Cosmology

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The Hubble eXtreme Deep Field (XDF) was completed in September 2012 and shows the farthest galaxies ever photographed. Except for the few stars in the foreground (which are bright and easily recognizable because only they have diffraction spikes), every speck of light in the photo is an individual galaxy, some of them as old as 13.2 billion years; the observable universe is estimated to contain more than 2 trillion galaxies. The Hubble eXtreme Deep Field.jpg
The Hubble eXtreme Deep Field (XDF) was completed in September 2012 and shows the farthest galaxies ever photographed. Except for the few stars in the foreground (which are bright and easily recognizable because only they have diffraction spikes), every speck of light in the photo is an individual galaxy, some of them as old as 13.2 billion years; the observable universe is estimated to contain more than 2 trillion galaxies.

Cosmology (from Greek κόσμος, kosmos "world" and -λογία, -logia "study of") 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. [2]

Contents

The term cosmology was first used in English in 1656 in Thomas Blount's Glossographia, [3] and in 1731 taken up in Latin by German philosopher Christian Wolff, in Cosmologia Generalis. [4]

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; [5] [6] 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. [7]

Disciplines

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Physics 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. [8] 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 (the science of the heavens), aerology (the science of the air), geology (the science of the continents), and hydrology (the science of waters). [9]

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." [10]

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.

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" [11] (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 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 (1917 to 1922) 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. [12] [13] 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 [14] which was subsequently corroborated by Edwin Hubble's discovery of the redshift in 1929 [15] and later by the discovery of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson in 1964. [16] 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, [17] WMAP [18] and Planck satellites, [19] large new galaxy redshift surveys including 2dfGRS [20] and SDSS, [21] 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". [22]

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. [23] [24] [25] However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported. [26] [27] [28]

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. [29]

Religious or mythological cosmology

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

Philosophical cosmology

Representation of the observable universe on a logarithmic scale. Observable universe logarithmic illustration.png
Representation of the observable universe on a logarithmic scale.

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 found in religion. [30] 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: [23] [31]

Historical cosmologies

NameAuthor and dateClassificationRemarks
Hindu cosmology Rigveda (c. 1700–1100 BC)Cyclical or oscillating, Infinite in time Primal 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 cosmology Jain Agamas (written around 500 AD as per the teachings of Mahavira 599–527 BC)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 cosmology Babylonian literature (c. 2300–500 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.
Eleatic cosmology Parmenides (c. 515 BC)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 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.
Atomist universe Anaxagoras (500–428 BC) & later Epicurus Infinite in extentThe 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.[ citation needed ]
Pythagorean universe Philolaus (d. 390 BC)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. [32]
De Mundo Pseudo-Aristotle (d. 250 BC or between 350 and 200 BC)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." [33]
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, and pulsates in size and undergoes periodic upheavals and conflagrations.
Aristotelian universe Aristotle (384–322 BC) 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 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 Ptolemy (2nd century AD)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. Almagest (the Great System).
Aryabhatan model Aryabhata (499)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 universe Medieval philosophers (500–1200)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 Al-Kindi, the Jewish philosopher Saadia Gaon, and the Muslim theologian Al-Ghazali.
Multiversal cosmology Fakhr al-Din al-Razi (1149–1209) 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 models Maragha school (1259–1528)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 model Nilakantha Somayaji (1444–1544)Geocentric and heliocentricA 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 orbitsFirst described in De revolutionibus orbium coelestium .
Tychonic system Tycho Brahe (1546–1601)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 cosmology Giordano Bruno (1548–1600)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 (water, earth, fire, and air), and is atomistic, animistic and intelligent.
Keplerian Johannes Kepler (1571–1630)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 Newtonian Isaac Newton (1642–1727) Static (evolving), 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 universe René Descartes, 17th centuryStatic (evolving), steady state, infinite System 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 universe Immanuel Kant, Johann Lambert, 18th centuryStatic (evolving), steady state, infiniteMatter is clustered on ever larger scales of hierarchy. Matter is endlessly recycled.
Einstein Universe with a cosmological constant Albert Einstein, 1917Static (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 1920sStatic and steady stateNew matter is created from radiation; starlight perpetually recycled into new matter particles.
Friedmann universe, spherical space Alexander Friedmann 1922Spherical expanding space.

k = +1 ; no Λ

Positive curvature. Curvature constant k = +1

Expands then recollapses. Spatially closed (finite).

