Timeline of cosmological theories

From Wikipedia, the free encyclopedia

This timeline of cosmological theories and discoveries is a chronological record of the development of humanity's understanding of the cosmos over the last two-plus millennia. Modern cosmological ideas follow the development of the scientific discipline of physical cosmology.

For millennia, what today is known to be the Solar System was regarded as the contents of the "whole universe", so advances in the knowledge of both mostly paralleled. Clear distinction was not made until circa mid-17th century. See Timeline of Solar System astronomy for further details on this side.


Early Hebrew conception of the cosmos.[citation needed] The firmament, Sheol and tehom are depicted.
  • 6th century BCE – The Babylonian Map of the World shows the Earth surrounded by the cosmic ocean, with seven islands arranged around it so as to form a seven-pointed star. Contemporary Biblical cosmology reflects the same view of a flat, circular Earth swimming on water and overarched by the solid vault of the firmament to which are fastened the stars.
  • 6th–4th century BCE – Greek philosophers, as early as Anaximander,[2] introduce the idea of multiple or even infinite universes.[3] Democritus further detailed that these worlds varied in distance, size; the presence, number and size of their suns and moons; and that they are subject to destructive collisions.[4] Also during this time period, the Greeks established that the Earth is spherical rather than flat.[5][6]
  • 6th century BCE – Anaximander conceives a mechanical, non-mythological model of the world: the Earth floats very still in the centre of the infinite, not supported by anything.[7] Its curious shape is that of a cylinder[8] with a height one-third of its diameter. The flat top forms the inhabited world, which is surrounded by a circular oceanic mass. Anaximander considered the Sun as a huge object (larger than the land of Peloponnesus[9]), and consequently, he realized how far from Earth it might be. In his system the celestial bodies turned at different distances. At the origin, after the separation of hot and cold, a ball of flame appeared that surrounded Earth like bark on a tree. This ball broke apart to form the rest of the Universe. It resembled a system of hollow concentric wheels, filled with fire, with the rims pierced by holes like those of a flute. Consequently, the Sun was the fire that one could see through a hole the same size as the Earth on the farthest wheel, and an eclipse corresponded with the occlusion of that hole. The diameter of the solar wheel was twenty-seven times that of the Earth (or twenty-eight, depending on the sources)[10] and the lunar wheel, whose fire was less intense, eighteen (or nineteen) times. Its hole could change shape, thus explaining lunar phases. The stars and the planets, located closer,[11] followed the same model.[12]
  • 5th century BCE – Parmenides is credited to be the first Greek who declared that the Earth is spherical and is situated in the centre of the universe.[13]
  • 5th century BCE – Pythagoreans as Philolaus believed the motion of planets is caused by an out-of-sight "fire" at the centre of the universe (not the Sun) that powers them, and Sun and Earth orbit that Central Fire at different distances. The Earth's inhabited side is always opposite to the Central Fire, rendering it invisible to people. They also claimed that the Moon and the planets orbit the Earth.[14] This model depicts a moving Earth, simultaneously self-rotating and orbiting around an external point (but not around the Sun), thus not being geocentrical, contrary to common intuition. Due to philosophical concerns about the number 10 (a "perfect number" for the Pythagorians), they also added a tenth "hidden body" or Counter-Earth (Antichthon), always in the opposite side of the invisible Central Fire and therefore also invisible from Earth.[15]
  • 4th century BCE – Plato claimed in his Timaeus that circles and spheres are the preferred shape of the universe, that the Earth is at the center and is circled by, ordered in-to-outwards: Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and finally the fixed stars located on the celestial sphere.[16] In Plato's complex cosmogony,[17] the demiurge gave the primacy to the motion of Sameness and left it undivided; but he divided the motion of Difference in six parts, to have seven unequal circles. He prescribed these circles to move in opposite directions, three of them with equal speeds, the others with unequal speeds, but always in proportion. These circles are the orbits of the heavenly bodies: the three moving at equal speeds are the Sun, Venus and Mercury, while the four moving at unequal speeds are the Moon, Mars, Jupiter and Saturn.[18][19] The complicated pattern of these movements is bound to be repeated again after a period called a 'complete' or 'perfect' year.[20] However, others like Philolaus and Hicetas had rejected geocentrism.[21]
  • 4th century BCE – Eudoxus of Cnidus devised a geometric-mathematical model for the movements of the planets, the first known effort in this sense, based on (conceptual) concentric spheres centered on Earth.[22] To explain the complexity of the movements of the planets along with that of the Sun and the Moon, Eudoxus thought they move as if they were attached to a number of concentrical, invisible spheres, every of them rotating around its own and different axis and at different paces. His model had twenty-seven homocentric spheres with each sphere explaining a type of observable motion for each celestial object. Eudoxus emphasised that this is a purely mathematical construct of the model in the sense that the spheres of each celestial body do not exist, it just shows the possible positions of the bodies.[23] His model was later refined and expanded by Callippus.
