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The universe is commonly defined as the totality of everything that exists,including all physical space, time, matter and energy, the planets, stars, galaxies, and the contents of intergalactic space,although this usage may differ with the context (see definitions, below). Sony VAIO VGN-NS10E/S Battery

The term universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or nature. Observations of earlier stages in the development of the universe, which can be seen at great distances, suggest that the universe has been governed by the same physical laws and constants throughout most of its extent and history.Sony VAIO VGN-NS10J/S Battery


Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the universe. The earliest quantitative geocentric models were developed by theancient Greeks, who proposed that the universe possesses infinite space and has existed eternally,Sony VAIO VGN-NS10L/S Battery

but contains a single set of concentric spheres of finite size – corresponding to the fixed stars, theSun and various planets – rotating about a spherical but unmoving Earth. Over the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and theNewtonian model of the Solar System, respectively.Sony VAIO VGN-NS110E/L Battery

Further improvements in astronomy led to the realization that the Solar System is embedded in a galaxy composed of billions of stars, the Milky Way, and that other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology.Sony VAIO VGN-NS110E/S Battery

Discovery of the red shift and cosmic microwave background radiation revealed that the universe is expanding and apparently had a beginning.

According to the prevailing scientific model of the universe, known as the Big Bang, the universe expanded from an extremely hot,Sony VAIO VGN-NS110E/W Battery

dense phase called the Planck epoch, in which all the matter and energy of the observable universe was concentrated. Since the Planck epoch, the universe has been expanding to its present form, possibly with a brief period (less than 10?32 seconds) of cosmic inflation. Sony VAIO VGN-NS115N/S Battery

Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of dark energy, and that most of the matter in the universe may be in a form which cannot be detected by present instruments, Sony VAIO VGN-NS11J/S Battery

and so is not accounted for in the present models of the universe; this has been named dark matter. The imprecision of current observations has hindered predictions of the ultimate fate of the universe.

Current interpretations of astronomical observations indicate that the age of the universe is 13.75 ± 0.17 billion years,[4] and that the diameter of the observable universe is at least 93 billion light years or 8.80×1026 metres.Sony VAIO VGN-NS130E/L Battery

According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the universe is finite or infinite.Sony VAIO VGN-NS130E/S Battery

Etymology, synonyms and definitions

The word universe derives from the Old French word Univers, which in turn derives from the Latin word universum.[6] The Latin word was used byCicero and later Latin authors in many of the same senses as the modern English word is used.Sony VAIO VGN-NS130E/W Battery

The Latin word derives 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").Sony VAIO VGN-NS140E/L Battery

Lucretius used the word in the sense "everything rolled into one, everything combined into one".

An alternative interpretation of unvorsum is "everything rotated as one" or "everything rotated by one". In this sense, it may be considered a translation of an earlier Greek word for the universe, ????????,Sony VAIO VGN-NS140E/S Battery

"something transported in a circle", originally used to describe a course of a meal, the food being carried around the circle of dinner guests.This Greek word refers to an early Greek model of the universe, in which all matter was contained within rotating spheres centered on the Earth; Sony VAIO VGN-NS140E/W Battery

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 astronomicaland physical measurements (such as the Foucault pendulum) are required to prove otherwise.Sony VAIO VGN-NS190J/L Battery

The most common term for "universe" among the ancient Greek philosophers from Pythagoras onwards was ?? ??? (The All), defined as all matter (?? ????) and all space (?? ?????). Other synonyms for the universe among the ancient Greek philosophers included ?????? (meaning the world, the cosmos) and ?????(meaning Nature, from which we derive the word physics).Sony VAIO VGN-NS190J/S Battery

The same synonyms are found in Latin authors (totummundusnatura) and survive in modern languages, e.g., the German words Das AllWeltall, and Natur for universe. The same synonyms are found in English, such as everything (as in the theory of everything), Sony VAIO VGN-NS190J/W Battery

the cosmos (as in cosmology), the world (as in the many-worlds hypothesis), and Nature (as in natural laws or natural philosophy).

