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The Universe
Universe
is all of space and time[a] and their contents,[12] including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe
Universe
is still unknown,[6] it is possible to measure the observable universe. The earliest scientific models of the Universe
Universe
were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth
Earth
at the centre of the Universe.[13][14] Over the centuries, more precise astronomical observations led Nicolaus Copernicus
Nicolaus Copernicus
to develop the heliocentric model with the Sun
Sun
at the centre of the Solar System. In developing the law of universal gravitation, Sir Isaac Newton
Isaac Newton
built upon Copernicus's work as well as observations by Tycho Brahe
Tycho Brahe
and Johannes Kepler's laws of planetary motion. Further observational improvements led to the realization that our Solar System
Solar System
is located in the Milky Way
Milky Way
galaxy, which is one of many galaxies in the Universe. It is assumed that galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. Discoveries in the early 20th century have suggested that the Universe
Universe
had a beginning and that it is expanding[15] at an increasing rate.[16] Roughly eighty percent of mass in the Universe
Universe
appears to exist in an unknown form called dark matter which cannot be directly observed.[17] The Big Bang
Big Bang
theory is the prevailing cosmological description of the development of the Universe. Under this theory, space and time emerged together 7001137990000000000♠13.799±0.021 billion years ago[2] with a fixed amount of energy and matter that has become less dense as the Universe
Universe
has expanded. After the initial expansion, the Universe cooled, allowing the first subatomic particles to form and then simple atoms. Giant clouds later merged through gravity to form galaxies, stars, and everything else seen today. It is possible to see objects that are now further away than 13.799 billion light-years because space itself has expanded. This means that objects which are now 46 billion light years away can still be seen in their distant past, because at that time they were much closer to us. 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
Universe
might be one among many universes that likewise exist.[6][18][19]

Part of a series on

Physical cosmology

Big Bang · Universe Age of the universe Chronology of the universe

Early universe

Planck epoch Grand unification
Grand unification
epoch Quark
Quark
epoch Hadron
Hadron
epoch Lepton
Lepton
epoch Photon
Photon
epoch Big Bang
Big Bang
nucleosynthesis Inflation Dark Ages

Backgrounds

Cosmic background radiation
Cosmic background radiation
(CBR) Gravitational wave background
Gravitational wave background
(GWB) Cosmic microwave background
Cosmic microwave background
(CMB) · Cosmic neutrino background (CNB) Cosmic infrared background
Cosmic infrared background
(INB)

Expansion · Future

Hubble's law · Redshift Metric expansion of space FLRW metric · Friedmann equations Inhomogeneous cosmology Future of an expanding universe Ultimate fate of the universe Heat
Heat
death of the universe Big Rip Big Crunch Big Bounce

Components · Structure

Components

Lambda-CDM model Baryonic matter Energy Radiation Dark energy

Quintessence Phantom energy

Dark matter

Cold dark matter Warm dark matter Hot dark matter

Dark radiation

Structure

Shape of the universe Reionization · Structure formation Galaxy
Galaxy
formation Large-scale structure Large quasar group Galaxy
Galaxy
filament Supercluster Galaxy
Galaxy
cluster Galaxy
Galaxy
group Local Group Void

Experiments

Black Hole Initiative
Black Hole Initiative
(BHI) BOOMERanG Cosmic Background Explorer
Cosmic Background Explorer
(COBE) Illustris project Planck space observatory Sloan Digital Sky Survey
Sloan Digital Sky Survey
(SDSS) 2dF Galaxy
Galaxy
Redshift
Redshift
Survey ("2dF")

Wilkinson Microwave
Microwave
Anisotropy Probe (WMAP)

Scientists

Aaronson Alfvén Alpher Bharadwaj Copernicus de Sitter Dicke Ehlers Einstein Ellis Friedman Galileo Gamow Guth Hawking Hubble Lemaître Mather Newton Penrose Penzias Rubin Schmidt Smoot Suntzeff Sunyaev Tolman Wilson Zel'dovich

