about  Universe
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The Universe is all of spacetime and everything that exists therein, including all planets, stars, galaxies, the contents of intergalactic space, the smallest subatomic particles, and all matter and energy.[1][2][3][4][5][6] Similar terms include the cosmos, the world, reality, and nature.

The estimated diameter of the observable universe is about 28 billion parsecs or 93 billion light years.[7] Scientific observation of the Universe has led to inferences of its earlier stages. These observations suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and history. The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe, which is calculated to have begun 13.798 ± 0.037 billion years ago.[8][9] Observations of supernovae have shown that the Universe is expanding at an accelerating rate.[10]

There are many competing theories about the ultimate fate of the universe. Physicists remain unsure about what, if anything, preceded the Big Bang. Many refuse to speculate, doubting that any information from any such prior state could ever be accessible.[citation needed] There are various multiverse hypotheses, in which some physicists have suggested that the Universe might be one among many, or even an infinite number, of universes that likewise exist.[11][12]

Contents  [show] 
Historical observation
Hubble eXtreme Deep Field (XDF)

XDF size compared to the size of the Moon – several thousand galaxies, each consisting of billions of stars, are in this small view.

XDF (2012) view – each light speck is a galaxy – some of these are as old as 13.2 billion years[13] – the visible Universe is estimated to contain 200 billion galaxies.

XDF image shows fully mature galaxies in the foreground plane – nearly mature galaxies from 5 to 9 billion years ago – protogalaxies, blazing with young stars, beyond 9 billion years.
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 the ancient Greek philosophers and Indian philosophers.[14][15] Over the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and the Newtonian model of the Solar System, respectively. Further improvements in astronomy led to the realization that the Solar System is embedded in a galaxy composed of 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. Discovery of the red shift and cosmic microwave background radiation suggested that the Universe is expanding and had a beginning.[16]

Main article: Chronology of the universe
According to the prevailing scientific model of the Universe, known as the Big Bang, the Universe expanded from an extremely hot, dense phase called the Planck epoch, in which all the matter and energy of the observable universe was concentrated. Since the Planck epoch, the Universe has been expanding 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 theory. The universe is composed of ordinary matter (4.9%) including atoms, stars, and galaxies, dark matter (26.8%) which is a hypothetical particle that has not yet been detected, and dark energy (68.3%), which is a kind of energy density that seemingly exists even in completely empty space.[17] 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, called dark matter.[18] The common use of the "dark matter" and "dark energy" placeholder names for the unknown entities (purported to account for about 95% of the mass-energy density of the Universe) demonstrates the present observational and conceptual shortcomings and uncertainties concerning the nature and ultimate fate of the Universe.[19]

On 21 March 2013, the European research team behind the Planck cosmology probe released the mission's all-sky map of the cosmic microwave background.[20][21][22][23][24] The map suggests the universe is slightly older than thought. According to the map, subtle fluctuations in temperature were imprinted on the deep sky when the cosmos was about 370,000 years old. The imprint reflects ripples that arose as early, in the existence of the universe, as the first nonillionth (10−30) of a second. Apparently, these ripples gave rise to the present vast cosmic web of galaxy clusters and dark matter. According to the team, the universe is 13.798 ± 0.037 billion years old,[9][25] and contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Also, the Hubble constant was measured to be 67.80 ± 0.77 (km/s)/Mpc.[20][21][22][24][25]

An earlier interpretation of astronomical observations indicated that the age of the Universe was 13.772 ± 0.059 billion years,[26] and that the diameter of the observable universe is at least 93 billion light years or 8.80×1026 meters.[27] 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.

Etymology, synonyms and definitions
See also: Cosmos, Nature, World (philosophy) and Celestial spheres
The word Universe derives from the Old French word Univers, which in turn derives from the Latin word universum.[28] The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.[29] 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").[29]

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, περιφορά, (periforá, "circumambulation"), originally used to describe a course of a meal, the food being carried around the circle of dinner guests.[30] This Greek word refers to celestial spheres, an early Greek model of the Universe. Regarding Plato's Metaphor of the sun, Aristotle suggests that the rotation of the sphere of fixed stars inspired by the prime mover, motivates, in turn, terrestrial change via the Sun. Careful astronomical and physical measurements (such as the Foucault pendulum) are required to prove the Earth rotates on its axis.

