Cosmology


Cosmology is a branch of astronomy concerned with the studies of the origin and evolution of the universe, from the Big Bang to today and on into the future. It is the scientific study of the origin, evolution, and eventual fate of the universe. Physical cosmology is the scientific study of the universe's origin, its large-scale structures and dynamics, and its ultimate fate, as well as the laws of science that govern these areas.
The term cosmology was first used in English in 1656 in Thomas Blount's Glossographia, and in 1731 taken up in Latin by German philosopher Christian Wolff, in Cosmologia Generalis.
Religious or mythological cosmology is a body of beliefs based on mythological, religious, and esoteric literature and traditions of creation myths and eschatology.
Physical cosmology is studied by scientists, such as astronomers and physicists, as well as philosophers, such as metaphysicians, philosophers of physics, and philosophers of space and time. Because of this shared scope with philosophy, theories in physical cosmology may include both scientific and non-scientific propositions, and may depend upon assumptions that cannot be tested. Cosmology differs from astronomy in that the former is concerned with the Universe as a whole while the latter deals with individual celestial objects. Modern physical cosmology is dominated by the Big Bang theory, which attempts to bring together observational astronomy and particle physics; more specifically, a standard parameterization of the Big Bang with dark matter and dark energy, known as the Lambda-CDM model.
Theoretical astrophysicist David N. Spergel has described cosmology as a "historical science" because "when we look out in space, we look back in time" due to the finite nature of the speed of light.

Disciplines

and astrophysics have played a central role in shaping the understanding of the universe through scientific observation and experiment. Physical cosmology was shaped through both mathematics and observation in an analysis of the whole universe. The universe is generally understood to have begun with the Big Bang, followed almost instantaneously by cosmic inflation; an expansion of space from which the universe is thought to have emerged 13.799 ± 0.021 billion years ago. Cosmogony studies the origin of the Universe, and cosmography maps the features of the Universe.
In Diderot's Encyclopédie, cosmology is broken down into uranology, aerology, geology, and hydrology.
Metaphysical cosmology has also been described as the placing of humans in the universe in relationship to all other entities. This is exemplified by Marcus Aurelius's observation that a man's place in that relationship: "He who does not know what the world is does not know where he is, and he who does not know for what purpose the world exists, does not know who he is, nor what the world is."

Discoveries

Physical cosmology

Physical cosmology is the branch of physics and astrophysics that deals with the study of the physical origins and evolution of the Universe. It also includes the study of the nature of the Universe on a large scale. In its earliest form, it was what is now known as "celestial mechanics", the study of the heavens. Greek philosophers Aristarchus of Samos, Aristotle, and Ptolemy proposed different cosmological theories. The geocentric Ptolemaic system was the prevailing theory until the 16th century when Nicolaus Copernicus, and subsequently Johannes Kepler and Galileo Galilei, proposed a heliocentric system. This is one of the most famous examples of epistemological rupture in physical cosmology.
in the infant universe may have been uncovered by the microscopic examination of the focal plane of the BICEP2 radio telescope.
Isaac Newton's Principia Mathematica, published in 1687, was the first description of the law of universal gravitation. It provided a physical mechanism for Kepler's laws and also allowed the anomalies in previous systems, caused by gravitational interaction between the planets, to be resolved. A fundamental difference between Newton's cosmology and those preceding it was the Copernican principle—that the bodies on earth obey the same physical laws as all the celestial bodies. This was a crucial philosophical advance in physical cosmology.
Modern scientific cosmology is usually considered to have begun in 1917 with Albert Einstein's publication of his final modification of general relativity in the paper "Cosmological Considerations of the General Theory of Relativity". General relativity prompted cosmogonists such as Willem de Sitter, Karl Schwarzschild, and Arthur Eddington to explore its astronomical ramifications, which enhanced the ability of astronomers to study very distant objects. Physicists began changing the assumption that the Universe was static and unchanging. In 1922 Alexander Friedmann introduced the idea of an expanding universe that contained moving matter. Around the same time the Great Debate took place, with early cosmologists such as Heber Curtis and Ernst Öpik determining that some nebulae seen in telescopes were separate galaxies far distant from our own.
In parallel to this dynamic approach to cosmology, one long-standing debate about the structure of the cosmos was coming to a climax. Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Way star system only; while Heber D. Curtis argued for the idea that spiral nebulae were star systems in their own right as island universes. This difference of ideas came to a climax with the organization of the Great Debate on 26 April 1920 at the meeting of the U.S. National Academy of Sciences in Washington, D.C. The debate was resolved when Edwin Hubble detected Cepheid Variables in the Andromeda Galaxy in 1923 and 1924. Their distance established spiral nebulae well beyond the edge of the Milky Way.
Subsequent modelling of the universe explored the possibility that the cosmological constant, introduced by Einstein in his 1917 paper, may result in an expanding universe, depending on its value. Thus the Big Bang model was proposed by the Belgian priest Georges Lemaître in 1927 which was subsequently corroborated by Edwin Hubble's discovery of the redshift in 1929 and later by the discovery of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson in 1964. These findings were a first step to rule out some of many alternative cosmologies.
Since around 1990, several dramatic advances in observational cosmology have transformed cosmology from a largely speculative science into a predictive science with precise agreement between theory and observation. These advances include observations of the microwave background from the COBE, WMAP and Planck satellites, large new galaxy redshift surveys including 2dfGRS and SDSS, and observations of distant supernovae and gravitational lensing. These observations matched the predictions of the cosmic inflation theory, a modified Big Bang theory, and the specific version known as the Lambda-CDM model. This has led many to refer to modern times as the "golden age of cosmology".
On 17 March 2014, astronomers at the Harvard-Smithsonian Center for Astrophysics announced the detection of gravitational waves, providing strong evidence for inflation and the Big Bang. However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported.
On 1 December 2014, at the Planck 2014 meeting in Ferrara, Italy, astronomers reported that the universe is 13.8 billion years old and is composed of 4.9% atomic matter, 26.6% dark matter and 68.5% dark energy.

