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The Universe is everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and physical constants that govern them. In a well-defined, mathematical sense, the universe can even be said to contain that which does not exist; according to the path-integral formulation of quantum mechanics, even unrealized possibilities contribute to the probability amplitudes of events in the universe. The universe is sometimes denoted as the cosmos or Nature, as in "cosmology" or "natural philosophy".

Scientific experiments have yielded several general facts about the observable universe. The age of the universe is estimated to be 13.7±0.2 billion years. The universe is very large, possibly infinite, being at least 93 billion light years across, and consisting mainly of matter, rather than antimatter. Only 4% of the matter and energy in the universe is luminous, that is, directly observable from its emitted electromagnetic radiation ("light" in its most general sense); the remainder consists of dark energy (73%) and dark matter (23%). The nature and composition of dark energy and dark matter are unknown. The luminous matter within the universe is sparse and consists principally of galaxies, which are distributed uniformly when averaged over length-scales longer than 300 million light years; on smaller length scales, galaxies tend to clump into clusters, superclusters and even larger structures. The light arriving from distant galaxies is detectably redshifted, with the redshift increasing with the galaxy's distance from Earth. The universe is bathed in a microwave radiation that is highly isotropic (uniform across different directions), and corresponds to a blackbody spectrum of roughly 2.7 Kelvin. The relative percentages of the lighter chemical elements — especially hydrogen, deuterium and helium — is apparently the same throughout the universe. The universe is believed to be expanding, in the sense that space itself is enlarging with time; even objects initially at rest to one another will appear to fly apart as the space between them is enlarged. The universe has at least three spatial dimensions and one temporal (time) dimension, although extremely small additional dimensions cannot be ruled out experimentally; spacetime appears to be smoothly and simply connected, with very small curvature, so that Euclidean geometry is accurate on the average throughout the universe. The universe appears to be governed by the same physical laws and constants throughout its extent and history.

Throughout their recorded history, humans have proposed several cosmologies and cosmogonies to account for their observations of the universe. The earliest quantitative models were developed by the ancient Greeks, who proposed that the universe possessed infinite space and had existed eternally, but contained 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 careful astronomical observations and improved theories of gravity led to Copernicus' heliocentric model and, eventually, to the present Lambda-CDM model of the Big Bang, which accounts for all the available data. According to such Big Bang theories, everything in the universe — all forms of matter and energy, and even spacetime itself — came into being at a single event, a gravitational singularity; as space expanded with time, the matter and energy cooled sufficiently to allow the stable condensation of elementary particles into the primordial nuclei and atoms. Once atoms formed, matter became transparent to most wavelengths of electromagnetic radiation; the ambient microwave radiation observed today is the residual radiation that decoupled from the matter.

According to the prevailing scientific models, the Universe is governed by the Standard Model of physics (which governs various forms of matter and fields), as well as special and general relativity (which govern spacetime and its interaction with matter and fields). On cosmological length scales, the dominant interaction in the universe is gravitation. Hence, the theory of general relativity (the most accurate description of gravity presently available) offers the best predictions for the overall development of the universe, including its origin, expansion (which mainly accounts for the observed redshift), large-scale structure and ultimate fate. However, components of the Standard Model — particularly those relating to nucleosynthesis, atomic spectra, and CP violation — are needed to account for other experimental observations, such as the distribution of chemical elements and microwave radiation throughout the universe, as well as the matter-antimatter asymmetry.

According to some speculations, this universe may be one of many disconnected universes, which are collectively denoted as the multiverse. In one theory, there is an infinite variety of universes, each with different values of the physical constants. In another theory, new universes are spawned with every quantum measurement. However, these speculations cannot be tested experimentally since, by definition, other universes cannot interact with our own.

Etymology, synonyms and definitions

The word universe derives from the Old French word univers, which in turn derives from the Latin word universum.^[1] The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.^[2] The Latin word derives from the poetic contraction unvorsum — first used by Lucretius in Book IV (line 262) of his masterpiece De rerum natura (On the Nature of Things) — which connects un, uni (the combining form of unus, or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning "something turned, rotated, rolled, changed").^[2] Lucretius used the word in the sense "everything rolled into one, everything combined into one".

An alternative interpretation of unvorsum is "everything turned as one" or "everything turned 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.^[3] This Greek word refers to an early Greek model of the universe, in which all matter was contained within rotating, concentric spheres; according to Aristotle, the rotation of the outermost sphere was responsible for the motion and change of everything within.