A term for "Universe" in ancient Greece was τὸ πᾶν (tò pán, The All, Pan (mythology)). Related terms were matter, (τὸ ὅλον, tò ólon, see also Hyle, lit. wood) and place (τὸ κενόν, tò kenón).[31][32] Other synonyms for the Universe among the ancient Greek philosophers included κόσμος (cosmos) and φύσις (meaning Nature, from which we derive the word physics).[33] The same synonyms are found in Latin authors (totum, mundus, natura)[34] 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 interpretation), and Nature (as in natural laws or natural philosophy).[35]

Broadest definition: reality and probability
See also: Essence–Energies distinction § Distinction between created and uncreated
The broadest definition of the Universe is found in De divisione naturae by the medieval philosopher and theologian Johannes Scotus Eriugena, who defined it as simply everything: everything that is created and everything that is not created.

Definition as reality
See also: Reality and Physics
More customarily, the Universe is defined as everything that exists, (has existed, and will exist).[36] According to our current understanding, the Universe consists of three principles: spacetime, forms of energy, including momentum and matter, and the physical laws that relate them.

Definition as connected space-time
See also: Eternal inflation
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.[37] 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 purely speculative.

Definition as observable reality
See also: Observable universe and Observational cosmology
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.[38] According to the general 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. 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.

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 the observable universe. 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. Nevertheless, even the most rapid traveler will 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
Main articles: Observable universe, Age of the universe and Abundance of the chemical elements
The size of the Universe is unknown; it may be infinite. The region visible from Earth (the observable universe) is a sphere with a radius of about 46 billion light years,[39] based on where the expansion of space has taken the most distant objects observed. For comparison, the diameter of a typical galaxy is 30,000 light-years, and the typical distance between two neighboring galaxies is 3 million light-years.[40] As an example, the Milky Way Galaxy is roughly 100,000 light years in diameter,[41] and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light years away.[42] There are probably more than 100 billion (1011) galaxies in the observable Universe.[43] Typical galaxies range from dwarfs with as few as ten million[44] (107) stars up to giants with one trillion[45] (1012) stars, all orbiting the galaxy's center of mass. A 2010 study by astronomers estimated that the observable Universe contains 300 sextillion (3×1023) stars.[46]

The observable matter is spread homogeneously (uniformly) throughout the Universe, when averaged over distances longer than 300 million light-years.[47] 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.[48] The Universe is also bathed in a highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725 kelvin.[49] The hypothesis that the large-scale Universe is homogeneous and isotropic is known as the cosmological principle,[50] 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 68.3% dark energy, 26.8% dark matter and 4.9% ordinary matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume.[51] 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 current estimate of the Universe's age is 13.798 ± 0.037 billion years old.[9] 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 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 — particularly the lightest atoms such as hydrogen, deuterium and helium — seem to be identical throughout the Universe and throughout its observable history.[52] The Universe seems to have much more matter than antimatter, an asymmetry possibly related to the observations of CP violation.[53] 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.[54]

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 is believed to confer mass on the particles with which it is connected. The graviton, a supposed gauge boson for gravity, is not shown.
The Universe appears to have a smooth space-time continuum consisting of three spatial 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.[55] 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.[56]

The Universe appears to behave in a manner that regularly follows a set of physical laws and physical constants.[57] 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. 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.

Fine tuning
Main article: Fine-tuned Universe
It appears that many of the properties of the Universe have special values in the sense that a Universe where these properties differ slightly would not be able to support intelligent life.[16][58] Not all scientists agree that this fine-tuning exists.[59][60] 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. 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).

Historical models
See also: Cosmology and Timeline of cosmology
Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the then-available data and conceptions of the Universe. Historically, cosmologies and cosmogonies were based on narratives of gods acting in various ways. Theories of an impersonal Universe governed by physical laws were first proposed by the Greeks and Indians.[15] 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.

Main articles: Creation myth and Creator deity
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 Universe is created 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), the Aztec goddess Coatlicue myth, the ancient Egyptian god Atum story, or the Genesis creation narrative. In another type of story, the Universe 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 other stories, the Universe emanates from fundamental principles, such as Brahman and Prakrti, the creation myth of the Serers,[61] or the yin and yang of the Tao.

