Utente:Vale maio/Sandbox2

DISCLAIMER

Questa è una sottopagina personale dove lavoro alla stesura di nuove voci o alla modifica di voci già esistenti.
Gentilmente, non modificate niente, a meno di espressa autorizzazione del sottoscritto.

Sandbox n° 1 · Sandbox n°2 · Sandbox n°3 · Sandbox n°4 · Sandbox n°5 · Sandbox n°6

Theoretical models

High-precision test of general relativity by the Cassini space probe (artist's impression): radio signals sent between the Earth and the probe (green wave) are delayed by the warping of space and time (blue lines) due to the Sun's mass.

Of the four fundamental interactions, gravitation is 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 and strong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.

General theory of relativity

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 spacetime metric (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),^[1]^[2] that corresponds to an energy density of empty space.^[3] Depending on its sign, the cosmological constant can either slow (negative Λ) or accelerate (positive Λ) the expansion of the universe. Although many scientists, including Einstein, had speculated that Λ was zero,^[4] recent astronomical observations of type Ia supernovae have detected a large amount of "dark energy" that is accelerating the universe's expansion.^[5] Preliminary studies suggest that this dark energy corresponds to a positive Λ, although alternative theories cannot be ruled out as yet.^[6] Russian physicist Zel'dovich suggested that Λ is a measure of the zero-point energy associated with virtual particles of quantum field theory, a pervasive vacuum energy that exists everywhere, even in empty space.^[7] Evidence for such zero-point energy is observed in the Casimir effect.

Special relativity and space-time

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 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 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

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

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

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

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

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

where c is the speed of light. According to special relativity, one can change a spatial and time separation (L₁, Δt₁) into another (L₂, Δt₂) by changing one's reference frame, as long as the change maintains the spacetime interval s. Such a change in reference frame corresponds to 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

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.

Animation illustrating the metric expansion of the universe

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

ds^{2}=-c^{2}dt^{2}+R(t)^{2}\left({\frac {dr^{2}}{1-kr^{2}}}+r^{2}d\theta ^{2}+r^{2}\sin ^{2}\theta \,d\phi ^{2}\right)

where (r, θ, φ) correspond to a spherical coordinate system. This 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.^[8]

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.

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)

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 years. If k=1, the curvature is positive and the universe has a finite volume. Such universes are often visualized as a three-dimensional sphere S³ 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; a traveler crossing an outer "boundary" of space going outwards would reappear instantly at another point on the boundary moving inwards.

Prevailing model of the origin and expansion of spacetime and all that it contains.

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 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

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.

Chief nuclear reactions responsible for the relative abundances of light atomic nuclei observed throughout the universe.

Other experimental observations can be explained by combining the overall expansion of space with 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 particles of matter could associate stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then associated into atomic nuclei. At this stage, the matter in the universe was mainly a hot, dense plasma of negative electrons, neutral neutrinos and positive nuclei. Nuclear reactions 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 physicists. Although the matter and antimatter mostly annihilated one another, producing photons, a small residue of matter survived, giving the present matter-dominated universe. Several lines of evidence also suggest that a rapid cosmic inflation of the universe occurred very early in its history (roughly 10⁻³⁵ seconds after its creation). Recent observations also suggest that the cosmological constant (Λ) is not zero and that the net mass-energy content of the universe is dominated by a dark energy and dark matter that have not been characterized scientifically. They differ in their gravitational effects. Dark matter gravitates as ordinary matter does, and thus slows the expansion of the universe; by contrast, dark energy serves to accelerate the universe's expansion.

Untestable proposals

Multiverse theory

Depiction of a multiverse of seven "bubble" universes, which are separate spacetime continua, each having different physical laws, physical constants, and perhaps even different numbers of dimensions or topologies.

Some speculative theories have proposed that this universe is but one of a set of disconnected universes, collectively denoted as the multiverse, altering the concept that the universe encompasses everything.^[9]^[10] 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 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 metaphysical conception of 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.^[11]

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 chaotic inflation model of the early universe.^[12] 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 unscientific; no known experimental test in one universe could reveal the existence or properties of another non-interacting universe.

Shape of the universe

Template:Details

The shape or geometry of the universe includes both local geometry in the observable universe and 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 space-time of the Universe. Analysis of data from WMAP implies that the universe is spatially flat with only a 2% margin of error.^[13]

Cosmologists normally work with a given space-like slice of spacetime called the 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.^[14] NASA released the first WMAP cosmic background radiation data in February 2003. In 2009 the 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.

