Historically, there have been many ideas of the cosmos (cosmologies) and its origin (cosmogonies). Theories of an impersonal universe governed by physical laws were first proposed by the Greeks and Indians. Over the centuries, improvements in astronomical observations and theories of motion and gravitation led to ever more accurate descriptions of the universe. The modern era of cosmology began with
Albert Einstein's 1915
general theory of relativity, which made it possible to quantitatively predict the origin, evolution, and conclusion of the universe as a whole. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted
Big Bang.
Mythologies Many cultures have
stories describing the origin of the world and universe. Cultures generally regard these stories as having some
truth. There are however many differing beliefs in how these stories apply amongst those believing in a supernatural origin, ranging from a god directly creating the universe as it is now to a god just setting the "wheels in motion" (for example via mechanisms such as the big bang and evolution). Ethnologists and anthropologists who study myths have developed various classification schemes for the various themes that appear in creation stories. For example, 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, and the
Judeo-Christian Genesis creation narrative in which the
Abrahamic God created the universe. In another type of story, the universe is created from the union of male and female deities, as in the
Māori 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, and the
creation myth of the
Serers.
Philosophical models The
pre-Socratic Greek philosophers and
Indian philosophers developed some of the earliest philosophical concepts of the universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the physical materials in the world are different forms of a single primordial material, or
arche. The first to do so was
Thales, who proposed this material to be
water. Thales' student,
Anaximander, proposed that everything came from the limitless
apeiron.
Anaximenes proposed the primordial material to be
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), while
Heraclitus proposed
fire (and spoke of
logos).
Empedocles proposed the elements to be earth, water, air and fire. His four-element model became very popular. Like
Pythagoras,
Plato believed that all things were composed of
number, with Empedocles' elements taking the form of the
Platonic solids.
Democritus, and later philosophers—most notably
Leucippus—proposed that the universe is composed of indivisible
atoms moving through a
void (
vacuum), although
Aristotle did not believe that to be 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. his contemporary
Parmenides emphasized changelessness. Parmenides' poem
On Nature has been read as saying that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature, or at least that the essential feature of each thing that exists must exist eternally, without origin, change, or end. His student
Zeno of Elea challenged everyday ideas about motion 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. The
Indian philosopher Kanada, founder of the
Vaisheshika school, developed a notion of
atomism and proposed that
light and
heat were varieties of the same substance. 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. The notion 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).
Pantheism is the
philosophical religious belief that the universe itself is identical to
divinity and a
supreme being or entity. The physical universe is thus understood as an all-encompassing,
immanent deity. The term 'pantheist' designates one who holds both that everything constitutes a unity and that this unity is divine, consisting of an all-encompassing, manifested
god or
goddess.
Astronomical concepts on the relative sizes of, from left to right, the Sun, Earth, and Moon, from a 10th-century AD Greek copy The earliest written records of identifiable
predecessors to modern astronomy come from
Ancient Egypt and
Mesopotamia from around 3000 to 1200
BCE.
Babylonian astronomers of the 7th century BCE viewed the world as a
flat disk surrounded by the ocean. Later
Greek philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the universe based more profoundly on
empirical evidence. Some of the earliest
cosmological models of the universe were developed by
ancient Greek and
Indian philosophers and were
geocentric, placing Earth at the center. The first coherent model was proposed by
Eudoxus of Cnidos, a student of Plato who followed Plato's idea that heavenly motions had to be circular. In order to account for the known complications of the planets' motions, particularly
retrograde movement, Eudoxus' model included 27 different
celestial spheres: four for each of the planets visible to the naked eye, three each for the Sun and the Moon, and one for the stars. All of these spheres were centered on the Earth, which remained motionless while they rotated eternally. Aristotle elaborated upon this model, increasing the number of spheres to 55 in order to account for further details of planetary motion. For Aristotle, normal
matter was entirely contained within the terrestrial sphere, and it obeyed fundamentally different rules from
heavenly material. The post-Aristotle treatise
De Mundo (of uncertain authorship and date) stated, "Five elements, situated in spheres in five regions, the less being in each case surrounded by the greater—namely, earth surrounded by water, water by air, air by fire, and fire by ether—make up the whole universe". 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 (according to
Stobaeus' account) 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. Though the original text has been lost, a reference in
Archimedes' book
The Sand Reckoner describes Aristarchus's heliocentric model. Archimedes wrote: 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
stellar parallax had not been observed, that is, the stars had not been observed to move relative 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 the explanation for the unobservability of stellar parallax. , Paris 1888 The only other astronomer from antiquity known by name who supported Aristarchus's heliocentric model was
Seleucus of Seleucia, a
Hellenistic astronomer who lived a century after Aristarchus. According to Plutarch, Seleucus was the first to prove the heliocentric system through
reasoning, but it is not known what arguments he used. Seleucus' arguments for a heliocentric cosmology were probably related to the phenomenon of
tides. According to
Strabo (1.1.9), Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun. Alternatively, he may have proved heliocentricity by determining the constants of a
geometric model for it, and by developing methods to compute planetary positions using this model, similar to
Nicolaus Copernicus in the 16th century. During the
Middle Ages,
heliocentric models were also proposed by the
Persian astronomers Albumasar and
Al-Sijzi. 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's perspective that the astronomical data could be explained more plausibly if the
Earth rotated on its axis and if the
Sun were placed at the center of the universe. As noted by Copernicus, the notion that the
Earth rotates is very old, dating at least to
Philolaus (),
Heraclides Ponticus () and
Ecphantus the Pythagorean. Roughly a century before Copernicus, the Christian scholar
Nicholas of Cusa also proposed that the Earth rotates on its axis in his book,
On Learned Ignorance (1440). Al-Sijzi also proposed that the Earth rotates on its axis.
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). This cosmology was accepted by
Isaac Newton,
Christiaan Huygens and later scientists. Newton demonstrated that the same
laws of motion and gravity apply to earthly and to celestial matter, making Aristotle's division between the two obsolete.
Edmund Halley (1720) and
Jean-Philippe de Chéseaux (1744) noted independently that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the Sun itself; this became known as
Olbers' paradox in the 19th century. 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. 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.
Deep space astronomy During the 18th century,
Immanuel Kant speculated that
nebulae could be entire galaxies separate from the Milky Way, In 1919, when the
Hooker Telescope was completed, the prevailing view was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope,
Edwin Hubble identified
Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively that
Andromeda Nebula and
Triangulum among others, were entire galaxies outside our own, thus proving that the universe consists of a multitude of galaxies. With this Hubble formulated the
Hubble constant, which allowed for the first time a calculation of the age of the universe and size of the Observable Universe, which became increasingly precise with better meassurements, starting at 2 billion years and 280 million light-years, until 2006 when data of the
Hubble Space Telescope allowed a very accurate calculation of the age of the universe and size of the Observable Universe. 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. The discoveries of this era, and the questions that remain unanswered, are outlined in the sections above. == See also ==