Universe Overview

Universe comprises everything we perceive Perception to exist Existence physically, the entirety of space and time, all forms of matter and energy. The term Universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world world (philosophy) , or Nature.

The word Universe is usually defined as encompassing everything. However, using an alternative definition, some cosmologists have speculated that the "Universe" composed of expanding space-as-we-know-it, is just one of many disconnected "universes", which are collectively called the multiverse. For example, in the many-worlds hypothesis, new "universes" are spawned with every quantum measurement . These universes are usually thought to be completely disconnected from our own and therefore impossible to detect experimentally . Observations of older parts of the universe (which are far away) suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and history. However, in bubble universe theory, there may be an infinite variety of "universes" created in various ways, and perhaps each with different physical constants.

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

shows a diverse range of galaxies Galaxy , each consisting of billions of stars. The equivalent area of sky that the picture occupies is shown in the lower left corner. The smallest, reddest galaxies, about 100, are some of the most distant galaxies to have been imaged by an optical telescope, existing at the time shortly after the Big Bang.

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

Current interpretations of astronomical observations observable universe indicate that the age of the Universe is 13.73 (± 0.12) billion years, and that the diameter of the observable universe is at least 93 billion light years, or 8.80 Scientific Notation metres. According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. It is uncertain whether the size of the Universe is finite or infinite.


 
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The word Universe derives from the Old French word Univers, which in turn derives from the Latin word universum. The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English English language word is used. Lucretius used the word in the sense "everything rolled into one, everything combined into one".

showing that the Earth is not stationary, but rotates.

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

The most common term for "Universe" among the ancient Greek philosophers Greek philosophy from Pythagoras onwards was το παν (The All), defined as all matter (το ολον) and all space (το κενον). Other synonyms for the Universe among the ancient Greek philosophers included κοσμος (meaning the world world (philosophy) , the cosmos) and φυσις (meaning Nature, from which we derive the word physics). The same synonyms are found in Latin authors (totum, mundus, natura) 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 world (philosophy) (as in the many-worlds hypothesis), and Nature (as in natural laws or natural philosophy).


Broadest definition: reality and probability



The broadest definition of the Universe is found in De divisione naturae by the medieval Middle Ages 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 something not entirely dissimilar reappears in quantum physics, perhaps most obviously in the path-integral formulation path integral formulation of Feynman Richard Feynman . According to that formulation, the probability amplitudes 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 quantum measurement measurement in quantum mechanics , also known as the collapse of the wavefunction wavefunction collapse (but see the many-worlds hypothesis below in the Multiverse section). In this well-defined mathematical sense, even that which does not exist (all possible paths) can influence that which does finally exist (the experimental measurement). As a specific example, every electron is intrinsically identical to every other; therefore, probability amplitudes must be computed allowing for the possibility that they exchange positions, something known as exchange symmetry. This conception of the Universe embracing both the existent and the non-existent loosely parallels the Buddhist Buddhism doctrines of shunyata and interdependent development of reality pratitya-samutpada , and Gottfried Leibniz's more modern concepts of contingency and the identity of indiscernibles.


Definition as reality



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 laws that govern the first two. These elements will be discussed in greater detail below. A related definition of the term Universe is everything that exists at a single moment of cosmological time, such as the present, as in the sentence "The Universe is now bathed uniformly in microwave radiation cosmic microwave background radiation ".

The three elements of the Universe (spacetime, matter-energy, and physical law) correspond roughly to the ideas of Aristotle. In his book The Physics Physics (Aristotle) (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 laws 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.


Definition as connected space-time



It is possible to conceive of disconnected space-times, each existing but unable to interact with one another. An easily visualized metaphor is a group of separate soap bubbles, 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. In principle, the other unconnected universes may have different dimensionalities and topologies topology of space-time, different forms of matter and energy, and different physical laws and physical constants, although such possibilities are currently speculative.


Definition as observable reality



According to a still-more-restrictive definition, the Universe is everything within our connected space-time that could have a chance to interact with us and vice versa. According to the general theory of relativity 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 ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the Universe would live forever; space may expand faster than light can traverse it. It is worth emphasizing that those distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with them. The spatial region within which we can affect and be affected is denoted as the observable universe. Strictly speaking, the observable universe depends on the location of the observer. By traveling, an observer can come into contact with a greater region of space-time than an observer who remains still, so that the observable universe for the former is larger than for the latter. Nevertheless, even the most rapid traveler may not be able to interact with all of space. Typically, the observable universe is taken to mean the universe observable from our vantage point in the Milky Way Galaxy.The Universe is very large and possibly infinite in volume; the observable matter is spread over a space at least 93 billion light years across. Independent estimates (based on measurements such as radioactive dating) agree, although they are less precise, ranging from 11–20 billion years
to 13–15 billion years. The universe has not been the same at all times in its history; for example, the relative po***tions of quasars and galaxies have changed and space itself appears to have expanded metric expansion of space . 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 redshifted; the photons emitted have been stretched to longer wavelengths and lower frequency during their journey. The rate of this spatial expansion is accelerating accelerating universe , based on studies of Type Ia supernovae and corroborated by other data.