The most common term for "universe" among the ancient Greek philosophers from Pythagoras onwards was το παν (The All), defined as all matter (το ολον) and all space (το κενον).^[4]^[5] Other synonyms for the universe among the ancient Greek philosophers included κοσμος (meaning the world, the cosmos) and φυσις (meaning Nature, from which we derive the word physics).^[6] The same synonyms are found in Latin authors (totum, mundus, natura)^[7] 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 (as in the many-worlds hypothesis), and Nature (as in natural laws or natural philosophy).^[8]

Broadest definition: reality and probability

The broadest definition of the universe is found in De divisione naturae by the medieval philosopher Johannes Scotus Eriugena, who defined it as simply everything: everything that exists and everything that does not exist. Time is not considered in Eriugena's definition; thus, his definition includes everything that exists, has existed and will exist, as well as everything that does not exist, has never existed and will never exist. This all-embracing definition was not adopted by most later philosophers, but it is relevant in quantum physics, particularly the path-integral formulation of Feynman.^[9] 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, also known as the collapse of the wavefunction (but see the many-worlds hypothesis below in the Multiverse section). In this well-defined mathematical sense, even that which does not exist (all possible paths) can influence that which does finally exist (the experimental measurement). As a specific example, every electron is intrinsically identical to every other; therefore, probability amplitudes must be computed allowing for the possibility that they exchange positions, something known as exchange symmetry. This conception of the universe embracing both the existent and the non-existent is loosely related to the Buddhist doctrines of shunyata and interdependent development of reality, and to 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 "universe" is everything that exists at a single moment of time, such as the present, as in the sentence "The universe is now bathed uniformly in microwave 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 (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.^[10] In principle, the other unconnected universes may have different dimensionalities and topologies of space-time, different forms of matter and energy, and different physical laws and physical constants.

Definition as observable reality

According to a still more restrictive definition, the universe is everything within our connected space-time that could ever interact with us and vice versa. According to the theory of general relativity, some regions of space may never interact with ours even in the lifetime of the universe, due to the finite speed of light and the expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the universe lives forever; space may expand faster than light can cover it. It is worth emphasizing that those distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with them. The spatial region within which we can affect and be affected is denoted as the observable universe. Strictly speaking, the observable universe depends on the observer. By traveling, an observer can come into contact with a greater region of space-time than an observer who remains still, so that the observable universe for the former is larger than for the latter; nevertheless, even the most rapid traveler may not be able to interact with all of space. Typically, the observable universe is taken to mean the universe observable from a stationary observer on Earth.

Scientific observations

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.^[11] For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years.^[12] As an example, our Milky Way galaxy is roughly 100,000 light years in diameter,^[13] and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away.^[14]

The universe is mostly composed of dark energy and dark matter, both of which are poorly understood at present. Only ≈4% of the universe is ordinary matter, a relatively small perturbation.

The observable matter is spread uniformly (homogeneously) throughout the universe, when averaged over distances longer than 300 million light-years.^[15] However, on smaller length-scales, matter is observed to form "clumps", i.e., to cluster hierarchically; many atoms are condensed into stars, most stars into galaxies, most galaxies into clusters, superclusters and, finally, the largest-scale structures such as the Great Wall of galaxies. The observable matter of the universe is also spread isotropically, meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content.^[16] The universe is also bathed in a highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725 Kelvin.^[17] The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle,^[18] which is supported by astronomical observations.

The present overall density of the universe is very low, roughly 9.9 × 10^-30 grams per cubic centimetre. This mass-energy appears to consist of 73% dark energy, 23% cold dark matter and 4% ordinary matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume.^[19] The properties of dark energy and dark matter are largely unknown. Dark matter gravitates as ordinary matter, and thus works to slow the expansion of the universe; by contrast, dark energy accelerates its expansion.

The universe is old and evolving. The most precise estimate of the universe's age is 13.7±0.2 billion years old, based on observations of the cosmic microwave background radiation.^[20] Independent estimates (based on measurements such as radioactive dating) agree, although they are less precise, ranging from 11-20 billions years^[21] to 13–15 billion years.^[22] The universe has not been the same at all times in its history; for example, the relative populations of quasars and galaxies have changed and space itself appears to have expanded. This expansion accounts for how Earth-bound scientists can observe the light from a galaxy 40 billion light years away, even if that light has traveled for only 13.7 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, based on studies of Type Ia supernovae and corroborated by other data.

The relative fractions 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.^[23] The universe seems to have much more matter than antimatter, an asymmetry possibly related to the observations of CP violation.^[24] The universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. The universe appears to have no net momentum and angular momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the universe were finite.^[25]

The universe appears to have a smooth spacetime continuum consisting of three spatial dimensions and one temporal (time) dimension. On the average, spacetime 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.^[26] Spacetime also appears to have a simply connected topology, at least on the length-scale of the observable universe. However, present observations cannot exclude the possibilities that the universe has more dimensions and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroidal topologies of two-dimensional spaces.^[27]

The universe appears to be governed throughout by the same physical laws and physical constants.^[28] 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 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, angular momentum and energy; in many cases, these conservation laws can be related to symmetries or mathematical identities.

Historical models

Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the then available data and conceptions of the universe.

