# Cosmology

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**Cosmology** is the study of the origins and eventual fate of the universe. Physical cosmology is the scholarly and scientific study of the origin, evolution, structure, dynamics, and ultimate fate of the universe, as well as the natural laws that keep it in order. Religious cosmology (or mythological cosmology) is a body of beliefs based on the historical, mythological, religious, and esoteric literature and traditions of creation and eschatology.

Physical cosmology is studied by scientists, such as astronomers, and theoretical physicists; and academic philosophers, such as metaphysicians, philosophers of physics, and philosophers of space and time. Modern cosmology is dominated by the Big Bang theory, which attempts to bring together observational astronomy and particle physics.

Although the word *cosmology* is recent (first used in 1730 in Christian Wolff's *Cosmologia Generalis*), the study of the universe has a long history involving science, philosophy, esotericism and religion. Related studies include cosmogony, which focuses on the origin of the Universe, and cosmography, which maps the features of the Universe. Cosmology is also connected to astronomy. However, they are contrasted in that while the former is concerned with the Universe as a whole, the latter deals with individual celestial objects.

## Disciplines

In old times, physics and astrophysics have played a central role in shaping the understanding of the universe through scientific observation and experiment. What is known as physical cosmology shaped through both mathematics and observation the analysis of the whole universe. It is generally understood to begin with the Big Bang, followed almost instantaneously by cosmic inflation - an expansion of space from which the universe is thought to have emerged ~13.7±0.2×10^{9} (roughly 13.5–13.9 billion) years ago.

Physical cosmologists propose that the history of the universe has been governed entirely by physical laws. Between the domains of religion and science stands the philosophical perspective of metaphysical cosmology. This ancient field of study seeks to draw intuitive conclusions about the nature of the universe, man, a supernatural creator, and/or their relationships based on the extension of some set of presumed facts borrowed from spiritual experience and/or observation.

Metaphysical cosmology has also been described as the placing of man in the universe in relationship to all other entities. This is exemplified by the observation made by Marcus Aurelius of 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."

Religious creation myths explain existence and nature of reality. Views about the origin ( cosmogony) and final condition ( eschatology) of the universe are foundational elements of a religious understanding of humanity's role in the universe.

Esoteric cosmology is distinguished from religion in its less tradition-bound construction and reliance on modern "intellectual understanding" rather than faith, and from philosophy in its emphasis on spirituality as a formative concept.

## Historical cosmologies

Name | Author and date | Classification | Remarks |
---|---|---|---|

Hindu cosmology | Hindu Rigveda (2000 BC) | Cyclical or oscillating, Infinite in time | One cycle of existence is around 311 trillion years and the life of one universe around 8 billion years. This Universal cycle is preceded by an infinite number of universes and to be followed by another infinite number of universes. Includes an infinite number of universes at one given time. |

Jain cosmology | Jain Agamas (written around 500 AD as per the teachings of Mahavira 599-527 BC) | Cyclical or oscillating, eternal and finite | 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 narrow at the top, broad at the middle and once again becomes broad at the bottom. |

Babylonian cosmology | Babylonian literature (c. 3000 BC) | 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 (the "firmament") keeps out the outer "chaos"-ocean. |

Biblical cosmology | Genesis creation narrative (c.500 BC) | Flat earth floating in infinite "waters of chaos" | Based on Babylonian cosmology. The Earth and the Heavens form a unit within infinite "waters of chaos"; the earth is flat and circular, and a solid dome (the " firmament") keeps out the outer "chaos"-ocean. |

Atomist universe | Anaxagoras (500–428 BC) & later Epicurus | Infinite in extent | The universe contains only two things: an infinite number of tiny seeds, or atoms, 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 universe | Philolaus (d. 390 BC) | Existence of a "Central Fire" at the centre of the Universe. | At the centre 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. This is the first known non-geocentric model of the Universe. |

Stoic universe | Stoics (300 BC – 200 AD) | Island universe | The cosmos is finite and surrounded by an infinite void. It is in a state of flux, as it pulsates in size and periodically passes through upheavals and conflagrations. |

Aristotelian universe | Aristotle (384–322 BC) | Geocentric, static, steady state, finite extent, infinite time | Spherical earth is surrounded by concentric celestial spheres. Universe exists unchanged throughout eternity. Contains a fifth element, called aether (later known as quintessence), added to the four Classical elements. |

Aristarchean universe | Aristarchus (circa 280 BC) | Heliocentric | Earth 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 model (based on Aristotelian universe) | Ptolemy (2nd century AD) | Geocentric | Universe orbits about a stationary Earth. Planets move in circular epicycles, each having a center that moved in a larger circular orbit (called an eccentric or a deferent) around a centre-point near the 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 (the Great System). |