Philosophical models
Further information: Cosmology
See also: Pre-Socratic philosophy, Physics (Aristotle), Hindu cosmology, Islamic cosmology and Time
The pre-Socratic Greek philosophers and Indian philosophers developed the earliest known philosophical models of the Universe.[15][62] 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 are different forms of a single primordial material, or arche. The first to do so was Thales, who proposed this material is Water. Thales' student, Anaximander, proposed that everything came from the limitless apeiron. Anaximenes proposed Air on account of its perceived attractive and repulsive qualities that cause the arche to condense or dissociate into different forms. Anaxagoras, proposed the principle of Nous (Mind). Heraclitus proposed fire (and spoke of logos). Empedocles proposed the elements: earth, water, air and fire. His four element theory became very popular. Like Pythagoras, Plato believed that all things were composed of number, with the Empedocles' elements taking the form of the Platonic solids. Democritus, and later philosophers—most notably Leucippus—proposed that the Universe was composed of indivisible atoms moving through void (vacuum). Aristotle did not believe that was feasible because air, like water, offers resistance to motion. Air will immediately rush in to fill a void, and moreover, without resistance, it would do so indefinitely fast.

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 τὸ ἐν (The One). Parmenides' theory seemed implausible to many Greeks, but his student Zeno of Elea challenged them with several famous paradoxes. Aristotle responded to these paradoxes by developing the notion of a potential countable infinity, as well as the infinitely divisible continuum. Unlike the eternal and unchanging cycles of time, he believed the world was bounded by the celestial spheres, and thus magnitude was only finitely multiplicative.

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.[63] 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.[64]

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 and future. 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). Borrowing from Aristotle's Physics and Metaphysics, 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:[65]

"An actual infinite cannot exist."
"An infinite temporal regress of events is an actual infinite."
"\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:[65]

"An actual infinite cannot be completed by successive addition."
"The temporal series of past events has been completed by successive addition."
"\therefore The temporal series of past events cannot be an actual infinite."
Both arguments were adopted by 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.[65]

Astronomical models
Main article: History of astronomy

Aristarchus's 3rd century BCE calculations on the relative sizes of from left the Sun, Earth and Moon, from a 10th-century AD Greek copy
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.

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 Aristotle's physical interpretation of the model, celestial spheres eternally rotate with uniform motion around a stationary Earth. Normal matter, is entirely contained within the terrestrial sphere. This model was also refined by Callippus and after concentric spheres were abandoned, it was brought into nearly perfect agreement with astronomical observations by Ptolemy. The success of such a 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). Other Greek scientists, such as 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.[66] 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 parallax apparent, that is, no 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 precision instruments. 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 Hellenistic astronomer who lived a century after Aristarchus.[67][68][69] 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.[70] 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.[71] 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.[72] During the Middle Ages, heliocentric models were also been proposed by the Indian astronomer, Aryabhata,[73] and by the Persian astronomers, Albumasar[74] and Al-Sijzi.[75]

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 more plausibly 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? ”
—Nicolaus 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).[76] Aryabhata (476–550), Brahmagupta (598–668), and Al-Sijzi,[77] 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).[citation needed]

Johannes Kepler published the Rudolphine Tables containing a star catalog and planetary tables using Tycho Brahe's measurements.
This cosmology was accepted by Isaac Newton, Christiaan Huygens and later scientists.[78] Edmund Halley (1720)[79] and Jean-Philippe de Cheseaux (1744)[80] 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.[81] Newton believed 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.[78] This instability was clarified in 1902 by the Jeans instability criterion.[82] One solution to these 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.[40][83] A significant astronomical advance of the 18th century was the realization by Thomas Wright, Immanuel Kant and others of nebulae.[79]

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.[84]

Theoretical models

High-precision test of general relativity by the Cassini space probe (artist's impression): radio signals sent between the Earth and the probe (green wave) are delayed by the warping of space and time (blue lines) due to the Sun's mass.
Of the four fundamental interactions, gravitation is dominant at cosmological length scales; that is, the other three forces play a negligible role in determining structures at the level of planetary systems, galaxies and larger-scale structures. Because all matter and energy gravitate, gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on cosmological length scales. The remaining two interactions, the weak and strong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.

General theory of relativity
Main articles: Introduction to general relativity, General relativity and Einstein's field equations
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 to date. However, because 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 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. Because 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),[84][85] that corresponds to an energy density of empty space.[86] 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,[87] recent astronomical observations of type Ia supernovae have detected a large amount of "dark energy" that is accelerating the Universe's expansion.[88] Preliminary studies suggest that this dark energy corresponds to a positive Λ, although alternative theories cannot be ruled out as yet.[89] 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.[90] Evidence for such zero-point energy is observed in the Casimir effect.