Religious or mythological cosmology

or mythological cosmology is a body of beliefs based on mythological, religious, and esoteric literature and traditions of creation and eschatology.

Philosophical cosmology

Cosmology deals with the world as the totality of space, time and all phenomena. Historically, it has had quite a broad scope, and in many cases was founded in religion. In modern use metaphysical cosmology addresses questions about the Universe which are beyond the scope of science. It is distinguished from religious cosmology in that it approaches these questions using philosophical methods like dialectics. Modern metaphysical cosmology tries to address questions such as:
NameAuthor and dateClassificationRemarks
Hindu cosmologyRigveda Cyclical or oscillating, Infinite in timePrimal matter remains manifest for 311.04 trillion years and unmanifest for an equal length. The universe remains manifest for 4.32 billion years and unmanifest for an equal length. Innumerable universes exist simultaneously. These cycles have and will last forever, driven by desires.
Jain cosmologyJain Agamas Cyclical or oscillating, eternal and finiteJain 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.
Babylonian cosmologyBabylonian literature Flat earth floating in infinite "waters of chaos"The Earth and the Heavens form a unit within infinite "waters of chaos"; the earth is flat and circular, and a solid dome keeps out the outer "chaos"-ocean.
Eleatic cosmologyParmenides Finite and spherical in extentThe Universe is unchanging, uniform, perfect, necessary, timeless, and neither generated nor perishable. Void is impossible. Plurality and change are products of epistemic ignorance derived from sense experience. Temporal and spatial limits are arbitrary and relative to the Parmenidean whole.
Biblical cosmologyGenesis creation narrativeEarth floating in infinite "waters of chaos"The Earth and the Heavens form a unit within infinite "waters of chaos"; the "firmament" keeps out the outer "chaos"-ocean.
Atomist universeAnaxagoras & later EpicurusInfinite in extentThe universe contains only two things: an infinite number of tiny seeds and the void of infinite extent. All atoms are made of the same substance, but differ in size and shape. Objects are formed from atom aggregations and decay back into atoms. Incorporates Leucippus' principle of causality: "nothing happens at random; everything happens out of reason and necessity". The universe was not ruled by gods.
Pythagorean universePhilolaus Existence of a "Central Fire" at the center of the Universe.At the center of the Universe is a central fire, around which the Earth, Sun, Moon and planets revolve uniformly. The Sun revolves around the central fire once a year, the stars are immobile. The earth in its motion maintains the same hidden face towards the central fire, hence it is never seen. First known non-geocentric model of the Universe.
De MundoPseudo-Aristotle The Universe then is a system made up of heaven and earth and the elements which are contained in them.There are "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."
Stoic universeStoics Island universeThe cosmos is finite and surrounded by an infinite void. It is in a state of flux, and pulsates in size and undergoes periodic upheavals and conflagrations.
Aristotelian universeAristotle Geocentric, static, steady state, finite extent, infinite timeSpherical earth is surrounded by concentric celestial spheres. Universe exists unchanged throughout eternity. Contains a fifth element, called aether, that was added to the four classical elements.
Aristarchean universeAristarchus HeliocentricEarth rotates daily on its axis and revolves annually about the sun in a circular orbit. Sphere of fixed stars is centered about the sun.
Ptolemaic modelPtolemy Geocentric Universe orbits around a stationary Earth. Planets move in circular epicycles, each having a center that moved in a larger circular orbit around a center-point near Earth. The use of equants added another level of complexity and allowed astronomers to predict the positions of the planets. The most successful universe model of all time, using the criterion of longevity. Almagest.
