universe

 The universe is all of space and time[a] and their contents.[10] It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from sub-atomic particles to entire galactic filaments. Since the early 20th century, the field of cosmology establishes that space and time emerged together at the Big Bang 13.787±0.020 billion years ago[11] and that the universe has been expanding since then. The portion of the universe that can be seen by humans is approximately 93 billion light-years in diameter at present, but the total size of the universe is not known.[3]

Some of the earliest cosmological models of the universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center.[12][13] Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus's work as well as Johannes Kepler's laws of planetary motion and observations by Tycho Brahe.

Further observational improvements led to the realization that the Sun is one of a few hundred billion stars in the Milky Way, which is one of a few hundred billion galaxies in the observable universe. Many of the stars in a galaxy have planets. At the largest scale, galaxies are distributed uniformly and the same in all directions, meaning that the universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure.[14] Discoveries in the early 20th century have suggested that the universe had a beginning and has been expanding since then.[15]

According to the Big Bang theory, the energy and matter initially present have become less dense as the universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10−32 seconds, and the separation of the four known fundamental forces, the universe gradually cooled and continued to expand, allowing the first subatomic particles and simple atoms to form. Giant clouds of hydrogen and helium were gradually drawn to the places where matter was most dense, forming the first galaxies, stars, and everything else seen today.

From studying the effects of gravity on both matter and light, it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known as dark matter.[16] In the widely accepted ΛCDM cosmological model, dark matter accounts for about 25.8%±1.1% of the mass and energy in the universe while about 69.2%±1.2% is dark energy, a mysterious form of energy responsible for the acceleration of the expansion of the universe.[17] Ordinary ('baryonic') matter therefore composes only 4.84%±0.1% of the universe.[17] Stars, planets, and visible gas clouds only form about 6% of this ordinary matter.[18]

There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which the universe might be one among many.[3][19][20]

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Definition

সময়কাল: 50 সেকেন্ড।0:50

Hubble Space Telescope – Ultra-Deep Field galaxies to Legacy field zoom out

(video 00:50; May 2, 2019)

The physical universe is defined as all of space and time[a] (collectively referred to as spacetime) and their contents.[10] Such contents comprise all of energy in its various forms, including electromagnetic radiation and matter, and therefore planets, moons, stars, galaxies, and the contents of intergalactic space.[21][22][23] The universe also includes the physical laws that influence energy and matter, such as conservation laws, classical mechanics, and relativity.[24]

The universe is often defined as "the totality of existence", or everything that exists, everything that has existed, and everything that will exist.[24] In fact, some philosophers and scientists support the inclusion of ideas and abstract concepts—such as mathematics and logic—in the definition of the universe.[26][27][28] The word universe may also refer to concepts such as the cosmos, the world, and nature.[29][30]

Etymology

The word universe derives from the Old French word univers, which in turn derives from the Latin word universus, meaning 'combined into one'.[31] The Latin word 'universum' was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.[32]

Synonyms

A term for universe among the ancient Greek philosophers from Pythagoras onwards was τὸ πᾶν (tò pân) 'the all', defined as all matter and all space, and τὸ ὅλον (tò hólon) 'all things', which did not necessarily include the void.[33][34] Another synonym was ὁ κόσμος (ho kósmos) meaning 'the world, the cosmos'.[35] Synonyms are also found in Latin authors (totum, mundus, natura)[36] and survive in modern languages, e.g., the German words Das All, Weltall, and Natur for universe. The same synonyms are found in English, such as everything (as in the theory of everything), the cosmos (as in cosmology), the world (as in the many-worlds interpretation), and nature (as in natural laws or natural philosophy).[37]

Chronology and the Big Bang

Main articles: Big Bang and Chronology of the universe

Nature timeline

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The prevailing model for the evolution of the universe is the Big Bang theory.[38][39] The Big Bang model states that the earliest state of the universe was an extremely hot and dense one, and that the universe subsequently expanded and cooled. The model is based on general relativity and on simplifying assumptions such as the homogeneity and isotropy of space. A version of the model with a cosmological constant (Lambda) and cold dark matter, known as the Lambda-CDM model, is the simplest model that provides a reasonably good account of various observations about the universe.

In this schematic diagram, time passes from left to right, with the universe represented by a disk-shaped "slice" at any given time. Time and size are not to scale. To make the early stages visible, the time to the afterglow stage (really the first 0.003%) is stretched and the subsequent expansion (really by 1,100 times to the present) is largely suppressed.

The initial hot, dense state is called the Planck epoch, a brief period extending from time zero to one Planck time unit of approximately 10−43 seconds. During the Planck epoch, all types of matter and all types of energy were concentrated into a dense state, and gravity—currently the weakest by far of the four known forces—is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. The physics controlling this very early period (including quantum gravity in the Planck epoch) is not understood, so we cannot say what, if anything, happened before time zero. Since the Planck epoch, the universe has been expanding to its present scale, with a very short but intense period of cosmic inflation speculated to have occurred within the first 10−32 seconds.[40] This initial period of inflation would explain why space appears to be very flat.

Within the first fraction of a second of the universe's existence, the four fundamental forces had separated. As the universe continued to cool from its inconceivably hot state, various types of subatomic particles were able to form in short periods of time known as the quark epoch, the hadron epoch, and the lepton epoch. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. These elementary particles associated stably into ever larger combinations, including stable protons and neutrons, which then formed more complex atomic nuclei through nuclear fusion.[41][42]

This process, known as Big Bang nucleosynthesis, lasted for about 17 minutes and ended about 20 minutes after the Big Bang, so only the fastest and simplest reactions occurred. About 25% of the protons and all the neutrons in the universe, by mass, were converted to helium, with small amounts of deuterium (a form of hydrogen) and traces of lithium. Any other element was only formed in very tiny quantities. The other 75% of the protons remained unaffected, as hydrogen nuclei.[41][42]: 27–42 

After nucleosynthesis ended, the universe entered a period known as the photon epoch. During this period, the universe was still far too hot for matter to form neutral atoms, so it contained a hot, dense, foggy plasma of negatively charged electrons, neutral neutrinos and positive nuclei. After about 377,000 years, the universe had cooled enough that electrons and nuclei could form the first stable atoms. This is known as recombination for historical reasons; electrons and nuclei were combining for the first time. Unlike plasma, neutral atoms are transparent to many wavelengths of light, so for the first time the universe also became transparent. The photons released ("decoupled") when these atoms formed can still be seen today; they form the cosmic microwave background (CMB).[42]: 15–27 

As the universe expands, the energy density of electromagnetic radiation decreases more quickly than does that of matter because the energy of each photon decreases as it is cosmologically redshifted. At around 47,000 years, the energy density of matter became larger than that of photons and neutrinos, and began to dominate the large scale behavior of the universe. This marked the end of the radiation-dominated era and the start of the matter-dominated era.[43]: 390 

In the earliest stages of the universe, tiny fluctuations within the universe's density led to concentrations of dark matter gradually forming. Ordinary matter, attracted to these by gravity, formed large gas clouds and eventually, stars and galaxies, where the dark matter was most dense, and voids where it was least dense. After around 100–300 million years,[43]: 333  the first stars formed, known as Population III stars. These were probably very massive, luminous, non metallic and short-lived. They were responsible for the gradual reionization of the universe between about 200–500 million years and 1 billion years, and also for seeding the universe with elements heavier than helium, through stellar nucleosynthesis.[44]

The universe also contains a mysterious energy—possibly a scalar field—called dark energy, the density of which does not change over time. After about 9.8 billion years, the universe had expanded sufficiently so that the density of matter was less than the density of dark energy, marking the beginning of the present dark-energy-dominated era.[45] In this era, the expansion of the universe is accelerating due to dark energy.

Physical properties

Main articles: Observable universe, Age of the universe, and Expansion of the universe

Of the four fundamental interactions, gravitation is the dominant at astronomical length scales. Gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on astronomical length scales. The remaining two interactions, the weak and strong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.[46]: 1470 

The universe appears to have much more matter than antimatter, an asymmetry possibly related to the CP violation.[47] This imbalance between matter and antimatter is partially responsible for the existence of all matter existing today, since matter and antimatter, if equally produced at the Big Bang, would have completely annihilated each other and left only photons as a result of their interaction.[48] These laws are Gauss's law and the non-divergence of the stress–energy–momentum pseudotensor.[49]

Size and regions

See also: Observational cosmology

Illustration of the observable universe, centered on the Sun. The distance scale is logarithmic. Due to the finite speed of light, we see more distant parts of the universe at earlier times.

Due to the finite speed of light, there is a limit (known as the particle horizon) to how far light can travel over the age of the universe. The spatial region from which we can receive light is called the observable universe. The proper distance (measured at a fixed time) between Earth and the edge of the observable universe is 46 billion light-years[50][51] (14 billion parsecs), making the diameter of the observable universe about 93 billion light-years (28 billion parsecs).[50] Although the distance traveled by light from the edge of the observable universe is close to the age of the universe times the speed of light, 13.8 billion light-years (4.2×109 pc), the proper distance is larger because the edge of the observable universe and the Earth have since moved further apart.[52]

For comparison, the diameter of a typical galaxy is 30,000 light-years (9,198 parsecs), and the typical distance between two neighboring galaxies is 3 million light-years (919.8 kiloparsecs).[53] As an example, the Milky Way is roughly 100,000–180,000 light-years in diameter,[54][55] and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light-years away.[56]

Because humans cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite.[3][57][58] Estimates suggest that the whole universe, if finite, must be more than 250 times larger than a Hubble sphere.[59] Some disputed[60] estimates for the total size of the universe, if finite, reach as high as {\displaystyle 10^{10^{10^{122}}}} megaparsecs, as implied by a suggested resolution of the No-Boundary Proposal.[61][b]

Age and expansion

Main articles: Age of the universe and Expansion of the universe

Assuming that the Lambda-CDM model is correct, the measurements of the parameters using a variety of techniques by numerous experiments yield a best value of the age of the universe at 13.799 ± 0.021 billion years, as of 2015.[2]

Over time, the universe and its contents have evolved. For example, the relative population of quasars and galaxies has changed[62] and the universe has expanded. This expansion is inferred from the observation that the light from distant galaxies has been redshifted, which implies that the galaxies are receding from us. Analyses of Type Ia supernovae indicate that the expansion is accelerating.[63][64]

The more matter there is in the universe, the stronger the mutual gravitational pull of the matter. If the universe were too dense then it would re-collapse into a gravitational singularity. However, if the universe contained too little matter then the self-gravity would be too weak for astronomical structures, like galaxies or planets, to form. Since the Big Bang, the universe has expanded monotonically. Perhaps unsurprisingly, our universe has just the right mass–energy density, equivalent to about 5 protons per cubic meter, which has allowed it to expand for the last 13.8 billion years, giving time to form the universe as observed today.[65][66]

There are dynamical forces acting on the particles in the universe which affect the expansion rate. Before 1998, it was expected that the expansion rate would be decreasing as time went on due to the influence of gravitational interactions in the universe; and thus there is an additional observable quantity in the universe called the deceleration parameter, which most cosmologists expected to be positive and related to the matter density of the universe. In 1998, the deceleration parameter was measured by two different groups to be negative, approximately −0.55, which technically implies that the second derivative of the cosmic scale factor {\displaystyle {\ddot {a}}} has been positive in the last 5–6 billion years.[67][68]

Spacetime

Main articles: Spacetime and World line

See also: Lorentz transformation

Modern physics regards events as being organized into spacetime.[69] This idea originated with the special theory of relativity, which predicts that if one observer sees two events happening in different places at the same time, a second observer who is moving relative to the first will see those events happening at different times.[70]: 45–52  The two observers will disagree on the time {\displaystyle T} between the events, and they will disagree about the distance {\displaystyle D} separating the events, but they will agree on the speed of light {\displaystyle c}, and they will measure the same value for the combination {\displaystyle c^{2}T^{2}-D^{2}}.[70]: 80  The square root of the absolute value of this quantity is called the interval between the two events. The interval expresses how widely separated events are, not just in space or in time, but in the combined setting of spacetime.[70]: 84, 136 [71]

The special theory of relativity cannot account for gravity. Its successor, the general theory of relativity, explains gravity by recognizing that spacetime is not fixed but instead dynamical. In general relativity, gravitational force is reimagined as curvature of spacetime. A curved path like an orbit is not the result of a force deflecting a body from an ideal straight-line path, but rather the body's attempt to fall freely through a background that is itself curved by the presence of other masses. A remark by John Archibald Wheeler that has become proverbial among physicists summarizes the theory: "Spacetime tells matter how to move; matter tells spacetime how to curve",[72][73] and therefore there is no point in considering one without the other.[15] The Newtonian theory of gravity is a good approximation to the predictions of general relativity when gravitational effects are weak and objects are moving slowly compared to the speed of light.[74]: 327 [75]

The relation between matter distribution and spacetime curvature is given by the Einstein field equations, which require tensor calculus to express.[76]: 43 [77] The universe appears to be a smooth spacetime continuum consisting of three spatial dimensions and one temporal (time) dimension. Therefore, an event in the spacetime of the physical universe can be identified by a set of four coordinates: (x, y, z, t). On average, space is observed to be very nearly flat (with a curvature close to zero), meaning that Euclidean geometry is empirically true with high accuracy throughout most of the universe.[78] Spacetime also appears to have a simply connected topology, in analogy with a sphere, at least on the length scale of the observable universe. However, present observations cannot exclude the possibilities that the universe has more dimensions (which is postulated by theories such as string theory) and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroidal topologies of two-dimensional spaces.[79][80]

Shape

Main article: Shape of the universe

The three possible options for the shape of the universe

General relativity describes how spacetime is curved and bent by mass and energy (gravity). The topology or geometry of the universe includes both local geometry in the observable universe and global geometry. Cosmologists often work with a given space-like slice of spacetime called the comoving coordinates. The section of spacetime which can be observed is the backward light cone, which delimits the cosmological horizon. The cosmological horizon, also called the particle horizon or the light horizon, is the maximum distance from which particles can have traveled to the observer in the age of the universe. This horizon represents the boundary between the observable and the unobservable regions of the universe.[81][82]

