# User:PiRSquared17/Universe

The Universe is everything that exists. The word Universe can be used in ways that do not mean the same thing. Examples of this are cosmos, the world, or Nature.

Most of the time, Universe means "the area around everything." But, using a different word meaning, some people have thought that the "Universe" made of getting bigger space-as-we-know-it, is just one of many "universes", which, when put together, are called the multiverse[1]. For example, in the many-worlds hypothesis, new "universes" are created with every quantum measurement[source?]. These universes are usually thought to not be touching our own universe and that makes it not possible for us to find experimentally[who?]. What we have seen of older parts of the universe (which are far away) make it seem that the Universe has had the same physical laws and constants throughout most of its history. However, in bubble universe idea, there may be an infinite amount of types of "universes" created in ways that are not the same, and maybe each with different physical constants.

In recorded history, some cosmologies and cosmogonies have been proposed to make up for sights of the Universe. The most early geocentric models were made by the ancient Greeks, who thought that the Universe has infinite space and has existed forever, but contains a single set of concentric spheres of finite size – relating to the fixed stars, the Sun and various planets – circling about a spherical but not moving Earth. Over the centuries, more precise viewings and better ideas of gravity led to Copernicus's heliocentric model and the Newtonian model of the Solar System. More improvements in astronomy led to people realizing that the Solar System is in a galaxy made of millions of stars, the Milky Way, and that other galaxies exist outside it, as far as machines can reach. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. Discovery of the red shift and cosmic microwave background radiation revealed that the Universe is getting bigger and had a beginning.

This high-resolution image of the Hubble ultra deep field shows a diverse range of galaxies, each made 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 far away galaxies to have been imaged by an optical telescope, existing at the time shortly after the Big Bang.

According to the most used scientific model of the Universe, known as the Big Bang, the Universe expanded from a very 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 getting bigger 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 and, more generally, the Big Bang idea. Recent observations show that this expansion is happening because of dark energy, and that most of the matter in the Universe may be in a form which cannot be detected by present computers (and other machines), and so is not accounted for in the present models of the universe; this has been named dark matter.

Current ways of thinking of astronomical observations show that the universe has existed for 13.73 (± 0.12) billion years,[2] and that the diameter of the observable universe is at least 93 billion light years, or 8.80 ×1026 metres.[3] According to general relativity, space can get bigger faster than the speed of light, but we can view only a small part of the universe because of the speed of light. Since we cannot see space beyond the limitations of light (or any electromagnetic radiation), is not completely certain whether the size of the Universe is finite or infinite.

## Etymology, synonyms and meaning

The word Universe comes from the Old French word Univers, which comes from the Latin word universum.[4] The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.[5] The Latin word is from the poetic contraction Unvorsum — first used by Lucretius in Book IV (line 262) of his De rerum natura (On the Nature of Things) — which connects un, uni (the combining form of unus, or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning "something rotated, rolled, changed").[5] Lucretius used the word in the sense "everything rolled into one, everything combined into one".

A different interpretation (way to interpret) of unvorsum is "everything rotated as one" or "everything rotated by one". In this sense, it may be thought of as 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.[6] This Greek word refers to an early Greek model of the Universe. In that model, all matter was in rotating spheres centered on the Earth; according to Aristotle, the rotation of the outermost sphere was responsible for the motion and change of everything within. It was natural for the Greeks to assume that the Earth was stationary and that the heavens rotated about the Earth, because careful astronomical and physical measurements (such as the Foucault pendulum) are required to prove otherwise.

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).[7][8] 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).[9] The same synonyms are found in Latin authors (totum, mundus, natura)[10] 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 idea of everything), the cosmos (as in cosmology), the world (as in the many-worlds hypothesis), and Nature (as in natural laws or natural philosophy).[11]

### Broadest meaning: reality and probability

The broadest word meaning 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 something similar is in quantum physics, maybe most easily recognized in the path-integral formulation of Feynman.[12]

### Definition as reality

More normally, the Universe is thought to be everything that exists, has existed, and will exist.[13]This definition says that the Universe is made of three elements: space and time, together known as space-time or the vacuum; matter and different forms of energy and momentum occupying space-time; and the physical laws that govern the first two. A similar definition of the term Universe is everything that exists at a single moment of cosmological time, such as the present or the beginning of time, as in the sentence "The Universe was of size 0".