Geocentric celestial spheres; Peter Apian's Cosmographia (Antwerp, 1539)
  • 4th century BCE – Aristotle follows the Plato's Earth-centered universe in which the Earth is stationary and the cosmos (or universe) is finite in extent but infinite in time. He argued for a spherical Earth using lunar eclipses[24] and other observations. Aristotle adopted and expanded even more the previous Eudoxus' and Callippus' model, but by supposing the spheres were material and crystalline.[25] Aristotle also tried to determine whether the Earth moves and concluded that all the celestial bodies fall towards Earth by natural tendency and since Earth is the centre of that tendency, it is stationary.[26] Plato seems to have obscurely argued that the universe did have a beginning, but Aristotle and others interpreted his words differently.[27]
  • 4th century BCE – De MundoFive 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 aether – make up the whole Universe.[28]
  • 4th century BCE – Heraclides Ponticus is said to be the first Greek who proposes that the Earth rotates on its axis, from west to east, once every 24 hours, contradicting Aristotle's teachings. Simplicius says that Heraclides proposed that the irregular movements of the planets can be explained if the Earth moves while the Sun stays still,[29] but these statements are disputed.[30]
  • 3rd century BCE – Aristarchus of Samos proposes a Sun-centered universe and Earth's rotation in its own axis. He also provides evidences for his theory from his own observations.[31]
  • 3rd century BCE – Archimedes in his essay The Sand Reckoner, estimates the diameter of the cosmos to be the equivalent in stadia of what would in modern times be called two light years, if Aristarcus' theories were correct.
  • 2nd century BCE – Seleucus of Seleucia elaborates on Aristarchus' heliocentric universe, using the phenomenon of tides to explain heliocentrism. Seleucus was the first to prove the heliocentric system through reasoning. 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.[32]
  • 2nd century BCE – Apollonius of Perga shows the equivalence of two descriptions of the apparent retrograde planet motions (assuming the geocentric model), one using eccentrics and another deferent and epicycles.[33] The latter will be a key feature for future models. The epicycle is described as a small orbit within a greater one, called the deferent: as a planet orbits the Earth, it also orbits the original orbit, so its trajectory resembles a curve known as an epitrochoid. This could explain how the planet seems to move as viewed from Earth.
  • 2nd century BCE – Eratosthenes determines that the radius of the Earth is roughly 6,400 km.[34]
  • 2nd century BCE – Hipparchus uses parallax to determine that the distance to the Moon is roughly 380,000 km.[35] The work of Hipparchus about the Earth-Moon system was so accurate that he could forecast solar and lunar eclipses for the next six centuries. Also, he discovers the precession of the equinoxes, and compiles a star catalog of about 850 entries.[36]
  • c. 2nd century BCE–3rd century CE – In Hindu cosmology, the Manusmriti (1.67–80) and Puranas describe time as cyclical, with a new universe (planets and life) created by Brahma every 8.64 billion years. The universe is created, maintained, and destroyed within a kalpa (day of Brahma) period lasting for 4.32 billion years, and is followed by a pralaya (night) period of partial dissolution equal in duration. In some Puranas (e.g. Bhagavata Purana), a larger cycle of time is described where matter (mahat-tattva or universal womb) is created from primal matter (prakriti) and root matter (pradhana) every 622.08 trillion years, from which Brahma is born.[37] The elements of the universe are created, used by Brahma, and fully dissolved within a maha-kalpa (life of Brahma; 100 of his 360-day years) period lasting for 311.04 trillion years containing 36,000 kalpas (days) and pralayas (nights), and is followed by a maha-pralaya period of full dissolution equal in duration.[38][39][40][41] The texts also speak of innumerable worlds or universes.[42]
  • 2nd century CE – Ptolemy proposes an Earth-centered universe, with the Sun, Moon, and visible planets revolving around the Earth. Based on Apollonius' epicycles,[43] he calculates the positions, orbits and positional equations of the Heavenly bodies along with instruments to measure these quantities. Ptolemy emphasised that the epicycle motion does not apply to the Sun. His main contribution to the model was the equant points. He also re-arranged the heavenly spheres in a different order than Plato did (from Earth outward): Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn and fixed stars, following a long astrological tradition and the decreasing orbital periods. His book The Almagest, which also cataloged 1,022 stars and other astronomical objects (largely based upon Hipparchus'), remained the most authoritative text on astronomy and largest astronomical catalogue until the 17th century.[44][45]