An important parameter determining the future evolution of the universe theory is the density parameter, Omega (Ω), defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible geometries depending on whether Ω is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes.[83]

Observations, including the Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe (WMAP), and Planck maps of the CMB, suggest that the universe is infinite in extent with a finite age, as described by the Friedmann–Lemaître–Robertson–Walker (FLRW) models.[84][79][85][86] These FLRW models thus support inflationary models and the standard model of cosmology, describing a flat, homogeneous universe presently dominated by dark matter and dark energy.[87][88]

Support of life

Main article: Fine-tuned universe

The fine-tuned universe hypothesis is the proposition that the conditions that allow the existence of observable life in the universe can only occur when certain universal fundamental physical constants lie within a very narrow range of values. According to this hypothesis, if any of several fundamental constants were only slightly different, the universe would have been unlikely to be conducive to the establishment and development of matter, astronomical structures, elemental diversity, or life as it is understood. Whether this is true, and whether that question is even logically meaningful to ask, are subjects of much debate.[89] The proposition is discussed among philosophers, scientists, theologians, and proponents of creationism.[90]

Composition

See also: Galaxy formation and evolution, Galaxy cluster, and Nebula

The universe is composed almost completely of dark energy, dark matter, and ordinary matter. Other contents are electromagnetic radiation (estimated to constitute from 0.005% to close to 0.01% of the total mass–energy of the universe) and antimatter.[91][92][93]

The proportions of all types of matter and energy have changed over the history of the universe.[94] The total amount of electromagnetic radiation generated within the universe has decreased by 1/2 in the past 2 billion years.[95][96] Today, ordinary matter, which includes atoms, stars, galaxies, and life, accounts for only 4.9% of the contents of the universe.[8] The present overall density of this type of matter is very low, roughly 4.5 × 10−31 grams per cubic centimeter, corresponding to a density of the order of only one proton for every four cubic meters of volume.[6] The nature of both dark energy and dark matter is unknown. Dark matter, a mysterious form of matter that has not yet been identified, accounts for 26.8% of the cosmic contents. Dark energy, which is the energy of empty space and is causing the expansion of the universe to accelerate, accounts for the remaining 68.3% of the contents.[8][97][98]

The formation of clusters and large-scale filaments in the cold dark matter model with dark energy. The frames show the evolution of structures in a 43 million parsecs (or 140 million light-years) box from redshift of 30 to the present epoch (upper left z=30 to lower right z=0).

A map of the superclusters and voids nearest to Earth

Matter, dark matter, and dark energy are distributed homogeneously throughout the universe over length scales longer than 300 million light-years (ly) or so.[99] However, over shorter length-scales, matter tends to clump hierarchically; many atoms are condensed into stars, most stars into galaxies, most galaxies into clusters, superclusters and, finally, large-scale galactic filaments. The observable universe contains as many as an estimated 2 trillion galaxies[100][101][102] and, overall, as many as an estimated 1024 stars[103][104] – more stars (and earth-like planets) than all the grains of beach sand on planet Earth;[105][106][107] but less than the total number of atoms estimated in the universe as 1082;[108] and the estimated total number of stars in an inflationary universe (observed and unobserved), as 10100.[109] Typical galaxies range from dwarfs with as few as ten million[110] (107) stars up to giants with one trillion[111] (1012) stars. Between the larger structures are voids, which are typically 10–150 Mpc (33 million–490 million ly) in diameter. The Milky Way is in the Local Group of galaxies, which in turn is in the Laniakea Supercluster.[112] This supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years.[113] The universe also has vast regions of relative emptiness; the largest known void measures 1.8 billion ly (550 Mpc) across.[114]

Comparison of the contents of the universe today to 380,000 years after the Big Bang, as measured with 5 year WMAP data (from 2008).[115] Due to rounding, the sum of these numbers is not 100%.

The observable universe is isotropic on scales significantly larger than superclusters, meaning that the statistical properties of the universe are the same in all directions as observed from Earth. The universe is bathed in highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.72548 kelvins.[7] The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle.[116] A universe that is both homogeneous and isotropic looks the same from all vantage points and has no center.[117][118]

Dark energy

Main article: Dark energy

An explanation for why the expansion of the universe is accelerating remains elusive. It is often attributed to the gravitational influence of "dark energy", an unknown form of energy that is hypothesized to permeate space.[119] On a mass–energy equivalence basis, the density of dark energy (~ 7 × 10−30 g/cm3) is much less than the density of ordinary matter or dark matter within galaxies. However, in the present dark-energy era, it dominates the mass–energy of the universe because it is uniform across space.[120][121]

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,[122] and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space while still permeating them enough to cause the observed rate of expansion. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy.

Dark matter

Main article: Dark matter

Dark matter is a hypothetical kind of matter that is invisible to the entire electromagnetic spectrum, but which accounts for most of the matter in the universe. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. Other than neutrinos, a form of hot dark matter, dark matter has not been detected directly, making it one of the greatest mysteries in modern astrophysics. Dark matter neither emits nor absorbs light or any other electromagnetic radiation at any significant level. Dark matter is estimated to constitute 26.8% of the total mass–energy and 84.5% of the total matter in the universe.[97][123]

Ordinary matter

Main article: Matter

The remaining 4.9% of the mass–energy of the universe is ordinary matter, that is, atoms, ions, electrons and the objects they form. This matter includes stars, which produce nearly all of the light we see from galaxies, as well as interstellar gas in the interstellar and intergalactic media, planets, and all the objects from everyday life that we can bump into, touch or squeeze.[124] The great majority of ordinary matter in the universe is unseen, since visible stars and gas inside galaxies and clusters account for less than 10 percent of the ordinary matter contribution to the mass–energy density of the universe.[125][126][127]

Ordinary matter commonly exists in four states (or phases): solid, liquid, gas, and plasma.[128] However, advances in experimental techniques have revealed other previously theoretical phases, such as Bose–Einstein condensates and fermionic condensates.[129][130] Ordinary matter is composed of two types of elementary particles: quarks and leptons.[131] For example, the proton is formed of two up quarks and one down quark; the neutron is formed of two down quarks and one up quark; and the electron is a kind of lepton. An atom consists of an atomic nucleus, made up of protons and neutrons (both of which are baryons), and electrons that orbit the nucleus.[46]: 1476 

Soon after the Big Bang, primordial protons and neutrons formed from the quark–gluon plasma of the early universe as it cooled below two trillion degrees. A few minutes later, in a process known as Big Bang nucleosynthesis, nuclei formed from the primordial protons and neutrons. This nucleosynthesis formed lighter elements, those with small atomic numbers up to lithium and beryllium, but the abundance of heavier elements dropped off sharply with increasing atomic number. Some boron may have been formed at this time, but the next heavier element, carbon, was not formed in significant amounts. Big Bang nucleosynthesis shut down after about 20 minutes due to the rapid drop in temperature and density of the expanding universe. Subsequent formation of heavier elements resulted from stellar nucleosynthesis and supernova nucleosynthesis.[132]

Particles

A four-by-four table of particles. Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (ν sub e) and electron (e), muon neutrino (ν sub μ) and muon (μ), and tau neutrino (ν sub τ) and tau (τ), and Z sup 0 and W sup ± weak force. Mass, charge, and spin are listed for each particle.

Standard model of elementary particles: the 12 fundamental fermions and 4 fundamental bosons. Brown loops indicate which bosons (red) couple to which fermions (purple and green). Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (νe) and electron (e), muon neutrino (νμ) and muon (μ), tau neutrino (ντ) and tau (τ), and the Z0 and W± carriers of the weak force. Mass, charge, and spin are listed for each particle.

Main article: Particle physics

Ordinary matter and the forces that act on matter can be described in terms of elementary particles.[133] These particles are sometimes described as being fundamental, since they have an unknown substructure, and it is unknown whether or not they are composed of smaller and even more fundamental particles.[134][135] In most contemporary models they are thought of as points in space.[136] All elementary particles are currently best explained by quantum mechanics and exhibit wave–particle duality: their behavior has both particle-like and wave-like aspects, with different features dominating under different circumstances.[137]

Of central importance is the Standard Model, a theory that is concerned with electromagnetic interactions and the weak and strong nuclear interactions.[138] The Standard Model is supported by the experimental confirmation of the existence of particles that compose matter: quarks and leptons, and their corresponding "antimatter" duals, as well as the force particles that mediate interactions: the photon, the W and Z bosons, and the gluon.[134] The Standard Model predicted the existence of the recently discovered Higgs boson, a particle that is a manifestation of a field within the universe that can endow particles with mass.[139][140] Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a "theory of almost everything".[138] The Standard Model does not, however, accommodate gravity. A true force–particle "theory of everything" has not been attained.[141]

Hadrons

Main article: Hadron

A hadron is a composite particle made of quarks held together by the strong force. Hadrons are categorized into two families: baryons (such as protons and neutrons) made of three quarks, and mesons (such as pions) made of one quark and one antiquark. Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions and are thus insignificant constituents of the modern universe.[142]: 118–123 

From approximately 10−6 seconds after the Big Bang, during a period known as the hadron epoch, the temperature of the universe had fallen sufficiently to allow quarks to bind together into hadrons, and the mass of the universe was dominated by hadrons. Initially, the temperature was high enough to allow the formation of hadron–anti-hadron pairs, which kept matter and antimatter in thermal equilibrium. However, as the temperature of the universe continued to fall, hadron–anti-hadron pairs were no longer produced. Most of the hadrons and anti-hadrons were then eliminated in particle–antiparticle annihilation reactions, leaving a small residual of hadrons by the time the universe was about one second old.[142]: 244–266 

Leptons

Main article: Lepton

A lepton is an elementary, half-integer spin particle that does not undergo strong interactions but is subject to the Pauli exclusion principle; no two leptons of the same species can be in exactly the same state at the same time.[143] Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Electrons are stable and the most common charged lepton in the universe, whereas muons and taus are unstable particles that quickly decay after being produced in high energy collisions, such as those involving cosmic rays or carried out in particle accelerators.[144][145] Charged leptons can combine with other particles to form various composite particles such as atoms and positronium. The electron governs nearly all of chemistry, as it is found in atoms and is directly tied to all chemical properties. Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the universe but rarely interact with normal matter.[146]

The lepton epoch was the period in the evolution of the early universe in which the leptons dominated the mass of the universe. It started roughly 1 second after the Big Bang, after the majority of hadrons and anti-hadrons annihilated each other at the end of the hadron epoch. During the lepton epoch the temperature of the universe was still high enough to create lepton–anti-lepton pairs, so leptons and anti-leptons were in thermal equilibrium. Approximately 10 seconds after the Big Bang, the temperature of the universe had fallen to the point where lepton–anti-lepton pairs were no longer created.[147] Most leptons and anti-leptons were then eliminated in annihilation reactions, leaving a small residue of leptons. The mass of the universe was then dominated by photons as it entered the following photon epoch.[148][149]

Photons

Main article: Photon epoch

See also: Photino

A photon is the quantum of light and all other forms of electromagnetic radiation. It is the carrier for the electromagnetic force. The effects of this force are easily observable at the microscopic and at the macroscopic level because the photon has zero rest mass; this allows long distance interactions.[46]: 1470 

The photon epoch started after most leptons and anti-leptons were annihilated at the end of the lepton epoch, about 10 seconds after the Big Bang. Atomic nuclei were created in the process of nucleosynthesis which occurred during the first few minutes of the photon epoch. For the remainder of the photon epoch the universe contained a hot dense plasma of nuclei, electrons and photons. About 380,000 years after the Big Bang, the temperature of the universe fell to the point where nuclei could combine with electrons to create neutral atoms. As a result, photons no longer interacted frequently with matter and the universe became transparent. The highly redshifted photons from this period form the cosmic microwave background. Tiny variations in the temperature of the CMB correspond to variations in the density of the universe that were the early "seeds" from which all subsequent structure formation took place.[142]: 244–266 

প্রসারণvte

Timeline of the Big Bang

Habitability

The frequency of life in the universe has been a frequent point of investigation in astronomy and astrobiology, being the issue of the Drake equation and the different views on it, from identifying the Fermi paradox, the situation of not having found any signs of extraterrestrial life, to arguments for a biophysical cosmology, a view of life being inherent to the physical cosmology of the universe.[150]

Cosmological models

Model of the universe based on general relativity

Main article: Solutions of the Einstein field equations

See also: Big Bang and Ultimate fate of the universe

General relativity is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. It is the basis of current cosmological models of the universe. General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present.[151]

The relation is specified by the Einstein field equations, a system of partial differential equations. In general relativity, the distribution of matter and energy determines the geometry of spacetime, which in turn describes the acceleration of matter. Therefore, solutions of the Einstein field equations describe the evolution of the universe. Combined with measurements of the amount, type, and distribution of matter in the universe, the equations of general relativity describe the evolution of the universe over time.[151]

With the assumption of the cosmological principle that the universe is homogeneous and isotropic everywhere, a specific solution of the field equations that describes the universe is the metric tensor called the Friedmann–Lemaître–Robertson–Walker metric,

{\displaystyle ds^{2}=-c^{2}dt^{2}+R(t)^{2}\left({\frac {dr^{2}}{1-kr^{2}}}+r^{2}d\theta ^{2}+r^{2}\sin ^{2}\theta \,d\phi ^{2}\right)}

where (r, θ, φ) correspond to a spherical coordinate system. This metric has only two undetermined parameters. An overall dimensionless length scale factor R describes the size scale of the universe as a function of time (an increase in R is the expansion of the universe),[152] and a curvature index k describes the geometry. The index k is defined so that it can take only one of three values: 0, corresponding to flat Euclidean geometry; 1, corresponding to a space of positive curvature; or −1, corresponding to a space of positive or negative curvature.[153] The value of R as a function of time t depends upon k and the cosmological constant Λ.[151] The cosmological constant represents the energy density of the vacuum of space and could be related to dark energy.[98] The equation describing how R varies with time is known as the Friedmann equation after its inventor, Alexander Friedmann.[154]

The solutions for R(t) depend on k and Λ, but some qualitative features of such solutions are general. First and most importantly, the length scale R of the universe can remain constant only if the universe is perfectly isotropic with positive curvature (k = 1) and has one precise value of density everywhere, as first noted by Albert Einstein.[151]

Second, all solutions suggest that there was a gravitational singularity in the past, when R went to zero and matter and energy were infinitely dense. It may seem that this conclusion is uncertain because it is based on the questionable assumptions of perfect homogeneity and isotropy (the cosmological principle) and that only the gravitational interaction is significant. However, the Penrose–Hawking singularity theorems show that a singularity should exist for very general conditions. Hence, according to Einstein's field equations, R grew rapidly from an unimaginably hot, dense state that existed immediately following this singularity (when R had a small, finite value); this is the essence of the Big Bang model of the universe. Understanding the singularity of the Big Bang likely requires a quantum theory of gravity, which has not yet been formulated.[155]

Third, the curvature index k determines the sign of the curvature of constant-time spatial surfaces[153] averaged over sufficiently large length scales (greater than about a billion light-years). If k = 1, the curvature is positive and the universe has a finite volume.[156] A universe with positive curvature is often visualized as a three-dimensional sphere embedded in a four-dimensional space. Conversely, if k is zero or negative, the universe has an infinite volume.[156] It may seem counter-intuitive that an infinite and yet infinitely dense universe could be created in a single instant when R = 0, but exactly that is predicted mathematically when k is nonpositive and the cosmological principle is satisfied. By analogy, an infinite plane has zero curvature but infinite area, whereas an infinite cylinder is finite in one direction and a torus is finite in both.