Broadest definition: reality and probability

The broadest definition of the universe can be found in De divisione naturae by the medieval philosopher and theologian Johannes Scotus Eriugena, Sony VAIO VGN-NS235J/L Battery

who defined it as simply everything: everything that is created and everything that is not created. 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.Sony VAIO VGN-NS235J/P Battery

This all-embracing definition was not adopted by most later philosophers, but something not entirely dissimilar reappears in quantum physics, perhaps most obviously in the path-integral formulation ofFeynman.Sony VAIO VGN-NS235J/S Battery

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. Sony VAIO VGN-NS235J/W Battery

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 themany-worlds hypothesis below in the Multiverse section).Sony VAIO VGN-NS240E/L Battery

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;Sony VAIO VGN-NS290J/L Battery

therefore, probability amplitudes must be computed allowing for the possibility that they exchange positions, something known as exchange symmetry. This conception of the universe embracing both the existent and the non-existent loosely parallels the Buddhist doctrines of shunyataand interdependent development of reality, Sony VAIO VGN-NS290J/S Battery

and Gottfried Leibniz's concept of the existence of incompossibles.

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;Sony VAIO VGN-NS50B/L Battery

matter and various forms of energy and momentum occupying space-time; and the physical laws that govern the first two. A related definition of the term universe is everything that exists at a single moment of cosmological time, such as the present, as in the sentence "The universe is now bathed uniformly in microwave radiation".Sony VAIO VGN-NS50B/W Battery

The three elements of the universe (spacetime, matter-energy, and physical law) correspond roughly to the ideas of Aristotle. In his book The Physics (???????, from which we derive the word "physics"), Aristotle divided ?? ??? (everything) into three roughly analogous elements: matter (the stuff of which the universe is made), Sony VAIO VGN-NS51B/L Battery

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, Sony VAIO VGN-NS51B/P Battery

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.Sony VAIO VGN-NS51B/W Battery

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.Sony VAIO VGN-NS52JB/L Battery

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.Sony VAIO VGN-NS52JB/P Battery

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.Sony VAIO VGN-NS52JB/W Battery

Definition as observable reality

According to a still-more-restrictive definition, the universe is everything within our connected space-time that could have a chance to interact with us and vice versa. According to thegeneral theory 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. Sony VAIO VGN-NS70B/W Battery

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. Sony VAIO VGN-NS71B/W Battery

The spatial region within which we can affect and be affected is denoted as theobservable 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,Sony VAIO VGN-NS72JB/W Battery

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.Sony VAIO VGN-NS90HS Battery

Size, age, contents, structure, and laws

The universe is immensely large and possibly infinite in volume. The region visible from Earth (the observable universe) is a sphere with a radius of about 46 billion light years, based on where theexpansion of space has taken the most distant objects observed. Sony VAIO VGN-NS92JS Battery

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. As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away.Sony VAIO VGN-NS92XS Battery

There are probably more than 100 billion (1011) galaxies in the observable universe. Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with one trillion (1012) stars, all orbiting the galaxy's center of mass. Sony VAIO VGN-NW21EF/S Battery

Thus, a very rough estimate from these numbers would suggest there are around one sextillion (1021) stars in the observable universe; though a 2010 study by astronomers resulted in a figure of 300 sextillion (3×1023).Sony VAIO VGN-NW21JF Battery

The observable matter is spread homogeneously (uniformly) throughout the universe, when averaged over distances longer than 300 million light-years. However, on smaller length-scales, matter is observed to form "clumps", i.e., to cluster hierarchically;Sony VAIO VGN-NW21MF Battery

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.Sony VAIO VGN-NW21MF/W Battery

The universe is also bathed in a highly isotropic microwaveradiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725-kelvins.[26] The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle,[27] which issupported by astronomical observations.Sony VAIO VGN-NW21ZF Battery

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.Sony VAIO VGN-NW31EF/W Battery

The properties of dark energy and dark matter are largely unknown. Dark matter gravitates as ordinary matter, and thus works to slow theexpansion of the universe; by contrast, dark energy accelerates its expansion.