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 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 (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 1927–29Expansion

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, 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. (Einstein's first choice after he rejected his 1917 model.)
Eddington universe Arthur 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 relativity Edward 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 (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, 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-state Hermann 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-state Fred 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.
Ambiplasma Hannes Alfvén 1965 Oskar Klein Cellular 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 theory Carl H. Brans, Robert H. Dicke ExpandingBased on Mach's principle. G varies with time as universe expands. "But nobody is quite sure what Mach's principle actually means."[ citation needed ]
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, 1983Big 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 our entire cosmos. Bubbles nucleate in a spacetime foam.
Cyclic model Paul 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 model Lauris Baum; Paul Frampton 2007Solution 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.

See also

Related Research Articles

Big Bang Cosmological model

The Big Bang theory is the prevailing cosmological model of the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from an initial state of high density and temperature, and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.

Physical cosmology Branch of astronomy

Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood. Physical cosmology, as it is now understood, began with the development in 1915 of Albert Einstein's general theory of relativity, followed by major observational discoveries in the 1920s: first, Edwin Hubble discovered that the universe contains a huge number of external galaxies beyond the Milky Way; then, work by Vesto Slipher and others showed that the universe is expanding. These advances made it possible to speculate about the origin of the universe, and allowed the establishment of the Big Bang theory, by Georges Lemaître, as the leading cosmological model. A few researchers still advocate a handful of alternative cosmologies; however, most cosmologists agree that the Big Bang theory best explains the observations.

Inflation (cosmology) Theory of rapid universe expansion

In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from 10−36 seconds after the conjectured Big Bang singularity to some time between 10−33 and 10−32 seconds after the singularity. Following the inflationary period, the universe continued to expand, but at a slower rate. The acceleration of this expansion due to dark energy began after the universe was already over 9 billion years old.

Cosmic microwave background Electromagnetic radiation as a remnant from an early stage of the universe in Big Bang cosmology

The cosmic microwave background, in Big Bang cosmology, is electromagnetic radiation which is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all space. It is an important source of data on the early universe because it is the oldest electromagnetic radiation in the universe, dating to the epoch of recombination. With a traditional optical telescope, the space between stars and galaxies is completely dark. However, a sufficiently sensitive radio telescope shows a faint background noise, or glow, almost isotropic, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1965 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned the discoverers the 1978 Nobel Prize in Physics.

Edwin Hubble American astronomer

Edwin Powell Hubble was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.

Redshift Eventual increase of wavelength in radiation during travel

In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation. The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum.

Universe All of space and time and their contents

The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. According to estimation of this theory, space and time emerged together 13.799±0.021 billion years ago, and the universe has been expanding ever since. While the spatial size of the entire universe is unknown, the cosmic inflation equation tells us that it must have a minimum diameter of 23 trillion light years, and it is possible to measure the size of the observable universe, which is approximately 93 billion light-years in diameter at the present day.

Hubbles law Observation in physical cosmology

Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance. In other words, the farther they are the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the spectrum.

Timeline of cosmological theories Timeline of theories about physical cosmology

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.

Ultimate fate of the universe Range of cosmological hypotheses and scenarios describing the eventual fate of the universe as we know it

The ultimate fate of the universe is a topic in physical cosmology, whose theoretical restrictions allow possible scenarios for the evolution and ultimate fate of the universe to be described and evaluated. Based on available observational evidence, deciding the fate and evolution of the universe has become a valid cosmological question, being beyond the mostly untestable constraints of mythological or theological beliefs. Several possible futures have been predicted by different scientific hypotheses, including that the universe might have existed for a finite and infinite duration, or towards explaining the manner and circumstances of its beginning.