Subject history

Discovery of cosmic microwave background radiation

History of the Big Bang
Big Bang
theory

Religious interpretations of the Big Bang
Big Bang
theory

Timeline of cosmological theories

 Category Cosmology
Cosmology
portal Astronomy
Astronomy
portal

v t e

Contents

1 Definition 2 Etymology

2.1 Synonyms

3 Chronology and the Big Bang 4 Physical properties

4.1 Size and regions 4.2 Age and expansion 4.3 Spacetime 4.4 Shape 4.5 Support of life

5 Composition

5.1 Dark energy 5.2 Dark matter 5.3 Ordinary Matter 5.4 Particles

5.4.1 Hadrons 5.4.2 Leptons 5.4.3 Photons

6 Cosmological models

6.1 Model of the Universe
Universe
based on general relativity 6.2 Multiverse
Multiverse
hypothesis

7 Historical conceptions

7.1 Mythologies 7.2 Philosophical models 7.3 Astronomical concepts

8 See also 9 Notes 10 References 11 Bibliography 12 External links

Definition The physical Universe
Universe
is defined as all of space and time[a] (collectively referred to as spacetime) and their contents.[12] Such contents encompass all of energy in its various forms, including electromagnetic radiation and matter, and therefore planets, moons, stars, galaxies, and the contents of intergalactic space.[20][21][22] The Universe
Universe
also includes the physical laws that influence energy and matter, such as conservation laws, classical mechanics, and relativity.[23] The Universe
Universe
is often defined as "the totality of existence", or everything that exists, everything that has existed, and everything that will exist.[23] 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.[24][25][26] The word universe may also refer to concepts such as the cosmos, the world, and nature.[27][28] Etymology The word universe derives from the Old French
Old French
word univers, which in turn derives from the Latin
Latin
word universum.[29] The Latin
Latin
word was used by Cicero
Cicero
and later Latin
Latin
authors in many of the same senses as the modern English word is used.[30] Synonyms A term for "universe" among the ancient Greek philosophers from Pythagoras
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.[31][32] Another synonym was ὁ κόσμος ho kósmos (meaning the world, the cosmos).[33] Synonyms are also found in Latin
Latin
authors (totum, mundus, natura)[34] 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).[35] Chronology and the Big Bang Main articles: Big Bang
Big Bang
and Chronology of the universe

Nature
Nature
timeline

view • discuss • edit

-13 — – -12 — – -11 — – -10 — – -9 — – -8 — – -7 — – -6 — – -5 — – -4 — – -3 — – -2 — – -1 — – 0 —

cosmic expansion

Earliest light

cosmic speed-up

Solar System

water

Single-celled life

photosynthesis

Multicellular life

Land life

Earliest gravity

Dark energy

Dark matter

Earliest universe (−13.80)

Earliest stars

Earliest galaxy

Earliest quasar/sbh

Omega Centauri
Omega Centauri
forms

Andromeda Galaxy
Galaxy
forms

Milky Way
Milky Way
Galaxy spiral arms form

Alpha Centauri
Alpha Centauri
forms

Earliest Earth
Earth
(−4.54)

Earliest life

Earliest oxygen

Atmospheric oxygen

Earliest sexual reproduction

Cambrian explosion

Earliest humans

L i f e

P r i m o r d i a l

Axis scale: billion years Also see: Human
Human
timeline and Life
Life
timeline

The prevailing model for the evolution of the Universe
Universe
is the Big Bang theory.[36][37] The Big Bang
Big Bang
model states that the earliest state of the Universe
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 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. The Big Bang
Big Bang
model accounts for observations such as the correlation of distance and redshift of galaxies, the ratio of the number of hydrogen to helium atoms, and the microwave radiation background.

In this diagram, time passes from left to right, so at any given time, the Universe
Universe
is represented by a disk-shaped "slice" of the diagram.

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, where gravitation is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. Since the Planck epoch, the Universe
Universe
has been expanding to its present form, possibly with a very brief period of cosmic inflation which caused the Universe
Universe
to reach a much larger size in less than 10−32 seconds.[38] After the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. The observed abundance of the elements can be explained by combining the overall expansion of space with nuclear and atomic physics. As the Universe
Universe
expands, the energy density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength. As the Universe
Universe
expanded and cooled, elementary particles associated stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then formed atomic nuclei through nuclear reactions. This process, known as Big Bang
Big Bang
nucleosynthesis, led to the present abundances of lighter nuclei, particularly hydrogen, deuterium, and helium. Big Bang
Big Bang
nucleosynthesis ended about 20 minutes after the Big Bang, when the Universe
Universe
had cooled enough so that nuclear fusion could no longer occur. At this stage, matter in the Universe
Universe
was mainly a hot, dense plasma of negatively charged electrons, neutral neutrinos and positive nuclei. This era, called the photon epoch, lasted about 380 thousand years. Eventually, at a time known as recombination, electrons and nuclei formed stable atoms, which are transparent to most wavelengths of radiation. With photons decoupled from matter, the Universe
Universe
entered the matter-dominated era. Light
Light
from this era could travel freely, and it can still be seen in the Universe
Universe
as the cosmic microwave background (CMB). After around 100 million years, the first stars formed; these were likely very massive, luminous, and responsible for the reionization of the Universe. Having no elements heavier than lithium, these stars also produced the first heavy elements through stellar nucleosynthesis.[39] The Universe
Universe
also contains a mysterious energy called dark energy, the density of which does not change over time. After about 9.8 billion years, the Universe
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.[40] In this era, the expansion of the Universe
Universe
is accelerating due to dark energy. Physical properties Main articles: Observable universe, Age of the Universe, and Metric expansion of space 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. The Universe
Universe
appears to have much more matter than antimatter, an asymmetry possibly related to the CP violation.[41] This imbalance between matter and antimatter is partially responsible for the existence of the Universe
Universe
in itself, since matter and antimatter, if equally produced at the Big Bang, would have completely annihilated each other and, as a result, the universe would not have existed.[42][43] The Universe
Universe
also appears to have neither net momentum nor angular momentum, which follows accepted physical laws if the Universe
Universe
is finite. These laws are the Gauss's law
Gauss's law
and the non-divergence of the stress-energy-momentum pseudotensor.[44]

Constituent spatial scales of the observable universe

This diagram shows Earth's location in the Universe.