The relative fractions a***nce of the chemical elements of different chemical elements — particularly the lightest atoms such as hydrogen, deuterium and helium — seem to be identical throughout the universe and throughout its observable history. The universe seems to have much more matter than antimatter, an asymmetry possibly related to the observations of CP violation. The Universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. The Universe also appears to have neither net momentum nor angular momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the universe were finite.

s from which the Universe is constructed. Six leptons and six quarks comprise most of the matter; for example, the protons and neutrons of atomic nuclei atomic nucleus are composed of quarks, and the ubiquitous electron is a lepton. These particles interact via the gauge bosons shown in the middle row, each corresponding to a particular type of gauge symmetry. The Higgs boson (as yet unobserved) is believed to confer mass on the particles with which it is connected. The graviton, a supposed gauge boson for gravity, is not shown.

The Universe appears to have a smooth space-time continuum consisting of three spatial space dimensions and one temporal (time) dimension. On the average, space 3-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. Spacetime also appears to have a simply connected simply connected space topology, at least on the length-scale of the observable universe. However, present observations cannot exclude the possibilities that the universe has more dimensions and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroidal topologies of two-dimensional spaces.

The Universe appears to be governed throughout by the same physical laws and physical constants. According to the prevailing Standard Model of physics, all matter is composed of three generations of leptons and quarks, both of which are fermions. These elementary particles interact via at most three fundamental interactions: 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 renormalized renormalization quantum field theory, and are mediated by gauge bosons 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 constants appear to have throughout our Universe, such as Planck's constant h or the gravitational constant G. Several conservation laws have been identified, such as the conservation of charge, momentum conservation of momentum , angular momentum conservation of angular momentum and energy conservation of energy ; in many cases, these conservation laws can be related to symmetries symmetry or mathematical identities Bianchi identity .Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the then-available data and conceptions of the Universe. Historically, cosmologies and cosmogonies were based on narratives of gods acting in various ways. Theories of an impersonal Universe governed by physical laws were first proposed by the Greeks and Indians. Over the centuries, improvements in astronomical observations and theories of motion and gravitation led to ever more accurate descriptions of the Universe. The modern era of cosmology began with Albert Einstein's Albert Einstein 1915 general theory of relativity general relativity , which made it possible to quantitatively predict the origin, evolution, and conclusion of the Universe as a whole. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted Big Bang; however, still more careful measurements are required to determine which theory is correct.


Creation myths


ian account of the creatrix goddess Nammu, the precursor of the Assyrian goddess Tiamat; perhaps the earliest surviving creation myth.

Many cultures have stories describing the origin of the world creation myth , 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 Finnish people epic poem epic poetry Kalevala, the Chinese China story of Pangu or the Indian History of India Brahmanda Purana. In related stories, the creation is caused by a single entity emanating or producing something by his or herself, as in the Tibetan Buddhism concept of Adi-Buddha, the ancient Greek ancient Greece story of Gaia Gaia (mythology) (Mother Earth), the Aztec Aztec mythology goddess Coatlicue myth, the ancient Egyptian ancient Egyptian religion god Ennead Atum story, or the Genesis creation myth. In another type of story, the world is created from the union of male and female deities, as in the Maori story Maori mythology 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 Babylonian 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 divinity god , as in the ancient Egyptian story of Ptah or the Genesis creation myth as a part of Jewish Jewish mythology and Christian mythology. In other stories, the universe emanates from fundamental principles, such as Brahman and Prakrti, or the yin and yang of the Tao.


Philosophical models




From the 6th century BCE, the pre-Socratic Greek philosophers pre-Socratic philosophy 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 Water (classical element) . Following him, Anaximenes called it Air Air (classical element) , and posited that there must be attractive and repulsive forces 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 numbers, whereas Thales' student, Anaximander, proposed that everything was composed of a chaotic substance known as apeiron Apeiron (cosmology) , 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 atoms moving through empty space, a vacuum; Aristotle opposed this view ("Nature abhors a vacuum") on the grounds that resistance to motion Drag (physics) increases with density; hence, empty space should offer no resistance to motion, leading to the possibility of infinite speed.

