TheInfoList

The universe ( la, universus) is all of
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

and
time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

and their contents, including
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

s,
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

s,
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

, and all other forms of
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
and
energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

. The
Big Bang The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

theory is the prevailing
cosmological Cosmology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...
description of the development of the universe. According to this theory, space and time emerged together ago, and the universe has been expanding ever since. While the spatial size of the entire universe is unknown, the cosmic inflation equation indicates that it must have a minimum diameter of 23 trillion light years, and it is possible to measure the size of the
observable universe The observable universe is a ball-shaped region of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...
, which is approximately 93 billion
light-year A light-year, alternatively spelt lightyear, is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metric un ...
s in diameter at the present day. The earliest cosmological models of the universe were developed by
ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the diale ...
and Indian philosophers and were
geocentric In , the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a description of the with at the center. Under the geocentric model, the , , s, and all Earth. The geocentric model was the pre ...
, placing
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

at the center. Over the centuries, more precise astronomical observations led
Nicolaus Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, link=no, Niclas Koppernigk, modern: ''Nikolaus Kopernikus''; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic Church, C ...

to develop the heliocentric model with the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

at the center of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

. In developing the
law of universal gravitation Newton's law of universal gravitation is usually stated as that every particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which can ...
,
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

built upon Copernicus's work as well as
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer An astronomer is a in the field of who focuses their studies on a specific question or field outside the scope of . They observe s such as s, s, , s and ...

's laws of planetary motion and observations by
Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; 14 December 154624 October 1601) was a Danish astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. T ...

. Further observational improvements led to the realization that the Sun is one of hundreds of billions of stars in the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

, which is one of a few hundred billion galaxies in the universe. Many of the stars in a galaxy . 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
supercluster A supercluster is a large group of smaller galaxy clusters or galaxy groups; it is among the largest known structures of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, ...
s which form immense filaments and
voids Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a s ...
in space, creating a vast foam-like structure. Discoveries in the early 20th century have suggested that the universe had a beginning and that space has been expanding since then at an increasing rate. 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 __NOTOC__ In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the ...
at around 10−32 seconds, and the separation of the four known
fundamental forces In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Sp ...
, the universe gradually cooled and continued to expand, allowing the first
subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ...
s and simple
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s to form.
Dark matter Dark matter is a hypothetical form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, ...

foam Foam is an object formed by trapping pockets of gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

-like structure of filaments and
voids Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a s ...
under the influence of
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

. Giant clouds of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

and
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

were gradually drawn to the places where dark matter was most
dense The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or ...

, forming the first galaxies, stars, and everything else seen today. From studying the movement of galaxies, it has been discovered that the universe contains much more
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known as dark matter (''dark'' means that there is a wide range of strong that it exists, but we have not yet detected it directly). The ΛCDM model is the most widely accepted model of the universe. It suggests that about
015 Fifteen or 15 may refer to: *15 (number) 15 (fifteen) is a number, numeral, and glyph. It is the natural number following 14 and preceding 16. Mathematics 15 is: * a lucky number In number theory Number theory (or arithmetic or higher ...
of the mass and energy in the universe is a cosmological constant (or, in extensions to ΛCDM, other forms of
dark energy In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the unive ...

, such as a
scalar field In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...