The most precise estimate of the universe's age is 13.73±0.12 billion years old, based on observations of the cosmic microwave background radiation.Sony VAIO VGN-NW31JF Battery

Independent estimates (based on measurements such as radioactive dating) agree, although they are less precise, ranging from 11 to 20 billion years  to 13–15 billion years. The universe has not been the same at all times in its history; for example, the relative populations of quasars and galaxies have changed and space itself appears to have expanded.Sony VAIO VGN-NW320F/B Battery

This expansion accounts for how Earth-bound scientists can observe the light from a galaxy 30 billion light years away, even if that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; Sony VAIO VGN-NW320F/TC Battery

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, Sony VAIO VGN-NW35E Battery

deuterium and helium — seem to be identical throughout the universe and throughout its observable history. The universe seems to have much more matter than antimatter, an asymmetry possibly related to the observations of CP violation. The universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. Sony VAIO VGN-NW380F/S Battery

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.Sony VAIO VGN-NW380F/T Battery

The universe appears to have a smooth space-time continuum consisting 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.Sony VAIO VGN-NW50JB Battery

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.Sony VAIO VGN-NW51FB/N Battery

The universe appears to behave in a manner that regularly follows a set of physical laws and physical constants. According to the prevailingStandard Model of physics, all matter is composed of three generations of leptons and quarks, both of which are fermions.Sony VAIO VGN-NW51FB/W Battery

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. Sony VAIO VGN-NW70JB Battery

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. Sony VAIO VGN-NW71FB/N Battery

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,Sony VAIO VGN-NW71FB/W Battery

such as Planck's constant h or the gravitational constant G. Severalconservation 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.Sony VAIO VGN-NW91FS Battery

Fine tuning

It appears that many of the properties of the universe have special values in the sense that a universe where these properties only differ slightly would not be able to support intelligent life. Not all scientists agree that this fine-tuning exists.Sony VAIO VGN-NW91GS Battery

In particular, it is not known under what conditions intelligent life could form and what form or shape that would take. A relevant observation in this discussion is that for an observer to exist to observe fine-tuning, the universe must be able to support intelligent life. Sony VAIO VGN-NW91VS Battery

As such the conditional probability of observing a universe that is fine-tuned to support intelligent life is 1. This observation is known as the anthropic principle and is particularly relevant if the creation of the universe was probabilistic or if multiple universes with a variety of properties exist (see below).Sony VAIO VPC CW2MFX/PU Battery

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. Historically, cosmologies and cosmogonies were based on narratives of gods acting in various ways.Sony VAIO VPC S11V9E/B Battery

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. Sony VAIO VPCB119GJ/B Battery

The modern era of cosmology began with Albert Einstein's 1915general 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; Sony VAIO VPCB11AGJ Battery

however, still more careful measurements are required to determine which theory is correct.


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; Sony VAIO VPCB11AVJ Battery

such stories include the Finnishepic poem Kalevala, the Chinese story of Pangu or the Indian Brahmanda Purana. In related stories, the creation idea is caused by a single entity emanating or producing something by him- or herself, as in the Tibetan Buddhism concept of Adi-Buddha, the ancient Greek story of Gaia (Mother Earth), Sony VAIO VPCB11V9E Battery

the Aztec goddess Coatlicue myth, the ancient Egyptian god Atum story, or the Genesis creation narrative. 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 — Sony VAIO VPCB11X9E Battery

as from Tiamat in the Babylonian epic Enuma Elish or from the giant Ymir in Norse mythology – or from chaotic materials, as in Izanagi andIzanami in Japanese mythology. In other stories, the universe emanates from fundamental principles, such as Brahman and Prakrti, or the yin and yang of the Tao.Sony VAIO VPCCW18FJ/P Battery

Philosophical models

From the 6th century BCE, the pre-Socratic Greek philosophers developed the earliest known philosophical models of the universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances.Sony VAIO VPCCW18FJ/R Battery

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.Sony VAIO VPCCW18FJ/W Battery

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, Sony VAIO VPCCW19FJ/W Battery

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;Sony VAIO VPCCW1AFJ Battery

Heraclitus argued for aLogos, 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. Sony VAIO VPCCW1AHJ Battery

Various modifications of the apeiron theory were proposed, most notably that of Anaxagoras, which proposed that the various matter in the world was spun off from a rapidly rotating apeiron, set in motion by the principle of Nous (Mind). Still other philosophers — most notably Leucippus and Democritus —Sony VAIO VPCCW1S1E Battery

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.Sony VAIO VPCCW1S1E/B Battery

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 ?? ?? (The One). Sony VAIO VPCCW1S1E/L Battery

Parmenides' theory seemed implausible to many Greeks, but his student Zeno of Elea challenged them with several famousparadoxes. Aristotle attempted to resolve these paradoxes by developing the notion of an infinitely divisible continuum, and applying it to space and time.Sony VAIO VPCCW1S1E/P Battery

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. In the 5th century AD, theBuddhist atomist philosopher Dign?ga proposed atoms to be point-sized, durationless, and made of energy. Sony VAIO VPCCW1S1E/R Battery

They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.