Notes and references

^ Errore nelle note: Errore nell'uso del marcatore <ref>: non è stato indicato alcun testo per il marcatore einstein_1917
^ Rindler (1977), pp. 226–229.
^ Landau and Lifshitz (1975), pp. 358–359.
^ A Einstein, Zum kosmologischen Problem der allgemeinen Relativitätstheorie, in Sitzungsberichte der Preussischen Akademie der Wissenschaften, Physikalisch-mathematische Klasse, vol. 1931, 1931, pp. 235–237.
Einstein A., de Sitter W., On the relation between the expansion and the mean density of the universe, in Proceedings of the National Academy of Sciences, vol. 18, n. 3, 1932, pp. 213–214, DOI:10.1073/pnas.18.3.213.
^ Hubble Telescope news release
^ BBC News story: Evidence that dark energy is the cosmological constant
^ Zel'dovich YB, Cosmological constant and elementary particles, in Zh. Eksp. & Teor. Fiz. Pis'ma, vol. 6, 1967, pp. 883–884. English translation in Sov. Phys. — JTEP Lett., 6, pp. 316–317 (1967).
^ Friedmann A., Über die Krümmung des Raumes, in Zeitschrift für Physik, vol. 10, 1922, pp. 377–386, DOI:10.1007/BF01332580.
^ George F.R. Ellis, U. Kirchner, W.R. Stoeger, Multiverses and physical cosmology, in Monthly Notices of the Royal Astronomical Society, vol. 347, 2004, pp. 921–936, DOI:10.1111/j.1365-2966.2004.07261.x. URL consultato il 9 gennaio 2007.
^ Munitz MK, One Universe or Many?, in Journal of the History of Ideas, vol. 12, n. 2, 1959, pp. 231–255, DOI:10.2307/2707516.
^ Misner, Thorne and Wheeler (1973), p.753.
^ Linde A., Eternal chaotic inflation, in Mod. Phys. Lett., A1, 1986, p. 81.
Linde A., Eternally existing self-reproducing chaotic inflationary universe, in Phys. Lett., B175, 1986, pp. 395–400.
^ Shape of the Universe, WMAP website at NASA.
^ http://en.wikipedia.org/wiki/Homology_sphere#Cosmology

External links

Template:Spoken Wikipedia

Wikimedia Commons contiene immagini o altri file su Vale maio/Sandbox2

Wikiquote contiene citazioni di o su Vale maio/Sandbox2
Template:HSW
Age of the Universe at Space.Com
Stephen Hawking's Universe – Why is the universe the way it is?
Cosmology FAQ
Cosmos – An "illustrated dimensional journey from microcosmos to macrocosmos"
Illustration comparing the sizes of the planets, the sun, and other stars
Logarithmic Maps of the Universe
My So-Called Universe – Arguments for and against an infinite and parallel universes
Parallel Universes by Max Tegmark
The Dark Side and the Bright Side of the Universe Princeton University, Shirley Ho
Richard Powell: An Atlas of the Universe – Images at various scales, with explanations
Multiple Big Bangs
Universe – Space Information Centre
Exploring the Universe at Nasa.gov
The Size Of The Universe, understand the size of the universe by starting with humans and going up by powers of ten