) which is responsible for the current
expansion of space The expansion of the universe is the increase in proper length, distance between any two given Gravitational binding energy, gravitationally unbound parts of the observable universe with time. It is an intrinsic and extrinsic properties (philosoph ...
015 Fifteen or 15 may refer to: *15 (number) 15 (fifteen) is a number, numeral, and glyph. It is the natural number following 14 and preceding 16. Mathematics 15 is: * a lucky number In number theory Number theory (or arithmetic or higher ...
is dark matter.Planck 2015 results, table 9
/ref> Ordinary ('
baryonic In particle physics, a baryon is a type of composite particle, composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron list of particles, family of particles; hadrons are composed of ...
') matter is therefore only
015 Fifteen or 15 may refer to: *15 (number) 15 (fifteen) is a number, numeral, and glyph. It is the natural number following 14 and preceding 16. Mathematics 15 is: * a lucky number In number theory Number theory (or arithmetic or higher ...
of the physical universe. Stars, planets, and visible gas clouds only form about 6% of the ordinary matter. There are many competing hypotheses about the
ultimate fate of the universe The ultimate fate of the universe is a topic in physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest ...
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 The multiverse is a hypothetical A hypothesis (plural hypotheses) is a proposed explanation An explanation is a set of statements usually constructed to describe a set of facts which clarifies the causes, context Context may refer ...

hypotheses, in which our universe might be one among many universes that likewise exist.

# Definition

The physical universe is defined as all of
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

and
time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

(collectively referred to as
spacetime In physics, spacetime is any mathematical model which fuses the three-dimensional space, three dimensions of space and the one dimension of time into a single four-dimensional manifold. Minkowski diagram, Spacetime diagrams can be used to visuali ...
) and their contents. Such contents comprise all of energy in its various forms, including
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

and
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
, and therefore planets,
moons A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...

, stars, galaxies, and the contents of
intergalactic space Outer space is the expanse that exists beyond Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen Hydrogen is the c ...
. The universe also includes the
physical law Scientific laws or laws of science are statements, based on repeated experiment An experiment is a procedure carried out to support or refute a , or determine the or of something previously untried. Experiments provide insight into by d ...
s that influence energy and matter, such as
conservation law In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...
s, classical mechanics, and relativity. The universe is often defined as "the totality of existence", or
everything Everything (or Every thing) is all that exists; the opposite of nothing "Nothing", used as a pronoun subject, is the absence of a something or particular thing that one might expect or desire to be present ("We found nothing", "Nothing was ther ...
that exists, everything that has existed, and everything that will exist. 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. The word ''universe'' may also refer to concepts such as ''the cosmos'', ''the world'', and ''nature''.

# Etymology

The word ''universe'' derives from the
Old French Old French (, , ; Modern French French ( or ) is a Romance language The Romance languages, less commonly Latin or Neo-Latin languages, are the modern languages that evolved from Vulgar Latin Vulgar Latin, also known as Popular o ...
word , which in turn derives from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

word . The Latin word was used by
Cicero Marcus Tullius Cicero ( ; ; 3 January 106 BC – 7 December 43 BC) was a Ancient Rome, Roman statesman, lawyer, scholar, philosopher and Academic skepticism, Academic Skeptic, who tried to uphold optimate principles during crisis of ...

and later Latin authors in many of the same senses as the modern
English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, which has eventually become the World language, leading lan ...

word is used.Lewis, C.T. and Short, S (1879) ''A Latin Dictionary'', Oxford University Press, , pp. 1933, 1977–1978.

## Synonyms

A term for ''universe'' among the ancient Greek philosophers from
Pythagoras Pythagoras of Samos, or simply ; in Ionian Greek () was an ancient Ionians, Ionian Ancient Greek philosophy, Greek philosopher and the eponymous founder of Pythagoreanism. His political and religious teachings were well known in Magna Graec ...

onwards was () 'the all', defined as all matter and all space, and () 'all things', which did not necessarily include the void. Another synonym was () meaning 'the
world In its most general sense, the term "world" refers to the totality of entities, to the whole of reality or to everything that is. The nature of the world has been conceptualized differently in different fields. Some conceptions see the world ...
, the
cosmos The cosmos (, ) is another name for the Universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prev ...

