | ___location = New York | isbn = 0-387-10090-3 | pages = 193–244}}
* {{cite book | author = [[Steven Weinberg|Weinberg, S.]] | year = 1993 | title = The First Three Minutes: A Modern View of the Origin of the Universe | edition = 2nd updated | publisher = Basic Books | ___location = New York | isbn = 978-0465024377 | oclc = 28746057}} For lay readers.
* -------- (2008) ''Cosmology''. Oxford University Press. Challenging.
* Oscar Monchito (1987) ''Universe. What a concept''. Colton, 23rd edition. For advanced readers.
==Footnotes and references==
<references />
==External links==
* {{Cita pubblicazione|autore= Paré M, Behets C, Cornu O |titolo= Paucity of presumptive ruffini corpuscles in the index finger pad of humans. |rivista= J Comp Neurol |volume= 456 |numero= 3 |pp= 260–6 |anno= 2003 | pmid = 12528190 | doi = 10.1002/cne.10519}}
{{Spoken Wikipedia|En-Universe.ogg|2007-07-07}}
<nowiki>
{{Commons category|Universe}}
[[Category:Sensory receptors]]
{{wikiquote}}
* {{HSW|hole-in-universe|Is there a hole in the universe?}}
* [http://www.space.com/scienceastronomy/age_universe_030103.html Age of the Universe] at Space.Com
* [http://www.pbs.org/wnet/hawking/html/home.html ''Stephen Hawking's Universe''] – Why is the universe the way it is?
* [http://www.astro.ucla.edu/~wright/cosmology_faq.html Cosmology FAQ]
* [http://www.shekpvar.net/~dna/Publications/Cosmos/cosmos.html Cosmos – An "illustrated dimensional journey from microcosmos to macrocosmos"]
* [http://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets, the sun, and other stars]
* [http://www.astro.princeton.edu/~mjuric/universe/ Logarithmic Maps of the Universe]
* [http://www.slate.com/id/2087206/nav/navoa/ My So-Called Universe] – Arguments for and against an infinite and parallel universes
* [http://www.hep.upenn.edu/~max/multiverse1.html Parallel Universes] by Max Tegmark
* [http://cosmology.lbl.gov/talks/Ho_07.pdf The Dark Side and the Bright Side of the Universe] Princeton University, Shirley Ho
* [http://www.atlasoftheuniverse.com/ Richard Powell: ''An Atlas of the Universe''] – Images at various scales, with explanations
* [http://www.npr.org/templates/story/story.php?storyId=1142346 Multiple Big Bangs]
* [http://www.exploreuniverse.com/ic/ Universe – Space Information Centre]
* [http://www.nasa.gov/topics/universe/index.html Exploring the Universe] at Nasa.gov
* [http://www.zideo.nl/index.php?option=com_podfeed&zideo=6c4947596d673d3d&playzideo=6c3461566f56593d The Size Of The Universe, understand the size of the universe by starting with humans and going up by powers of ten ]
===Videos===
* [http://www.youtube.com/watch?v=17jymDn0W6U The Known Universe] created by the [[American Museum of Natural History]]
{{Earth's ___location}}
{{Nature nav}}
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