Special relativity and space-time
Main articles: Introduction to special relativity and Special relativity

Only its length L is intrinsic to the rod (shown in black); coordinate differences between its endpoints (such as Δx, Δy or Δξ, Δη) depend on their frame of reference (depicted in blue and red, respectively).
The Universe has at least three spatial and one temporal (time) dimension. It was long thought that the spatial and temporal dimensions were different in nature and independent of one another. However, according to the special theory of relativity, spatial and temporal separations are interconvertible (within limits) by changing one's motion.

To understand this interconversion, it is helpful to consider the analogous interconversion of spatial separations along the three spatial dimensions. Consider the two endpoints of a rod of length L. The length can be determined from the differences in the three coordinates Δx, Δy and Δz of the two endpoints in a given reference frame

L^{2} = \Delta x^{2} + \Delta y^{2} + \Delta z^{2}
using the Pythagorean theorem. In a rotated reference frame, the coordinate differences differ, but they give the same length

L^{2} = \Delta \xi^{2} + \Delta \eta^{2} + \Delta \zeta^{2}.
Thus, the coordinates differences (Δx, Δy, Δz) and (Δξ, Δη, Δζ) are not intrinsic to the rod, but merely reflect the reference frame used to describe it; by contrast, the length L is an intrinsic property of the rod. The coordinate differences can be changed without affecting the rod, by rotating one's reference frame.

The analogy in spacetime is called the interval between two events; an event is defined as a point in spacetime, a specific position in space and a specific moment in time. The spacetime interval between two events is given by

s^{2} = L_{1}^{2} - c^{2} \Delta t_{1}^{2} = L_{2}^{2} - c^{2} \Delta t_{2}^{2}
where c is the speed of light. According to special relativity, one can change a spatial and time separation (L1, Δt1) into another (L2, Δt2) by changing one's reference frame, as long as the change maintains the spacetime interval s. Such a change in reference frame corresponds to changing one's motion; in a moving frame, lengths and times are different from their counterparts in a stationary reference frame. The precise manner in which the coordinate and time differences change with motion is described by the Lorentz transformation.

Solving Einstein's field equations
See also: Big Bang and Ultimate fate of the Universe
File:Closed Friedmann universe zero Lambda.ogg
Animation illustrating the metric expansion of the universe
The distances between the spinning galaxies increase with time, but the distances between the stars within each galaxy stay roughly the same, due to their gravitational interactions. This animation illustrates a closed Friedmann Universe with zero cosmological constant Λ; such a Universe oscillates between a Big Bang and a Big Crunch.

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

ds^2 = -c^{2} dt^2 +
R(t)^2 \left( \frac{dr^2}{1-k r^2} + r^2 d\theta^2 + r^2 \sin^2 \theta \, d\phi^2 \right)
where (r, θ, φ) correspond to a spherical coordinate system. This metric has only two undetermined parameters: an overall length scale R that can vary with time, and a curvature index k that can be only 0, 1 or −1, corresponding to flat Euclidean geometry, or spaces of positive or negative curvature. In cosmology, solving for the history of the Universe is done by calculating R as a function of time, given k and the value of the cosmological constant Λ, which is a (small) parameter in Einstein's field equations. The equation describing how R varies with time is known as the Friedmann equation, after its inventor, Alexander Friedmann.[91]

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 because the Universe is known to be inhomogeneous on smaller scales, R must change, according to general relativity. When R changes, all the spatial distances in the Universe change in tandem; there is an overall expansion or contraction of space itself. This accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point 13.8 billion years ago[92] 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 because it is based on the questionable assumptions of perfect homogeneity and isotropy (the cosmological principle) and that only the gravitational interaction is significant. However, the Penrose–Hawking singularity theorems show that a singularity should exist for very general conditions. Hence, according to Einstein's field equations, R grew rapidly from an unimaginably hot, dense state that existed immediately following this singularity (when R had a small, finite value); this is the essence of the Big Bang model of the Universe. A common misconception is that the Big Bang model predicts that matter and energy exploded from a single point in space and time; that is false. Rather, space itself was created in the Big Bang and imbued with a fixed amount of energy and matter distributed uniformly throughout; as space expands (i.e., as R(t) increases), the density of that matter and energy decreases.