Non-standard cosmology

A non-standard cosmology is any physical cosmological model of the universe that was, or still is, proposed as an alternative to the then-current standard model of cosmology. The term non-standard is applied to any theory that does not conform to the scientific consensus. Because the term depends on the prevailing consensus, the meaning of the term changes over time. For example, hot dark matter would not have been considered non-standard in 1990, but would be in 2010. Conversely, a non-zero cosmological constant resulting in an accelerating universe would have been considered non-standard in 1990, but is part of the standard cosmology in 2010.

Georges Lemaître Belgian scientist and priest (1894-1966)

Georges Henri Joseph Édouard Lemaître was a Belgian Catholic priest, mathematician, astronomer, and professor of physics at the Catholic University of Louvain. He was the first to theorize that the recession of nearby galaxies can be explained by an expanding universe, which was observationally confirmed soon afterwards by Edwin Hubble. He first derived "Hubble's law", now called the Hubble–Lemaître law by the IAU, and published the first estimation of the Hubble constant in 1927, two years before Hubble's article. Lemaître also proposed the "Big Bang theory" of the origin of the universe, calling it the "hypothesis of the primeval atom", and later calling it "the beginning of the world".

Observable universe All matter that can be observed from the Earth at the present time

The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There may be 2 trillion galaxies in the observable universe, although that number has recently been estimated at only several hundred billion based on new data from New Horizons. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe has a spherical volume centered on the observer. Every location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.

Astrophysics is a science that employs the methods and principles of physics in the study of astronomical objects and phenomena. Among the subjects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. Emissions from these objects are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists apply concepts and methods from many disciplines of physics, including classical mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

Observational cosmology

Observational cosmology is the study of the structure, the evolution and the origin of the universe through observation, using instruments such as telescopes and cosmic ray detectors.

Inflationary epoch theoretical period when the early universe expanded extremely rapidly

In physical cosmology the inflationary epoch was the period in the evolution of the early universe when, according to inflation theory, the universe underwent an extremely rapid exponential expansion. This rapid expansion increased the linear dimensions of the early universe by a factor of at least 1026 (and possibly a much larger factor), and so increased its volume by a factor of at least 1078. Expansion by a factor of 1026 is equivalent to expanding an object 1 nanometer (10−9 m, about half the width of a molecule of DNA) in length to one approximately 10.6 light years (about 62 trillion miles) long.

History of the Big Bang theory History of a cosmological theory

The history of the Big Bang theory began with the Big Bang's development from observations and theoretical considerations. Much of the theoretical work in cosmology now involves extensions and refinements to the basic Big Bang model. The theory itself was originally formalised by Belgian Catholic priest, mathematician, astronomer, and professor of physics Georges Lemaître.

Expansion of the universe Increase in distance between parts of the universe over time

The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale. As the spatial part of the universe's spacetime metric increases in scale, objects move apart from one another at ever-increasing speeds. To any observer in the universe, it appears that all of space is expanding while all but the nearest galaxies recede at speeds that are proportional to their distance from the observer – at great enough distances the speeds exceed even the speed of light.

Gravitational-wave astronomy

Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves to collect observational data about objects such as neutron stars and black holes, events such as supernovae, and processes including those of the early universe shortly after the Big Bang.

Dark energy Unknown property in cosmology that causes the expansion of the universe to accelerate.

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovae, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating. Understanding the evolution of the universe requires knowledge of its starting conditions and its composition. Prior to these observations, the only forms of matter–energy known to exist were ordinary matter, antimatter, dark matter, and radiation. Measurements of the cosmic microwave background suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Without introducing a new form of energy, there was no way to explain how an accelerating universe could be measured. Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion. As of 2021, there are active areas of cosmology research aimed at understanding the fundamental nature of dark energy.

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