Creation stories

Many cultures have stories describing the creation of the world, which may be roughly grouped into common types. In one type of story, the world is born from a world egg; such stories include the Finnish epic poem Kalevala, the Chinese story of Pangu or the Indian Brahmanda Purana. In related stories, the creation is caused by a single god emanating or producing something by themselves, as in Buddhist concept of Adi-Buddha, the ancient Greek story of Gaia (Mother Earth), the Aztec goddess Coatlicue or the ancient Egyptian god Atum. In another type of story, the world is created from the union of male and female deities, as in the Maori story of Rangi and Papa. In other stories, the universe is created by crafting it from pre-existing materials, such as the corpse of a dead god - as from Tiamat in the 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, as in the ancient Egyptian story of Ptah or the Biblical account in Genesis. In other stories, the universe is emanates from fundamental principles, such as Brahman and Prakrti, or the yin and yang of the Tao.

Philosophical models

The first philosophical models of the universe were developed by the pre-Socratic philosophers. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the apparently different materials of the world (wood, metal, etc.) are all different forms of a single material, the arche. The first to do so was Thales, who called this material Water. Following him, Anaximenes called it Air, 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, 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 is 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 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 rough contemporary Parmenides made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides denoted this reality as το εν (The One). Parmenides' theory seemed implausible to many Greeks, but his student Zeno of Elea challenged them with several famous paradoxes. Aristotle resolved these paradoxes by developing the notion of an infinitely divisible continuum, and applying it to space and time.

Astronomical models

Hand-colored version of the Flammarion woodcut, depicting the Aristotelian conception of the universe that preceded the models of Copernicus and Thomas Digges.

More practical Greek philosophers were concerned with developing models of the universe that would account for the observed motion of the stars and planets. The first coherent model was proposed by Eudoxus of Cnidos. According to this model, space and time are infinite and eternal, but the distribution of matter is confined to certain rotating concentric spheres. This model was refined by Callippus and Aristotle, and brought to its highest level by Ptolemy. The success of this model is due to the mathematical fact that any function (such as the position of a planet) can be decomposed into a set of Fourier modes.

Model of the 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 planets.

The Aristotelian model was accepted for roughly two millennia, until Copernicus proposed that the astronomical data could be explained better if the sun were placed at the center of the universe, rather than the earth.

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?
— Copernicus, Chapter 10, Book 1

This heliocentric model allowed the stars to be placed uniformly through the (infinite) space surrounding the planets, as first proposed by Thomas Digges and considered by Isaac Newton.

The Newtonian cosmology, however, 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 existed for an infinite time; however, since stars are constantly radiating energy, a finite star seems inconsistent with the radiation of infinite energy. Secondly, Jean-Philippe de Cheseaux noted that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nightime sky would be as bright as the sun itself;^[29] this became known as Olber's paradox in the 19th century.^[30] 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. This instability was clarified by the Jeans instability criterion. One solution to these latter two paradoxes is the 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.^[31]

The modern era of physical cosmology began in 1917, when Albert Einstein first applied his theory of general relativity to model the structure and dynamics of the universe.^[32] This theory and its implications will be discussed in more detail in the following section.

Theoretical models

Of the four fundamental interactions, gravity 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. The effects of the two nuclear interactions, the weak and strong nuclear forces, are mainly confined to sub-atomic length scales, because their strengths decrease very rapidly with distance. In principle, electromagnetism could be significant, being equally as long-ranged as gravity; however, the effects of positive and negative charges tend to cancel one another, leaving only a small residue. By contrast, all matter and energy gravitate, causing gravity's effects to be cumulative.

General relativity

Given gravity's predominance in shaping cosmological structures, accurate predictions of the universe's past and future require an accurate theory of gravitation. The best available theory is general relativity, which has passed all experimental tests hitherto. However, such experiments have not been carried out on cosmological length scales so, in principle, general relativity might be inaccurate. Nevertheless, its cosmological predictions appear to be consistent with observations, so there is no compelling reason to adopt another theory.

General relativity consists of a set of equations (the 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, the various galaxies making up the universe are replaced by a fine uniform dust distributed equally throughout the universe. Assuming the cosmological principle makes it easy to solve the Einstein equations and predict the past and future of the universe on cosmological time scales.

Einstein's field equations include a cosmological constant Λ,^[32]^[33] that corresponds to an energy density of empty space.^[34] 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,^[35] recent astronomical observations of type Ia supernovae have detected a large amount of "dark energy" that is accelerating the universe's expansion.^[36] Preliminary studies suggest that this dark energy corresponds to a positive Λ, although competing theories cannot be ruled out as yet.^[37] 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.^[38]

Special relativity and space-time

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 theory of special relativity, spatial and temporal separations are interconvertible (within limits) by changing one's reference frame.