'. Synonyms are also found in Latin authors (, , ) and survive in modern languages, e.g., the
German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also German nationality law * German language The German la ...

words , , and for ''universe''. The same synonyms are found in English, such as everything (as in the
theory of everything A theory of everything (TOE or ToE), final theory, ultimate theory, or master theory is a hypothetical single, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the univers ...
), the cosmos (as in
cosmology Cosmology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...
), the world (as in the
many-worlds interpretation The many-worlds interpretation (MWI) is an that asserts that the is , and that there is no . This implies that all outcomes of quantum measurements are physically realized in some "world" or universe. In contrast to some other interpretations ...
), and
nature Nature, in the broadest sense, is the natural, physical, material world or universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter an ...

(as in
natural law Natural law ( la, ius naturale, ''lex naturalis'') is a system of law based on a close observation of human nature Human nature is a concept that denotes the fundamental disposition A disposition is a quality of character, a habit A habit (or ...
s or
natural philosophy Natural philosophy or philosophy of nature (from Latin ''philosophia naturalis'') was the philosophy, philosophical study of nature and the physical universe that was dominant before the development of modern science. From the ancient world, a ...
).

# Chronology and the Big Bang

The prevailing model for the evolution of the universe is the Big Bang theory. 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 General relativity, also known as the general theory of relativity, is the geometric Geometry (from the grc, γεωμετρία; '' geo-'' "earth", '' -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathema ...
and on simplifying assumptions such as the
homogeneity Homogeneity and heterogeneity are concepts often used in the sciences Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts ...
and
isotropy Isotropy is uniformity in all orientations; it is derived from the Greek ''isos'' (ἴσος, "equal") and ''tropos'' (τρόπος, "way"). Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by t ...
of space. A version of the model with a cosmological constant (Lambda) and
cold dark matter In Physical cosmology, cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. Observations indicate that approximately 85% of the matter in the universe is dark matter, with only a small fraction being the ordinary b ...
, 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 model accounts for observations such as the correlation of distance and
redshift In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...

of galaxies, the ratio of the number of hydrogen to helium atoms, and the microwave radiation background. The initial hot, dense state is called the
Planck epoch The chronology of the universe describes the history and Future of an expanding universe, future of the universe according to Big Bang cosmology. The earliest stages of the universe's existence are estimated as taking place 13.8 billion years ag ...
, a brief period extending from time zero to one
Planck time In particle physics Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various types of very small objects (e.g. , gas particl ...
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 Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

—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. Since the Planck epoch, space has been expanding to its present scale, with a very short but intense period of
cosmic inflation In physical cosmology Physical cosmology is a branch of cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the ch ...
believed to have occurred within the first 10−32 seconds. This was a kind of expansion different from those we can see around us today. Objects in space did not physically move; instead the
metric METRIC (Mapping EvapoTranspiration at high Resolution with Internalized Calibration) is a computer model Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of or th ...
that defines space itself changed. Although objects in
spacetime In physics, spacetime is any mathematical model which fuses the three-dimensional space, three dimensions of space and the one dimension of time into a single four-dimensional manifold. Minkowski diagram, Spacetime diagrams can be used to visuali ...
cannot move faster than the
speed of light The speed of light in vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
, this limitation does not apply to the metric governing spacetime itself. This initial period of inflation is believed to explain why space appears to be very flat, and much larger than light could travel since the start of the universe. Within the first fraction of a second of the universe's existence, the four fundamental forces had separated. As the universe continued to cool down from its inconceivably hot state, various types of
subatomic particles In physics, physical sciences, subatomic particles are smaller than atoms. They can be composite particles, such as the neutron and proton; or elementary particles, which according to the standard model are not made of other particles. Particle p ...
were able to form in short periods of time known as the
quark epoch In physical cosmology the Quark epoch was the period in the evolution of the Chronology_of_the_universe#Early_universe, early universe when the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interacti ...
, the
hadron epoch In physical cosmology, the hadron epoch started 20 microseconds after the Big Bang. The temperature of the universe had fallen sufficiently to allow the quarks from the preceding quark epoch to bind together into hadrons. Initially the temperature ...
, and the
lepton epoch In physical cosmology, 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 Hadron, hadrons and Antimatt ...
. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. These
elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fundamental fermions (quarks, leptons, antiquarks, and a ...
s associated stably into ever larger combinations, including stable
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

s and
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

s, which then formed more complex
atomic nuclei The atomic nucleus is the small, dense region consisting of proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutr ...
through
nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products ...