Middle Ages[edit]

  • 2nd century CE-5th century CE – Jain cosmology considers the loka, or universe, as an uncreated entity, existing since infinity, the shape of the universe as similar to a man standing with legs apart and arm resting on his waist. This Universe, according to Jainism, is broad at the top, narrow at the middle and once again becomes broad at the bottom.
  • 5th century (or earlier) – Buddhist texts speak of "hundreds of thousands of billions, countlessly, innumerably, boundlessly, incomparably, incalculably, unspeakably, inconceivably, immeasurably, inexplicably many worlds" to the east, and "infinite worlds in the ten directions".[46][47]
  • 5th century – Several Indian astronomers propose a rudimentary Sun-centered universe, including Aryabhata. He also writes a treatise on motion of planets, Sun and Moon and stars. Aryabhatta puts forward the theory of rotation of the Earth in its own axis and explained day and night was caused by the diurnal rotation of the Earth. He also provided empirical evidence for his notion from his astronomical experiments and observation.[48]
  • 5th century – The Jewish talmud gives an argument for finite universe theory along with explanation.
Naboth's representation of Martianus Capella's geo-heliocentric astronomical model (1573)
  • 5th centuryMartianus Capella describes a modified geocentric model, in which the Earth is at rest in the center of the universe and circled by the Moon, the Sun, three planets and the stars, while Mercury and Venus circle the Sun, all surrounded by the sphere of fixed stars.[49]
  • 6th century – John Philoponus proposes a universe that is finite in time and argues against the ancient Greek notion of an infinite universe
  • 7th century – The Quran says in Chapter 21: Verse 30 – "Have those who disbelieved not considered that the Heavens and the Earth were a joined entity, and We separated them".
  • 9th–12th centuries – Al-Kindi (Alkindus), Saadia Gaon (Saadia ben Joseph) and Al-Ghazali (Algazel) support a universe that has a finite past and develop two logical arguments for the notion.
  • 12th century – Fakhr al-Din al-Razi discusses Islamic cosmology, rejects Aristotle's idea of an Earth-centered universe, and, in the context of his commentary on the Quranic verse, "All praise belongs to God, Lord of the Worlds," and proposes that the universe has more than "a thousand worlds beyond this world."[50]
  • 12th century – Robert Grosseteste described the birth of the Universe in an explosion and the crystallisation of matter. He also put forward several new ideas such as rotation of the Earth around its axis and the cause of day and night. His treatise De Luce is the first attempt to describe the heavens and Earth using a single set of physical laws.[51]
  • 14th century – Jewish astronomer Levi ben Gershon (Gersonides) estimates the distance to the outermost orb of the fixed stars to be no less than 159,651,513,380,944 Earth radii, or about 100,000 light-years in modern units.[52]
  • 14th century – Several European mathematicians and astronomers develop the theory of Earth's rotation including Nicole Oresme. Oresme also give logical reasoning, empirical evidence and mathematical proofs for his notion.[53][54]
  • 15th century – Nicholas of Cusa proposes that the Earth rotates on its axis in his book, On Learned Ignorance (1440).[55] Like Oresme, he also wrote about the possibility of the plurality of worlds.[56]