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 will differ, although they naturally give the same length

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

Thus, the coordinates differences are not intrinsic to the rod, but reflect the reference frame used to describe it; by contrast, the length l is intrinsic. The coordinate differences can be changed freely without affecting the rod, merely 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. Thus, by changing one's reference frame, one can change a spatial and time separation (L₁, Δt₁) into another (L₂, Δt₂), 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.

Solving Einstein's equations

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, 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 zero, one 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.^[39]

The solutions for R(t) depend on k and Λ, but some qualitative features of such solutions are general. First and most importantly, the length scale R of the universe can remain constant only if the universe is perfectly isotropic with positive curvature (k=1) and has one precise value of density everywhere, as first noted by Albert Einstein. However, this equilibrium is unstable and since the universe is known to be inhomogeneous on smaller scales, R must change, according to general relativity. When R changes, all the spatial distances in the universe change in tandem; there is an overall expansion or contraction of space itself. The accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point 13.7 billion years ago and never moved faster than the speed of light.

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 seems based on the assumptions of homogeneity and isotropy (the cosmological principle) and that only the gravitational interaction matters. However, the Penrose-Hawking singularity theorems show that a singularity should exist for very general conditions. According to the equations, R should grow 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.

Die Unbegrenztheit des Raumes besitzt...eine gröszere empirische Gewiszheit als irgend eine äuszere Erfahrung. Hieraus folgt aber die Unendlichkeit keineswegs... [Space has no boundary - that is empirically more certain than any external observation. However, that does not imply that space is infinite...]

Bernhard Riemann (Habilitationsvortrag, 1854)

Third, the curvature index k determines the sign of the mean 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 in 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 traveller crossing an outer "boundary" of space going outwards would reappear instantly at another point on the boundary moving inwards.

The ultimate fate of the universe is still unknown, since it depends critically on the curvature index k and the cosmological constant Λ. If the universe is sufficiently dense, k equals +1, meaning that its average curvature throughout is positive and the universe will eventually recollapse in a Big Crunch, possibly starting a new universe in a Big Bounce. Conversely, if the universe is insufficiently dense, k equals 0 or -1 and the universe will expand forever, cooling off and eventually becoming inhospitable for all life, as the stars die and all matter coalesces into black holes (the Big Freeze and the heat death of the universe). As noted above, recent data suggests that the expansion of the universe is not decreasing as originally expected, but accelerating; if this continues indefinitely, the universe will eventually rip itself to shreds (the Big Rip). Experimentally, the universe has an overall density that is very close to the critical value between recollapse and eternal expansion; more careful astronomical observations are needed to decide the question.

Prevailing 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.

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 other, 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.

Alternative cosmologies

Despite its experimental verification, some scientists find the theory of general relativity implausible and have suggested alternatives. Such theories can only be considered scientific if they offer testable predictions that differ from those of general relativity. The main scientific alternative is the Brans-Dicke theory, which augments general relativity with a scalar field that determines the local value of the gravitational constant G.^[40]^[41] Other, more radical suggestions include the variable G cosmologies (in which the universe's physical constants vary with the age or size of the universe),^[42] the tired light hypothesis of Fritz Zwicky,^[43] and the plasma cosmology theory.^[44] The validity of most such theories seems unlikely, given the available data.^[45]^[46]

The steady-state theory of cosmology was popular in the 1950s; according to this theory, the universe has been the same at all times, as well as being homogeneous and isotropic (the perfect cosmological principle).^[47] This theory predicts that the conservation of energy is violated, albeit very slightly. This theory made admirably precise predictions, which have since been disproven by experimental observations.^[48]

Multiverse

Some speculative theories have proposed that this universe is but one of a set of disconnected universes, collectively denoted as the multiverse.^[10]^[49] 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 a loose usage of the term "universe". The disconnected universes should also be distinguished from the metaphysical conception of alternate planes of consciousness, which are not thought to be physical places.

There are two scientific senses in which multiple universes can occur. 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.^[50] Second, according to the many-worlds hypothesis, a parallel universe is born with every quantum measurement; the universe "forks" into parallel copies, each one corresponding to a different outcome of the quantum measurement. Authors have explored this concept in some fiction, most notably Jorge Borges' short story The Garden of Forking Paths. However, both senses of the term "multiverse" are speculative and may be considered unscientific; the fact that universes cannot interact makes it impossible to test experimentally in this universe whether another universe exists.