Space has no boundary – that is empirically more certain than any external observation. However, that does not imply that space is infinite... (translated, original German)
Bernhard Riemann (Habilitationsvortrag, 1854)
Third, the curvature index k determines the sign of the mean spatial curvature of spacetime averaged over length scales greater than a billion light years. If k=1, the curvature is positive and the Universe has a finite volume. Such universes are often visualized as a three-dimensional sphere S3 embedded in a four-dimensional space. Conversely, if k is zero or negative, the Universe may have infinite volume, depending on its overall topology. It may seem counter-intuitive that an infinite and yet infinitely dense Universe could be created in a single instant at the Big Bang when R=0, but exactly that is predicted mathematically when k does not equal 1. For comparison, an infinite plane has zero curvature but infinite area, whereas an infinite cylinder is finite in one direction and a torus is finite in both. A toroidal Universe could behave like a normal Universe with periodic boundary conditions, as seen in "wrap-around" video games such as Asteroids; a traveler crossing an outer "boundary" of space going outwards would reappear instantly at another point on the boundary moving inwards.

The Discovery of the New World 
and the End of the Old

Preliminary Thoughts

American textbooks often carry the history of Europe up into the Renaissance, and then plunge into the Age of Discovery and Exploration as a preliminary to the study of United States history. As a result, we are much more aware of the effect of the Discovery of the New World, as the Europeans conceived it, upon the Americas, than the effect that the opening up of new lands had upon Europe. If we were more aware of the changes that the discoveries caused, we might be willing to concede that these discoveries were a basic factor in the end of the Middle Ages.
Gold and Silver

Columbus' voyage of 1492 was intended to discover a shorter all-water route to China and India than the route around Africa that was being opened up by the Portuguese, and the aim of both was to be able to by-pass the Muslim and Byzantine middle-men through which the spices of the East reached Western Europe. Although Columbus died still believing that he had opened up the Indies to Spain -- which is why Europeans called the native inhabitants of the Americas "Indians" -- most realized that a great land mass lay between them and the spices of the East, and also began to realize that there were sources of GOLD AND SILVER there.
The natives had amassed a great deal of golden treasure over the centuries, and the first flood of "new" gold into Spain and Europe came as a result of the conquistadores [Spanish for "conquerors] seizing this accumulation. With the conquest of Peru by Francisco Pizarro, new gold began to be mined; and, with the discovery of the silver veins of San Luis Potosi in Mexico, vast amounts of silver began to appear. The European explorers began to search primarily for gold, for the "Land of El Dorado," a fabled land where, after the king bathed each morning, his subjects would cover his body with gold dust until he shone like the sun. Since the time of the conquistadores, a series of new sources of gold strikes have been made -- bonanzas, from the Spanish word meaning "PROSPERITY" -- Colorado, California, South Africa, the Canadian Klondike. Well over 95% of the gold in use today was mined since 1500.

Gold is like anything else: the more there is of it, the less valuable it is. And so, as gold and silver arrived in Europe from the Americas, the price of everything began to rise steadily. Just to explain why that happened, consider that if a hundred people have one ounce of gold apiece, they all want to buy wheat, and there are only one hundred bushels of wheat for sale, the price of a bushel of wheat will be one ounce of gold. If those same people find a pirate treasure and divide it up so that each of them has two ounces of gold, but there is still only one hundred bushels of wheat for sale, the price of a bushel of wheat will be two ounces of gold. You can look at it another way. When the amount of gold (or any other medium of exchange) in circulation increases, the value of salaries, rents, and debts drops. There is a simple equation for all of this:

Price = the amount of currency divided by the supply of goods
The steady increase of gold and silver in Europe brought about what historians call The Price Revolution. People on FIXED INCOMES were impoverished; it became more advantageous to owe money than to be solvent. Money lost value every day it stayed in one's pocket, so the only way to prosper was through trade. Nobles could no longer depend on their income from the rents paid by their tenants, and began to use their lands to raise sheep for wool and meat, or to produce other goods for sale. Land was no longer the basis of wealth, and the land-owners no longer the dominant economic class.