Size and regions See also: Observable universe
Observable universe
and Observational cosmology The size of the Universe
Universe
is somewhat difficult to define. According to the general theory of relativity, some regions of space may never interact with ours even in the lifetime of the Universe
Universe
due to the finite speed of light and the ongoing expansion of space. For example, radio messages sent from Earth
Earth
may never reach some regions of space, even if the Universe
Universe
were to exist forever: space may expand faster than light can traverse it.[45] Distant regions of space are assumed to exist and to be part of reality as much as we are, even though we can never interact with them. The spatial region that we can affect and be affected by is the observable universe. The observable universe depends on the location of the observer. By traveling, an observer can come into contact with a greater region of spacetime than an observer who remains still. Nevertheless, even the most rapid traveler will not be able to interact with all of space. Typically, the observable universe is taken to mean the portion of the Universe
Universe
that is observable from our vantage point in the Milky Way. The proper distance—the distance as would be measured at a specific time, including the present—between Earth
Earth
and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the diameter of the observable universe about 91 billion light-years (28×10^9 pc). The distance the light from the edge of the observable universe has travelled is very close to the age of the Universe
Universe
times the speed of light, 13.8 billion light-years (4.2×10^9 pc), but this does not represent the distance at any given time because the edge of the observable universe and the Earth
Earth
have since moved further apart.[46] 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).[47] As an example, the Milky Way
Milky Way
is roughly 100,000–180,000 light years in diameter,[48][49] and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light years away.[50] Because we cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the Universe
Universe
in its totality is finite or infinite.[6][51][52] Estimates for the total size of the universe, if finite, reach as high as

10

10

10

122

displaystyle 10^ 10^ 10^ 122

megaparsecs, implied by one resolution of the No-Boundary Proposal.[53][b] Age and expansion Main articles: Age of the universe
Age of the universe
and Metric expansion of space Astronomers calculate the age of the Universe
Universe
by assuming that the Lambda-CDM model
Lambda-CDM model
accurately describes the evolution of the Universe from a very uniform, hot, dense primordial state to its present state and measuring the cosmological parameters which constitute the model.[citation needed] This model is well understood theoretically and supported by recent high-precision astronomical observations such as WMAP
WMAP
and Planck.[citation needed] Commonly, the set of observations fitted includes the cosmic microwave background anisotropy, the brightness/redshift relation for Type Ia supernovae, and large-scale galaxy clustering including the baryon acoustic oscillation feature.[citation needed] Other observations, such as the Hubble constant, the abundance of galaxy clusters, weak gravitational lensing and globular cluster ages, are generally consistent with these, providing a check of the model, but are less accurately measured at present.[citation needed] With the prior that the Lambda-CDM model
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
Universe
as of 2015 of 13.799 ± 0.021 billion years.[2] Over time, the Universe
Universe
and its contents have evolved; for example, the relative population of quasars and galaxies has changed[54] and space itself has expanded. Due to this expansion, scientists on Earth can observe the light from a galaxy 30 billion light years away even though that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; the photons emitted have been stretched to longer wavelengths and lower frequency during their journey. Analyses of Type Ia supernovae indicate that the spatial expansion is accelerating.[55][56] The more matter there is in the Universe, the stronger the mutual gravitational pull of the matter. If the Universe
Universe
were too dense then it would re-collapse into a gravitational singularity. However, if the Universe
Universe
contained too little matter then the expansion would accelerate too rapidly for planets and planetary systems to form. Since the Big Bang, the universe has expanded monotonically. Perhaps unsurprisingly, our universe has just the right mass density of 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.[57] There are dynamical forces acting on the particles in the Universe which affect the expansion rate. Before 1998, it was expected that the rate of increase of the Hubble Constant 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
Universe
called the deceleration parameter which cosmologists expected to be directly related to the matter density of the Universe. In 1998, the deceleration parameter was measured by two different groups to be consistent with −1 but not zero, which implied that the present-day rate of increase of the Hubble Constant is increasing over time.[16][58] Spacetime Main articles: Spacetime
Spacetime
and World line See also: Lorentz transformation Spacetimes are the arenas in which all physical events take place. The basic elements of spacetimes are events. In any given spacetime, an event is defined as a unique position at a unique time. A spacetime is the union of all events (in the same way that a line is the union of all of its points), formally organized into a manifold.[59] The Universe
Universe
appears to be a smooth spacetime continuum consisting of three spatial dimensions and one temporal (time) dimension (an event in the spacetime of the physical Universe
Universe
can therefore be identified by a set of four coordinates: (x, y, z, t) ). On the average, space is observed to be very nearly flat (with a curvature close to zero), meaning that Euclidean geometry
Euclidean geometry
is empirically true with high accuracy throughout most of the Universe.[60] Spacetime
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
Universe
has more dimensions (which is postulated by theories such as the 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.[61][62] The spacetime of the Universe
Universe
is usually interpreted from a Euclidean perspective, with space as consisting of three dimensions, and time as consisting of one dimension, the "fourth dimension".[63] By combining space and time into a single manifold called Minkowski space, physicists have simplified a large number of physical theories, as well as described in a more uniform way the workings of the Universe
Universe
at both the supergalactic and subatomic levels. Spacetime
Spacetime
events are not absolutely defined spatially and temporally but rather are known to be relative to the motion of an observer. Minkowski space
Minkowski space
approximates the Universe
Universe
without gravity; the pseudo-Riemannian manifolds of general relativity describe spacetime with matter and gravity. Shape Main article: Shape of the universe

The three possible options for the shape of the Universe.