Videos

The Known Universe created by the American Museum of Natural History

Template:Earth's ___location Template:Nature nav

[[Category:Environments]] [[Category:Universe]] [[af:Heelal]] [[ar:فضاء كوني]] [[an:Universo]] [[ast:Universu]] [[az:Kainat]] [[bn:মহাবিশ্ব]] [[zh-min-nan:Ú-tiū]] [[be:Сусвет]] [[be-x-old:Сусьвет]] [[bs:Svemir]] [[bg:Вселена]] [[ca:Univers]] [[cv:Çут Тĕнче]] [[cs:Vesmír]] [[cy:Bydysawd (seryddiaeth)]] [[da:Universet]] [[de:Universum]] [[nv:Yágháhookáán]] [[dsb:Uniwersum]] [[et:Universum]] [[el:Σύμπαν]] [[es:Universo]] [[eo:Universo]] [[eu:Unibertso]] [[fa:گیتی]] [[fr:Univers]] [[fy:Hielal]] [[gl:Universo]] [[gu:બ્રહ્માંડ]] [[hak:Yî-chhiu]] [[ko:우주]] [[hi:ब्रह्माण्ड]] [[hr:Svemir]] [[io:Universo]] [[id:Alam semesta]] [[ia:Universo]] [[is:Alheimurinn]] [[it:Universo]] [[he:היקום]] [[jv:Alam semesta]] [[kn:ಬ್ರಹ್ಮಾಂಡ]] [[pam:Sikluban]] [[ka:სამყარო]] [[csb:Swiatnica]] [[sw:Ulimwengu]] [[ku:Gerdûn]] [[la:Universum]] [[lv:Visums]] [[lt:Visata]] [[li:Universum]] [[lmo:Ünivers]] [[hu:Világegyetem]] [[mk:Вселена]] [[ml:പ്രപഞ്ചം]] [[mr:विश्व]] [[arz:كون]] [[ms:Alam semesta]] [[mwl:Ouniberso]] [[mn:Ертөнц]] [[my:စကြာဝဠာ]] [[nah:Cemānāhuac]] [[nl:Heelal]] [[nds-nl:Hielal]] [[ne:ब्रह्माण्ड]] [[ja:宇宙]] [[nap:Annevierzo]] [[no:Universet]] [[nn:Universet]] [[nrm:Eunivers]] [[nov:Universe]] [[oc:Univèrs]] [[uz:Olam]] [[pap:Universo]] [[nds:Weltruum]] [[pl:Wszechświat]] [[pt:Universo]] [[ksh:Weltall]] [[ro:Univers]] [[qu:Ch'askancha]] [[ru:Вселенная]] [[stq:Al]] [[sq:Gjithësia]] [[scn:Universu]] [[simple:Universe]] [[sk:Vesmír]] [[sl:Vesolje]] [[ckb:گەردوون]] [[sr:Свемир]] [[sh:Svemir]] [[su:Jagat]] [[fi:Maailmankaikkeus]] [[sv:Universum]] [[tl:Sanlibutan]] [[ta:அண்டம்]] [[tt:Космос]] [[te:విశ్వం]] [[th:เอกภพ]] [[tg:Коинот]] [[tr:Evren]] [[uk:Всесвіт]] [[ur:کائنات]] [[vi:Vũ trụ]] [[zh-classical:宇宙]] [[war:Sangkalibutan]] [[yi:אוניווערס]] [[zh-yue:宇宙]] [[bat-smg:Vėsata]] [[zh:宇宙]]

[einstein_1917-1] Errore nelle note: Errore nell'uso del marcatore <ref>: non è stato indicato alcun testo per il marcatore einstein_1917

[2] Rindler (1977), pp. 226–229.

[3] Landau and Lifshitz (1975), pp. 358–359.

[4] A Einstein, Zum kosmologischen Problem der allgemeinen Relativitätstheorie, in Sitzungsberichte der Preussischen Akademie der Wissenschaften, Physikalisch-mathematische Klasse, vol. 1931, 1931, pp. 235–237.
Einstein A., de Sitter W., On the relation between the expansion and the mean density of the universe, in Proceedings of the National Academy of Sciences, vol. 18, n. 3, 1932, pp. 213–214, DOI:10.1073/pnas.18.3.213.

[5] Hubble Telescope news release

[6] BBC News story: Evidence that dark energy is the cosmological constant

[7] Zel'dovich YB, Cosmological constant and elementary particles, in Zh. Eksp. & Teor. Fiz. Pis'ma, vol. 6, 1967, pp. 883–884. English translation in Sov. Phys. — JTEP Lett., 6, pp. 316–317 (1967).

[8] Friedmann A., Über die Krümmung des Raumes, in Zeitschrift für Physik, vol. 10, 1922, pp. 377–386, DOI:10.1007/BF01332580.

[EllisKS03-9] George F.R. Ellis, U. Kirchner, W.R. Stoeger, Multiverses and physical cosmology, in Monthly Notices of the Royal Astronomical Society, vol. 347, 2004, pp. 921–936, DOI:10.1111/j.1365-2966.2004.07261.x. URL consultato il 9 gennaio 2007.

[10] Munitz MK, One Universe or Many?, in Journal of the History of Ideas, vol. 12, n. 2, 1959, pp. 231–255, DOI:10.2307/2707516.

[11] Misner, Thorne and Wheeler (1973), p.753.

[chaotic_inflation-12] Linde A., Eternal chaotic inflation, in Mod. Phys. Lett., A1, 1986, p. 81.
Linde A., Eternally existing self-reproducing chaotic inflationary universe, in Phys. Lett., B175, 1986, pp. 395–400.

[13] Shape of the Universe, WMAP website at NASA.

[14] ttp://en.wikipedia.org/wiki/Homology_sphere#Cosmology

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]