We have said that most of the population of medieval Europe went to bed hungry and that their DIET was unbalanced and boring at best. The new plants that were introduced from the New World changed that situation. A medieval peasant could expect to harvest about 600 pounds of wheat from an acre of land. It took a long time for the Europeans to get used to these new plants, but when that same acre of land was planted in potatoes -- native to South America -- the peasant could plan on harvesting 50,000 pounds of food. It was even harder for the Europeans to get adjusted to corn -- eating it made many of them sick, and they weren't accustomed to planting row crops in their fields -- but they could harvest 1800 pounds of corn on the acre that had given them only 600 pounds of wheat. Some Europeans, such as the Italians, eventually became used to corn, but it was used primarily as food for chicken, geese and other fowl, and for pigs. If the introduction of potatoes produced a caloric revolution, the acceptance of corn brought about a protein revolution. Since the land of Europe could now produce more food, the relative price of food began to drop. The productive capacity of the land had caught up with the population, and the average European could now eat more. The Europeans, in turn, introduced corn into Africa and sweet potatoes in China, where these new foods also changed conditions dramatically

He could also eat better, since a number of lesser food crops arrived from the New World that made possible a more varied DIET. The French imported tomatoes, which they called "apples of love," and used them for ornamental purposes in their FLOWER GARDENS. They thought that they were poisonous, which, in fact, many of the early varieties were. In time, however, the poison-producing capacities of the tomato were bred out, and the tomato became one of the most popular additions to European cuisine. There were many other food plants brought back to Europe -- particularly many varieties of squash, beans, PUMPKINS, peppers -- that introduced a welcome variety, as well as a wide range of vitamins, into the European diet. The health of the average European began to IMPROVE, and his height, weight, and strength increased. As this occurred, his resistance to disease grew.


A great deal of attention is paid to the terrible death toll among the native inhabitants of the New World caused by the European's introduction of new diseases for which they had no immunity. It should also be noted that over half of the Europeans coming to the Americas died within a year of their arrival, usually from some fever, and that the death toll among Europeans in the interior of Africa was so great that it remained largely unexplored by them until well into the 19th century. The Europeans were quick to use native remedies for their ailments, and the bark of the chincona tree -- from which quinine was extracted -- was of great help to them. The medical establishment of Europe resisted the introduction of these new drugs, however, and it was not until the 1830's, for instance, that quinine was brought into general use. This lag has CONTINUED to be the case. It was only in 1952, for instance, that Western medical researchers recognized the value of Rauwolfia, a root that the inhabitants of India had chewed to relieve nervousness for centuries. The active substance was extracted from the root and sold as miltown, the first tranquilizer. Given this general resistance to "native remedies," the medicines and medical techniques of the new lands had relatively little effect on Europe. The importance of the drugs of the new worlds lay in another direction.
We have noted that medieval Europeans displayed violent swings of emotions. Part of this may have been simply a difference in cultural norms, but it should be noted that the men and women of medieval Europe had relatively little personal control over their states of mind. Like most other parts of the world, the Europeans had an effective DEPRESSANT in alcohol, but, unlike any other of the world's civilization, they did not have an alkaloid stimulant. These were quickly important from their native lands, and their use swiftly spread. The first was cocoa from the Aztecs, a rich source of caffeine, and Europeans began their long love affair with chocolate. Coming next were coffee, another source of caffeine, from the Near East, and tobacco, adding nicotine to the Europeans' personal stash of drugs. Finally tea from the Far East introduced another potent source of caffeine. The Europeans developed the custom of mixing caffeine with sugar, an import from India and the Near East, a practice that cut the bitterness of the drink and enhanced its effectiveness.

At the same time, coca leaves from South America yielded cocaine, opium from Far Eastern poppies provided both opium itself and morphine, and hashish from the Near East offered a potent form of marijuana. The use of these narcotics and DEPRESSANTS was widespread until well into the 19th century. It's said that Coca-Cola started out as a medicinal concoction laced with cocaine, and was guaranteed to slow you down, but, when such patent medicines became illegal, the company substituted caffeine for cocaine and guaranteed that their drink would pep you up.

In any event, the exploitation of lands beyond the sea gave Europeans a variety of potent stimulants and depressants, and they now had some control over their moods. Western culture has continued this practice, and few of us go through a day without a smoke, a coke, a cup of coffee, or a candy bar. It is difficult to imagine what people might be like if they did not have easy access to these New World drugs.