. This process, known as
Big Bang nucleosynthesis In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the univer ...
, only 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
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

s and all the
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

s in the universe, by mass, were converted to
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

, with small amounts of
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific elemen ...

(a
form Form is the shape A shape or figure is the form of an object or its external boundary, outline, or external Surface (mathematics), surface, as opposed to other properties such as color, Surface texture, texture, or material type. A plane shape, ...
of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

) and traces of
lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: the ...

. Any other
element Element may refer to: Science * Chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all ...
was only formed in very tiny quantities. The other 75% of the protons remained unaffected, as
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

nuclei. 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
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s, so it contained a hot, dense, foggy Plasma (physics), 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
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s. This is known as recombination (cosmology), recombination for historical reasons; in fact electrons and nuclei were combining for the first time. Unlike plasma, neutral atoms are Opacity (optics), transparent to many wavelengths of light, so for the first time the universe also became transparent. The photons released ("photon decoupling, decoupled") when these atoms formed can still be seen today; they form the cosmic microwave background (CMB). As the universe expands, the energy density of
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

decreases more quickly than does that of
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
because the energy of a photon decreases with its wavelength. 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. In the earliest stages of the universe, tiny fluctuations within the universe's density led to filament (cosmology), concentrations of dark matter gradually forming. Ordinary matter, attracted to these by
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

, formed large gas clouds and eventually, stars and galaxies, where the dark matter was most dense, and Void (astronomy), voids where it was least dense. After around 100 - 300 million years, the first
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

s formed, known as Population III stars. These were probably very massive, luminous, metallicity, 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. The universe also contains a mysterious energy—possibly a
scalar field In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...

—called
dark energy In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the unive ...

, 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. In this era, the expansion of the universe is accelerating expansion of the universe, accelerating due to dark energy.

# Physical properties

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 nuclear force, weak and strong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales. The universe appears to have much more
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
than antimatter, an asymmetry possibly related to the CP violation. 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 The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

, would have completely annihilated each other and left only photons as a result of their interaction. The universe also appears to have neither net momentum nor angular momentum, which follows accepted physical laws if the universe is finite. These laws are Gauss's law and the non-divergence of the stress-energy-momentum pseudotensor.

## Size and regions

According to the general theory of relativity, far regions of
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

may never interact with ours even in the lifetime of the universe due to the finite
speed of light The speed of light in vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
and the ongoing expansion of space. For example, radio messages sent from
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

may never reach some regions of space, even if the universe were to exist forever: space may expand faster than light can traverse it. The spatial region that can be observed with telescopes is called the
observable universe The observable universe is a ball-shaped region of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...
, which depends on the location of the observer. The Comoving distance, proper distance—the distance as would be measured at a specific time, including the present—between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the Observable universe#Size, diameter of the observable universe about 93 billion light-years (28 billion parsecs). The distance the light from the edge of the observable universe has travelled is very close to the age of the universe times the speed of light, , but this does not represent the distance at any given time because the edge of the observable universe and the Earth have since moved further apart. 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).#Rindler, Rindler, p. 196. As an example, the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

is roughly 100,000–180,000 light-years in diameter, and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light-years away. Because we 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. Estimates suggest that the whole universe, if finite, must be more than 250 times larger than the observable universe. Some disputed estimates for the total size of the universe, if finite, reach as high as $10^$ megaparsecs, as implied by a suggested resolution of the No-Boundary Proposal.