Aryabhatan model | Aryabhata (499) | Geocentric or Heliocentric | The Earth rotates and the planets move in elliptical orbits, possibly around either the Earth or the Sun. It is uncertain whether the model is geocentric or heliocentric due to planetary orbits given with respect to both the Earth and the Sun. |

Medieval universe | Medieval philosophers (500–1200) | Finite in time | A 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 cosmology | Fakhr al-Din al-Razi (1149–1209) | Multiverse, multiple worlds & universes | There 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 models | Maragha school (1259–1528) | Geocentric | Various 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 model | Nilakantha Somayaji (1444–1544) | Geocentric and Heliocentric | A universe in which the planets orbit the Sun and the Sun orbits the Earth, similar to the later Tychonic system. |

Copernican universe | Nicolaus Copernicus (1473–1543) | Heliocentric with circular planetary orbits | First clearly described heliocentric model, in De revolutionibus orbium coelestium. |

Tychonic system | Tycho Brahe (1546–1601) | Geocentric and Heliocentric | A universe in which the planets orbit the Sun and the Sun orbits the Earth, similar to the earlier Nilakanthan model. |

Keplerian | Johann Kepler (1571–1630) | Heliocentric with elliptical planetary orbits | Kepler'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 Newtonian | Sir Isaac Newton (1642–1727) | Static (evolving), steady state, infinite | Every particle in the universe attracts every other particle. Matter on the large scale is uniformly distributed. Gravitationally balanced but unstable. |

Cartesian Vortex universe | René Descartes 17th century |
Static (evolving), steady state, infinite | A system of huge swirling whirlpools of aethereal or fine matter produces what we would call gravitational effects. His vacuum was not empty. All space was filled with matter that swirled around in large and small vortices. |

Hierarchical universe | Immanuel Kant, Johann Lambert 18th century | Static (evolving), steady state, infinite | Matter is clustered on ever larger scales of hierarchy. Matter is endlessly being recycled. |

Einstein Universe with a cosmological constant | Albert Einstein 1917 | Static (nominally). Bounded (finite) | "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 universe | Willem de Sitter 1917 | Expanding 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 (radius of universe) increases exponentially, i.e. constant inflation. |

MacMillan universe | William Duncan MacMillan 1920s | Static & steady state |
New matter is created from radiation. Starlight is perpetually recycled into new matter particles. |

Friedmann universe of spherical space | Alexander Friedmann 1922 | Spherical expanding space. k= +1 ; no Λ |
Positive curvature. Curvature constant k = +1 Expands then recollapses. Spatially closed (finite). |

Friedmann universe of hyperbolic space | Alexander Friedmann 1924 | Hyperbolic expanding space. k= -1 ; no Λ |
Negative curvature. Said to be infinite (but ambiguous). Unbounded. Expands forever. |

Dirac large numbers hypothesis | Paul Dirac 1930s | Expanding | Demands a large variation in G, which decreases with time. Gravity weakens as universe evolves. |

Friedmann zero-curvature, a.k.a. the Einstein-DeSitter universe | Einstein & DeSitter 1932 | Expanding flat space. k= 0 ; Λ = 0 Critical density |
Curvature constant k = 0. Said to be infinite (but ambiguous). 'Unbounded cosmos of limited extent.' Expands forever. 'Simplest' of all known universes. Named after but not considered by Friedmann. Has a deceleration term q =½ which means that its expansion rate slows down. |

The original Big Bang. a.k.a. Friedmann-Lemaître Model | Georges Lemaître 1927–29 | Expansion Λ > 0 Λ > |Gravity| |
Λ is positive and has a magnitude greater than Gravity. Universe has initial high density state ('primeval atom'). Followed by a two stage expansion. Λ is used to destabilize the universe. (Lemaître is considered to be the father of the big bang model.) |

Oscillating universe (a.k.a. Friedmann-Einstein; was latter's 1st choice after rejecting his own 1917 model) |
Favored by Friedmann 1920s |
Expanding and contracting in cycles | Time is endless and beginningless; thus avoids the beginning-of-time paradox. Perpetual cycles of big bang followed by big crunch. |

Eddington | Arthur Eddington 1930 | First Static then Expands |
Static Einstein 1917 universe with its instability disturbed into expansion mode; with relentless matter dilution becomes a DeSitter universe. Λ dominates gravity. |