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

he Indian philosopher Indian philosophy Kanada, founder of the Vaisheshika school, developed a theory of atomism and proposed that light and heat were varieties of the same substance.



Al-Razi disagreed with the Aristotelian Aristotelianism and Avicennian Avicennism view of a single universe. This disagreement arose from his affirmation of atomism, as advocated by the Ash'ari school of Islamic theology, which entails the existence of vacant space, or void, in which the atoms move, combine and separate.
The Greek astronomer Greek astronomy Aristarchus of Samos was the first known individual to propose a heliocentric Heliocentrism 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)


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.


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):


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]


The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was Seleucus of Seleucia, a Hellenized Hellenization Babylonian astronomer who lived a century after Aristarchus. 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 tides. According to Strabo (1.1.9), Seleucus was the first to state that the tides 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. Alternatively, he may have proved the heliocentric theory by determining the constants of a geometric Geometry 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. During the Middle Ages, heliocentric models may have also been proposed by the Indian astronomer Indian astronomy , Aryabhata, and by the Persian astronomers Islamic astronomy , Albumasar Ja'far ibn Muhammad Abu Ma'shar al-Balkhi and Al-Sijzi.

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 planets.

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.



As noted by Copernicus himself, the suggestion that the Earth rotates Earth's rotation 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). Aryabhata (476–550), Brahmagupta (598–668), Albumasar and Al-Sijzi, also proposed that the Earth rotates on its axis. The first empirical evidence Empirical research for the Earth's rotation on its axis, using the phenomenon of comets, was given by Tusi Nasīr al-Dīn al-Tūsī (1201–1274) and Ali Kuşçu (1403–1474). 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.

published the Rudolphine Tables containing a star catalog and planetary tables using Tycho Brahe's measurements.

Copernicus' heliocentric model Heliocentrism 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). 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 burned at the stake execution by burning in the Campo dei Fiori Campo de' Fiori in Rome on 17 February 1600. 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) and Jean-Philippe de Cheseaux (1744) 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. 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. One solution to these latter two paradoxes is the Charlier universe Carl Charlier , 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. A significant astronomical advance of the 18th century was the realization by Thomas Wright Thomas Wright (astronomer) , Immanuel Kant and others that stars are not distributed uniformly throughout space; rather, they are grouped into galaxies galaxy .

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. This theory and its implications will be discussed in more detail in the following section.space probe (artist's impression): radio signals sent between the Earth and the probe (green wave) are delayed Shapiro effect 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 weak nuclear force 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 metric tensor (general relativity) (Einstein's field equations Einstein 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), that corresponds to an energy density of empty space. Depending on its sign, the cosmological constant can either slow (negative Λ) or accelerate (positive Λ) the expansion of the universe metric expansion of space . Although many scientists, including Einstein, had speculated that Λ was zero, recent astronomical observations of type Ia supernovae have detected a large amount of "dark energy" that is accelerating the universe's expansion. Preliminary studies suggest that this dark energy corresponds to a positive Λ, although alternative theories cannot be ruled out as yet. Russian physicist physics Zel'dovich Yakov Borisovich 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. Evidence for such zero-point energy is observed in the Casimir effect.


Special relativity and space-time




The universe has at least three spatial space 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 special 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.


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 gμν, 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-k r^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 metric (mathematics) 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.

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.



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 3-sphere . 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 Asteroids (arcade game) ; a traveler crossing an outer "boundary" of space going outwards would reappear instantly at another point on the boundary moving inwards.

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 Future of an expanding universe 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 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.

of light atomic nuclei atomic nucleus observed throughout the universe.

Other experimental observations can be explained by combining the overall expansion of space with nuclear nuclear physics 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 Plasma (physics) of negative electrons, neutral neutrinos and positive nuclei. Nuclear reactions among the nuclei led to the present a***nces 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 particle physics . 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.of seven "bubble" universes bubble universe theory , which are separate spacetime continua, each having different physical laws, physical constants, and perhaps even different numbers of dimensions or topologies topology .

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. 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" parallel universe (fiction) , 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 metaphysics conception of alternate planes of consciousness plane (esotericism) , 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.

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 bubble universe theory model of the early universe. 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 Falsifiability ; no experimental test in one universe could reveal the existence or properties of another non-interacting universe.
 

 

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