The three elements of the Universe (spacetime, matter-energy, and physical law) correspond 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[source?]; 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 form space-times, each existing but not able to touch, move, or change (interact with each other. An easy way to think of this is a group of separate soap bubbles, in which people living on one soap bubble cannot interact with those on other soap bubbles. 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.[14] 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, 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.[source?] According to the general idea of 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.

## Size, age, contents, structure, and laws

The Universe is very large and possibly infinite in volume; the matter that can be seen is spread over a space at least 93 billion light years across.[15] 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.[16] As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter,[17] and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away.[18] There are probably more than 100 billion (1011) galaxies in the observable universe.[19] Typical galaxies range from dwarfs with as few as ten million[20] (107) stars up to giants with one trillion[21] (1012) stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around one sextillion (1021) stars in the observable universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion (7 x 1022)[22].

The universe is thought to be mostly made of dark energy and dark matter, both of which are not understood very right now. Less than 5% of the universe is ordinary matter.

The matter that can be seen is spread throughout the universe, when averaged over distances longer than 300 million light-years.[23] However, on smaller length-scales, matter is observed to form "clumps", many atoms are condensed into stars, most stars into galaxies, most galaxies into clusters, superclusters and, lastly, the largest-scale structures such as the Great Wall of galaxies. 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.[24] The properties of dark energy and dark matter are not known. Dark matter gravitates as normal matter, so it works to slow the expansion of the Universe.Dark energy makes its expansion faster.

The Universe is old and changing. The best good guess of the Universe's age is 13.73±0.12 billion years old, based on what was seen of the cosmic microwave background radiation.[25] Independent estimates (based on measurements such as radioactive dating) agree, although they are less precise, ranging from 11–20 billion years[26] to 13–15 billion years.[27] The universe has not been the same at all times in its history. This getting bigger accounts for how Earth-bound pople can see 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, based on studies of Type Ia supernovae and corroborated by other data.

The relative fractions of different chemical elements — especially the lightest atoms such as hydrogen, deuterium and helium — seem to be identical in all of the universe and throughout all of the history of it that we know of.[28] The universe seems to have much more matter than antimatter, an.[29] 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.[30]

The elementary particles 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 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 made of three spatial dimensions and one temporal (time) dimension. On the average, 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.[31] 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.[32]

The Universe seems to be governed throughout by the same physical laws and physical constants.[33] 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 idea, and are mediated by gauge bosons that correspond to a particular type of gauge symmetry. A renormalized quantum field idea of general relativity has not yet been achieved, although various forms of string idea seem promising. The idea of special relativity is thoght to hold in all of the universe, provided that the spatial and temporal length scales are sufficiently short; otherwise, the more general idea 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 thought of (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 1915 general theory of 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

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

Many cultures have stories describing the origin 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 entity emanating or producing something by his or herself, as in the Tibetan Buddhism concept of Adi-Buddha, the ancient Greek story of Gaia (Mother Earth), the Aztec goddess Coatlicue myth, the ancient Egyptian god 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 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 Genesis creation myth as a part of Jewish 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.

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. Aristotle resolved these paradoxes by developing the notion of an infinitely divisible continuum, and applying it to space and time.

The Indian philosopher Kanada, founder of the Vaisheshika school, developed a theory of atomism and proposed that light and heat were varieties of the same substance.[34] In the 5th century AD, the Buddhist atomist philosopher Dignāga proposed atoms to be point-sized, durationless, and made of energy. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.[35]

The theory of temporal finitism was inspired by the doctrine of creation shared by the three Abrahamic religions: Judaism, Christianity and Islam. The Christian philosopher, John Philoponus, presented the philosophical arguments against the ancient Greek notion of an infinite past. Philoponus' arguments against an infinite past were used by the early Muslim philosopher, Al-Kindi (Alkindus); the Jewish philosopher, Saadia Gaon (Saadia ben Joseph); and the Muslim theologian, Al-Ghazali (Algazel). They employed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:[36]

"An actual infinite amount cannot exist."
"An infinite temporal regress of events is an actual infinite."
"${\displaystyle \therefore }$ An infinite temporal regress of events cannot exist."

The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:[36]

"An actual infinite cannot be completed by successive addition."
"The temporal series of past events has been completed by successive addition."
"${\displaystyle \therefore }$ The temporal series of past events cannot be an actual infinite."