General relativity
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.[64][65] The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model. An important parameter determining the future evolution of the Universe
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.[66] Observations, including the Cosmic Background Explorer
Cosmic Background Explorer
(COBE), Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe
(WMAP), and Planck maps of the CMB, suggest that the Universe
Universe
is infinite in extent with a finite age, as described by the Friedmann–Lemaître–Robertson–Walker (FLRW) models.[67][61][68][69] 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.[70][71] Support of life Main article: Fine-tuned Universe The Universe
Universe
may be fine-tuned; the Fine-tuned Universe hypothesis is the proposition that the conditions that allow the existence of observable life in the Universe
Universe
can only occur when certain universal fundamental physical constants lie within a very narrow range of values, so that if any of several fundamental constants were only slightly different, the Universe
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.[72] The proposition is discussed among philosophers, scientists, theologians, and proponents of creationism. Composition See also: Galaxy
Galaxy
formation and evolution, Galaxy
Galaxy
cluster, Illustris project, and Nebula The Universe
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 of the Universe) and antimatter.[73][74][75] The proportions of all types of matter and energy have changed over the history of the Universe.[76] The total amount of electromagnetic radiation generated within the universe has decreased by 1/2 in the past 2 billion years.[77][78] Today, ordinary matter, which includes atoms, stars, galaxies, and life, accounts for only 4.9% of the contents of the Universe.[10] The present overall density of this type of matter is very low, roughly 4.5 × 10−31 grams per cubic centimetre, corresponding to a density of the order of only one proton for every four cubic meters of volume.[8] 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
Universe
to accelerate, accounts for the remaining 68.3% of the contents.[10][79][80]

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
Universe
over length scales longer than 300 million light-years or so.[81] 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 approximately 300 sextillion (3×1023) stars[82] and more than 100 billion (1011) galaxies.[83] Typical galaxies range from dwarfs with as few as ten million[84] (107) stars up to giants with one trillion[85] (1012) stars. Between the larger structures are voids, which are typically 10–150 Mpc (33 million–490 million ly) in diameter. The Milky Way
Milky Way
is in the Local Group
Local Group
of galaxies, which in turn is in the Laniakea Supercluster.[86] This supercluster spans over 500 million light years, while the Local Group
Local Group
spans over 10 million light years.[87] The Universe
Universe
also has vast regions of relative emptiness; the largest known void measures 1.8 billion ly (550 Mpc) across.[88]

Comparison of the contents of the Universe
Universe
today to 380,000 years after the Big Bang
Big Bang
as measured with 5 year WMAP
WMAP
data (from 2008).[89] (Due to rounding errors, the sum of these numbers is not 100%). This reflects the 2008 limits of WMAP's ability to define dark matter and dark energy.

The observable Universe
Universe
is isotropic on scales significantly larger than superclusters, meaning that the statistical properties of the Universe
Universe
are the same in all directions as observed from Earth. The Universe
Universe
is bathed in highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.72548 kelvin.[9] The hypothesis that the large-scale Universe
Universe
is homogeneous and isotropic is known as the cosmological principle.[90] A Universe
Universe
that is both homogeneous and isotropic looks the same from all vantage points[91] and has no center.[92] Dark energy Main article: Dark energy An explanation for why the expansion of the Universe
Universe
is accelerating remains elusive. It is often attributed to "dark energy", an unknown form of energy that is hypothesized to permeate space.[93] 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.[94][95] Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,[96] and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. 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. Scalar fields having only a slight amount of spatial inhomogeneity would be difficult to distinguish from a cosmological constant. Dark matter Main article: Dark matter 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
Dark matter
neither emits nor absorbs light or any other electromagnetic radiation at any significant level. Dark matter
Dark matter
is estimated to constitute 26.8% of the total mass–energy and 84.5% of the total matter in the Universe.[79][97] Ordinary Matter Main article: Matter The remaining 4.9% of the mass–energy of the Universe
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.[98] As a matter of fact, 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 per cent of the ordinary matter contribution to the mass-energy density of the universe.[99] Ordinary matter commonly exists in four states (or phases): solid, liquid, gas, and plasma. However, advances in experimental techniques have revealed other previously theoretical phases, such as Bose–Einstein condensates and fermionic condensates. Ordinary matter is composed of two types of elementary particles: quarks and leptons.[100] 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, and electrons that orbit the nucleus. Because most of the mass of an atom is concentrated in its nucleus, which is made up of baryons, astronomers often use the term baryonic matter to describe ordinary matter, although a small fraction of this "baryonic matter" is electrons. Soon after the Big Bang, primordial protons and neutrons formed from the quark–gluon plasma of the early Universe
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.[101] Particles