The ultimate fate of the universe is still unknown because it depends critically on the curvature index k and the cosmological constant Λ. If the universe were sufficiently dense, k would equal +1, meaning that its average curvature throughout is positive and the universe will eventually recollapse in a Big Crunch,[157] possibly starting a new universe in a Big Bounce. Conversely, if the universe were insufficiently dense, k would equal 0 or −1 and the universe would expand forever, cooling off and eventually reaching the Big Freeze and the heat death of the universe.[151] Modern data suggests that the expansion of the universe is accelerating; if this acceleration is sufficiently rapid, the universe may eventually reach a Big Rip. Observationally, the universe appears to be flat (k = 0), with an overall density that is very close to the critical value between recollapse and eternal expansion.[158]

Multiverse hypotheses

Main articles: Multiverse, Many-worlds interpretation, and Bubble universe theory

See also: Eternal inflation

Some speculative theories have proposed that our universe is but one of a set of disconnected universes, collectively denoted as the multiverse, challenging or enhancing more limited definitions of the universe.[19][159] Max Tegmark developed a four-part classification scheme for the different types of multiverses that scientists have suggested in response to various problems in physics. An example of such multiverses is the one resulting from the chaotic inflation model of the early universe.[160]

Another is the multiverse resulting from the many-worlds interpretation of quantum mechanics. In this interpretation, parallel worlds are generated in a manner similar to quantum superposition and decoherence, with all states of the wave functions being realized in separate worlds. Effectively, in the many-worlds interpretation the multiverse evolves as a universal wavefunction. If the Big Bang that created our multiverse created an ensemble of multiverses, the wave function of the ensemble would be entangled in this sense.[161] Whether scientifically meaningful probabilities can be extracted from this picture has been and continues to be a topic of much debate, and multiple versions of the many-worlds interpretation exist.[162][163][164] The subject of the interpretation of quantum mechanics is in general marked by disagreement.[165][166][167]

The least controversial, but still highly disputed, category of multiverse in Tegmark's scheme is Level I. The multiverses of this level are composed by distant spacetime events "in our own universe". Tegmark and others[168] have argued that, if space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entire Hubble volume occur every so often, simply by chance. Tegmark calculated that our nearest so-called doppelgänger is 1010115 metres away from us (a double exponential function larger than a googolplex).[169][170] However, the arguments used are of speculative nature.[171]

It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another.[169][172] An easily visualized metaphor of this concept is a group of separate soap bubbles, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle.[173] According to one common terminology, each "soap bubble" of spacetime is denoted as a universe, whereas humans' particular spacetime is denoted as the universe,[19] just as humans call Earth's moon the Moon. The entire collection of these separate spacetimes is denoted as the multiverse.[19]

With this terminology, different universes are not causally connected to each other.[19] In principle, the other unconnected universes may have different dimensionalities and topologies of spacetime, different forms of matter and energy, and different physical laws and physical constants, although such possibilities are purely speculative.[19] Others consider each of several bubbles created as part of chaotic inflation to be separate universes, though in this model these universes all share a causal origin.[19]

Historical conceptions

See also: Cosmology, Timeline of cosmological theories, Nicolaus Copernicus § Copernican system, and Philosophiæ Naturalis Principia Mathematica § Beginnings of the Scientific Revolution

Historically, there have been many ideas of the cosmos (cosmologies) and its origin (cosmogonies). Theories of an impersonal universe governed by physical laws were first proposed by the Greeks and Indians.[13] Ancient Chinese philosophy encompassed the notion of the universe including both all of space and all of time.[174] Over the centuries, improvements in astronomical observations and theories of motion and gravitation led to ever more accurate descriptions of the universe. The modern era of cosmology began with Albert Einstein's 1915 general theory of relativity, which made it possible to quantitatively predict the origin, evolution, and conclusion of the universe as a whole. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted Big Bang.[175]

Mythologies

Main articles: Creation myth, Cosmogony, and Religious cosmology

Many cultures have stories describing the origin of the world and universe. Cultures generally regard these stories as having some truth. There are however many differing beliefs in how these stories apply amongst those believing in a supernatural origin, ranging from a god directly creating the universe as it is now to a god just setting the "wheels in motion" (for example via mechanisms such as the big bang and evolution).[176]

Ethnologists and anthropologists who study myths have developed various classification schemes for the various themes that appear in creation stories.[177][178] For example, in one type of story, the world is born from a world egg; such stories include the Finnish epic poem Kalevala, the Chinese story of Pangu or the Indian Brahmanda Purana. In related stories, the universe is created by a single entity emanating or producing something by him- or herself, as in the Tibetan Buddhism concept of Adi-Buddha, the ancient Greek story of Gaia (Mother Earth), the Aztec goddess Coatlicue myth, the ancient Egyptian god Atum story, and the Judeo-Christian Genesis creation narrative in which the Abrahamic God created the universe. In another type of story, the universe is created from the union of male and female deities, as in the Maori story of Rangi and Papa. In other stories, the universe is created by crafting it from pre-existing materials, such as the corpse of a dead god—as from Tiamat in the Babylonian epic Enuma Elish or from the giant Ymir in Norse mythology—or from chaotic materials, as in Izanagi and Izanami in Japanese mythology. In other stories, the universe emanates from fundamental principles, such as Brahman and Prakrti, and the creation myth of the Serers.[179]

Philosophical models

Further information: Cosmology

See also: Pre-Socratic philosophy, Physics (Aristotle), Hindu cosmology, Islamic cosmology, and Philosophy of space and time

The pre-Socratic Greek philosophers and Indian philosophers developed some of the earliest philosophical concepts of the universe.[13][180] The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the physical materials in the world are different forms of a single primordial material, or arche. The first to do so was Thales, who proposed this material to be water. Thales' student, Anaximander, proposed that everything came from the limitless apeiron. Anaximenes proposed the primordial material to be air on account of its perceived attractive and repulsive qualities that cause the arche to condense or dissociate into different forms. Anaxagoras proposed the principle of Nous (Mind), while Heraclitus proposed fire (and spoke of logos). Empedocles proposed the elements to be earth, water, air and fire. His four-element model became very popular. Like Pythagoras, Plato believed that all things were composed of number, with Empedocles' elements taking the form of the Platonic solids. Democritus, and later philosophers—most notably Leucippus—proposed that the universe is composed of indivisible atoms moving through a void (vacuum), although Aristotle did not believe that to be feasible because air, like water, offers resistance to motion. Air will immediately rush in to fill a void, and moreover, without resistance, it would do so indefinitely fast.[13]

Although Heraclitus argued for eternal change,[181] his contemporary Parmenides emphasized changelessness. Parmenides' poem On Nature has been read as saying that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature, or at least that the essential feature of each thing that exists must exist eternally, without origin, change, or end.[182] His student Zeno of Elea challenged everyday ideas about motion with several famous paradoxes. Aristotle responded to these paradoxes by developing the notion of a potential countable infinity, as well as the infinitely divisible continuum.[183][184]

The Indian philosopher Kanada, founder of the Vaisheshika school, developed a notion of atomism and proposed that light and heat were varieties of the same substance.[185] In the 5th century AD, the Buddhist atomist philosopher Dignāga proposed atoms to be point-sized, durationless, and made of energy. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.[186]

The notion of temporal finitism was inspired by the doctrine of creation shared by the three Abrahamic religions: Judaism, Christianity and Islam. The Christian philosopher, John Philoponus, presented the philosophical arguments against the ancient Greek notion of an infinite past and future. Philoponus' arguments against an infinite past were used by the early Muslim philosopher, Al-Kindi (Alkindus); the Jewish philosopher, Saadia Gaon (Saadia ben Joseph); and the Muslim theologian, Al-Ghazali (Algazel).[187]

Pantheism is the philosophical religious belief that the universe itself is identical to divinity and a supreme being or entity.[188] The physical universe is thus understood as an all-encompassing, immanent deity.[189] The term 'pantheist' designates one who holds both that everything constitutes a unity and that this unity is divine, consisting of an all-encompassing, manifested god or goddess.[190][191]

Astronomical concepts

Main articles: History of astronomy and Timeline of astronomy

3rd century BCE calculations by Aristarchus on the relative sizes of, from left to right, the Sun, Earth, and Moon, from a 10th-century AD Greek copy

The earliest written records of identifiable predecessors to modern astronomy come from Ancient Egypt and Mesopotamia from around 3000 to 1200 BCE.[192][193] Babylonian astronomers of the 7th century BCE viewed the world as a flat disk surrounded by the ocean.[194][195]

Later Greek philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the universe based more profoundly on empirical evidence. The first coherent model was proposed by Eudoxus of Cnidos, a student of Plato who followed Plato's idea that heavenly motions had to be circular. In order to account for the known complications of the planets' motions, particularly retrograde movement, Eudoxus' model included 27 different celestial spheres: four for each of the planets visible to the naked eye, three each for the Sun and the Moon, and one for the stars. All of these spheres were centered on the Earth, which remained motionless while they rotated eternally. Aristotle elaborated upon this model, increasing the number of spheres to 55 in order to account for further details of planetary motion. For Aristotle, normal matter was entirely contained within the terrestrial sphere, and it obeyed fundamentally different rules from heavenly material.[196][197]

The post-Aristotle treatise De Mundo (of uncertain authorship and date) stated, "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".[198] This model was also refined by Callippus and after concentric spheres were abandoned, it was brought into nearly perfect agreement with astronomical observations by Ptolemy.[199] The success of such a model is largely due to the mathematical fact that any function (such as the position of a planet) can be decomposed into a set of circular functions (the Fourier modes). Other Greek scientists, such as the Pythagorean philosopher Philolaus, postulated (according to Stobaeus' account) that at the center of the universe was a "central fire" around which the Earth, Sun, Moon and planets revolved in uniform circular motion.[200]

The Greek astronomer Aristarchus of Samos was the first known individual to propose a heliocentric model of the universe. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus's heliocentric model. Archimedes wrote:

You, King Gelon, are aware the universe is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the universe just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.[201]

Aristarchus thus believed the stars to be very far away, and saw this as the reason why stellar parallax had not been observed, that is, the stars had not been observed to move relative each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times, which is why stellar parallax is only detectable with precision instruments. The geocentric model, consistent with planetary parallax, was assumed to be the explanation for the unobservability of stellar parallax.[202]

Flammarion engraving, Paris 1888

The only other astronomer from antiquity known by name who supported Aristarchus's heliocentric model was Seleucus of Seleucia, a Hellenistic astronomer who lived a century after Aristarchus.[203][204][205] According to Plutarch, Seleucus was the first to prove the heliocentric system through reasoning, but it is not known what arguments he used. Seleucus' arguments for a heliocentric cosmology were probably related to the phenomenon of tides.[206] According to Strabo (1.1.9), Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.[207] Alternatively, he may have proved heliocentricity by determining the constants of a geometric model for it, and by developing methods to compute planetary positions using this model, similar to Nicolaus Copernicus in the 16th century.[208] During the Middle Ages, heliocentric models were also proposed by the Persian astronomers Albumasar[209] and Al-Sijzi.[210]

Model of the Copernican Universe by Thomas Digges in 1576, with the amendment that the stars are no longer confined to a sphere, but spread uniformly throughout the space surrounding the planets

The Aristotelian model was accepted in the Western world for roughly two millennia, until Copernicus revived Aristarchus's perspective that the astronomical data could be explained more plausibly if the Earth rotated on its axis and if the Sun were placed at the center of the universe.[211]

In the center rests the Sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can illuminate everything at the same time?