Industrial Materials

Less dramatic than the influx of GOLD AND SILVER, but perhaps more important in the long run were the raw materials extracted from the new lands. The most important single industry in medieval Europe was the manufacture of cloth, and the manufacturers were always looking for colorful dyes that would not fade or wash out. They found them in the New World. Brazil is named after a tree in the Near East, the bark of which produced a good red dye; and the islands off the Carolina coast in North America were found to be a good source of a rich and relatively permanent blue dye called indigo. Europe was almost deforested, and was quick to import American wood. Most North American colonists were expected to unload their belongings from their ship and then fill it with shingles for its return voyage. Tall oaks and pines allowed the Europeans to build larger ships, and they were quick to extract barrels of pitch and turpentine from the pines and spruces of the New World. American furs were popular for both clothing and the making of felt. All of the colonial powers anxiously sought for deposits of salt, and most were able to find them.
This list could be extended greatly, but the point should be obvious. European manufacture had been woefully short of industrial materials. The resources of the New World gave it the supplies it needed to produce the surplus necessary to begin a profitable trade with the other parts of the world, parts that Europe had not be able to conquer as it had the Americas.


We have discussed how medieval philosophers and "proto-scientists" based their search for knowledge upon logic, and how the basis of that logic lay in the manipulation of categories. The discoverers and explorers began to bring back reports and specimens of phenomena and things that did not fit easily into the categories with which the European intellectuals were accustomed. It was easy enough to say "Socrates is a man," but where [a duck-billed platypus]does one put the gorilla? Is he also a man? What about the duck-billed platypus? It has a bill, feathers, and webbed feet, so it's a bird, right? But it has scales and swims around underwater, so it's a fish? Or is it a mammal, since it has hair and gives birth to living young? These things were not easy to answer, and it took time to sort them out. Until that was done, however, the logic based upon categories was almost useless. European intellectuals turned from the practice of logical investigation to observing and recording, measuring, and arranging. The patterns that had dominated European thought since Peter Abelard fell into disuse, and the logic of categories did not emerge again until the mid-19th century with the publication of Darwin's The Origin of Species
A Conclusion

It's easy to look back upon the men and women of medieval times with a feeling of moral and intellectual superiority. Certainly they were capable of great cruelty and seemed curiously passive in the face of a social organization in which a wealthy and powerful few proclaimed that everyone else was innately inferior. You might ask why the "people" did not demand liberty and equality, why they did not establish education for all, why they kept women in a generally subordinate role, and why a whole lot of other things. They were different in many ways from us, and, by our standards, they were inferior, but it is important to ask the reasons for those differences.
One difference is exemplified by the "Birkenhead Rule." When the the British liner Birkenhead was sinking and everyone was trying to get into the lifeboats, someone shouted out Women and children first!, and this has been the custom of the sea ever since. A medieval man or woman would never have thought of raising such a cry. A child is a burden upon society, consuming more than it produces for at least the first ten or twelve years of its life. Able-bodied men, however, are an investment that society has already made and from whom it must gain a return. Young women are necessary to restore the losses of population due to wars, famines, plagues, and the other dangers of life, but they do not produce as much as mature men and so are less valuable to society -- unless of course, they fall into short supply. In the middle ages, young men and women would have had first call on the lifeboats, and the young and aged would have been left behind. This may strike you as cruel and inhumane, but that is only because you are rich enough to afford such luxuries as believing that the Birkenhead Rule is the only proper way to behave.

The gulf that lies between you and the men and women of medieval Europe is mostly the difference between your wealth and their poverty. Many of you drive an auto with a hundred horse-power engine. The work of a SINGLE MAN is rated at about 1/8 horse-power, so you have the equivalent of 800 slaves to carry you from place to place. Your rooms are lighted by the equivalent of hundreds of candles, and your closet has more clothes than the entire population of a medieval village possessed. The knives in your kitchen are made of a steel so fine that, in medieval times, only a king could have afforded their equal. You look back upon the men and women of medieval Europe and see their ignorance, dirt, and heartlessness; if they could look at you, they would see only a person wealthy beyond their comprehension. They would also wonder why you should enjoy such riches since they worked much harder and longer than you and had so much less to show for it. And if you could speak to them and tell them how you felt that people should behave, they would think to themselves Sure, it's easy to make sacrifices and be generous and kind when you are wealthy. I wonder what would happen to their high principles if they were hungry and cold most of their lives?

I suppose that the basic question is why you are so much wealthier than they. The usual answer is that you are enjoying the fruits of global commerce and the Industrial Revolution. But neither of those things would have occurred without the discovery and exploitation of the New World. That's one reason to consider that 1492 is as good a date as any and better than most to mark the end of the middle ages. It also marked the beginning of a 500-year boom economy for Europeans and their descendants, but that's another matter.
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