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.[102] 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.[103][104] Of central importance is the Standard Model, a theory that is concerned with electromagnetic interactions and the weak and strong nuclear interactions.[105] The Standard Model
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.[103] The Standard Model
Standard Model
predicted the existence of the recently discovered Higgs boson, a particle that is a manifestation of a field within the Universe
Universe
that can endow particles with mass.[106][107] Because of its success in explaining a wide variety of experimental results, the Standard Model
Standard Model
is sometimes regarded as a "theory of almost everything".[105] The Standard Model does not, however, accommodate gravity. A true force-particle "theory of everything" has not been attained.[108] 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. From approximately 10−6 seconds after the Big Bang, during a period is 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
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
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
Universe
was about one second old.[109]:244–66 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.[110] 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 particle that quickly decay after being produced in high energy collisions, such as those involving cosmic rays or carried out in particle accelerators.[111][112] 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
Neutrinos
rarely interact with anything, and are consequently rarely observed. Neutrinos
Neutrinos
stream throughout the Universe
Universe
but rarely interact with normal matter.[113] 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
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
Universe
had fallen to the point where lepton/anti-lepton pairs were no longer created.[114] Most leptons and anti-leptons were then eliminated in annihilation reactions, leaving a small residue of leptons. The mass of the Universe
Universe
was then dominated by photons as it entered the following photon epoch.[115][116] Photons Main article: Photon
Photon
epoch See also: Photino A photon is the quantum of light and all other forms of electromagnetic radiation. It is the force carrier for the electromagnetic force, even when static via virtual photons. 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. Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of waves and of particles. 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
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
Universe
contained a hot dense plasma of nuclei, electrons and photons. About 380,000 years after the Big Bang, the temperature of the Universe
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
Universe
became transparent. The highly redshifted photons from this period form the cosmic microwave background. Tiny variations in temperature and density detectable in the CMB were the early "seeds" from which all subsequent structure formation took place.[109]:244–66

v t e

Timeline of the Big Bang

Chronology of the universe

Big Bang Planck epoch Grand unification
Grand unification
epoch Electroweak epoch
Electroweak epoch
(Inflationary epoch, Reheating, Baryogenesis) Quark
Quark
epoch Hadron
Hadron
epoch Lepton
Lepton
epoch Photon epoch
Photon epoch
( Big Bang
Big Bang
nucleosynthesis, Matter
Matter
domination, Recombination) Dark ages

Habitable epoch

Reionization

Fate of the universe

Big Crunch Big Rip Heat
Heat
death of the universe

Graphical timeline of the Big Bang     Cosmology portal

Cosmological models Model of the Universe
Universe
based on general relativity Main article: Solutions of the Einstein field equations See also: Big Bang
Big Bang
and Ultimate fate of the Universe General relativity
General relativity
is the geometric theory of gravitation published by Albert Einstein
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
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. 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
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
Universe
over time.[117] With the assumption of the cosmological principle that the Universe
Universe
is homogeneous and isotropic everywhere, a specific solution of the field equations that describes the Universe
Universe
is the metric tensor called the Friedmann–Lemaître–Robertson–Walker metric,

d

s

2

= −

c

2

d

t

2

+ R ( t

)

2

(

d

r

2

1 − k

r

2

+

r

2

d

θ

2

+

r

2

sin

2

⁡ θ

d

ϕ

2

)

displaystyle ds^ 2 =-c^ 2 dt^ 2 +R(t)^ 2 left( frac dr^ 2 1-kr^ 2 +r^ 2 dtheta ^ 2 +r^ 2 sin ^ 2 theta ,dphi ^ 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
Universe
as a function of time; an increase in R is the expansion of the Universe.[118] 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.[119] The value of R as a function of time t depends upon k and the cosmological constant Λ.[117] The cosmological constant represents the energy density of the vacuum of space and could be related to dark energy.[80] The equation describing how R varies with time is known as the Friedmann equation
Friedmann equation
after its inventor, Alexander Friedmann.[120] 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
Universe
can remain constant only if the Universe
Universe
is perfectly isotropic with positive curvature (k=1) and has one precise value of density everywhere, as first noted by Albert Einstein.[117] However, this equilibrium is unstable: because the Universe
Universe
is known to be inhomogeneous on smaller scales, R must change over time. When R changes, all the spatial distances in the Universe change in tandem; there is an overall expansion or contraction of space itself. This accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point 13.8 billion years ago[121] and never moved faster than the speed of light. 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
Big Bang
model of the Universe. Understanding the singularity of the Big Bang
Big Bang
likely requires a quantum theory of gravity, which has not yet been formulated.[122] Third, the curvature index k determines the sign of the mean spatial curvature of spacetime[119] averaged over sufficiently large length scales (greater than about a billion light years). If k=1, the curvature is positive and the Universe
Universe
has a finite volume.[123] A Universe
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
Universe
has an infinite volume.[123] It may seem counter-intuitive that an infinite and yet infinitely dense Universe
Universe
could be created in a single instant at the Big Bang
Big Bang
when R=0, but exactly that is predicted mathematically when k does not equal 1. 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. A toroidal Universe
Universe
could behave like a normal Universe
Universe
with periodic boundary conditions. The ultimate fate of the Universe
Universe
is still unknown, because it depends critically on the curvature index k and the cosmological constant Λ. If the Universe
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,[124] possibly starting a new Universe
Universe
in a Big Bounce. Conversely, if the Universe
Universe
were insufficiently dense, k would equal 0 or −1 and the Universe
Universe
would expand forever, cooling off and eventually reaching the Big Freeze and the heat death of the Universe.[117] Modern data suggests that the rate of expansion of the Universe
Universe
is not decreasing, as originally expected, but increasing; if this continues indefinitely, the Universe may eventually reach a Big Rip. Observationally, the Universe
Universe
appears to be flat (k = 0), with an overall density that is very close to the critical value between recollapse and eternal expansion.[125] Multiverse
Multiverse
hypothesis Main articles: Multiverse, Many-worlds interpretation, Bubble universe theory, and Parallel universe (fiction)

Depiction of a multiverse of seven "bubble" universes, which are separate spacetime continua, each having different physical laws, physical constants, and perhaps even different numbers of dimensions or topologies.