— Nicolaus Copernicus, in Chapter 10, Book 1 of De Revolutionibus Orbium Coelestrum (1543)

As noted by Copernicus, the notion that the Earth rotates is very old, dating at least to Philolaus (c. 450 BC), Heraclides Ponticus (c. 350 BC) and Ecphantus the Pythagorean. Roughly a century before Copernicus, the Christian scholar Nicholas of Cusa also proposed that the Earth rotates on its axis in his book, On Learned Ignorance (1440).[212] Al-Sijzi[213] also proposed that the Earth rotates on its axis. Empirical evidence for the Earth's rotation on its axis, using the phenomenon of comets, was given by Tusi (1201–1274) and Ali Qushji (1403–1474).[214]

This cosmology was accepted by Isaac Newton, Christiaan Huygens and later scientists.[215] Newton demonstrated that the same laws of motion and gravity apply to earthly and to celestial matter, making Aristotle's division between the two obsolete. Edmund Halley (1720)[216] and Jean-Philippe de Chéseaux (1744)[217] noted independently that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the Sun itself; this became known as Olbers' paradox in the 19th century.[218] Newton believed that an infinite space uniformly filled with matter would cause infinite forces and instabilities causing the matter to be crushed inwards under its own gravity.[215] This instability was clarified in 1902 by the Jeans instability criterion.[219] One solution to these paradoxes is the Charlier universe, in which the matter is arranged hierarchically (systems of orbiting bodies that are themselves orbiting in a larger system, ad infinitum) in a fractal way such that the universe has a negligibly small overall density; such a cosmological model had also been proposed earlier in 1761 by Johann Heinrich Lambert.[53][220]

Deep space astronomy

During the 18th century, Immanuel Kant speculated that nebulae could be entire galaxies separate from the Milky Way,[216] and in 1850, Alexander von Humboldt called these separate galaxies Weltinseln, or "world islands", a term that later developed into "island universes".[221][222] In 1919, when the Hooker Telescope was completed, the prevailing view was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope, Edwin Hubble identified Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively that Andromeda Nebula and Triangulum among others, were entire galaxies outside our own, thus proving that the universe consists of a multitude of galaxies.[223] With this Hubble formulated the Hubble constant, which allowed for the first time a calculation of the age of the Universe and size of the Observable Universe, which became increasingly precise with better meassurements, starting at 2 billion years and 280 million light-years, until 2006 when data of the Hubble Space Telescope allowed a very accurate calculation of the age of the Universe and size of the Observable Universe.[224]

The modern era of physical cosmology began in 1917, when Albert Einstein first applied his general theory of relativity to model the structure and dynamics of the universe.[225] The discoveries of this era, and the questions that remain unanswered, are outlined in the sections above.

Map of the observable universe with some of the notable astronomical objects known as of 2018. The scale of length increases exponentially toward the right. Celestial bodies are shown enlarged in size to be able to understand their shapes.

Location of the Earth in the universe

Earth

Solar System

Radcliffe Wave

Orion Arm

Milky Way

Local Group

Virgo SCl

Laniakea SCl

Observable universe

See also

Cosmic Calendar (scaled down timeline)

Cosmic latte

Detailed logarithmic timeline

Earth's location in the universe

False vacuum

Future of an expanding universe

Galaxy And Mass Assembly survey

Heat death of the universe

History of the center of the Universe

Illustris project

Non-standard cosmology

Nucleocosmochronology

Parallel universe (fiction)

Rare Earth hypothesis

Space and survival

Terasecond and longer

Timeline of the early universe

Timeline of the far future

Timeline of the near future

Zero-energy universe

References

Footnotes

^ ঝাঁপ দিন:a b According to modern physics, particularly the theory of relativity, space and time are intrinsically linked as spacetime.

^ Although listed in megaparsecs by the cited source, this number is so vast that its digits would remain virtually unchanged for all intents and purposes regardless of which conventional units it is listed in, whether it to be nanometers or gigaparsecs, as the differences would disappear into the error.

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মহাবিশ্ব

 মহাবিশ্ব বা বিশ্বব্রহ্মাণ্ড হলো অসীম স্থানকাল (spacetime) এবং তাতে যা কিছু আছে তার সবকিছুর সম্মিলিত রূপ। এর মধ্যে অন্তর্ভুক্ত যেকোনো ধরনের অস্তিত্ব, মৌলিক ক্রিয়া-বিক্রিয়া, ভৌত প্রক্রিয়া, ভৌত ধ্রুবক এবং সেগুলো যেসব কাঠামো তৈরি করে, যেমন- অতিপারমাণবিক কণা থেকে শুরু করে বিশাল ছায়াপথ। প্রচলিত বিগ ব্যাং তত্ত্ব অনুযায়ী, প্রায় ১৩.৭৮৭±০.০২০ বিলিয়ন বছর আগে স্থানকালের উদ্ভব হয় এবং তখন থেকেই মহাবিশ্ব প্রসারিত হতে থাকে। আজ যতটুকু পর্যবেক্ষণ করা সম্ভব, তার ব্যাস প্রায় ৯৩ বিলিয়ন আলোকবর্ষ, তবে মহাবিশ্বের পুরো আকার (যদি থেকে থাকে) সম্পর্কে আমরা অজ্ঞ।

মহাবিশ্ব সম্পর্কে সর্বপ্রথম ধারণাগুলো এসেছিল প্রাচীন গ্রিক এবং ভারতীয় দার্শনিকদের থেকে। তাদের মহাজাগতিক মডেলগুলোতে পৃথিবীকে কেন্দ্রে স্থাপন করা হয়েছিল। শতাব্দীর পর শতাব্দী জুড়ে সূক্ষ্ম জ্যোতির্বৈজ্ঞানিক পর্যবেক্ষণের ফলে নিকোলাস কোপারনিকাস সূর্যকেন্দ্রিক মডেল তৈরি করেন যেখানে সূর্য সৌরজগতের কেন্দ্রে অবস্থিত। বিশ্বজনীন মহাকর্ষ সূত্র প্রতিষ্ঠা করার সময় আইজ্যাক নিউটন কোপারনিকাসের কাজের উপর ভিত্তি করেই এগিয়েছিলেন, যেমন করেছিলেন জোহানেস কেপলারের গ্রহের গতির সূত্র এবং টাইকো ব্রাহের পর্যবেক্ষণগুলোর উপর।

আরও উন্নত পর্যবেক্ষণের ফলে জানা যায়, সূর্য আকাশগঙ্গার কয়েকশো বিলিয়ন তারার মধ্যে একটি মাত্র, এবং পর্যবেক্ষণযোগ্য মহাবিশ্বে আকাশগঙ্গার মতোই আরও কয়েকশো বিলিয়ন ছায়াপথ রয়েছে। একটি গ্যালাক্সির অধিকাংশ তারাই গ্রহসমূহ ধারণ করে। মহাবিশ্বের সবচেয়ে বড় স্কেলে, গ্যালাক্সিগুলি সুষমভাবে ও সকল দিকে সমানভাবে বিতরণ করা হয়েছে। এর অর্থ মহাবিশ্বের কোনো কিনারা বা কেন্দ্র নেই। ছোট স্কেলে গ্যালাক্সিগুলি ক্লাস্টার এবং সুপারক্লাস্টারে বিন্যস্ত যা মহাশূন্যতে বিশাল ফিলামেন্ট আর শূন্যতা তৈরি করে ফেনার মত বিশাল কাঠামো তৈরি করে। বিংশ শতাব্দীর গোড়ার দিকের আবিষ্কারগুলি থেকে জানা যায় যে মহাবিশ্বের একটি শুরু ছিল এবং এর প্রসারণ সেই সময় থেকে হচ্ছে।

বিগ ব্যাং তত্ত্ব অনুযায়ী, মূলত যে শক্তি এবং পদার্থ উপস্থিত ছিল তা মহাবিশ্বের প্রসারণের সাথে সাথে কম ঘন হয়ে এসেছে। মাত্র ১০^-৩২ সেকেন্ডের মধ্যে মহাবিশ্বের ত্বরিত প্রসারণ (যাকে inflationary epoch বলা হয়), এরপর চারটি মৌলিক শক্তি পৃথক হয়ে যাওয়ার পর মহাবিশ্ব ধীরে ধীরে শীতল হতে থাকে এবং প্রসারিত হতে থাকে। এর ফলে প্রথম উপ-পারমাণবিক কণা এবং সাধারণ পরমাণু তৈরি হতে শুরু করে। হাইড্রোজেন এবং হিলিয়ামের বিশাল মেঘ ধীরে ধীরে ঘনীভূত হয়ে সেইসব স্থানে প্রথম গ্যালাক্সি, তারা এবং বর্তমানে আমরা যা যা দেখি তৈরি করে।

পদার্থ এবং আলোর উপর মহাকর্ষের প্রভাব পর্যালোচনা করে জানা যায় যে, মহাবিশ্বে দৃশ্যমান বস্তুর চেয়ে অনেক বেশি পদার্থ রয়েছে; তারা, ছায়াপথ, নীহারিকা (nebulas) এবং আন্তঃনাক্ষত্রিক গ্যাস ইত্যাদি। এই অদৃশ্য পদার্থকে বলা হয় ডার্ক ম্যাটার (ডার্ক বলতে বোঝায় এর সম্পর্কে আমাদের পরোক্ষ প্রমাণ আছে কিন্তু সরাসরি পর্যবেক্ষণ করা সম্ভব হয়নি)। এই ডার্ক ম্যাটার সৃষ্টি হয় বাকি মহাবিশ্বের সাথে সাথেই, এবং ফিলামেন্ট ও শূন্যতার ফেনার মত কাঠামো তৈরি করে অন্যান্য পদার্থকে দৃশ্যমান কাঠামো গঠন করতে সহায়তা করে। ΛCDM মডেল হলো বর্তমানে মহাবিশ্বের সবচেয়ে স্বীকৃত মডেল। এটি বলে যে মহাবিশ্বের প্রায় 69.2%±1.2% ভর-শক্তি হলো ডার্ক এনার্জি, যা মহাবিশ্বের ত্বরিত প্রসারণের জন্য দায়ী এবং প্রায় 25.8%±1.1% হলো ডার্ক ম্যাটার। সাধারণ ('ব্যারিওনিক') পদার্থ মহাবিশ্বের মাত্র4.84%±0.1%। তারা, গ্রহ এবং দৃশ্যমান গ্যাস মেঘ সাধারণ পদার্থের প্রায় 6% গঠন করে।

মহাবিশ্বের চূড়ান্ত পরিণতি সম্পর্কে অনেক প্রতিদ্বন্দ্বিতামূলক অনুমান রয়েছে এবং বলা হয় বিগ ব্যাংয়ের আগে কী ছিল। অনেক পদার্থবিজ্ঞানী ও দার্শনিক এসব বিষয়ে জল্পনা-কল্পনা থেকে বিরত থাকেন কারণ তাঁরা মনে করেন সেই আদি অবস্থা সম্পর্কে তথ্য কখনোই পাওয়া সম্ভব হবে না। কিছু পদার্থবিজ্ঞানী বহুবিশ্বের (multiverse) হাইপোথিসিস তুলে ধরেন যেখানে আমাদের এই মহাবিশ্ব অনেকগুলো মহাবিশ্বের একটি।

ইতিহাস

প্রাচীন কালে মহাবিশ্বকে ব্যাখ্যা করার জন্য নানাবিধ বিশ্বতত্ত্বের আশ্রয় নেওয়া হত। পুরাতন গ্রিক দার্শনিকরাই প্রথম এই ধরনের তত্ত্বে গাণিতিক মডেলের সাহায্য নেন এবং পৃথিবী কেন্দ্রিক একটি মহাবিশ্বের ধারণা প্রণয়ন করেন। তাদের মডেলে পৃথিবীই মহাবিশ্বের কেন্দ্রে অবস্থিত এবং পৃথিবীকে কেন্দ্র করে সমস্ত গ্রহ, সূর্য ও নক্ষত্ররা ঘুরছে। গ্রীকদের এই মডেলে মহাবিশ্বের মোট আয়তন বর্তমানে জ্ঞাত বৃহস্পতি গ্রহের কক্ষপথের মধ্যেই ছিল। তারা ভেবেছিলেন আকাশের তারারা আমাদের থেকে খুব বেশি দূরে অবস্থিত নয়।

বেশ কয়েক জন জ্যোতির্বিদ পৃথিবীকেন্দ্রিক মহাবিশ্বের ব্যাপারে দ্বিমত পোষণ করলেও যতদিন না চৌদ্দশো শতকে কোপের্নিকাস সৌরকেন্দ্রিক মহাবিশ্বকে যৌক্তিক ভাবে তার বইয়ে উপস্থাপনা করলেন ততদিন পৃথিবীকেন্দ্রিক ধারণা মানুষের মনে দৃঢ়ভাবে গেঁড়ে ছিল। পরবর্তীকালে নিউটনের গতি ও মহাকর্ষ সংক্রান্ত গভীর ধারণা পর্যবেক্ষণের সাথে সৌরকেন্দ্রিক জগতের সামঞ্জস্য নির্ধারণ করে। ধীরে ধীরে জ্যোতির্বিদরা আবিষ্কার করেন সূর্যের মতই কোটি কোটি তারা দিয়ে একটি ছায়াপথ গঠিত। কয়েক শত বছর বিজ্ঞানীদের ধারণা ছিল সমগ্র মহাবিশ্ব মানে শুধুমাত্র আমাদের এই ছায়াপথ ছায়াপথটিই। ১৯২০র দশকে উন্নত দুরবীনের কল্যাণে জ্যোতির্বিদরা আবিষ্কার করলেন ছায়াপথের বাইরে অন্য ছায়াপথদের। [১০][১১]

সেই কোটি কোটি ছায়াপথদের মধ্যে ছায়াপথের মতই কোটি কোটি নক্ষত্রদের অবস্থান। সেই সমস্ত ছায়াপথদের থেকে আগত আলোর বর্ণালি বিশ্লেষণে বোঝা গেল সেই ছায়াপথগুলি আমাদের থেকে দূরে সরে যাচ্ছে। [১২]