Both arguments were adopted by later Christian philosophers and theologians, and the second argument in particular became more famous after it was adopted by Immanuel Kant in his thesis of the first antinomy concerning time.[36]

Fakhr al-Din al-Razi (1149–1209) criticized the idea of the Earth's centrality within the universe. In the context of his commentary on the Qur'anic verse, "All praise belongs to God, Lord of the Worlds," he raises the question of whether the term "worlds" in this verse refers to "multiple worlds within this single universe or cosmos, or to many other universes or a multiverse beyond this known universe." He rejected the Aristotelian and Avicennian notions of a single universe revolving around a single world, and instead argued that there are more than "a thousand thousand worlds (alfa alfi 'awalim) beyond this world such that each one of those worlds be bigger and more massive than this world as well as having the like of what this world has."[37] He argued that there exists an infinite outer space beyond the known world,[38] and that God has the power to fill the vacuum with an infinite number of universes.[39]

### Models

Models of the Universe were thought of and talked about soon after astronomy began with the Babylonian astronomers, who viewed the Universe as a flat disk floating in the ocean, and this forms the premise for early Greek maps like those of Anaximander and Hecataeus of Miletus.

Later Greek philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the Universe based more profoundly on empirical evidence. The first coherent model was proposed by Eudoxus of Cnidos. According to this model, space and time are infinite and eternal, the Earth is spherical and stationary, and all other matter is confined to rotating concentric spheres. This model was refined by Callippus and Aristotle, and brought into nearly perfect agreement with astronomical observations by Ptolemy. The success of this model is largely due to the mathematical fact that any function (such as the position of a planet) can be decomposed into a set of circular functions (the Fourier modes). However, not all Greek scientists accepted the geocentric model of the Universe. The Pythagorean philosopher Philolaus postulated that at the center of the Universe was a "central fire" around which the Earth, Sun, Moon and Planets revolved in uniform circular motion.[40] The Greek astronomer Aristarchus of Samos was the first known individual to propose a heliocentric model of the universe. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus' heliocentric theory. Archimedes wrote: (translated into English)

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 Babylonian astronomer who lived a century after Aristarchus.[41][42][43] 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.[44] 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.[45] Alternatively, he may have proved the heliocentric theory by determining the constants of a geometric model for the heliocentric theory and by developing methods to compute planetary positions using this model, like what Nicolaus Copernicus later did in the 16th century.[46] During the Middle Ages, heliocentric models may have also been proposed by the Indian astronomer, Aryabhata,[47] and by the Persian astronomers, Albumasar[48] and Al-Sijzi.[49]

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 in the Western world for roughly two millennia, until Copernicus revived Aristarchus' theory that the astronomical data could be explained in a more believable way if the earth rotated on its axis and if the sun were placed at the center of the Universe.

 “ 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, in Chapter 10, Book 1 of De Revolutionibus Orbium Coelestrum (1543)

As noted by Copernicus himself, the suggestion that the Earth rotates was very old, dating at least to Philolaus (c. 450 BC), Heraclides Ponticus (c. 350 BC) and Ecphantus the Pythagorean. Roughly a century before Copernicus, Christian scholar Nicholas of Cusa also proposed that the Earth rotates on its axis in his book, On Learned Ignorance (1440).[50] Aryabhata (476–550), Brahmagupta (598–668), Albumasar and Al-Sijzi, also proposed that the Earth rotates on its axis.[source?] The first empirical evidence for the Earth's rotation on its axis, using the phenomenon of comets, was given by Tusi (1201–1274) and Ali Kuşçu (1403–1474).[source?] 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.[51][52]

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

Copernicus' heliocentric model allowed the stars to be placed uniformly through the (infinite) space surrounding the planets, as first proposed by Thomas Digges in his Perfit Description of the Caelestiall Orbes according to the most aunciente doctrine of the Pythagoreans, latelye revived by Copernicus and by Geometricall Demonstrations approved (1576).[53] 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 in the Campo dei Fiori in Rome on 17 February 1600.[53]

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.[60] This theory and its implications will be discussed in more detail in the following section.

## Theoretical models

### General theory of relativity

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

General relativity provides of a set of ten nonlinear partial differential equations for the spacetime metric (Einstein's field equations) that must be solved from the distribution of mass-energy and momentum throughout the universe. Since these are unknown in exact detail, cosmological models have been based on the cosmological principle, which states that the universe is homogeneous and isotropic. In effect, this principle asserts that the gravitational effects of the various galaxies making up the universe are equivalent to those of a fine dust distributed uniformly throughout the universe with the same average density. The assumption of a uniform dust makes it easy to solve Einstein's field equations and predict the past and future of the universe on cosmological time scales.