## Age and expansion

Astronomers calculate the age of the universe by assuming that the 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. This model is well understood theoretically and supported by recent high-precision astronomical observations such as WMAP and Planck (satellite), Planck. 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. 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. 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 as of 2015 of 13.799 Measurement uncertainty, ± 0.021 billion years. Over time, the universe and its contents have evolved; for example, the relative population of quasars and galaxies has changed and
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

itself has metric expansion of space, 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
redshift In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...

ed; 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 expansion of the Universe, accelerating. 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. Anthropic principle#Anthropic coincidences, Perhaps unsurprisingly, our universe has Critical Mass Density of the Universe, just the right mass-energy density, equivalent to about 5 protons per cubic metre, which has allowed it to expand for the last 13.8 billion years, giving time to form the universe as observed today. 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 cosmology, scale factor $\ddot$ has been positive in the last 5-6 billion years. This acceleration does not, however, imply that the Hubble parameter is currently increasing; see deceleration parameter for details.

## Spacetime

Spacetimes are the arenas in which all physical events take place. The basic elements of spacetimes are event (relativity), 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. Events, such as matter and energy, bend spacetime. Curved spacetime, on the other hand, forces matter and energy to behave in a certain way. There is no point in considering one without the other. The universe appears to be a smooth spacetime continuum consisting of three space, spatial dimensions and one temporal (
time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

) dimension (an event in the spacetime of the physical universe can therefore be identified by a set of four coordinates: (''x'', ''y'', ''z'', ''t'') ). On average, 3-space, space is observed to be very nearly Shape of the universe, flat (with a curvature close to zero), meaning that Euclidean geometry is empirically true with high accuracy throughout most of the Universe.WMAP Mission: Results – Age of the Universe
Map.gsfc.nasa.gov. Retrieved November 28, 2011.
Spacetime also appears to have a simply connected space, 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 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
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

s. The spacetime of the universe is usually interpreted from a Euclidean space, Euclidean perspective, with space as consisting of Three-dimensional space, three dimensions, and time as consisting of one dimension, the "Four-dimensional space, fourth dimension". By combining space and time into a single manifold called Minkowski space, physicists have simplified a large number of Theoretical physics, physical theories, as well as described in a more uniform way the workings of the universe at both the Physical cosmology, supergalactic and quantum mechanics, subatomic levels. Spacetime event (relativity), events are not absolutely defined spatially and temporally but rather are known to be relative to the motion of an observer (special relativity), observer. Minkowski space approximates the universe without
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

; the pseudo-Riemannian manifolds of
general relativity General relativity, also known as the general theory of relativity, is the geometric Geometry (from the grc, γεωμετρία; '' geo-'' "earth", '' -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathema ...
describe spacetime with matter and gravity.

## Shape

General relativity describes how spacetime is curved and bent by mass and energy (gravity). The topology or geometry of the universe includes both Shape of the universe#Local geometry (spatial curvature), local geometry in the
observable universe The observable universe is a ball-shaped region of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...
and Shape of the universe#Global geometry, global geometry. Cosmologists often work with a given space-like slice of spacetime called the Comoving distance, 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 Elementary particle, particles can have traveled to the observation, observer in the age of the universe. This horizon represents the boundary between the observable and the unobservable regions of the universe. 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 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 Shape of the universe, geometries depending on whether Ω is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes. Observations, including the Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe (WMAP), and Planck (spacecraft), 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 metric, Friedmann–Lemaître–Robertson–Walker (FLRW) models.Will the Universe expand forever?
WMAP website at NASA.
These FLRW models thus support inflationary models and the standard model of cosmology, describing a Minkowski space, flat, homogeneous universe presently dominated by dark matter and
dark energy In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the unive ...

.

## Support of life

The 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 can only occur when certain universal physical constant, 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 would have been unlikely to be conducive to the establishment and development of
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
, astronomical structures, elemental diversity, or life as it is understood. The proposition is discussed among philosophy, philosophers, scientists, theology, theologians, and proponents of creationism.