  • 1501 – Indian astronomer Nilakantha Somayaji proposes a universe in which the planets orbit the Sun, but the Sun orbits the Earth.[57]
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica
  • 1543 – Nicolaus Copernicus publishes his heliocentric universe in his De revolutionibus orbium coelestium.[58]
  • 1576 – Thomas Digges modifies the Copernican system by removing its outer edge and replacing the edge with a star-filled unbounded space.[59]
  • 1584 – Giordano Bruno proposes a non-hierarchical cosmology, wherein the Copernican Solar System is not the center of the universe, but rather, a relatively insignificant star system, amongst an infinite multitude of others.[60]
  • 1588 – Tycho Brahe publishes his own Tychonic system, a blend between Ptolemy's classical geocentric model and Copernicus' heliocentric model, in which the Sun and the Moon revolve around the Earth, in the center of universe, and all other planets revolve around the Sun.[61] It is a geo-heliocentric model similar to that described by Somayaji.
  • 1600 – William Gilbert rejects the idea of a limiting sphere of the fixed stars for which no proof has been offered.[62]
  • 1609 – Galileo Galilei examines the skies and constellations through a telescope and concluded that the "fixed stars" which had been studied and mapped were only a tiny portion of the massive universe that lay beyond the reach of the naked eye.[63] When in 1610 he aimed his telescope to the faint strip of the Milky Way, he found it resolves into countless white star-like spots, presumably farther stars themselves.[64]
  • 1610 – Johannes Kepler uses the dark night sky to argue for a finite universe. Shortly after, it was proved by Kepler himself that the Jupiter's moons move around the planet the same way planets orbit the Sun, thus making Kepler's laws universal.[65]

Enlightenment to Victorian Era[edit]

William Herschel's model of the Milky Way, 1785
One of Andrew Ainslie Common's 1883 photographs of the Orion nebula, the first to show that a long exposure could record stars and nebulae invisible to the human eye.


The earliest known photograph of the Great Andromeda "Nebula" (with M110 to upper left), by Isaac Roberts, 1899.
Three steps to the Hubble constant[92]


The Karl G. Jansky Very Large Array, a radio interferometer in New Mexico, United States.
The sky at energies above 100 MeV observed by the Energetic Gamma Ray Experiment Telescope (EGRET) of the Compton Gamma Ray Observatory (CGRO) satellite (1991–2000).


  • 2001 – The 2dF Galaxy Redshift Survey (2dF) by an Australian/British team gave strong evidence that the matter density is near 25% of critical density. Together with the CMB results for a flat universe, this provides independent evidence for a cosmological constant or similar dark energy.
  • 2002 – The Cosmic Background Imager (CBI) in Chile obtained images of the cosmic microwave background radiation with the highest angular resolution of 4 arc minutes. It also obtained the anisotropy spectrum at high-resolution not covered before up to l ~ 3000. It found a slight excess in power at high-resolution (l > 2500) not yet completely explained, the so-called "CBI-excess".
  • 2003 – NASA's Wilkinson Microwave Anisotropy Probe (WMAP) obtained full-sky detailed pictures of the cosmic microwave background radiation. The images can be interpreted to indicate that the universe is 13.7 billion years old (within one percent error), and are very consistent with the Lambda-CDM model and the density fluctuations predicted by inflation.
Cosmic microwave background as measured by the Cosmic Background Imager experiment.

See also[edit]

Physical cosmology[edit]

Historical development of hypotheses[edit]

Belief systems[edit]



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