এর সহজতম ব্যাখ্যা হল ছায়াপথদের মধ্যে স্থানের প্রসারণ হচ্ছে এবং প্রতিটি ছায়াপথই অন্য ছায়াপথ থেকে দূরে সরছে। বিজ্ঞানীদের ধারণা হল সুদূর অতীতে সমস্ত ছায়াপথগুলি বা তাদের অন্তর্নিহিত সমস্ত পদার্থই একসাথে খুব ঘন অবস্থায় ছিল এবং কোন মহা বিস্ফোরণের ফলে বস্তুসমূহ একে অপর থেকে দূরে সরে যাচ্ছে। এই বিস্ফোরণের নাম দেওয়া হল বিগ ব্যাং। ১৯৬০এর দশকে বিজ্ঞানীরা বিগ ব্যাংএ সৃষ্ট উষ্ণ বিকিরণের শীতল অবশেষের সন্ধান পেলেন।[১৩] এই তরঙ্গ বিগ ব্যাং ঘটনার প্রায় ৪০০,০০০ বা চার লক্ষ বছর পরে, বস্তু ঘনত্বের হ্রাসের পর, মুক্ত হয়েছিল। এই মাইক্রোওয়েভ বিকিরণ মহাবিশ্বের প্রতিটি জায়গাতেই পাওয়া যায়। এক অর্থে বলা যায় এই তরঙ্গ দৃশ্যমান মহাবিশ্বের শেষ প্রান্ত থেকে আসছে। বিংশ শতাব্দীর শেষে এসে জ্যোতির্বিদরা আবিষ্কার করলেন মহাবিশ্বের প্রসারণ ত্বরাণ্বিত হচ্ছে।[১৪] এই আবিষ্কারটি বিশ্বতত্ত্বের কিছু প্রশ্নের উত্তর দিল।

বিগ ব্যাং মডেল অনুযায়ী মহাবিশ্বের শুরু হয়েছিল একটা ভীষণ ঘন ও উষ্ণ দশা থেকে। এই সময় বা অবস্থাকে প্ল্যাঙ্ক ইপোখ বলে অভিহিত করা যায়। সেই সময় থেকে মহাবিশ্বের সম্প্রসারণ হয়ে চলেছে। বিজ্ঞানীদের ধারণা শুরুর খুব অল্প সময়ের মধ্যেই (১০−-৩২ সেকেন্ডের মধ্যেই) মহাবিশ্বের অতি স্ফিতী (inflation) হয় যা কিনা দেশ বা স্থানের প্রতিটি অংশে প্রায় একই তাপমাত্রা স্থাপন করতে সাহায্য করে।[১৫] এই সময়ে সুসম ঘনত্বের মাঝে হ্রাস-বৃদ্ধির ফলে ভবিষ্যত ছায়াপথ সৃষ্টির বীজ তৈরি হয়। মহাকর্ষ শক্তি মাধ্যমে সম্প্রসারণের বিরূদ্ধে বস্তুজগতকে আকর্ষিত করে ছায়াপথ সৃষ্টির পেছনে কৃষ্ণ বা অন্ধকার বস্তুর বিশেষ ভূমিকা আছে। অন্যদিকে মহাবিশ্বের বর্তমান প্রসারণের মাত্রার ত্বরণের জন্য কৃষ্ণ বা অন্ধকার শক্তি বলে একটি জিনিসকে দায়ী করা হচ্ছে। তাত্ত্বিক ভাবে কৃষ্ণ বস্তু মহাকর্ষ ছাড়া অন্য বলগুলোর সাথে (তড়িৎ-চুম্বকীয়, সবল ও দুর্বল) খুব অল্পই বিক্রিয়া করে সেইজন্য ডিটেকটর দিয়ে তাকে দেখা মুশকিল। বর্তমান মহাবিশ্বের মূল অংশই হচ্ছে কৃষ্ণ শক্তি, বাকিটা কৃষ্ণ বস্তু। আমরা চোখে বা ডিটেকটর মাধ্যমে যা দেখি তা মহাবিশ্বের মাত্র ৫ শতাংশের কম। এই মডেলে মহাবিশ্বের বর্তমান বয়স ১,৩৭৫ কোটি বছর। এই মহাবিশ্বের দৃশ্যমান অংশের "এই মুহূর্তের" ব্যাস প্রায় ৯,৩০০ কোটি আলোকবর্ষ। যেহেতু মহাবিশ্বের প্রতিটি বিন্দু প্রতিটি বিন্দু থেকে প্রতি মুহূর্তে আরো দ্রুত সরছে, মহাবিশ্বের ব্যাস ১,৩৭৫ × ২ = ২,৭৫০ কোটি আলোক বছরের চাইতে বেশি। দেশ বা স্থানের প্রতিটি বিন্দু বহু দূরের কোন বিন্দুর তুলনায় আলোর গতির উর্ধে ভ্রমণ করে, যতক্ষণ না সেই বিন্দুগুলির মাঝে তথ্য আদানপ্রদান না হচ্ছে এই গতি বিশেষ বা সাধারণ আপেক্ষিকতার কোন নিয়ম ভঙ্গ করে না।

হাবল মহাকাশ দূরবীক্ষণ যন্ত্র মহাশূন্যের একটি মোটামুটি ফাঁকা অংশকে তাক করে বেশ কিছুদিন ধরে এই উচ্চ রেজোলিউশনের ছবিটি ধারণ করে। এই অংশটির মোট আকার চাঁদের কৌণিক ব্যাসের (অর্ধেক ডিগ্রি) সমান। নিচের বাঁদিকের বাক্সে সেটা দেখানো হয়েছে। এই ছবিতে দৃশ্যমান মহাবিশ্বের সবচেয়ে দূরবর্তী অঞ্চলের ছায়াপথসমূহ দেখা যাচ্ছে। এখানে প্রতিটি ছায়াপথে অন্তত ১০০ কোটি নক্ষত্র রয়েছে। এছাড়া এই ছবিতে প্রায় ১০০টি খুবই ছোট লাল ছায়াপথ আছে যাদের দূরত্ব হয়তো আমাদের জানা মতে সবচেয়ে বেশি, সেগুলো মহাবিস্ফোরণের পরে কয়েক শত কোটি বছরের মধ্যেই সৃষ্টি হয়েছিল।

বর্তমান মহাবিশ্বের উপাদানসমূহ

মহাবিশ্বের আকার বিশাল। বর্তমান বিশ্বতত্ত্বের মডেল অনুযায়ী মহাবিশ্বের বর্তমান বয়স ১,৩৭৫ কোটি বছর। এই মহাবিশ্বের দৃশ্যমান অংশের "এই মুহূর্তের" ব্যাস প্রায় ৯,৩০০ কোটি আলোকবর্ষ। যেহেতু মহাবিশ্বের প্রতিটি বিন্দু প্রতিটি বিন্দু থেকে প্রতি মুহূর্তে আরো দ্রুত সরছে, মহাবিশ্বের ব্যাস ১,৩৭৫ x ২ = ২,৭৫০ কোটি আলোকবর্ষের চাইতে বেশি। দেশ বা স্থানের প্রতিটি বিন্দু বহু দূরের কোন বিন্দুর তুলনায় আলোর গতির উর্ধে ভ্রমণ করে, যতক্ষণ না সেই বিন্দুগুলির মাঝে তথ্য আদানপ্রদান না হচ্ছে এই গতি বিশেষ বা সাধারণ আপেক্ষিকতার কোন নিয়ম ভঙ্গ করে না। কাজেই পৃথিবীকে কেন্দ্র করে মহাবিশ্বকে যদি একটা গোলক কল্পনা করা হও তবে তার ব্যাসার্ধ হবে প্রায় ৪,৬০০ কোটি আলোকবর্ষ। যদিও সেই দূরত্ত্বে অবস্থিত ছায়াপথ থেকে এই মুহূর্তে যে বিকিরণ বার হচ্ছে তা আমরা কখনই দেখতে পাব না।

জ্যোতির্বিদরা মনে করছেন দৃশ্যমান মহাবিশ্বে প্রায় ১০ হাজার কোটি (১০+১১) ছায়াপথ আছে। এই ছায়াপথগুলো খুব ছোট হতে পারে, যেমন মাত্র ১ কোটি তারা সংবলিত বামন ছায়াপথ অথবা খুব বড় হতে পারে, দৈত্যাকার ছায়াপথগুলিতে ১০০ হাজার কোটি নক্ষত্র থাকতে পারে (আমাদের ছায়াপথের ১০ গুণ বেশি)। দৃশ্যমান মহাবিশ্বে আনুমানিক ৩ x ১০+২৩টি নক্ষত্র থাকতে পারে।[১৬]

বর্তমানের মহাজাগতিক মডেল অনুযায়ী মহাবিশ্বের মূল উপাদান মূলতঃ কৃষ্ণ বা অন্ধকার শক্তি। ধরা হচ্ছে যে এই শক্তি সারা মহাবিশ্বে ছড়িয়ে আছে এবং মহাবিশ্বের প্রসারণের পিছনে মূল ভূমিকা পালন করছে। কৃষ্ণ শক্তির পরিমাণ যেখানে ৭৪% ভাগ ধরা হয়, মোট বস্তুর পরিমাণ সেখানে ২৬%। কিন্তু এই বস্তুর মধ্যে ২২% কৃষ্ণ বস্তু ও ৪% দৃশ্যমান বস্তু। কৃষ্ণ বস্তুর অস্তিত্ব পরোক্ষভাবে ছায়াপথর ঘূর্ণন, ছায়াপথপুঞ্জ, মহাকর্ষীয় লেন্সিং, ইত্যাদি পর্যবেক্ষণ মাধ্যমে নিশ্চিত করা হয়েছে। কৃষ্ণ বস্তু যেহেতু মহাকর্ষ ছাড়া অন্য কোন বলের সংঙ্গে পারতপক্ষে কোন মিথষ্ক্রিয়ায় অংশগ্রহণ করে না, সেই জন্য তাকে সরাসরি পর্যবেক্ষণ করা কঠিন।

মহাবিশ্বের সংঙ্গে আমাদের পরিচয় দৃশ্যমান বস্তুর আঙ্গিকে। পরমাণু ও পরমাণু দ্বারা গঠিত যৌগ পদার্থ দিয়ে এই দৃশ্যমান বিশ্ব গঠিত। পরমাণুর কেন্দ্রে নিউক্লিয়াস প্রোটন ও নিউট্রন দিয়ে গঠিত। প্রোটন ও নিউট্রনকে ব্যারিয়ন বলা হয়। ব্যারিয়ন তিনটি কোয়ার্ক কণা দিয়ে গঠিত। অন্যদিকে দুটি কোয়ার্ক কণা দিয়ে গঠিত কণাদের মেজন বলা হয়। অন্যদিকে লেপটন কণা কোয়ার্ক দিয়ে গঠিত নয়। সবেচেয়ে পরিচিত লেপটন কণা হচ্ছে ইলেকট্রন। প্রমিত মডেল বা স্ট্যান্ডার্ড মডেল কোয়ার্ক, লেপটন ও বিভিন্ন বলের মিথষ্ক্রিয়ায় সাহায্যকারী কণাসমূহ (যেমন ফোটন, বোজন ও গ্লুয়োন) দিয়ে তৈরি। বর্তমানের কণা পদার্থবিদ্যাকে ব্যাখ্যা করতে এই মডেল সফল হয়েছে।

মহাবিশ্বের গঠন ও আকার

সূর্য আমাদের নিকটবর্তী নক্ষত্র। সূর্য থেকে আলো আসতে ৮ মিনিট মত সময় লাগে, কাজেই সূর্যের দূরত্ত্ব হচ্ছে আনুমানিক ৮ আলোক মিনিট। আমাদের সৌর জগতের আকার হচ্ছে ১০ আলোক ঘন্টার মত। সূর্যের পরে আমাদের নিকটবর্তী তারা হচ্ছে ৪ আলোক বর্ষ দূরত্বে। নিচের চিত্রে ১৪ আলোক বর্ষের মধ্যে অবস্থিত সমস্ত তারাদের দেখানো হয়েছে।

আমাদের ১৪ আলোকবর্ষের মধ্যে যে সমস্ত তারা আছে

আমাদের ছায়াপথ

গ্যালাকটিক বারের মহাকর্ষীয় প্রভাবের বাইরে, আকাশগঙ্গা ছায়াপথের ডিস্কে আন্তঃনাক্ষত্রিক মাধ্যম এবং তারার গঠন চারটি সর্পিল বাহুতে সংগঠিত। [১৭] সর্পিল বাহুগুলো সাধারণত গ্যালাকটিক বার থেকে উচ্চ ঘনত্বের আন্তঃনাক্ষত্রিক গ্যাস এবং ধূলিকণার থাকে। এটি H II অঞ্চল এবং আণবিক মেঘ দ্বারা চিহ্নিত করা হয়েছে।[১৮]

নিচের ছবিতে ছায়াপথ ছায়াপথর বাহুসমূহ দেখানো হয়েছে। সূর্য থেকে ছায়াপথর কেন্দ্রের দূরত্ব প্রায় ৩০,০০০ আলোক বর্ষ। ছায়াপথর ব্যাস ১০০,০০০ বা এক লক্ষ আলোক বর্ষ। কেন্দ্রের উল্টোদিকের অংশকে আমরা দেখতে পাই না।

আমাদের ছায়াপথর সর্পিল বাহুসমূহ

স্থানীয় ছায়াপথপুঞ্জ

নিচের চিত্রে ছায়াপথের ৫ মিলিয়ন বা ৫০ লক্ষ আলোকবর্ষের মধ্যে অবস্থিত ছায়াপথগুলো দেখানো হয়েছে। এই স্থানীয় ছায়াপথ দলের মধ্যে বড় তিনটি সর্পিল ছায়াপথ - ছায়াপথ, অ্যান্ড্রোমিডা বা M31 এবং M33 একটি মহাকর্ষীয় ত্রিভুজ তৈরি করেছে। অ্যান্ড্রোমিডা ছায়াপথ আমাদের নিকটবর্তী বড় ছায়াপথ। এর দূরত্ব হচ্ছে ২.৫ মিলিয়ন বা ২৫ লক্ষ আলোকবর্ষ। স্থানীয় দলের মধ্যে বেশির ভাগ ছায়াপথই বড় ম্যাজিল্লান মেঘের মত অনিয়মিত ছায়াপথ।

