SafekipediaCosmology is the study of the universe and everything in it. The word comes from old words meaning "the world" and "the study of." People who study cosmology include scientists like astronomers and physicists, as well as philosophers who think about big questions about space and time. Cosmology can include scientific facts and ideas that are hard to test.
In science, physical cosmology looks at how the universe began, how it has changed over time, and what might happen in the future. Scientists use observations of stars, galaxies, and other objects to understand these big questions. One important idea in cosmology is the Big Bang Theory. This theory helps explain how the universe started and how it has changed, using ideas from astronomy and physics.
Because light can only travel so fast, when we look far into space, we are also looking back in time. This makes cosmology a special science that lets us explore the universe's history.
Disciplines
Physics and astrophysics help us learn about the universe by watching and testing. Physical cosmology studies the whole universe using math and what we see. We think the universe started with the Big Bang, and then cosmic inflation made space grow very fast, about 13.8 billion years ago. Cosmogony looks at how the universe began, and cosmography draws maps of its features.
Long ago, people split cosmology into different parts: uranology studied the sky, aerology the air, geology the land, and hydrology the water. Metaphysical cosmology thinks about how humans are part of the universe, like the idea that learning about the world's purpose helps us know ourselves.
Discoveries
Main article: Physical cosmology
See also: Observational cosmology
Physical cosmology is a part of science that studies how the universe began and how it has changed. It looks at the universe on very large scales. Long ago, people like Aristarchus of Samos, Aristotle, and Ptolemy thought Earth was the center of everything. This idea is called the geocentric Ptolemaic system.
Later, Nicolaus Copernicus, Johannes Kepler, and Galileo Galilei showed that the Sun, not Earth, is at the center of our solar system. This is called the heliocentric view.
Modern cosmology started with Albert Einstein in 1917. He used his theory of general relativity to describe the universe. This helped scientists think the universe might be changing. In the 1920s, Edwin Hubble showed that there are many galaxies besides our own Milky Way. Later, scientists found something called the cosmic microwave background radiation. This helped them understand that the universe began with a big event called the Big Bang. Today, we know the universe is very old and is made of different kinds of matter and energy.
See also: Religious cosmology
Different cultures have stories about how the world was made. These stories often talk about gods or powerful beings who created the world in many ways.
Cosmology also has big questions that science can’t always answer, like why the universe exists. People have thought about these questions in many ways throughout history.
Historical cosmologies
Further information: Timeline of cosmological theories
Historical cosmologies are the different ways people have thought about the universe over time. Long ago, many cultures had their own ideas about how the universe began and worked. These early ideas often included myths and stories about gods and natural forces. Over time, these ideas changed and grew into the science of cosmology we know today.
The term "static" in these early models means the universe was thought to stay the same, not growing bigger or smaller. Important ideas like Newton's gravitational constant and the cosmological constant later helped us understand how the universe is built and how it changes.
| Name | Author and date | Classification | Remarks |
|---|---|---|---|
| Hindu cosmology | Rigveda (c. 1700–1100 BCE) | Cyclical or oscillating, Infinite in time | Primal matter remains manifest for 311.04 trillion years and unmanifest for an equal length of time. The universe remains manifest for 4.32 billion years and unmanifest for an equal length of time. Innumerable universes exist simultaneously. These cycles have and will last forever, driven by desires. |
| Zoroastrian Cosmology | Avesta (c. 1500–600 BCE) | Dualistic Cosmology | According to Zoroastrian Cosmology, the universe is the manifestation of perpetual conflict between Existence and non-existence, Good and evil and light and darkness. the universe will remain in this state for 12000 years; at the time of resurrection, the two elements will be separated again. |
| Jain cosmology | Jain Agamas (written around 500 CE as per the teachings of Mahavira 599–527 BCE) | 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 broad at the top, narrow at the middle and once again becomes broad at the bottom. |
| Babylonian cosmology | Babylonian literature (c. 2300–500 BCE) | 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. |
| Eleatic cosmology | Parmenides (c. 515 BCE) | Finite and spherical in extent | The 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. |
| Samkhya Cosmic Evolution | Kapila (6th century BCE), pupil Asuri | Prakriti (Matter) and Purusha (Consiouness) Relation | Prakriti (Matter) is the source of the world of becoming. It is pure potentiality that evolves itself successively into twenty four tattvas or principles. The evolution itself is possible because Prakriti is always in a state of tension among its constituent strands known as gunas (Sattva (lightness or purity), Rajas (passion or activity), and Tamas (inertia or heaviness)). The cause and effect theory of Sankhya is called Satkaarya-vaada (theory of existent causes), and holds that nothing can really be created from or destroyed into nothingness—all evolution is simply the transformation of primal Nature from one form to another. |
| Biblical cosmology | Genesis creation narrative | Earth 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. |
| Anaximander's model | Anaximander (c. 560 BCE) | Geocentric, cylindrical Earth, infinite in extent, finite time; first purely mechanical model | The Earth floats very still in the centre of the infinite, not supported by anything. 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; no heavenly bodies as such, only light through the holes. Three wheels, in order outwards from Earth: stars (including planets), moon, and a large Sun. |
| Atomist universe | Anaxagoras (500–428 BCE) and later Epicurus | Infinite in extent | The universe contains only two things: an infinite number of tiny seeds (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 BCE) | 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 Mundo | Pseudo-Aristotle (d. 250 BCE or between 350 and 200 BCE) | The Universe 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 universe | Stoics (300 BCE – 200 CE) | Island universe | The 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. |