See also: Eternal inflation Some speculative theories have proposed that our Universe
Universe
is but one of a set of disconnected universes, collectively denoted as the multiverse, challenging or enhancing more limited definitions of the Universe.[18][126] Scientific multiverse models are distinct from concepts such as alternate planes of consciousness and simulated reality. Max Tegmark
Max Tegmark
developed a four-part classification scheme for the different types of multiverses that scientists have suggested in response to various Physics
Physics
problems. An example of such multiverses is the one resulting from the chaotic inflation model of the early universe.[127] 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
Big Bang
that created our multiverse created an ensemble of multiverses, the wave function of the ensemble would be entangled in this sense.[128] The least controversial 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". If space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entire Hubble volume
Hubble volume
occur every so often, simply by chance. Tegmark calculated that our nearest so-called doppelgänger, is 1010115 meters away from us (a double exponential function larger than a googolplex).[129][130] In principle, it would be impossible to scientifically verify the existence of an identical Hubble volume. However, this existence does follow as a fairly straightforward consequence from otherwise unrelated scientific observations and theories.[clarification needed (which ones?)] It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another.[129][131] 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.[132] According to one common terminology, each "soap bubble" of spacetime is denoted as a universe, whereas our particular spacetime is denoted as the Universe,[18] just as we call our moon the Moon. The entire collection of these separate spacetimes is denoted as the multiverse.[18] With this terminology, different Universes are not causally connected to each other.[18] 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.[18] 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.[18] 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.[14] Ancient Chinese philosophy encompassed the notion of the Universe
Universe
including both all of space and all of time.[133][134] 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
Universe
as a whole. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted Big Bang.[135] Mythologies Main articles: Creation myth, Creator deity, 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
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).[136] Ethnologists and anthropologists who study myths have developed various classification schemes for the various themes that appear in creation stories.[137][138] 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
Pangu
or the Indian Brahmanda Purana. In related stories, the Universe
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
Coatlicue
myth, the ancient Egyptian god Atum
Atum
story, and the Judeo-Christian Genesis creation narrative
Genesis creation narrative
in which the Abrahamic God created the Universe. In another type of story, the Universe
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
Universe
is created by crafting it from pre-existing materials, such as the corpse of a dead god — as from Tiamat
Tiamat
in the Babylonian epic Enuma Elish
Enuma Elish
or from the giant Ymir
Ymir
in Norse mythology – or from chaotic materials, as in Izanagi
Izanagi
and Izanami
Izanami
in Japanese mythology. In other stories, the Universe
Universe
emanates from fundamental principles, such as Brahman
Brahman
and Prakrti, the creation myth of the Serers,[139] or the yin and yang of the Tao. Philosophical models Further information: Cosmology See also: Pre-Socratic philosophy, Physics
Physics
(Aristotle), Hindu cosmology, Islamic cosmology, and Philosophy
Philosophy
of space and time The pre-Socratic Greek philosophers and Indian philosophers developed some of the earliest philosophical concepts of the Universe.[14][140] 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
Anaxagoras
proposed the principle of Nous
Nous
(Mind), while Heraclitus
Heraclitus
proposed fire (and spoke of logos). Empedocles
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
Universe
is composed of indivisible atoms moving through a void (vacuum), although Aristotle
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.[14] Although Heraclitus
Heraclitus
argued for eternal change, his contemporary Parmenides
Parmenides
made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides
Parmenides
denoted this reality as τὸ ἐν (The One). Parmenides' idea seemed implausible to many Greeks, but his student Zeno of Elea
Zeno of Elea
challenged them with several famous paradoxes. Aristotle
Aristotle
responded to these paradoxes by developing the notion of a potential countable infinity, as well as the infinitely divisible continuum. Unlike the eternal and unchanging cycles of time, he believed that the world is bounded by the celestial spheres and that cumulative stellar magnitude is only finitely multiplicative. 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.[141] In the 5th century AD, the Buddhist atomist philosopher Dignāga
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.[142] The notion of temporal finitism was inspired by the doctrine of creation shared by the three Abrahamic religions: Judaism, Christianity
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 Gaon
(Saadia ben Joseph); and the Muslim theologian, Al-Ghazali
Al-Ghazali
(Algazel).[143] Astronomical concepts Main articles: History of astronomy
History of astronomy
and Timeline of astronomy

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

Astronomical models of the Universe
Universe
were proposed soon after astronomy began with the Babylonian astronomers, who viewed the Universe
Universe
as a flat disk floating in the ocean, and this forms the premise for early Greek maps like those of Anaximander
Anaximander
and Hecataeus of Miletus. 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. According to Aristotle's physical interpretation of the model, celestial spheres eternally rotate with uniform motion around a stationary Earth. Normal matter is entirely contained within the terrestrial sphere. De Mundo
De Mundo
(composed before 250 BC or between 350 and 200 BC), 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".[144] 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. 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
Universe
was a "central fire" around which the Earth, Sun, Moon
Moon
and Planets revolved in uniform circular motion.[145] The Greek astronomer Aristarchus of Samos
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
Archimedes
wrote:

You, King Gelon, are aware the Universe
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
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
Universe
just mentioned. His hypotheses are that the fixed stars and the Sun
Sun
remain unmoved, that the Earth revolves about the Sun
Sun
on the circumference of a circle, the Sun
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
Earth
to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface

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 an explanation for the unobservability of the parallel phenomenon, stellar parallax. The rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests (On the Apparent Face in the Orb of the Moon):

Cleanthes [a contemporary of Aristarchus and head of the Stoics] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the Universe
Universe
[i.e. the Earth], ... supposing the heaven to remain at rest and the Earth
Earth
to revolve in an oblique circle, while it rotates, at the same time, about its own axis

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
Hellenistic astronomer
who lived a century after Aristarchus.[146][147][148] 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.[149] According to Strabo
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.[150] 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, like what Nicolaus Copernicus
Nicolaus Copernicus
later did in the 16th century.[151] During the Middle Ages, heliocentric models were also proposed by the Indian astronomer Aryabhata,[152] and by the Persian astronomers Albumasar[153] and Al-Sijzi.[154]

Model of the Copernican Universe
Universe
by Thomas Digges
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
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
Sun
were placed at the center of the Universe.

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 himself, the notion that the Earth
Earth
rotates is very old, dating at least to Philolaus
Philolaus
(c. 450 BC), Heraclides Ponticus (c. 350 BC) and Ecphantus the Pythagorean. Roughly a century before Copernicus, the Christian scholar Nicholas of Cusa
Nicholas of Cusa
also proposed that the Earth
Earth
rotates on its axis in his book, On Learned Ignorance (1440).[155] Al-Sijzi[156] also proposed that the Earth rotates on its axis. Empirical evidence for the Earth's rotation
Earth's rotation
on its axis, using the phenomenon of comets, was given by Tusi (1201–1274) and Ali Qushji
Ali Qushji
(1403–1474).[157] This cosmology was accepted by Isaac Newton, Christiaan Huygens
Christiaan Huygens
and later scientists.[158] Edmund Halley
Edmund Halley
(1720)[159] and Jean-Philippe de Chéseaux (1744)[160] 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
Olbers' paradox
in the 19th century.[161] 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.[158] This instability was clarified in 1902 by the Jeans instability
Jeans instability
criterion.[162] 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
Universe
has a negligibly small overall density; such a cosmological model had also been proposed earlier in 1761 by Johann Heinrich Lambert.[47][163] A significant astronomical advance of the 18th century was the realization by Thomas Wright, Immanuel Kant
Immanuel Kant
and others of nebulae.[159] In 1919, when Hooker Telescope
Hooker Telescope
was completed, the prevailing view still was that the Universe
Universe
consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope, Edwin Hubble
Edwin Hubble
identified Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively that Andromeda Nebula
Nebula
and Triangulum among others, were entire galaxies outside our own, thus proving that Universe
Universe
consists of multitude of galaxies.[164] 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.[165] See also

Astronomy
Astronomy
portal Cosmology
Cosmology
portal Space
Space
portal

Chronology of the Universe Cosmic Calendar
Cosmic Calendar
(scaled down timeline) Cosmic latte Cosmos Detailed logarithmic timeline Earth's location in the Universe Esoteric cosmology False vacuum Future of an expanding Universe Galaxy
Galaxy
And Mass Assembly survey Heat
Heat
death of the universe History of the Center of the Universe Illustris project Multiverse
Multiverse
(set theory) (Hyperverse, Megaverse or Omniverse) Non-standard cosmology Nucleocosmochronology Rare Earth
Earth
hypothesis Religious cosmology Space
Space
and survival Terasecond and longer Timeline of the far future Timeline of the formation of the universe Timeline of the near future Vacuum
Vacuum
genesis Zero-energy Universe

Notes

^ a b According to modern physics, space and time are intimately intertwined and physically meaningless if taken separately from each other. See Theory of relativity. ^ Although listed in megaparsecs by the cited source, this number is so vast that it's digits would remain virtually unchanged for all intents and purposes regardless of which conventional units it is listed in, whether it to be nanometres or gigaparsecs, as the differences would disappear into the error.