The theory of temporal finitism was inspired by the doctrine of creation shared by the three Abrahamic religions: Judaism, Christianity and Islam.Sony VAIO VPCCW1S1E/W Battery

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); theJewish philosopher, Saadia Gaon (Saadia ben Joseph);Sony VAIO VPCCW21FX/B Battery

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:

"An actual infinite cannot exist."

"An infinite temporal regress of events is an actual infinite." Sony VAIO VPCCW21FX/L Battery

" 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:

"An actual infinite cannot be completed by successive addition."

"The temporal series of past events has been completed by successive addition." Sony VAIO VPCCW21FX/R Battery

" 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.Sony VAIO VPCCW21FX/W Battery

Astronomical models

Astronomical models of the universe were proposed 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.Sony VAIO VPCCW26EC Battery

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. Sony VAIO VPCCW26FX/B Battery

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. Sony VAIO VPCCW28EC Battery

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. Sony VAIO VPCCW28FJ/P Battery

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. The Greek astronomer Aristarchus of Samos was the first known individual to propose a heliocentric model of the universe. Sony VAIO VPCCW28FJ/R Battery

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, Sony VAIO VPCCW28FJ/W Battery

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, Sony VAIO VPCCW29FJ/W Battery

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, Sony VAIO VPCCW2AFJ Battery

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.Sony VAIO VPCCW2AHJ Battery

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, Sony VAIO VPCCW2S1E Battery

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,Sony VAIO VPCCW2S1E/B Battery

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 Sony VAIO VPCCW2S1E/L Battery

, . . . 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.

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.Sony VAIO VPCCW2S1E/P Battery

According to Plutarch, Seleucus was the first to prove the heliocentric system through reasoning, but it is not known what arguments he used. Seleucus' arguments for a heliocentric theory were probably related to the phenomenon of tides. According to Strabo (1.1.9), Sony VAIO VPCCW2S1E/R Battery

Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun. Alternatively, he may have proved the heliocentric theory by determining the constants of a geometric model for the heliocentric theory and by developing methods to compute planetary positions using this model, Sony VAIO VPCCW2S1E/W Battery

like what Nicolaus Copernicus later did in the 16th century.[51] During the Middle Ages, heliocentric models may have also been proposed by the Indian astronomer, Aryabhata,[52]and by the Persian astronomers, Albumasar[53] and Al-Sijzi.Sony VAIO VPCCW2S5C CN1 Battery

The Aristotelian model was accepted in the Western world for roughly two millennia, until Copernicus revived Aristarchus' theory that the astronomical data could be explained more plausibly if the earth rotated on its axis and if the sun were placed at the center of the universe.Sony VAIO VPCEA20 Battery

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).Sony VAIO VPCEB10 Battery

Aryabhata (476–550), Brahmagupta (598–668), Albumasar and Al-Sijzi, also proposed that the Earth rotates on its axis.[citation needed] 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 Qushji (1403–1474).Sony VAIO VPCEB11FM Battery

Tusi, however, continued to support the Aristotelian universe, thus Qushji 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 "circularinertia" to explain the Earth's rotation, similar to how Galileo Galilei explained it.Sony VAIO VPCEB11FM/BI Battery

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).Sony VAIO VPCEB11FM/T Battery

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.