Notes and references

^ The Compact Edition of the Oxford English Dictionary, volume II, Oxford:Oxford University Press, 1971, p. 3518
^ ^a ^b Lewis and Short, A Latin Dictionary, Oxford University Press, ISBN 0-19-864201-6, pp. 1933, 1977–1978
^ Lidell and Scott, A Greek-English Lexicon, Oxford University Press, ISBN 0-19-864214-8, p.1392
^ Liddell and Scott, pp.1345–1346
^ Yonge, Charles Duke (1870). An English-greek lexicon. new York: American Bok Company. pp. p. 567. {{cite book}}: |pages= has extra text (help)
^ Liddell and Scott, pp.985, 1964.
^ Lewis and Short, pp. 1881–1882, 1175, 1189–1190
^ OED, pp. 909, 569, 3821–3822, 1900.
^ Feynman RP, Hibbs AR (1965). Quantum Physics and Path Integrals. New York: McGraw-Hill. ISBN 0-07-020650-3.
Zinn Justin J (2004). Path Integrals in Quantum Mechanics. Oxford University Press. ISBN 0-19-856674-3.
^ ^a ^b Ellis, George F.R. (2004). "Multiverses and physical cosmology". Monthly Notices of the Royal Astronomical Society. 347: 921–936. Retrieved 2007-01-09. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Lineweaver, Charles (2005). "Misconceptions about the Big Bang". Scientific American. Retrieved 2007-03-05. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Rindler (1977), p. 196.
^ Christian, Eric; Samar, Safi-Harb. "How large is the Milky Way?". Retrieved 2007-11-28.
^ I. Ribas, C. Jordi, F. Vilardell, E.L. Fitzpatrick, R.W. Hilditch, F. Edward (2005). "First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy". Astrophysical Journal. 635: L37–L40.{{cite journal}}: CS1 maint: multiple names: authors list (link)
McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N. (2005). "Distances and metallicities for 17 Local Group galaxies". Monthly Notices of the Royal Astronomical Society. 356 (4): 979–997.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ N. Mandolesi, P. Calzolari, S. Cortiglioni, F. Delpino, G. Sironi (1986). "Large-scale homogeneity of the Universe measured by the microwave background". Letters to Nature. 319: 751–753.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Hinshaw, Gary (November 29, 2006). "New Three Year Results on the Oldest Light in the Universe". NASA WMAP. Retrieved 2006-08-10.
^ Hinshaw, Gary (December 15, 2005). "Tests of the Big Bang: The CMB". NASA WMAP. Retrieved 2007-01-09.
^ Rindler (1977), p. 202.
^ Hinshaw, Gary (February 10, 2006). "What is the Universe Made Of?". NASA WMAP. Retrieved 2007-01-04.
^ "The Age of the Universe with New Accuracy". Retrieved 2006-12-29.
^ Britt RR (2003-01-03). "Age of Universe Revised, Again". space.com. Retrieved 2007-01-08. {{cite web}}: Check date values in: |date= (help)
^ Wright EL (2005). "Age of the Universe". UCLA. Retrieved 2007-01-08.
Krauss LM, Chaboyer B (3 January 2003). "Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology". Science. 299 (5603). American Association for the Advancement of Science: 65–69. Retrieved 2007-01-08. {{cite journal}}: Check date values in: |date= (help)
^ Wright, Edward L. (September 12, 2004). "Big Bang Nucleosynthesis". UCLA. Retrieved 2007-01-05.
M. Harwit, M. Spaans (2003). "Chemical Composition of the Early Universe". The Astrophysical Journal. 589 (1): 53–57.
C. Kobulnicky, E. D. Skillman (1997). "Chemical Composition of the Early Universe". Bulletin of the American Astronomical Society. 29: 1329.
^ "Antimatter". Particle Physics and Astronomy Research Council. October 28, 2003. Retrieved 2006-08-10.
^ Landau and Lifshitz (1975), p. 361.
^ http://map.gsfc.nasa.gov/m_mm/mr_content.html
^ Luminet, Jean-Pierre (1999). "Topology of the Universe: Theory and Observations". Proceedings of Cosmology School held at Cargese, Corsica, August 1998. Retrieved 2007-01-05. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
Luminet, Jean-Pierre (2003). "Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background". Nature. 425: 593. Retrieved 2007-01-09. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Strobel, Nick (May 23, 2001). "The Composition of Stars". Astronomy Notes. Retrieved 2007-01-04.
"Have physical constants changed with time?". Astrophysics (Astronomy Frequently Asked Questions). Retrieved 2007-01-04.
^ de Cheseaux JPL (1744). Traité de la Comète. Lausanne. pp. pp. 223ff. {{cite book}}: |pages= has extra text (help). Reprinted as Appendix II in Dickson FP (1969). The Bowl of Night: The Physical Universe and Scientific Thought. Cambridge, MA: M.I.T. Press. ISBN 978-0262540032.
^ Olbers HWM (1826). "Unknown title". Bode's Jahrbuch. 111.. Reprinted as Appendix I in Dickson FP (1969). The Bowl of Night: The Physical Universe and Scientific Thought. Cambridge, MA: M.I.T. Press. ISBN 978-0262540032.
^ Rindler, p. 196.
^ ^a ^b Einstein, A (1917). "Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie". Preussische Akademie der Wissenschaften, Sitzungsberichte. 1917 (part 1): 142–152.
^ Rindler (1977), pp. 226–229.
^ Landau and Lifshitz (1975), pp. 358–359.
^ Einstein, A (1931). "Zum kosmologischen Problem der allgemeinen Relativitätstheorie". Sitzungsberichte der Preussischen Akademie der Wissenschaften, Physikalisch-mathematische Klasse. 1931: 235–237.
Einstein A, de Sitter W (1932). "On the relation between the expansion and the mean density of the universe". Proceedings of the National Academy of Sciences. 18: 213–214.
^ Hubble Telescope news release
^ BBC News story: Evidence that dark energy is the cosmological constant
^ Zel'dovich YB (1967). "Cosmological constant and elementary particles". Zh. Eksp. & Teor. Fiz. Pis'ma. 6: 883–884. English translation in Sov. Phys. — JTEP Lett., 6, pp. 316–317 (1967).
^ Friedmann A (1922). "Über die Krümmung des Raumes". Zeitschrift für Physik. 10: 377–386.
^ Brans, C. H. (November 1 1961). "Mach's Principle and a Relativistic Theory of Gravitation". Physical Review. 124 (3): 925–935. doi:10.1103/PhysRev.124.925. Retrieved 2006-09-23. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Weinberg (1972), pp. 157–160, 622–631.
^ Weinberg (1972), pp. 619–631.
^ {{cite journal | author = [[Fritz Zwicky|Zwicky F | date = 1929 | title = On the Red Shift of Spectral Lines through Interstellar Space | journal = Proceedings of the National Academy of Science | volume = 15 | pages = 773–779}} Abstract (ADS) Full article (PDF)
^ Hannes Alfvén, "On hierarchical cosmology" (1983) Astrophysics and Space Science (ISSN 0004-640X), vol. 89, no. 2, Jan. 1983, p. 313-324.
^ Weinberg (1972), pp. 611–633.
^ Ned Wright; Errors in Tired Light Cosmology (2005)
^ Bondi H, Gold T (1948). "The Steady State Theory of the Expanding Universe". Monthly Notices of the Royal Astronomical Society. 108: 252–270.
Hoyle F (1948). "A New Model for the Expanding Universe". Monthly Notices of the Royal Astronomical Society. 108: 372–382.
^ Weinberg (1972), pp. 459–464, 616–620.
^ Munitz MK (1959). "One Universe or Many?". Journal of the History of Ideas. 12: 231–255.
^ Linde A (1986). "Eternal chaotic inflation". Mod. Phys. Lett. A1: 81.
Linde A (1986). "Eternally existing self-reproducing chaotic inflationary universe". Phys. Lett. B175: 395–400.