Milne universe of kinematic relativity | Edward Milne, 1933, 1935; William H. McCrea, 1930s |
Kinematic expansion with NO space expansion | Rejects general relativity and the expanding space paradigm. Gravity not included as initial assumption. Obeys cosmological principle & rules of special relativity. The Milne expanding universe consists of a finite spherical cloud of particles (or galaxies) that expands WITHIN flat space which is infinite and otherwise empty. It has a centre and a cosmic edge (the surface of the particle cloud) which expands at light speed. His explanation of gravity was elaborate and unconvincing. For instance, his universe has an infinite number of particles, hence infinite mass, within a finite cosmic volume. |

Friedmann-Lemaître-Robertson-Walker class of models | Howard Robertson, Arthur Walker, 1935 | Uniformly expanding | Class of universes that are homogenous and isotropic. Spacetime separates into uniformly curved space and cosmic time common to all comoving observers. The formulation system is now known as the FLRW or Robertson-Walker metrics of cosmic time and curved space. |

Steady-state expanding (Bondi & Gold) | Herman Bondi, Thomas Gold 1948 | Expanding, steady state, infinite | Matter creation rate maintains constant density. Continuous creation out of nothing from nowhere. Exponential expansion. Deceleration term q = -1. |

Steady-state expanding (Hoyle) | Fred Hoyle 1948 | Expanding, steady state; but unstable | Matter creation rate maintains constant density. But since matter creation rate must be exactly balanced with the space expansion rate the system is unstable. |

Ambiplasma | Hannes Alfvén 1965 Oskar Klein | Cellular universe, expanding by means of matter-antimatter annihilation | Based on the concept of plasma cosmology. The universe is viewed as meta-galaxies divided by double layers —hence its 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 | Carl H. Brans; Robert H. Dicke | Expanding | Based on Mach's principle. G varies with time as universe expands. "But nobody is quite sure what Mach's principle actually means." |

Cosmic inflation | Alan Guth 1980 | Big Bang with modification to solve horizon problem and flatness problem. | Based 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 preventing them from interacting. |

Eternal Inflation (a multiple universe model) | Andreï Linde 1983 | Big Bang with cosmic inflation | A multiverse, 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 model | Paul Steinhardt; Neil Turok 2002 | Expanding 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 model | Lauris Baum; Paul Frampton 2007 | Solution of Tolman's entropy problem | Phantom 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 (Lambda) is the cosmological constant.

## 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 its very largest scales. In its earliest form it was what is now known as celestial mechanics, the study of the heavens. The Greek philosophers Aristarchus of Samos, Aristotle and Ptolemy proposed different cosmological theories. In particular, the geocentric Ptolemaic system was the accepted theory to explain the motion of the heavens until Nicolaus Copernicus, and subsequently Johannes Kepler and Galileo Galilei proposed a heliocentric system in the 16th century. This is known as one of the most famous examples of epistemological rupture in physical cosmology.

With Isaac Newton and the 1687 publication of *Principia Mathematica*, the problem of the motion of the heavens was finally solved. Newton provided a physical mechanism for Kepler's laws and his law of universal gravitation 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," (although this paper was not widely available outside of Germany until the end of World War I). General relativity prompted cosmogonists such as Willem de Sitter, Karl Schwarzschild and Arthur Eddington to explore the astronomical consequences of the theory, which enhanced the growing ability of astronomers to study very distant objects. Prior to this (and for some time afterwards), physicists assumed that the Universe was static and unchanging.

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 – island universes. This difference of ideas came to a climax with the organization of the Great Debate at the meeting of the (US) National Academy of Sciences in Washington on 26 April 1920. The resolution of this debate came with the detection of novae in the Andromeda galaxy by Edwin Hubble in 1923 and 1924. Their distance established spiral nebulae well beyond the edge of the Milky Way and has galaxies of their own.

Subsequent modeling 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 red shift 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 physical cosmologies.

Recent observations made by the COBE and WMAP satellites observing this background radiation have effectively, in many scientists' eyes, transformed cosmology from a highly speculative science into a predictive science, as these observations matched predictions made by a theory called Cosmic inflation, which is a modification of the standard big bang model. This has led many to refer to modern times as the "Golden age of cosmology."

## Religious and mythological cosmology

Religious and mythological 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. The ancient Greeks did not draw a distinction between this use and their model for the cosmos. However, in modern use it 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 (e.g. dialectics). Modern metaphysical cosmology tries to address questions such as:

- What is the origin of the Universe? What is its first cause? Is its existence necessary? (see monism, pantheism, emanationism and creationism)
- What are the ultimate material components of the Universe? (see mechanism, dynamism, hylomorphism, atomism)
- What is the ultimate reason for the existence of the Universe? Does the cosmos have a purpose? (see teleology)
- Does the existence of consciousness have a purpose? How do we know what we know about the totality of the cosmos? Does cosmological reasoning reveal metaphysical truths? (see epistemology)