স্থানীয় ছায়াপথপুঞ্জ

স্থানীয় ছায়াপথ মহাপুঞ্জ

ডান দিকের চিত্রে স্থানীয় ছায়াপথ দল থেকে স্থানীয় ছায়াপথ মহাপুঞ্জের অন্যান্য দলের দূরত্ত্ব দেখানো হয়েছে। এই মহাপুঞ্জের কেন্দ্র কন্যা ছায়াপথ দল হওয়াতে তাকে কন্যা মহাপুঞ্জ বা মহাদল বলা হয়। কন্যা ছায়াপথ পুঞ্জ আমাদের থেকে প্রায় ৬৫ মিলিয়ন বা ৬.৫ কোটি আলোকবর্ষ দূরে অবস্থিত। এই ধরনের মহাপুঞ্জগুলো ফিতার আকারের মত। সাবানের বুদবুদ দিয়ে এই ধরনের ছায়াপথপুঞ্জ গঠনের মডেল করা যায়। দুটো বুদবুদের দেওয়াল যেখানে মেশে সেখানেই যেন ছায়াপথর ফিতা সৃষ্টি হয়েছে।

স্থানীয় ছায়াপথ মহাপুঞ্জ বা সুপারক্লাস্টার।

নিচের চিত্রে আমাদের ৫০০ মিলিয়ন বা ৫০ কোটি আলোকবর্ষের মধ্যে অবস্থিত প্রধান ছায়াপথপুঞ্জ ও ছায়াপথ দেওয়াল দেখানো হচ্ছে। এই চিত্রে বুদবুদগুলোর মাঝের শূন্যতা (void) খুব ভাল করেই বোঝা যাচ্ছে। কন্যা ছায়াপথ মহাপুঞ্জসহ ৫০ মেগাপার্সেকের (৫০ মিলিয়ন পার্সেক বা ১৬৩ মিলিয়ন আলোকবর্ষ)মধ্যে সমস্ত পদার্থ ৬৫ মেগাপার্সেক দূরের নর্মা পুঞ্জের (Abell 3627)দিকে ৬০০ কিমি/সেকেন্ডে ছুটে যাচ্ছে। বৃহত্তর স্কেলে ছায়াপথ সৃষ্টির জন্য ব্যারিয়ন ধ্বনি স্পন্দন (Baryon Acoustic Oscillation) যথেষ্ট সফল হয়েছে। এই মডেল অনুযায়ী ছায়াপথ পুঞ্জ মোটামুটি ১০০ মেগাপার্সেক (~৩০০ মিলিয়ন আলোকবর্ষ)স্কেলে বা স্থান জুড়ে সৃষ্টি হবে। স্লোয়ান ডিজিটাল স্কাই সার্ভের ডাটাতে ব্যারিয়ন স্পন্দন ২০০৫এ ধরা পড়ে।[১৯]

আমাদের ৫০০ মিলিয়ন আলোকবর্ষের মধ্যের মহাবিশ্ব। নিকটবর্তী ছায়াপথ দেওয়াল দেখা যাচ্ছে

আরও দেখুন

মহাজাগতিক বর্ষপঞ্জি

মেকি শূন্যস্থান

প্রসারমান মহাবিশ্বের ভবিষ্যৎ

মহাবিশ্বের তাপ মৃত্যু

তথ্যসূত্র

 "Hubble sees galaxies galore"। spacetelescope.org। মে ৪, ২০১৭ তারিখে মূল থেকে আর্কাইভ করা। সংগ্রহের তারিখ এপ্রিল ৩০, ২০১৭।

 উদ্ধৃতি ত্রুটি: <ref> ট্যাগ বৈধ নয়; Planck 2015 নামের সূত্রটির জন্য কোন লেখা প্রদান করা হয়নি

 উদ্ধৃতি ত্রুটি: <ref> ট্যাগ বৈধ নয়; Brian Greene 2011 নামের সূত্রটির জন্য কোন লেখা প্রদান করা হয়নি

 Bars, Itzhak; Terning, John (২০০৯)। Extra Dimensions in Space and Time। Springer। পৃষ্ঠা 27–। আইএসবিএন 978-0-387-77637-8। সংগ্রহের তারিখ মে ১, ২০১১।

 Davies, Paul (২০০৬)। The Goldilocks Enigmaবিনামূল্যে নিবন্ধন প্রয়োজন। First Mariner Books। পৃষ্ঠা 43ff। আইএসবিএন 978-0-618-59226-5।

 NASA/WMAP Science Team (জানুয়ারি ২৪, ২০১৪)। "Universe 101: What is the Universe Made Of?"। NASA। মার্চ ১০, ২০০৮ তারিখে মূল থেকে আর্কাইভ করা। সংগ্রহের তারিখ ফেব্রুয়ারি ১৭, ২০১৫।

 Fixsen, D.J. (২০০৯)। "The Temperature of the Cosmic Microwave Background"। The Astrophysical Journal। 707 (2): 916–920। arXiv:0911.1955 অবাধে প্রবেশযোগ্য। আইএসএসএন 0004-637X। এসটুসিআইডি 119217397। ডিওআই:10.1088/0004-637X/707/2/916। বিবকোড:2009ApJ...707..916F।

 উদ্ধৃতি ত্রুটি: <ref> ট্যাগ বৈধ নয়; planck2013parameters নামের সূত্রটির জন্য কোন লেখা প্রদান করা হয়নি

 NASA/WMAP Science Team (জানুয়ারি ২৪, ২০১৪)। "Universe 101: Will the Universe expand forever?"। NASA। মার্চ ৯, ২০০৮ তারিখে মূল থেকে আর্কাইভ করা। সংগ্রহের তারিখ এপ্রিল ১৬, ২০১৫।

 Curtis, H. D. (১৯৮৮)। "Novae in Spiral Nebulae and the Island Universe Theory"। Publications of the Astronomical Society of the Pacific। 100: 6। ডিওআই:10.1086/132128। বিবকোড:1988PASP..100....6C।

 Hubble, E. P. (১৯২৯)। "A spiral nebula as a stellar system, Messier 31"। Astrophysical Journal। 69: 103–158। ডিওআই:10.1086/143167। বিবকোড:1929ApJ....69..103H।

 Hubble, E. P.; Humason, M. L. (১৯৩১)। "The Velocity-Distance Relation among Extra-Galactic Nebulae"। Astrophysical Journal। 74: 43।

 Penzias, A. A. & Wilson, R. W. 1965, A Measurement of Excess Antenna Temperature at 4080 Mc/s, Astrophysical Journal, vol. 142, p.419-421

 Adam G. Riess et al. 1998, Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, The Astronomical Journal, 116, #3

 Guth A.,H.. 1981, Inflationary universe: A possible solution to the horizon and flatness problems, Phys. Rev. D 23, 347–356

 van Dokkum, Pieter G; Conroy, Charlie (২০১০)। "A substantial population of low-mass stars in luminous elliptical galaxies"। Nature। 468: 3।

 , E., Churchwell; ও অন্যান্য (২০০৯)। "The Spitzer/GLIMPSE surveys: a new view of the Milky Way"। Publications of the Astronomical Society of the Pacific। 121 (877): 213–230। ডিওআই:doi:10.1086/597811 |doi= এর মান পরীক্ষা করুন (সাহায্য)। বিবকোড:2009PASP..121..213C।

 Dame, T. M.; Hartmann, D.; Thaddeus, P. (২০০১)। "The Milky Way in molecular clouds: A new complete CO survey"। The Astrophysical Journal। 547 (2): 792–813। arXiv:astro-ph/0009217 অবাধে প্রবেশযোগ্য। এসটুসিআইডি 118888462। ডিওআই:10.1086/318388। বিবকোড:2001ApJ...547..792D।

 Eisenstein, D. J. et al. 2005, Detection of the Baryon Acoustic Peak in the Large-Scale Correlation Function of SDSS Luminous Red Galaxies, The Astrophysical Journal, Volume 633, Issue 2, pp. 560-574

 এটি জ্যোতির্বিজ্ঞান বিষয়ক অসম্পূর্ণ নিবন্ধ। আপনি চাইলে এটি পরিবর্ধন করে উইকিপিডিয়াকে সাহায্য করতে পারেন।

বিষয়শ্রেণীসমূহ: অসম্পূর্ণ জ্যোতির্বিজ্ঞান নিবন্ধজ্যোতির্বিজ্ঞানমহাবিশ্ব

continent

 A continent is any of several large geographical regions. Continents are generally identified by convention rather than any strict criteria. A continent could be a single landmass or a part of a very large landmass, as in the case of Asia or Europe. Due to this, the number of continents varies; up to seven or as few as four geographical regions are commonly regarded as continents. Most English-speaking countries recognize seven regions as continents. In order from largest to smallest in area, these seven regions are Asia, Africa, North America, South America, Antarctica, Europe, and Australia. Different variations with fewer continents merge some of these regions; examples of this are merging Asia and Europe into Eurasia,[1] North America and South America into America, and Africa, Asia, and Europe into Afro-Eurasia.

Oceanic islands are occasionally grouped with a nearby continent to divide all the world's land into geographical regions. Under this scheme, most of the island countries and territories in the Pacific Ocean are grouped together with the continent of Australia to form the geographical region of Oceania.[2]

In geology, a continent is defined as "one of Earth's major landmasses, including both dry land and continental shelves".[3] The geological continents correspond to seven large areas of continental crust that are found on the tectonic plates, but exclude small continental fragments such as Madagascar that are generally referred to as microcontinents. Continental crust is only known to exist on Earth.[4]

The idea of continental drift gained recognition in the 20th century. It postulates that the current continents formed from the breaking up of a supercontinent (Pangaea) that formed hundreds of millions of years ago.

Etymology

From the 16th century the English noun continent was derived from the term continent land, meaning continuous or connected land[5] and translated from the Latin terra continens.[6] The noun was used to mean "a connected or continuous tract of land" or mainland.[5] It was not applied only to very large areas of land—in the 17th century, references were made to the continents (or mainlands) of the Isle of Man, Ireland and Wales and in 1745 to Sumatra.[5] The word continent was used in translating Greek and Latin writings about the three "parts" of the world, although in the original languages no word of exactly the same meaning as continent was used.[7]

While continent was used on the one hand for relatively small areas of continuous land, on the other hand geographers again raised Herodotus's query about why a single large landmass should be divided into separate continents. In the mid-17th century, Peter Heylin wrote in his Cosmographie that "A Continent is a great quantity of Land, not separated by any Sea from the rest of the World, as the whole Continent of Europe, Asia, Africa." In 1727, Ephraim Chambers wrote in his Cyclopædia, "The world is ordinarily divided into two grand continents: the Old and the New." And in his 1752 atlas, Emanuel Bowen defined a continent as "a large space of dry land comprehending many countries all joined together, without any separation by water. Thus Europe, Asia, and Africa is one great continent, as America is another."[8] However, the old idea of Europe, Asia and Africa as "parts" of the world ultimately persisted with these being regarded as separate continents.

Definitions and application

Further information: Island § Differentiation from continents

By convention, continents "are understood to be large, continuous, discrete masses of land, ideally separated by expanses of water".[9] By this definition, all continents have to be an island of some metric. In modern schemes with five or more recognized continents, at least one pair of continents is joined by land in some fashion. The criterion "large" leads to arbitrary classification: Greenland, with a surface area of 2,166,086 square kilometres (836,330 sq mi), is only considered the world's largest island, while Australia, at 7,617,930 square kilometres (2,941,300 sq mi), is deemed the smallest continent.

Earth's major landmasses all have coasts on a single, continuous World Ocean, which is divided into several principal oceanic components by the continents and various geographic criteria.[10][11]

The geological definition of a continent has four criteria: high elevation relative to the ocean floor; a wide range of igneous, metamorphic and sedimentary rocks rich in silica; a crust thicker than the surrounding oceanic crust; and well-defined limits around a large enough area.[12]

Extent

The most restricted meaning of continent is that of a continuous[13][non-tertiary source needed] area of land or mainland, with the coastline and any land boundaries forming the edge of the continent. In this sense, the term continental Europe (sometimes referred to in Britain as "the Continent") is used to refer to mainland Europe, excluding islands such as Great Britain, Iceland, Ireland, and Malta, while the term continent of Australia may refer to the mainland of Australia, excluding New Guinea, Tasmania, and other nearby islands. Similarly, the continental United States refers to "the 49 States (including Alaska but excluding Hawaii) located on the continent of North America, and the District of Columbia."[14]

From the perspective of geology or physical geography, continent may be extended beyond the confines of continuous dry land to include the shallow, submerged adjacent area (the continental shelf)[15] and the islands on the shelf (continental islands), as they are structurally part of the continent.[16]

From this perspective, the edge of the continental shelf is the true edge of the continent, as shorelines vary with changes in sea level.[17] In this sense the islands of Great Britain and Ireland are part of Europe, while Australia and the island of New Guinea together form a continent. Taken to its limit, this view could support the view that there are only three continents: Antarctica, Australia-New Guinea, and a single mega-continent which joins Afro-Eurasia and America via the contiguous continental shelf in and around the Bering Sea. The vast size of the latter compared to the first two might even lead some to say it is the only continent, the others being more comparable to Greenland or New Zealand.[18]

Map of island nations depicting sovereign states and a de facto state (Taiwan) (tw). Those with land borders are shaded green, and those without shaded blue. Along with dependent or disputed island territories, which do not appear on the map, these states are often grouped geographically with a neighboring continental landmass.