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Stephen Hawking's Universe - why is the universe the way it is?
Richard Powell: An Atlas of the Universe - images at various scales, with explanations.
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My So-Called Universe arguments for and against an infinite and parallel universes
Parallel Universes by Max Tegmark
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Universe - Space Information Centre by Exploreuniverse.com
Size of the Universe at Space.Com
Illustration comparing the sizes of the planets, the sun, and other stars
Cosmology FAQ

Template:Earth location

[1] The Compact Edition of the Oxford English Dictionary, volume II, Oxford:Oxford University Press, 1971, p. 3518

[lewis_short-2] Lewis and Short, A Latin Dictionary, Oxford University Press, ISBN 0-19-864201-6, pp. 1933, 1977–1978

[3] Lidell and Scott, A Greek-English Lexicon, Oxford University Press, ISBN 0-19-864214-8, p.1392

[4] Liddell and Scott, pp.1345–1346

[5] Yonge, Charles Duke (1870). An English-greek lexicon. new York: American Bok Company. pp. p. 567. {{cite book}}: |pages= has extra text (help)

[6] Liddell and Scott, pp.985, 1964.

[7] Lewis and Short, pp. 1881–1882, 1175, 1189–1190

[8] OED, pp. 909, 569, 3821–3822, 1900.

[path_integral-9] Feynman RP, Hibbs AR (1965). Quantum Physics and Path Integrals. New York: McGraw-Hill. ISBN 0-07-020650-3.
Zinn Justin J (2004). Path Integrals in Quantum Mechanics. Oxford University Press. ISBN 0-19-856674-3.

[EllisKS03-10] Ellis, George F.R. (2004). "Multiverses and physical cosmology". Monthly Notices of the Royal Astronomical Society. 347: 921–936. Retrieved 2007-01-09. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[11] Lineweaver, Charles (2005). "Misconceptions about the Big Bang". Scientific American. Retrieved 2007-03-05. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[12] Rindler (1977), p. 196.

[13] Christian, Eric; Samar, Safi-Harb. "How large is the Milky Way?". Retrieved 2007-11-28.