# Composition

The universe is composed almost completely of dark energy, dark matter, and matter, ordinary matter. Other contents are
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

(estimated to constitute from 0.005% to close to 0.01% of the total mass-energy of the universe) and antimatter. The proportions of all types of matter and energy have changed over the history of the universe. The total amount of electromagnetic radiation generated within the universe has decreased by 1/2 in the past 2 billion years. Today, ordinary matter, which includes atoms, stars, galaxies, and life, accounts for only 4.9% of the contents of the Universe. 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 metres of volume. 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. Matter, dark matter, and dark energy are distributed homogeneously throughout the universe over length scales longer than 300 million light-years or so. However, over shorter length-scales, matter tends to clump hierarchically; many
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s are condensed into
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

s, most stars into galaxies, most galaxies into galaxy groups and clusters, clusters, superclusters and, finally, large-scale Galaxy filament, galactic filaments. The observable universe contains as many as 200 billion galaxies and, overall, as many as an estimated stars (more stars than all the Sand, grains of sand on planet
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

). Typical galaxies range from dwarf galaxy, dwarfs with as few as ten million (107) stars up to giants with one 10^12, trillion (1012) stars. Between the larger structures are
voids Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a s ...
, which are typically 10–150 Mpc (33 million–490 million ly) in diameter. The
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

is in the Local Group of galaxies, which in turn is in the Laniakea Supercluster. This supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years. The Universe also has vast regions of relative emptiness; the largest known void measures 1.8 billion ly (550 Mpc) across. 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 electromagnetic radiation, radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.72548 kelvins. The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle. A universe that is both homogeneous and isotropic looks the same from all vantage points. p. 2. and has no center.

## Dark energy

An explanation for why the expansion of the universe is accelerating remains elusive. It is often attributed to "dark energy", an unknown form of energy that is hypothesized to permeate space. 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. Two proposed forms for dark energy are the cosmological constant, a ''constant'' energy density filling space homogeneously, and Scalar field theory, scalar fields such as quintessence (physics), quintessence or moduli (physics), 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

Dark matter is a hypothetical kind of
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
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 Observable universe#Large-scale structure, 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 blackbody spectrum, emits nor absorbs light or any other
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

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.Sean Carroll, Ph.D., Caltech, 2007, The Teaching Company, ''Dark Matter, Dark Energy: The Dark Side of the Universe'', Guidebook Part 2 p. 46, Accessed October 7, 2013, "...dark matter: An invisible, essentially collisionless component of matter that makes up about 25 percent of the energy density of the universe... it's a different kind of particle... something not yet observed in the laboratory..."

## Ordinary matter

The remaining 4.9% of the mass–energy of the universe is ordinary matter, that is,
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s, ions, electrons and the objects they form. This matter includes
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

s, which produce nearly all of the light we see from galaxies, as well as interstellar gas in the interstellar medium, interstellar and intergalactic medium, intergalactic media,
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

s, and all the objects from everyday life that we can bump into, touch or squeeze. 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. Ordinary matter commonly exists in four state of matter, states (or phase (matter), phases): solid, liquid, gas, and plasma (physics), 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 particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fundamental fermions (quarks, leptons, antiquarks, and a ...
s: quarks and leptons. 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 The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

, 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 In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the univer ...
, nuclei formed from the primordial protons and neutrons. This nucleosynthesis formed lighter elements, those with small atomic numbers up to
lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: the ...

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 metallicity, heavier elements resulted from stellar nucleosynthesis and supernova nucleosynthesis.

## Particles

Ordinary matter and the forces that act on matter can be described in terms of
elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fundamental fermions (quarks, leptons, antiquarks, and a ...
s. 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. Of central importance is the Standard Model, a theory that is concerned with Electromagnetism, electromagnetic interactions and the Weak interaction, weak and Strong interaction, strong nuclear interactions. 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 fundamental interactions, interactions: the photon, the W and Z bosons, and the gluon. 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. Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a "theory of almost everything". The Standard Model does not, however, accommodate gravity. A true force-particle "theory of everything" has not been attained.