References

^ "Hubble sees galaxies galore". spacetelescope.org. Retrieved April 30, 2017.  ^ a b c Planck Collaboration (2015). "Planck 2015 results. XIII. Cosmological parameters (See Table 4 on page 31 of pfd)". Astronomy & Astrophysics. 594: A13. arXiv:1502.01589 . Bibcode:2016A&A...594A..13P. doi:10.1051/0004-6361/201525830.  ^ "MSU researcher recognized for discoveries about universe". December 21, 2004. Retrieved February 8, 2011.  ^ Itzhak Bars; John Terning (2009). Extra Dimensions in Space
Space
and Time. Springer. pp. 27ff. ISBN 978-0-387-77637-8. Retrieved May 1, 2011.  ^ SPACE.com – Universe
Universe
Measured: We're 156 Billion Light-years
Light-years
Wide! ^ a b c d Greene, Brian (2011). The Hidden Reality. Alfred A. Knopf.  ^ Paul Davies (2006). The Goldilocks Enigma. First Mariner Books. p. 43ff. ISBN 978-0-618-59226-5.  ^ a b NASA/ WMAP
WMAP
Science
Science
Team (January 24, 2014). " Universe
Universe
101: What is the Universe
Universe
Made Of?". NASA. Retrieved February 17, 2015.  ^ a b Fixsen, D. J. (2009). "The Temperature of the Cosmic Microwave Background". The Astrophysical Journal. 707 (2): 916–20. arXiv:0911.1955 . Bibcode:2009ApJ...707..916F. doi:10.1088/0004-637X/707/2/916.  ^ a b c "First Planck results: the Universe
Universe
is still weird and interesting". Matthew Francis. Ars technica. March 21, 2013. Retrieved August 21, 2015.  ^ NASA/ WMAP
WMAP
Science
Science
Team (January 24, 2014). " Universe
Universe
101: Will the Universe
Universe
expand forever?". NASA. Retrieved April 16, 2015.  ^ a b Zeilik, Michael; Gregory, Stephen A. (1998). Introductory Astronomy
Astronomy
& Astrophysics
Astrophysics
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and Scientific Thought. Cambridge, MA: M.I.T. Press. ISBN 978-0-262-54003-2.  ^ Jeans, J. H. (1902). "The Stability of a Spherical Nebula" (PDF). Philosophical Transactions of the Royal Society A. 199 (312–320): 1–53. Bibcode:1902RSPTA.199....1J. doi:10.1098/rsta.1902.0012. JSTOR 90845. Archived from the original (PDF) on July 20, 2011. Retrieved March 17, 2011.  ^ Misner, Thorne and Wheeler, p. 757. ^ Sharov, Aleksandr Sergeevich; Novikov, Igor Dmitrievich (1993). Edwin Hubble, the discoverer of the big bang universe. Cambridge University Press. p. 34. ISBN 978-0-521-41617-7. Retrieved December 31, 2011.  ^ Einstein, A (1917). "Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie". Preussische Akademie der Wissenschaften, Sitzungsberichte. 1917. (part 1): 142–52. 

Bibliography

Bartel, Leendert van der Waerden (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy". Annals of the New York Academy of Sciences. 500 (1): 525–45. Bibcode:1987NYASA.500..525V. doi:10.1111/j.1749-6632.1987.tb37224.x.  Landau L, Lifshitz E (1975). The Classical Theory of Fields (Course of Theoretical Physics). 2 (revised 4th English ed.). New York: Pergamon Press. pp. 358–97. ISBN 978-0-08-018176-9.  Liddell, H. G. & Scott, R. (1968). A Greek-English Lexicon. Oxford University Press. ISBN 0-19-864214-8.  Misner, C.W., Thorne, Kip, Wheeler, J.A. (1973). Gravitation. San Francisco: W. H. Freeman. pp. 703–816. ISBN 978-0-7167-0344-0.  Raine, D. J.; Thomas, E. G. (2001). An Introduction to the Science
Science
of Cosmology. Institute of Physics
Physics
Publishing.  Rindler, W. (1977). Essential Relativity: Special, General, and Cosmological. New York: Springer Verlag. pp. 193–244. ISBN 0-387-10090-3.  Martin Rees, ed. (2012). Smithsonian Universe
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NASA/IPAC Extragalactic Database (NED) NED-Distances

v t e

Cosmology

Background

Age of the universe Big Bang Chronology of the universe Universe

History of cosmological theories

Discovery of cosmic microwave background History of the Big Bang
Big Bang
theory Religious interpretations of the Big Bang Timeline of cosmological theories

Past universe

Cosmic microwave background Gravitational
Gravitational
wave background Neutrino
Neutrino
background Inflation Nucleosynthesis Habitable epoch

Present universe

FLRW metric Friedmann equations Hubble's law Metric expansion of space Redshift

Future universe

Future of an expanding universe Ultimate fate of the universe

Components

Dark energy Dark fluid Dark matter Lambda-CDM model

Structure formation

Galaxy
Galaxy
filament Galaxy
Galaxy
formation Large quasar group Large-scale structure Reionization Shape of the universe Structure formation

Experiments

2dF BOOMERanG COBE Illustris project Observational cosmology Planck SDSS WMAP

Cosmology
Cosmology
portal

v t e

The fundamental interactions of physics

Physical forces

Strong interaction Weak interaction Electromagnetism Gravitation

Radiations

Electromagnetic radiation Gravitational
Gravitational
radiation

Hypothetical forces

Quintessence Weak gravity conjecture

Glossary of physics Particle physics Philosophy
Philosophy
of physics Universe Weakless Universe

v t e

Earth's location in the Universe

Included

Earth → Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way
Milky Way
subgroup → Local Group → Virgo Supercluster → Laniakea Supercluster → Observable universe → Universe Each arrow (→) may be read as "within" or "part of".

Related

"Cosmic View" (1957 essay) To the Moon
Moon
and Beyond (1964 film) Cosmic Zoom (1968 film) Powers of Ten (1968 and 1977 films) Cosmic Voyage
Cosmic Voyage
(1996 documentary) Cosmic Eye (2012)

Astronomy
Astronomy
portal - Cosmology
Cosmology
portal

v t e

Elements of nature

Universe

Space Time Energy Matter Change

Earth

Earth
Earth
science History (geological) Structure Geology Plate tectonics Oceans Gaia hypothesis Future

Weather

Meteorology Atmosphere (Earth) Climate Clouds Sunlight Tides Wind

Natural environment

Ecology Ecosystem Field Radiation Wilderness Wildfires

Life

Origin (abiogenesis) Evolutionary history Biosphere Hierarchy Biology (astrobiology)

Organism Eukaryota

flora

plants

fauna

animals

fungi protista

Prokaryotes

archaea bacteria

Viruses

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