[14] I. Ribas, C. Jordi, F. Vilardell, E.L. Fitzpatrick, R.W. Hilditch, F. Edward (2005). "First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy". Astrophysical Journal. 635: L37–L40.{{cite journal}}: CS1 maint: multiple names: authors list (link)
McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N. (2005). "Distances and metallicities for 17 Local Group galaxies". Monthly Notices of the Royal Astronomical Society. 356 (4): 979–997.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[15] N. Mandolesi, P. Calzolari, S. Cortiglioni, F. Delpino, G. Sironi (1986). "Large-scale homogeneity of the Universe measured by the microwave background". Letters to Nature. 319: 751–753.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[16] Hinshaw, Gary (November 29, 2006). "New Three Year Results on the Oldest Light in the Universe". NASA WMAP. Retrieved 2006-08-10.

[17] Hinshaw, Gary (December 15, 2005). "Tests of the Big Bang: The CMB". NASA WMAP. Retrieved 2007-01-09.

[18] Rindler (1977), p. 202.

[19] Hinshaw, Gary (February 10, 2006). "What is the Universe Made Of?". NASA WMAP. Retrieved 2007-01-04.

[NASA_age-20] "The Age of the Universe with New Accuracy". Retrieved 2006-12-29.

[21] Britt RR (2003-01-03). "Age of Universe Revised, Again". space.com. Retrieved 2007-01-08. {{cite web}}: Check date values in: |date= (help)

[22] Wright EL (2005). "Age of the Universe". UCLA. Retrieved 2007-01-08.
Krauss LM, Chaboyer B (3 January 2003). "Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology". Science. 299 (5603). American Association for the Advancement of Science: 65–69. Retrieved 2007-01-08. {{cite journal}}: Check date values in: |date= (help)

[23] Wright, Edward L. (September 12, 2004). "Big Bang Nucleosynthesis". UCLA. Retrieved 2007-01-05.
M. Harwit, M. Spaans (2003). "Chemical Composition of the Early Universe". The Astrophysical Journal. 589 (1): 53–57.
C. Kobulnicky, E. D. Skillman (1997). "Chemical Composition of the Early Universe". Bulletin of the American Astronomical Society. 29: 1329.

[24] "Antimatter". Particle Physics and Astronomy Research Council. October 28, 2003. Retrieved 2006-08-10.

[25] Landau and Lifshitz (1975), p. 361.

[Shape-26] ttp://map.gsfc.nasa.gov/m_mm/mr_content.html

[_spacetime_topology-27] Luminet, Jean-Pierre (1999). "Topology of the Universe: Theory and Observations". Proceedings of Cosmology School held at Cargese, Corsica, August 1998. Retrieved 2007-01-05. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
Luminet, Jean-Pierre (2003). "Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background". Nature. 425: 593. Retrieved 2007-01-09. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[28] Strobel, Nick (May 23, 2001). "The Composition of Stars". Astronomy Notes. Retrieved 2007-01-04.
"Have physical constants changed with time?". Astrophysics (Astronomy Frequently Asked Questions). Retrieved 2007-01-04.

[29] Cheseaux JPL (1744). Traité de la Comète. Lausanne. pp. pp. 223ff. {{cite book}}: |pages= has extra text (help). Reprinted as Appendix II in Dickson FP (1969). The Bowl of Night: The Physical Universe and Scientific Thought. Cambridge, MA: M.I.T. Press. ISBN 978-0262540032.

[30] Olbers HWM (1826). "Unknown title". Bode's Jahrbuch. 111.. Reprinted as Appendix I in Dickson FP (1969). The Bowl of Night: The Physical Universe and Scientific Thought. Cambridge, MA: M.I.T. Press. ISBN 978-0262540032.

[31] Rindler, p. 196.

[einstein_1917-32] Einstein, A (1917). "Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie". Preussische Akademie der Wissenschaften, Sitzungsberichte. 1917 (part 1): 142–152.

[33] Rindler (1977), pp. 226–229.

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

[35] Einstein, A (1931). "Zum kosmologischen Problem der allgemeinen Relativitätstheorie". Sitzungsberichte der Preussischen Akademie der Wissenschaften, Physikalisch-mathematische Klasse. 1931: 235–237.
Einstein A, de Sitter W (1932). "On the relation between the expansion and the mean density of the universe". Proceedings of the National Academy of Sciences. 18: 213–214.

[36] Hubble Telescope news release

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

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

[39] Friedmann A (1922). "Über die Krümmung des Raumes". Zeitschrift für Physik. 10: 377–386.

[40] Brans, C. H. (November 1 1961). "Mach's Principle and a Relativistic Theory of Gravitation". Physical Review. 124 (3): 925–935. doi:10.1103/PhysRev.124.925. Retrieved 2006-09-23. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[41] Weinberg (1972), pp. 157–160, 622–631.

[42] Weinberg (1972), pp. 619–631.