This cosmology was accepted provisionally by Isaac Newton, Christiaan Huygens and later scientists,Sony VAIO VPCEB11FM/WI Battery

although it had several paradoxes that were resolved only with the development of general relativity. The first of these was that it assumed that space and time were infinite, and that the stars in the universe had been burning forever; however, since stars are constantly radiating energy, a finite star seems inconsistent with the radiation of infinite energy.Sony VAIO VPCEB11FX Battery

Secondly, Edmund Halley (1720)[59] and Jean-Philippe de Cheseaux (1744)[60] noted independently that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the sun itself; this became known as Olbers' paradox in the 19th century.Sony VAIO VPCEB11FX/BI Battery

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 in 1902 by the Jeans instability criterion.Sony VAIO VPCEB11FX/T Battery

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;Sony VAIO VPCEB11FX/WI Battery

such a cosmological model had also been proposed earlier in 1761 by Johann Heinrich Lambert.[63] A significant astronomical advance of the 18th century was the realization by Thomas Wright, Immanuel Kant and others that stars are not distributed uniformly throughout space; rather, they are grouped into galaxies.Sony VAIO VPCEB11GX Battery

The modern era of physical cosmology began in 1917, when Albert Einstein first applied his general theory of relativity to model the structure and dynamics of the universe. This theory and its implications will be discussed in more detail in the following section.Sony VAIO VPCEB11GX/BI Battery

Theoretical models

Of the four fundamental interactions, gravitation is dominant at cosmological length scales; that is, the other three forces are believed to play a negligible role in determining structures at the level of planets, stars, galaxies and larger-scale structures. Sony VAIO VPCEB11GX/T Battery

Since all matter and energy gravitate, gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on cosmological length scales. The remaining two interactions, the weak and strong nuclear forces, decline very rapidly with distance;Sony VAIO VPCEB11GX/WI Battery

their effects are confined mainly to sub-atomic length scales.

General theory of relativity

Given gravitation's predominance in shaping cosmological structures, accurate predictions of the universe's past and future require an accurate theory of gravitation.Sony VAIO VPCEB12FX Battery

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.Sony VAIO VPCEB12FX/BI Battery

Nevertheless, its cosmological predictions appear to be consistent with observations, so there is no compelling reason to adopt another theory.

General relativity provides 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. Sony VAIO VPCEB12FX/BIC Battery

Since these are unknown in exact detail, cosmological models have been based on thecosmological 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. Sony VAIO VPCEB12FX/T Battery

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 (?),  that corresponds to an energy density of empty space.Sony VAIO VPCEB14FX Battery

Depending on its sign, the cosmological constant can either slow (negative ?) or accelerate (positive ?) the expansion of the universe. Although many scientists, including Einstein, had speculated that ? was zero, recent astronomical observations of type Ia supernovae have detected a large amount of "dark energy" that is accelerating the universe's expansion.Sony VAIO VPCEB14FX/BI Battery

Preliminary studies suggest that this dark energy corresponds to a positive ?, although alternative theories cannot be ruled out as yet.Russian physicist Zel'dovich suggested that ? 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.Sony VAIO VPCEB14FX/T Battery

Evidence for such zero-point energy is observed in the Casimir effect.

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 dimension

s were different in nature and independent of one another. Sony VAIO VPCEB14FX/WI Battery

However, according to the special theory of relativity, spatial and temporal separations are interconvertible (within limits) by changing one's motion.

To understand this interconversion, it is helpful to consider the analogous interconversion of spatial separations along the three spatial dimensions. Sony VAIO VPCEB15FM Battery

Consider the two endpoints of a rod of length L. The length can be determined from the differences in the three coordinates ?x, ?y and ?z of the two endpoints in a given reference frame

L2 = ?x2 + ?y2 + ?z2

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

L2 = ??2 + ??2 + ??2.

Thus, the coordinates differences (?x, ?y, ?z) and (??, ??, ??) are not intrinsic to the rod, but merely reflect the reference frame used to describe it; Sony VAIO VPCEB15FM/BI Battery

by contrast, the length L is an intrinsic property of the rod. The coordinate differences can be changed without affecting the rod, by rotating one's reference frame.