As a cultural construct, the concept of a continent may go beyond the continental shelf to include oceanic islands and continental fragments. In this way, Iceland is considered a part of Europe, and Madagascar a part of Africa. Extrapolating the concept to its extreme, some geographers group the Australian continental landmass with other islands in the Pacific Ocean into Oceania, which is usually considered a region rather than a continent. This divides the entire land surface of Earth into continents, regions, or quasi-continents.[19]

Separation

Main article: Boundaries between the continents

See also: List of transcontinental countries

The criterion that each continent is a discrete landmass is commonly relaxed due to historical conventions and practical use. Of the seven most globally recognized continents, only Antarctica and Australia are completely separated from other continents by the ocean. Several continents are defined not as absolutely distinct bodies but as "more or less discrete masses of land".[20] Africa and Asia are joined by the Isthmus of Suez, and North America and South America by the Isthmus of Panama. In both cases, there is no complete separation of these landmasses by water (disregarding the Suez Canal and the Panama Canal, which are both narrow and shallow, as well as human-made). Both of these isthmuses are very narrow compared to the bulk of the landmasses they unite.

North America and South America are treated as separate continents in the seven-continent model. However, they may also be viewed as a single continent known as America. This viewpoint was common in the United States until World War II, and remains prevalent in some Asian six-continent models.[21] The single American continent model remains a common view in European countries like France, Greece, Hungary, Italy, Malta, Portugal, Spain, Latin American countries and some Asian countries.

The criterion of a discrete landmass is completely disregarded if the continuous landmass of Eurasia is classified as two separate continents (Asia and Europe). Physiographically, Europe and the Indian subcontinent are large peninsulas of the Eurasian landmass. However, Europe is considered a continent with its comparatively large land area of 10,180,000 square kilometres (3,930,000 sq mi), while the Indian subcontinent, with less than half that area, is considered a subcontinent. The alternative view—in geology and geography—that Eurasia is a single continent results in a six-continent view of the world. Some view the separation of Eurasia into Asia and Europe as a residue of Eurocentrism: "In physical, cultural and historical diversity, China and India are comparable to the entire European landmass, not to a single European country. [...]."[22] However, for historical and cultural reasons, the view of Europe as a separate continent continues in almost all categorizations.

If continents are defined strictly as discrete landmasses, embracing all the contiguous land of a body, then Africa, Asia, and Europe form a single continent which may be referred to as Afro-Eurasia.[23] Combined with the consolidation of the Americas, this would produce a four-continent model consisting of Afro-Eurasia, America, Antarctica, and Australia.

When sea levels were lower during the Pleistocene ice ages, greater areas of the continental shelf were exposed as dry land, forming land bridges between Tasmania and the Australian mainland.[24] At those times, Australia and New Guinea were a single, continuous continent known as Sahul. Likewise, Afro-Eurasia and the Americas were joined by the Bering Land Bridge. Other islands, such as Great Britain, were joined to the mainlands of their continents. At that time, there were just three discrete landmasses in the world: Africa-Eurasia-America, Antarctica, and Australia-New Guinea (Sahul).

Number

There are several ways of distinguishing the continents:

Color-coded map showing the various continents.

Similar shades exhibit areas that may be consolidated or subdivided.

Number Continents Sources Comment

Four     Afro-Eurasia (Old World or World Island)    America (New World)   Antarctica   Australia [25][26][27][28][29][30][31][32] Continuous landmasses

Five   Africa    Eurasia    America   Antarctica   Australia [33][34][35] Physiographic regions

Six   Africa    Eurasia   North America   South America   Antarctica   Australia [36][37] Geological continents

  Africa   Asia   Europe    America   Antarctica   Australia [38] UNSD continental regions

Seven   Africa   Asia   Europe   North America   South America   Antarctica   Australia [36][39][40][41][42][43] "Parts" of the world

The seven-continent model is taught in most English-speaking countries, including Australia,[44] Canada, the United Kingdom,[45] and the United States, and also in Bangladesh, China, India, Indonesia, Pakistan, the Philippines, Sri Lanka, Suriname, parts of Europe and Africa.

The six-continent combined-Eurasia model is mostly used in Russia and some parts of Eastern Europe.[46][47]

The six-continent combined-America model is taught in Greece and many Romance-speaking countries—including Latin America.[38]

The Olympic flag's five rings represent the five inhabited continents of the combined-America model but excludes the uninhabited Antarctica.[48]

In the English-speaking countries, geographers often use the term Oceania to denote a geographical region which includes most of the island countries and territories in the Pacific Ocean, as well as the continent of Australia.[49]

Eighth continent

Zealandia (a submerged continent) has been called the eighth continent.[50]

Area and population

For a more detailed list of populations by continental regions and subregions, see List of continents and continental subregions by population.

The following table provides areas given by the Encyclopædia Britannica for each continent in accordance with the seven-continent model, including Australasia along with Melanesia, Micronesia, and Polynesia as parts of Oceania. It also provides populations of continents according to 2021 estimates by the United Nations Statistics Division based on the United Nations geoscheme, which includes all of Egypt (including the Isthmus of Suez and the Sinai Peninsula) as a part of Africa, all of Armenia, Azerbaijan, Cyprus, Georgia, Indonesia, Kazakhstan, and Turkey (including East Thrace) as parts of Asia, all of Russia (including Siberia) as a part of Europe, all of Panama and the United States (including Hawaii) as parts of North America, and all of Chile (including Easter Island) as a part of South America.

Land areas and population estimates

Continent Land area[51][52][53][54][55][56][57][58] Population[59][60]

km2 sq mi % of

world 2021

(estimate) % of

world

Earth 149,733,926 57,812,592 100.0 7,909,295,151 100.0

Asia 44,614,000 17,226,000 29.8 4,694,576,167 59.4

Africa 30,365,000 11,724,000 20.3 1,393,676,444 17.6

North America 24,230,000 9,360,000 16.2 595,783,465 7.5

South America 17,814,000 6,878,000 11.9 434,254,119 5.5

Antarctica 14,200,000 5,500,000 9.5 0 0

Europe 10,000,000 3,900,000 6.7 745,173,774 9.4

Oceania[α] 8,510,926 3,286,087 5.7 44,491,724 0.6

^ Not usually considered to be a continent in the English-speaking world. Its land area and population includes Australia, New Zealand, and Papua New Guinea, but excludes the Aru Islands and Western New Guinea.

Other divisions

Supercontinents

Main article: Supercontinent

Further information: Geological history of Earth

Reconstruction of the supercontinent Pangaea approximately 200 million years ago

Apart from the current continents, the scope and meaning of the term continent includes past geological ones. Supercontinents, largely in evidence earlier in the geological record, are landmasses that comprise most of the world's cratons or continental cores.[61] These have included Vaalbara, Kenorland, Columbia, Rodinia, Pannotia, and Pangaea. Over time, these supercontinents broke apart into large landmasses which formed the present continents.

Subcontinents

Further information: Indian subcontinent and Arabian Peninsula

The Indian subcontinent

Certain parts of continents are recognized as subcontinents, especially the large peninsulas separated from the main continental landmass by geographical features. The most widely recognized example is the Indian subcontinent.[62] The Arabian Peninsula, Southern Africa, the Southern Cone of South America, and Alaska in North America might be considered further examples.[62][63]

In many of these cases, the "subcontinents" concerned are on different tectonic plates from the rest of the continent, providing a geological justification for the terminology.[64] Greenland, generally considered the world's largest island on the northeastern periphery of the North American Plate, is sometimes referred to as a subcontinent.[65][66] This is a significant departure from the more conventional view of a subcontinent as comprising a very large peninsula on the fringe of a continent.[62]

Where the Americas are viewed as a single continent (America), it is divided into two subcontinents (North America and South America)[67][68][69] or three (Central America being the third).[70][71] When Eurasia is regarded as a single continent, Asia and Europe are treated as subcontinents.[62]

Submerged continents

Main article: Submerged continent

Further information: List of lost lands

Zealandia, the largest submerged landmass or continent

Some areas of continental crust are largely covered by the ocean and may be considered submerged continents. Notable examples are Zealandia, emerging from the ocean primarily in New Zealand and New Caledonia,[72][non-tertiary source needed] and the almost completely submerged Kerguelen Plateau in the southern Indian Ocean.[73]

Microcontinents

Further information: Continental fragment

See also: Madagascar

Some islands lie on sections of continental crust that have rifted and drifted apart from a main continental landmass. While not considered continents because of their relatively small size, they may be considered microcontinents. Madagascar, the largest example, is usually considered an island of Africa, but its divergent evolution has caused it to be referred to as "the eighth continent" from a biological perspective.[74]

Geological continents

See also: Zealandia

Geologists use four key attributes to define a continent:[75]

Elevation – The landmass, whether dry or submerged beneath the ocean, should be elevated above the surrounding ocean crust.

Geology – The landmass should contain different types of rock: igneous, metamorphic, and sedimentary.

Crustal structure – The landmass should consist of the continental crust, which is thicker and has a lower seismic velocity than the oceanic crust.

Limits and area – The landmass should have clearly defined boundaries and an area of more than one million square kilometres.[a]

With the addition of Zealandia in 2017, Earth currently has seven recognized geological continents:

Africa

Antarctica

Australia

Eurasia

North America

South America

Zealandia[76]

Due to a seeming lack of Precambrian cratonic rocks, Zealandia's status as a geological continent has been disputed by some geologists.[77] However, a study conducted in 2021 found that part of the submerged continent is indeed Precambrian, twice as old as geologists had previously thought, which is further evidence that supports the idea of Zealandia being a geological continent.[78][79]

All seven geological continents are spatially isolated by geologic features.[80]

History of the concept

Early concepts of the Old World continents

The Ancient Greek geographer Strabo holding a globe showing Europa and Asia

The term "continent" translates the Greek word ἤπειρος, meaning "landmass, terra firma", the proper name of Epirus and later especially used for Asia (i.e. Asia Minor).[81]

The first distinction between continents was made by ancient Greek mariners who gave the names Europe and Asia to the lands on either side of the waterways of the Aegean Sea, the Dardanelles strait, the Sea of Marmara, the Bosporus strait and the Black Sea.[82] The names were first applied just to lands near the coast and only later extended to include the hinterlands.[83][84] But the division was only carried through to the end of navigable waterways and "... beyond that point the Hellenic geographers never succeeded in laying their finger on any inland feature in the physical landscape that could offer any convincing line for partitioning an indivisible Eurasia ..."[82]

Ancient Greek thinkers subsequently debated whether Africa (then called Libya) should be considered part of Asia or a third part of the world. Division into three parts eventually came to predominate.[85] From the Greek viewpoint, the Aegean Sea was the center of the world; Asia lay to the east, Europe to the north and west, and Africa to the south.[86] The boundaries between the continents were not fixed. Early on, the Europe–Asia boundary was taken to run from the Black Sea along the Rioni River (known then as the Phasis) in Georgia. Later it was viewed as running from the Black Sea through Kerch Strait, the Sea of Azov and along the Don River (known then as the Tanais) in Russia.[87] The boundary between Asia and Africa was generally taken to be the Nile River. Herodotus[88] in the 5th century BCE objected to the whole of Egypt being split between Asia and Africa ("Libya") and took the boundary to lie along the western border of Egypt, regarding Egypt as part of Asia.[89][90][91][92] He also questioned the division into three of what is really a single landmass,[93] a debate that continues nearly two and a half millennia later. Herodotus believed Europe to be larger (at least in width) than the other two continents:

I wonder, then, at those who have mapped out and divided the world into Libya, Asia, and Europe; for the difference between them is great, seeing that in length Europe stretches along both the others together, and it appears to me to be wider beyond all comparison.[94]

Eratosthenes, in the 3rd century BCE, noted that some geographers divided the continents by rivers (the Nile and the Don), thus considering them "islands". Others divided the continents by isthmuses, calling the continents "peninsulas". These latter geographers set the border between Europe and Asia at the isthmus between the Black Sea and the Caspian Sea, and the border between Asia and Africa at the isthmus between the Red Sea and the mouth of Lake Bardawil on the Mediterranean Sea.[95]

The Roman author Pliny the Elder, writing in the 1st century CE, stated that "The whole globe is divided into three parts, Europe, Asia, and Africa", adding:

I shall first then speak of Europe, the foster-mother of that people which has conquered all other nations, and itself by far the most beauteous portion of the earth. Indeed, many persons have, not without reason, considered it, not as a third part only of the earth, but as equal to all the rest, looking upon the whole of our globe as divided into two parts only, by a line drawn from the river Tanais to the Straits of Gades.[96]

Medieval T and O map showing the three continents as domains of the sons of Noah—Asia to Sem (Shem), Europe to Iafeth (Japheth), and Africa to Cham (Ham).

Following the fall of the Western Roman Empire, the culture that developed in its place, linked to Latin and the Catholic church, began to associate itself with the concept of Europe.[84] Through the Roman period and the Middle Ages, a few writers took the Isthmus of Suez as the boundary between Asia and Africa, but most writers continued to consider it the Nile or the western border of Egypt (Gibbon).[citation needed] In the Middle Ages, the world was usually portrayed on T and O maps, with the T representing the waters dividing the three continents. By the middle of the 18th century, "the fashion of dividing Asia and Africa at the Nile, or at the Great Catabathmus [the boundary between Egypt and Libya] farther west, had even then scarcely passed away".[97]

European arrival in the Americas

Christopher Columbus sailed across the Atlantic Ocean to the Caribbean in 1492, sparking a period of European exploration of the Americas. But despite four voyages to the Americas, Columbus never believed he had reached a new continent—he always thought it was part of Asia.