| Platonic universe | Plato (c. 360 BCE) | Geocentric, complex cosmogony, finite extent, implied finite time, cyclical | Static Earth at center, surrounded by heavenly bodies which move in perfect circles, arranged by the will of the Demiurge in order: Moon, Sun, planets and fixed stars. Complex motions repeat every 'perfect' year. |
| Eudoxus' model | Eudoxus of Cnidus (c. 340 BCE) and later Callippus | Geocentric, first geometric-mathematical model | The heavenly bodies move as if attached to a number of Earth-centered concentrical, invisible spheres, each of them rotating around its own and different axis and at different paces. There are 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. |
| Aristotelian universe | Aristotle (384–322 BCE) | Geocentric (based on Eudoxus' model), static, steady state, finite extent, infinite time | Static and spherical Earth is surrounded by 43 to 55 concentric celestial spheres, which are material and crystalline. Universe exists unchanged throughout eternity. Contains a fifth element, called aether, that was added to the four classical elements. |
| Aristarchean universe | Aristarchus (c. 280 BCE) | 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 | Ptolemy (2nd century CE) | Geocentric (based on Aristotelian universe) | Universe orbits around 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 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. The Almagest (the Great System). |
| Capella's model | Martianus Capella (c. 420) | Geocentric and Heliocentric | 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. |
| Aryabhatan model | Aryabhata (499) | Geocentric or Heliocentric | The 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. |
| Quranic cosmology | Quran (610–632 CE) | Flat-earth (Sometimes interpreted as a geocentric globe) | The universe consists of stacked flat layers, including seven levels of heaven and in some interpretations seven levels of earth (including hell) |
| 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 Al-Kindi, the Jewish philosopher Saadia Gaon, and the Muslim theologian Al-Ghazali. |
| Non-Parallel Multiverse | Bhagvata Puran (800–1000) | Multiverse, Non Parallel | Innumerable universes is comparable to the multiverse theory, except nonparallel where each universe is different and individual jiva-atmas (embodied souls) exist in exactly one universe at a time. All universes manifest from the same matter, and so they all follow parallel time cycles, manifesting and unmanifesting at the same time. |
| Multiversal cosmology | Fakhr al-Din al-Razi (1149–1209) | Multiverse, multiple worlds and 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, which orbits the Earth; similar to the later Tychonic system. |
| Copernican universe | Nicolaus Copernicus (1473–1543) | Heliocentric with circular planetary orbits, finite extent | First described in De revolutionibus orbium coelestium. The Sun is in the center of the universe, planets including Earth orbit the Sun, but the Moon orbits the Earth. The universe is limited by the sphere of the fixed stars. |
| 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. |
| Bruno's cosmology | Giordano Bruno (1548–1600) | Infinite extent, infinite time, homogeneous, isotropic, non-hierarchical | Rejects 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 (water, earth, fire, and air), and is atomistic, animistic and intelligent. |
| De Magnete | William Gilbert (1544–1603) | Heliocentric, indefinitely extended | Copernican heliocentrism, but he rejects the idea of a limiting sphere of the fixed stars for which no proof has been offered. |
| Keplerian | Johannes Kepler (1571–1630) | Heliocentric with elliptical planetary orbits | Kepler's discoveries, marrying mathematics and physics, provided the foundation for the present conception of the Solar System, but distant stars were still seen as objects in a thin, fixed celestial sphere. |
| Static Newtonian | 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 | System of huge swirling whirlpools of aethereal or fine matter produces gravitational effects. But his vacuum was not empty; all space was filled with matter. |
| 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 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 increases exponentially (constant inflation). |
| MacMillan universe | William Duncan MacMillan 1920s | Static and steady state | New matter is created from radiation; starlight perpetually recycled into new matter particles. |
| Friedmann universe, spherical space | Alexander Friedmann 1922 | Spherical expanding space. k = +1 ; no Λ | Positive curvature. Curvature constant k = +1 |
| Friedmann universe, 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 | Einstein and De Sitter 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 = 1/2, which means that its expansion rate slows down. |
| The original Big Bang (Friedmann-Lemaître) | Georges Lemaître 1927–1929 | 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 the father of the Big Bang model.) |
| Oscillating universe (Friedmann-Einstein) | 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. (Einstein's first choice after he rejected his 1917 model.) |
| Eddington universe | Arthur Eddington 1930 | First static then expands | Static 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 relativity | Edward Milne 1933, 1935; William H. McCrea 1930s | Kinematic expansion without space expansion | Rejects 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 (or galaxies) that expands within an infinite and otherwise empty flat space. It has a center and a cosmic edge (surface of the particle cloud) that expands at light speed. Explanation of gravity was elaborate and unconvincing. |
| Friedmann–Lemaître–Robertson–Walker class of models | Howard Robertson, Arthur Walker 1935 | Uniformly expanding | Class 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-state | Hermann 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 | 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 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 theory | 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 modified to solve horizon and flatness problems | 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. |
| Eternal inflation (a multiple universe model) | Andreï Linde 1983 | Big Bang with cosmic inflation | 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 the 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. The observable patch contracts containing only dark energy with zero entropy. |
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