The analogy in spacetime is called the interval between two events; an event is defined as a point in spacetime, a specific position in space and a specific moment in time. Sony VAIO VPCEB15FM/T Battery

The spacetime interval between two events is given by


where c is the speed of light. According to special relativity, one can change a spatial and time separation (L1, ?t1) into another (L2, ?t2) by changing one's reference frame, as long as the change maintains the spacetime interval sSony VAIO VPCEB15FM/WI Battery

Such a change in reference frame corresponds to changing one's motion; in a moving frame, lengths and times are different from their counterparts in a stationary reference frame. The precise manner in which the coordinate and time differences change with motion is described by the Lorentz transformation.Sony VAIO VPCEB15FX Battery

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 ?; such a universe oscillates between a Big Bang and a Big Crunch.Sony VAIO VPCEB15FX/BI Battery

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??, which can vary from place to place and which describes the local geometry in the particular coordinate system. Sony VAIO VPCEB15FX/T Battery

However, assuming the cosmological principle that the universe is homogeneous and isotropic everywhere, every point in space is like every other point; hence, the metric tensor must be the same everywhere. That leads to a single form for the metric tensor, called the Friedmann-Lemaître-Robertson-Walker metric .Sony VAIO VPCEB15FX/WI Battery


where (r, ?, ?) correspond to a spherical coordinate system. This metric has only two undetermined parameters: an overall length scale R that can vary with time, and a curvature index k that can be only 0, 1 or ?1, corresponding to flat Euclidean geometry, or spaces of positive or negative curvature.Sony VAIO VPCEB16FX Battery

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 ?, 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.Sony VAIO VPCEB16FX/B Battery

The solutions for R(t) depend on k and ?, 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. Sony VAIO VPCEB16FX/G Battery

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 Rchanges, all the spatial distances in the universe change in tandem; there is an overall expansion or contraction of space itself. Sony VAIO VPCEB16FX/L Battery

This accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point 13.7 billion years ago and never moved faster than the speed of light.Sony VAIO VPCEB16FX/P Battery

Second, all solutions suggest that there was a gravitational singularity in the past, when Rgoes to zero and matter and energy became infinitely dense. It may seem that this conclusion is uncertain since it is based on the questionable assumptions of perfect homogeneity and isotropy (the cosmological principle) and that only the gravitational interaction is significant. Sony VAIO VPCEB16FX/W Battery

However, the Penrose-Hawking singularity theorems show that a singularity should exist for very general conditions. Hence, according to Einstein's field equations, R grew rapidly from an unimaginably hot, dense state that existed immediately following this singularity (when R had a small, finite value); Sony VAIO VPCEB17FX Battery

this is the essence of the Big Bang model of the universe. A common misconception is that the Big Bang model predicts that matter and energy exploded from a single point in space and time; that is false. Rather, space itself was created in the Big Bang and imbued with a fixed amount of energy and matter distributed uniformly throughout;Sony VAIO VPCEB17FX/B Battery

as space expands (i.e., as R(t) increases), the density of that matter and energy decreases.

Third, the curvature index k determines the sign of the mean spatial curvature of spacetime averaged over length scales greater than a billion light years. If k=1, the curvature is positive and the universe has a finite volume. Sony VAIO VPCEB17FX/G Battery

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, Sony VAIO VPCEB17FX/L Battery

but exactly that is predicted mathematically when k does not equal 1. For comparison, an infinite plane has zero curvature but infinite area, whereas an infinite cylinder is finite in one direction and a torus is finite in both. A toroidal universe could behave like a normal universe with periodic boundary conditions, Sony VAIO VPCEB17FX/P Battery

as seen in "wrap-around" video games such as Asteroids; a traveler crossing an outer "boundary" of space going outwards would reappear instantly at another point on the boundary moving inwards.

The ultimate fate of the universe is still unknown, because it depends critically on the curvature index k and the cosmological constant ?Sony VAIO VPCEB17FX/W Battery

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 ?Sony VAIO VPCEB190X Battery

1 and the universe will expand forever, cooling off and eventually becoming inhospitable for all life, as the stars die and all matter coalesces into black holes (the Big Freeze and the heat death of the universe). As noted above, recent data suggests that the expansion speed of the universe is not decreasing as originally expected, but increasing;Sony VAIO VPCEB19FX Battery

if this continues indefinitely, the universe will eventually rip itself to shreds (the Big Rip). Experimentally, the universe has an overall density that is very close to the critical value between recollapse and eternal expansion; more careful astronomical observations are needed to resolve the question.Sony VAIO VPCEB19GX Battery

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