In 1501, Amerigo Vespucci and Gonçalo Coelho attempted to sail around what they considered the southern end of the Asian mainland into the Indian Ocean, passing through Fernando de Noronha. After reaching the coast of Brazil, they sailed along the coast of South America much farther south than Asia was known to extend, confirming that this was a land of continental proportions.[98] On return to Europe, an account of the voyage, called Mundus Novus ("New World"), was published under Vespucci's name in 1502 or 1503,[99] although it seems that it had additions or alterations by another writer.[100] Regardless of who penned the words, Mundus Novus credited Vespucci with saying, "I have discovered a continent in those southern regions that is inhabited by more numerous people and animals than our Europe, or Asia or Africa",[101] the first known explicit identification of part of the Americas as a continent like the other three.

Within a few years, the name "New World" began appearing as a name for South America on world maps, such as the Oliveriana (Pesaro) map of around 1504–1505. Maps of this time, though, still showed North America connected to Asia and showed South America as a separate land.[100]

Universalis Cosmographia, Waldseemüller's 1507 world map—the first to show the Americas separate from Asia

In 1507 Martin Waldseemüller published a world map, Universalis Cosmographia, which was the first to show North and South America as separate from Asia and surrounded by water. A small inset map above the main map explicitly showed for the first time the Americas being east of Asia and separated from Asia by an ocean, as opposed to just placing the Americas on the left end of the map and Asia on the right end. In the accompanying book Cosmographiae Introductio, Waldseemüller noted that the earth is divided into four parts, Europe, Asia, Africa, and the fourth part, which he named "America" after Amerigo Vespucci's first name.[102] On the map, the word "America" was placed on part of South America.

Beyond four continents

The Sanskrit text Rig Veda often dated 1500 BCE [note 1] has the earliest mention of seven continents in the Earth, the text claims that the Earth has seven continents and Lord Vishnu Measured the entire universe from his first foot from the land of Earth which has 7 continents.[109]

ato devā avantu no yato viṣṇurvicakrame |

pṛthivyāḥ saptadhāmabhiḥ ||

idaṃ viṣṇurvi cakrame tredhā ni dadhe padam |

samūḷhamasya pāṃsure ||

trīṇi padā vi cakrame viṣṇurghopā adābhyaḥ |

ato dharmāṇi dhārayan ||

The Gods be gracious unto us even from the place whence Vishnu strode

Through the seven regions of the earth!

Through all this world strode Vishnu; thrice his foot he planted, and the whole

Was gathered in his footstep's dust.

Vishnu, the Guardian, he whom none deceiveth, made three steps; thenceforth

Establishing his high decrees.

—RigVeda transliteration of Book 1, Hymn 22, Verses 16-18[110] —RigVeda translation by Ralph T.H. Griffith (1896) of Book 1, Hymn 22, Verses 16–18[111]

Rigveda page in Sanskrit

In regard to the above-quoted verses, it is commonly accepted that there are Seven Continents or 'regions of the earth'. A. Glucklich adds that 'In the Matsya Purana, for instance, there is a seven-part map of the world ... [it has] one centre, where an immense mountain – Mount Meru (or Maha Meru, Great Meru) – stands ... The continents encircle the mountain in seven concentric circles ... It seems clear that the Himalayas were the approximate location of Mt. Meru and the text is clear that the earth has seven continents.[109]

Hollandia Nova, 1659 map prepared by Joan Blaeu based on voyages by Abel Tasman and Willem Jansz, this image shows a French edition of 1663

From the late 18th century, some geographers started to regard North America and South America as two parts of the world, making five parts in total. Overall though, the fourfold division prevailed well into the 19th century.[112]

Europeans discovered Australia in 1606, but for some time it was taken as part of Asia. By the late 18th century, some geographers considered it a continent in its own right, making it the sixth (or fifth for those still taking America as a single continent).[112] In 1813, Samuel Butler wrote of Australia as "New Holland, an immense island, which some geographers dignify with the appellation of another continent" and the Oxford English Dictionary was just as equivocal some decades later.[113] It was in the 1950s that the concept of Oceania as a "great division" of the world was replaced by the concept of Australia as a continent.[114]

Antarctica was sighted in 1820 during the First Russian Antarctic Expedition and described as a continent by Charles Wilkes on the United States Exploring Expedition in 1838, the last continent identified, although a great "Antarctic" (antipodean) landmass had been anticipated for millennia. An 1849 atlas labelled Antarctica as a continent but few atlases did so until after World War II.[115]

Over time, the western concept of dividing the world into continents spread globally, replacing conceptions in other areas of the world. The idea of continents continued to become imbued with cultural and political meaning. In the 19th century during the Meiji period, Japanese leaders began to self-identify with the concept of being Asian, and renew relations with other "Asian" countries while conceiving of the idea of Asian solidarity against western countries. This conception of an Asian identity, as well as the idea of Asian solidarity, was later taken up by others in the region, such as Republican China and Vietnam.[116]

From the mid-19th century, atlases published in the United States more commonly treated North and South America as separate continents, while atlases published in Europe usually considered them one continent. However, it was still not uncommon for American atlases to treat them as one continent up until World War II.[117] From the 1950s, most U.S. geographers divided the Americas into two continents.[117] With the addition of Antarctica, this made the seven-continent model. However, this division of the Americas never appealed to Latin Americans, who saw their region spanning an América as a single landmass, and there the conception of six continents remains dominant, as it does in scattered other countries.[citation needed]

Some geographers regard Europe and Asia together as a single continent, dubbed Eurasia.[118] In this model, the world is divided into six continents, with North America and South America considered separate continents.

Geology

Further information: Continental crust and Plate tectonics

Geologists use the term continent in a different manner from geographers. In geology, a continent is defined by continental crust, which is a platform of metamorphic and igneous rocks, largely of granitic composition. Continental crust is less dense and much thicker than oceanic crust, which causes it to "float" higher than oceanic crust on the dense underlying mantle. This explains why the continents form high platforms surrounded by deep ocean basins.[119][3]

Some geologists restrict the term continent to portions of the crust built around stable regions called cratons. Cratons have largely been unaffected by mountain-building events (orogenies) since the Precambrian. A craton typically consists of a continental shield surrounded by a continental platform. The shield is a region where ancient crystalline basement rock (typically 1.5 to 3.8 billion years old) is widely exposed at the surface. The platform surrounding the shield is also composed of ancient basement rock, but with a cover of younger sedimentary rock.[120] The continents are accretionary crustal "rafts" that, unlike the denser basaltic crust of the ocean basins, are not subjected to destruction through the plate tectonic process of subduction. This accounts for the great age of the rocks comprising the continental cratons.[121]

The margins of geologic continents are either active or passive. An active margin is characterised by mountain building, either through a continent-on continent collision or a subduction zone. Continents grow by accreting lighter volcanic island chains and microcontinents along these active margins, forming orogens. At a passive margin, the continental crust is stretched thin by extension to form a continental shelf, which tapers off with a gradual slope covered in sediment, connecting it directly to the oceanic crust beyond. Most passive margins eventually transition into active margins: where the oceanic plate becomes too heavy due to cooling, it disconnects from the continental crust, and starts subducting below it, forming a new subduction zone.[122]

Sixteen principal tectonic plates of the continents and the floor of the oceans

There are many microcontinents, or continental fragments, that are built of continental crust but do not contain a craton. Some of these are fragments of Gondwana or other ancient cratonic continents: Zealandia,[75] which includes New Zealand and New Caledonia; Madagascar; the northern Mascarene Plateau, which includes the Seychelles. Other islands, such as several in the Caribbean Sea, are composed largely of granitic rock as well, but all continents contain both granitic and basaltic crust, and there is no clear boundary as to which islands would be considered microcontinents under such a definition. The Kerguelen Plateau, for example, is largely volcanic, but is associated with the breakup of Gondwanaland and is considered a microcontinent,[123][124] whereas volcanic Iceland and Hawaii are not. The British Isles, Sri Lanka, Borneo, and Newfoundland were on the margins of the Laurasian continent—only separated from the main continental landmass by inland seas flooding its margins.

The movement of plates has caused the continual formation and breakup of continents, and occasionally supercontinents, in a process called the Wilson Cycle. The supercontinent Columbia or Nuna formed during a period of 2.0–1.8 billion years ago and broke up about 1.5–1.3 billion years ago.[125][126] The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later reassembled into another supercontinent called Pangaea; Pangaea broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).[127]

Criticism

Some academics, such as the historical geographer Martin W. Lewis, argue that the systems we understand today are more rooted in social, political, and cultural history than in geological fact, a view particularly outlined in his book The Myth of Continents: A Critique of Metageography.[128]

See also

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Boundaries between the continents

Dvipa

Forgotten continent

List of continent name etymologies

List of continents and continental subregions by population

List of sovereign states and dependent territories by continent

List of transcontinental countries

Subregion

Notes

^ It is certain that the hymns of the Rig Veda post-date Indo-Iranian separation of c. 2000 BCE and probably that of the relevant Mitanni documents of c. 1400 BCE. Philological estimates tend to date the bulk of the text to the second half of the second millennium:

Max Müller: "the hymns of the Rig-Veda are said to date from 1500 B.C."[103]

The EIEC (s.v. Indo-Iranian languages, p. 306) gives 1500–1000 BCE.

Flood and Witzel both mention c. 1500–1200 BCE.[104][105]

Anthony mentions c. 1500–1300 BCE.[106]

Thomas Oberlies (Die Religion des Rgveda, 1998, p. 158) based on 'cumulative evidence' sets a wide range of 1700–1100 BCE.[107] Oberlies 1998, p. 155 gives an estimate of 1100 BCE for the youngest hymns in book 10.[108]

Witzel 1995, p. 4 mentions c. 1500–1200 BCE. According to Witzel 1997, p. 263, the whole Rig Vedic period may have lasted from c. 1900 BCE to c. 1200 BCE: "the bulk of the RV represents only 5 or 6 generations of kings (and of the contemporary poets) of the Pūru and Bharata tribes. It contains little else before and after this "snapshot" view of contemporary Rgvedic history, as reported by these contemporary "tape recordings." On the other hand, the whole Rgvedic period may have lasted even up to 700 years, from the infiltration of the Indo-Aryans into the subcontinent, c. 1900 B.C. (at the utmost, the time of collapse of the Indus civilization), up to c. 1200 B.C., the time of the introduction of iron which is first mentioned in the clearly post-gvedic hymns of the Atharvaveda."

^ In accordance with these attributes, Eurasia and North America are connected by a bridge of continental crust at least 2 thousand kilometers wide. And with Africa, Eurasia is connected by such a bridge (interrupted by internal sections of the oceanic crust) with a width of at least 5 thousand kilometers.

References

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^ Murray, Warwick E. (2016). "Changing Rural Worlds – A Global View". In Daniels, Peter; Bradshaw, Michael; Shaw, Denis; Sidaway, James; Hall, Tim (eds.). An Introduction To Human Geography (5th ed.). Pearson. p. 231. ISBN 978-1-292-12939-6.

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^ Lewis & Wigen, The Myth of Continents (1997), p. 29.

^ Bowen, Emanuel. (1752). A Complete Atlas, or Distinct View of the Known World. London, p. 3.

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^ "continent [2, n] 6" (1996) Webster's Third New International Dictionary, Unabridged. ProQuest Information and Learning. "a large segment of the earth's outer shell including a terrestrial continent and the adjacent continental shelf"

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^ Lewis & Wigen, The Myth of Continents (1997), p. 35.

^ Lewis & Wigen, The Myth of Continents (1997), Chapter 1: "While it might seem surprising to find North and South America still joined into a single continent in a book published in the United States in 1937, such a notion remained fairly common until World War II. [...] By the 1950s, however, virtually all American geographers had come to insist that the visually distinct landmasses of North and South America deserved separate designations."

^ Lewis & Wigen, The Myth of Continents (1997), pp. 104–123.

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^ [1] Archived 26 March 2023 at the Wayback Machine "In some textbooks, North and South America are combined into 'America' and/or Europe and Asia are combined into 'Eurasia', for a grand total of 6 or even 5."scienceline

^ Martin W. Lewis and Kären E. Wigen, The Myth of Continents: A Critique of Metageography (Berkeley and Los Angeles: University of California Press, 1997)Wigen

^ [2] Archived 16 May 2023 at the Wayback Machine "There are even geographical views that prefer the presence of both a Eurasian as well as one American continent. These geographers therefore contend that there should only be 5 continents."universetoday

^ ঝাঁপ দিন:a b "Continent Archived 19 June 2019 at the Wayback Machine". Encyclopædia Britannica. 2006. Chicago: Encyclopædia Britannica, Inc.

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p. 116: The Arabian Subcontinent later, approximately 35 million years ago, collided with southern Eurasia to form the Zagros Mountains of southwestern Iran.

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^ Tozer, H. F. (1897). A History of Ancient Geography. Cambridge: Cambridge University Press. p. 67.

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^ ঝাঁপ দিন:a b Lewis & Wigen, The Myth of Continents (1997), p. 30

^ "continent n. 5. a." (1989) Oxford English Dictionary, 2nd edition. Oxford University Press. "the great island of Australia is sometimes reckoned as another [continent]"

^ Lewis & Wigen, The Myth of Continents (1997), p. 32: "...the 1950s... was also the period when... Oceania as a "great division" was replaced by Australia as a continent along with a series of isolated and continentally attached islands. [Footnote 78: When Southeast Asia was conceptualized as a world region during World War II..., Indonesia and the Philippines were perforce added to Asia, which reduced the extent of Oceania, leading to a reconceptualization of Australia as a continent in its own right. This maneuver is apparent in postwar atlases]"

^ Lewis, Martin W.; Wigen, Kären E. (1997). The Myth of Continents: a Critique of Metageography. Berkeley: University of California Press. ISBN 978-0-520-20743-1.

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External links

Wikimedia Commons has media related to:

Continents (category)

Continent at the Encyclopædia Britannica

"Continent" . Encyclopædia Britannica (11th ed.). 1911.

"What Are Continents?" on YouTube by CGP Grey

Lost continent revealed in new reconstruction of geologic history