Aryabhatan modelAryabhata Geocentric or HeliocentricThe Earth rotates and the planets move in elliptical orbits around either the Earth or Sun; uncertain whether the model is geocentric or heliocentric due to planetary orbits given with respect to both the Earth and Sun.
Medieval universeMedieval philosophers Finite in timeA universe that is finite in time and has a beginning is proposed by the Christian philosopher John Philoponus, who argues against the ancient Greek notion of an infinite past. Logical arguments supporting a finite universe are developed by the early Muslim philosopher Alkindus, the Jewish philosopher Saadia Gaon, and the Muslim theologian Algazel.
Multiversal cosmologyFakhr al-Din al-Razi Multiverse, multiple worlds and universesThere exists an infinite outer space beyond the known world, and God has the power to fill the vacuum with an infinite number of universes.
Maragha modelsMaragha school GeocentricVarious modifications to Ptolemaic model and Aristotelian universe, including rejection of equant and eccentrics at Maragheh observatory, and introduction of Tusi-couple by Al-Tusi. Alternative models later proposed, including the first accurate lunar model by Ibn al-Shatir, a model rejecting stationary Earth in favour of Earth's rotation by Ali Kuşçu, and planetary model incorporating "circular inertia" by Al-Birjandi.
Nilakanthan modelNilakantha Somayaji Geocentric and heliocentricA universe in which the planets orbit the Sun, which orbits the Earth; similar to the later Tychonic system
Copernican universeNicolaus Copernicus Heliocentric with circular planetary orbitsFirst described in De revolutionibus orbium coelestium.
Tychonic systemTycho Brahe Geocentric and HeliocentricA universe in which the planets orbit the Sun and the Sun orbits the Earth, similar to the earlier Nilakanthan model.
Bruno's cosmologyGiordano Bruno Infinite extent, infinite time, homogeneous, isotropic, non-hierarchicalRejects the idea of a hierarchical universe. Earth and Sun have no special properties in comparison with the other heavenly bodies. The void between the stars is filled with aether, and matter is composed of the same four elements, and is atomistic, animistic and intelligent.
KeplerianJohannes Kepler Heliocentric with elliptical planetary orbitsKepler's discoveries, marrying mathematics and physics, provided the foundation for our present conception of the Solar system, but distant stars were still seen as objects in a thin, fixed celestial sphere.
Static NewtonianIsaac Newton Static, steady state, infiniteEvery particle in the universe attracts every other particle. Matter on the large scale is uniformly distributed. Gravitationally balanced but unstable.
Cartesian Vortex universeRené Descartes, 17th centuryStatic, steady state, infiniteSystem of huge swirling whirlpools of aethereal or fine matter produces what we would call gravitational effects. But his vacuum was not empty; all space was filled with matter.
Hierarchical universeImmanuel Kant, Johann Lambert, 18th centuryStatic, steady state, infiniteMatter is clustered on ever larger scales of hierarchy. Matter is endlessly recycled.
Einstein Universe with a cosmological constantAlbert Einstein, 1917Static. Bounded "Matter without motion". Contains uniformly distributed matter. Uniformly curved spherical space; based on Riemann's hypersphere. Curvature is set equal to Λ. In effect Λ is equivalent to a repulsive force which counteracts gravity. Unstable.
De Sitter universeWillem de Sitter, 1917Expanding flat space.
Steady state.
Λ > 0		"Motion without matter." Only apparently static. Based on Einstein's general relativity. Space expands with constant acceleration. Scale factor increases exponentially.
MacMillan universeWilliam Duncan MacMillan 1920sStatic and steady stateNew matter is created from radiation; starlight perpetually recycled into new matter particles.
Friedmann universe, spherical spaceAlexander Friedmann 1922Spherical expanding space.
k = +1 ; no Λ		Positive curvature. Curvature constant k = +1
Expands then recollapses. Spatially closed.
Friedmann universe, hyperbolic spaceAlexander Friedmann, 1924Hyperbolic expanding space.
k = −1 ; no Λ		Negative curvature. Said to be infinite. Unbounded. Expands forever.
Dirac large numbers hypothesisPaul Dirac 1930sExpandingDemands a large variation in G, which decreases with time. Gravity weakens as universe evolves.