Einstein's field equations include a cosmological constant (Λ),[60][61] that is related to an energy density of empty space.[62] 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,[63] recent astronomical observations of type Ia supernovae have detected a large amount of "dark energy" that is accelerating the universe's expansion.[64] Preliminary studies suggest that this dark energy is related to a positive Λ, although alternative theories cannot be ruled out as yet.[65] 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.[66] Evidence for such zero-point energy is observed in the Casimir effect.

### Solving Einstein's field equations

The distances between the spinning galaxies increase with time, but the distances between the stars within each galaxy stay roughly the same, due to their gravitational interactions. This animation illustrates a closed Friedmann universe with zero cosmological constant Λ; such a universe oscillates between a Big Bang and a Big Crunch.

Animation illustrating the metric expansion of the universe

### Big Bang model

The prevailing Big Bang model accounts for many of the experimental observations described above, such as the correlation of distance and redshift of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous, isotropic microwave radiation background. As noted above, the redshift arises from the metric expansion of space; as the space itself expands, the wavelength of a photon traveling through space likewise increases, decreasing its energy. The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an important problem in experimental physical cosmology.

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

Other experimental observations can be explained by combining the overall expansion of space with nuclear and atomic physics. As the universe expands, the energy density of the electromagnetic radiation decreases more quickly than does that of matter, since the energy of a photon decreases with its wavelength. Thus, although the energy density of the universe is now dominated by matter, it was once dominated by radiation; poetically speaking, all was light. As the universe expanded, its energy density decreased and it became cooler; as it did so, the elementary particles of matter could associate stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then associated into atomic nuclei. At this stage, the matter in the universe was mainly a hot, dense plasma of negative electrons, neutral neutrinos and positive nuclei. Nuclear reactions among the nuclei led to the present abundances of the lighter nuclei, particularly hydrogen, deuterium, and helium. Eventually, the electrons and nuclei combined to form stable atoms, which are transparent to most wavelengths of radiation; at this point, the radiation decoupled from the matter, forming the ubiquitous, isotropic background of microwave radiation observed today.

Other observations are not answered definitively by known physics. According to the prevailing theory, a slight imbalance of matter over antimatter was present in the universe's creation, or developed very shortly thereafter, possibly due to the CP violation that has been observed by particle physicists. Although the matter and antimatter mostly annihilated one another, producing photons, a small residue of matter survived, giving the present matter-dominated universe. Several lines of evidence also suggest that a rapid cosmic inflation of the universe occurred very early in its history (roughly 10−35 seconds after its creation). Recent observations also suggest that the cosmological constant (Λ) is not zero and that the net mass-energy content of the universe is dominated by a dark energy and dark matter that have not been characterized scientifically. They differ in their gravitational effects. Dark matter gravitates as ordinary matter does, and thus slows the expansion of the universe; by contrast, dark energy serves to accelerate the universe's expansion.

## Multiverse

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

Some people[who?] think that there is more than one Universe. They think that there is a set of universes called the multiverse. By definition, there is no way for anything in one universe to affect smething in another.

## Notes and references

1. Linde, Andrei; Vanchurin, Vitaly (December 8, 2009). "How many universes are in the multiverse?": 12. Cite journal requires `|journal=` (help)
2. Chang, Kenneth (2008-03-09). "Gauging Age of Universe Becomes More Precise". New York Times. Retrieved 2008-09-24.
3. Lineweaver, Charles (2005). "Misconceptions about the Big Bang". Scientific American. Retrieved 2008-11-06. Unknown parameter `|coauthors=` ignored (`|author=` suggested) (help)
4. The Compact Edition of the Oxford English Dictionary, volume II, Oxford: Oxford University Press, 1971, p.3518.
5. Lewis and Short, A Latin Dictionary, Oxford University Press, ISBN 0-19-864201-6, pp. 1933, 1977–1978.
6. Liddell and Scott, A Greek-English Lexicon, Oxford University Press, ISBN 0-19-864214-8, p.1392.
7. Liddell and Scott, pp.1345–1346.
8. Yonge, Charles Duke (1870). An English-Greek lexicon. New York: American Bok Company. p. 567.
9. Liddell and Scott, pp.985, 1964.
10. Lewis and Short, pp. 1881–1882, 1175, 1189–1190.
11. OED, pp. 909, 569, 3821–3822, 1900.
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