[43] {{cite journal | author = [[Fritz Zwicky|Zwicky F | date = 1929 | title = On the Red Shift of Spectral Lines through Interstellar Space | journal = Proceedings of the National Academy of Science | volume = 15 | pages = 773–779}} Abstract (ADS) Full article (PDF)

[scaling-44] Hannes Alfvén, "On hierarchical cosmology" (1983) Astrophysics and Space Science (ISSN 0004-640X), vol. 89, no. 2, Jan. 1983, p. 313-324.

[45] Weinberg (1972), pp. 611–633.

[46] Ned Wright; Errors in Tired Light Cosmology (2005)

[47] Bondi H, Gold T (1948). "The Steady State Theory of the Expanding Universe". Monthly Notices of the Royal Astronomical Society. 108: 252–270.
Hoyle F (1948). "A New Model for the Expanding Universe". Monthly Notices of the Royal Astronomical Society. 108: 372–382.

[48] Weinberg (1972), pp. 459–464, 616–620.

[49] Munitz MK (1959). "One Universe or Many?". Journal of the History of Ideas. 12: 231–255.

[chaotic_inflation-50] Linde A (1986). "Eternal chaotic inflation". Mod. Phys. Lett. A1: 81.
Linde A (1986). "Eternally existing self-reproducing chaotic inflationary universe". Phys. Lett. B175: 395–400.

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@@ Line 2: / Line 2: @@
 [[Image:Hubble ultra deep field.jpg|thumb|right|350px|The [[Hubble Ultra Deep Field]] image of a small region of the sky, near the [[constellation]] [[Fornax]].   The light from the smallest, most [[redshift]]ed galaxies originated roughly 13 billion years ago.]]
-The '''Universe''' is everything that exists: the entirety of [[space]] and [[time]], all forms of [[matter]], [[energy]] and [[momentum]], and the [[physical law]]s and [[physical constant]]s that govern them.  In a well-defined, mathematical sense, the universe can even be said to contain that which does not exist; according to the [[path integral formulation|path-integral formulation]] of [[quantum mechanics]], even unrealized possibilities contribute to the [[probability amplitude]]s of events in the universe.  The universe is sometimes denoted as the ''[[cosmos]]'' or ''[[Nature]]'', as in "[[cosmology]]" or "[[natural philosophy]]".
+The '''Universe''' is everything that physically exists: the entirety of [[space]] and [[time]], all forms of [[matter]], [[energy]] and [[momentum]], and the [[physical law]]s and [[physical constant]]s that govern them.  In a well-defined, mathematical sense, the universe can even be said to contain that which does not exist; according to the [[path integral formulation|path-integral formulation]] of [[quantum mechanics]], even unrealized possibilities contribute to the [[probability amplitude]]s of events in the universe.  The universe is sometimes denoted as the ''[[cosmos]]'' or ''[[Nature]]'', as in "[[cosmology]]" or "[[natural philosophy]]".
 Scientific experiments have yielded several general facts about the [[observable universe]].  The [[age of the universe]] is estimated to be 13.7±0.2 billion years.  The universe is very large, possibly infinite, being at least 93 billion [[light year]]s across, and consisting mainly of [[matter]], rather than [[antimatter]].  Only 4% of the matter and energy in the universe is luminous, that is, directly observable from its emitted [[electromagnetic radiation]] ("light" in its most general sense); the remainder consists of [[dark energy]] (73%) and [[dark matter]] (23%). The nature and composition of dark energy and dark matter are unknown.  The luminous matter within the universe is sparse and consists principally of [[galaxy|galaxies]], which are distributed uniformly when averaged over length-scales longer than 300 million light years; on smaller length scales, galaxies tend to clump into [[galaxy groups and clusters|clusters, superclusters]] and even [[large-scale structure of the universe|larger structures]].  The light arriving from distant galaxies is detectably [[redshift]]ed, with the redshift increasing with the galaxy's distance from Earth.  [[Cosmic microwave background radiation|The universe is bathed]] in a [[microwave]] [[electromagnetic radiation|radiation]] that is highly isotropic (uniform across different directions), and  corresponds to a [[blackbody spectrum]] of roughly 2.7 [[Kelvin]].   The relative percentages of the lighter [[chemical element]]s — especially [[hydrogen]], [[deuterium]] and [[helium]] — is apparently the same throughout the universe.  The universe is believed to be expanding, in the sense that [[space]] itself is enlarging with time; even objects initially at rest to one another will appear to fly apart as the space between them is enlarged.  The universe has at least three spatial [[dimension]]s and one temporal ([[time]]) dimension, although [[compactification (physics)|extremely small]] additional dimensions cannot be ruled out experimentally; [[spacetime]] appears to be [[differentiable manifold|smoothly]] and [[simply connected]], with very small [[Riemann curvature tensor|curvature]], so that [[Euclidean geometry]] is accurate ''on the average'' throughout the universe.  The universe appears to be governed by the same [[physical law]]s and [[physical constant|constant]]s throughout its extent and history.