Friedmann zero-curvatureEinstein and De Sitter, 1932Expanding flat space
k = 0 ; Λ = 0
Critical density		Curvature constant k = 0. Said to be infinite. "Unbounded cosmos of limited extent". Expands forever. "Simplest" of all known universes. Named after but not considered by Friedmann. Has a deceleration term q = 1/2, which means that its expansion rate slows down.
The original Big Bang Georges Lemaître 1927–29Expansion
Λ > 0 ; Λ > |Gravity|		Λ is positive and has a magnitude greater than gravity. Universe has initial high-density state. Followed by a two-stage expansion. Λ is used to destabilize the universe.
Oscillating universe Favored by Friedmann, 1920sExpanding and contracting in cyclesTime is endless and beginningless; thus avoids the beginning-of-time paradox. Perpetual cycles of Big Bang followed by Big Crunch.
Eddington universeArthur Eddington 1930First static then expandsStatic Einstein 1917 universe with its instability disturbed into expansion mode; with relentless matter dilution becomes a De Sitter universe. Λ dominates gravity.
Milne universe of kinematic relativityEdward Milne, 1933, 1935;
William H. McCrea, 1930s		Kinematic expansion without space expansionRejects general relativity and the expanding space paradigm. Gravity not included as initial assumption. Obeys cosmological principle and special relativity; consists of a finite spherical cloud of particles that expands within an infinite and otherwise empty flat space. It has a center and a cosmic edge that expands at light speed. Explanation of gravity was elaborate and unconvincing.
Friedmann–Lemaître–Robertson–Walker class of modelsHoward Robertson, Arthur Walker, 1935Uniformly expandingClass of universes that are homogeneous and isotropic. Spacetime separates into uniformly curved space and cosmic time common to all co-moving observers. The formulation system is now known as the FLRW or Robertson–Walker metrics of cosmic time and curved space.
Steady-stateHermann Bondi, Thomas Gold, 1948Expanding, steady state, infiniteMatter creation rate maintains constant density. Continuous creation out of nothing from nowhere. Exponential expansion. Deceleration term q = −1.
Steady-stateFred Hoyle 1948Expanding, steady state; but unstableMatter creation rate maintains constant density. But since matter creation rate must be exactly balanced with the space expansion rate the system is unstable.
AmbiplasmaHannes Alfvén 1965 Oskar KleinCellular universe, expanding by means of matter–antimatter annihilationBased on the concept of plasma cosmology. The universe is viewed as "meta-galaxies" divided by double layers and thus a bubble-like nature. Other universes are formed from other bubbles. Ongoing cosmic matter-antimatter annihilations keep the bubbles separated and moving apart preventing them from interacting.
Brans–Dicke theoryCarl H. Brans, Robert H. DickeExpandingBased on Mach's principle. G varies with time as universe expands. "But nobody is quite sure what Mach's principle actually means."
Cosmic inflationAlan Guth 1980Big Bang modified to solve horizon and flatness problemsBased on the concept of hot inflation. The universe is viewed as a multiple quantum flux – hence its bubble-like nature. Other universes are formed from other bubbles. Ongoing cosmic expansion kept the bubbles separated and moving apart.
Eternal inflation Andreï Linde, 1983Big Bang with cosmic inflationMultiverse based on the concept of cold inflation, in which inflationary events occur at random each with independent initial conditions; some expand into bubble universes supposedly like our entire cosmos. Bubbles nucleate in a spacetime foam.
Cyclic modelPaul Steinhardt; Neil Turok 2002Expanding and contracting in cycles; M-theory.Two parallel orbifold planes or M-branes collide periodically in a higher-dimensional space. With quintessence or dark energy.
Cyclic modelLauris Baum; Paul Frampton 2007Solution of Tolman's entropy problemPhantom dark energy fragments universe into large number of disconnected patches. Our patch contracts containing only dark energy with zero entropy.

Table notes: the term "static" simply means not expanding and not contracting. Symbol G represents Newton's gravitational constant; Λ is the cosmological constant.