A hadron is a composite particle made of quarks bound state, held together by the strong force. Hadrons are categorized into two families: baryons (such as
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

s and
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

s) made of three quarks, and mesons (such as pions) made of one quark and one antiparticle, 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 The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

, during a period is known as the
hadron epoch In physical cosmology, the hadron epoch started 20 microseconds after the Big Bang. The temperature of the universe had fallen sufficiently to allow the quarks from the preceding quark epoch to bind together into hadrons. Initially the temperature ...

### Leptons

A lepton is an elementary particle, 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. Two main classes of leptons exist: electric charge, 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 tau (particle), taus are unstable particle that quickly decay after being produced in high energy physics, high energy collisions, such as those involving cosmic rays or carried out in particle accelerators. Charged leptons can combine with other particles to form various composite particles such as
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s and positronium. The electron governs nearly all of chemistry, as it is found in
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s and is directly tied to all chemical property, chemical properties. Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the universe but rarely interact with normal matter. The
lepton epoch In physical cosmology, 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 Hadron, hadrons and Antimatt ...
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 The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

, after the majority of hadrons and anti-hadrons annihilated each other at the end of the
hadron epoch In physical cosmology, the hadron epoch started 20 microseconds after the Big Bang. The temperature of the universe had fallen sufficiently to allow the quarks from the preceding quark epoch to bind together into hadrons. Initially the temperature ...
. 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. 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.

### Photons

A photon is the quantum of light and all other forms of
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

. It is the force carrier for the electromagnetic force, even when Static forces and virtual-particle exchange, static via virtual photons. The effects of this force are easily observable at the microscopic scale, microscopic and at the macroscopic scale, macroscopic level because the photon has zero rest mass; this allows long distance fundamental interaction, interactions. Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of waves and of wikt:particle, particles. The photon epoch started after most leptons and anti-leptons were annihilation, 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 (physics), 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 temperature and density detectable in the CMB were the early "seeds" from which all subsequent structure formation took place.

# Cosmological models

## Model of the universe based on general relativity

General relativity is the Differential geometry, geometric Theoretical physics, theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. It is the basis of current Physical cosmology, 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 Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

and Time in physics, time, or spacetime. In particular, the curvature of spacetime is directly related to the
energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

and momentum of whatever
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
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 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. 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, :$ds^2 = -c^ dt^2 + R\left(t\right)^2 \left\left( \frac + r^2 d\theta^2 + r^2 \sin^2 \theta \, d\phi^2 \right\right)$ where (''r'', θ, φ) correspond to a spherical coordinate system. This metric (mathematics), metric has only two undetermined parameters. An overall dimensionless length scale factor (cosmology), 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. 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. The value of ''R'' as a function of time ''t'' depends upon ''k'' and the cosmological constant ''Λ''. The cosmological constant represents the energy density of the vacuum of space and could be related to dark energy. The equation describing how ''R'' varies with time is known as the Friedmann equation after its inventor, Alexander Friedmann. 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. However, this equilibrium is unstable: because the universe is 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 and never moved faster than the
speed of light The speed of light in vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
. 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 The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

model of the universe. Understanding the singularity of the Big Bang likely requires a quantum theory of gravity, which has not yet been formulated. Third, the curvature index ''k'' determines the sign of the mean spatial curvature of spacetime averaged over sufficiently large length scales (greater than about a billion
light-year A light-year, alternatively spelt lightyear, is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metric un ...
s). If ''k''=1, the curvature is positive and the universe has a finite volume. A universe with positive curvature is often visualized as a 3-sphere, three-dimensional sphere embedded in a four-dimensional space. Conversely, if ''k'' is zero or negative, the universe has an infinite volume. It may seem counter-intuitive that an infinite and yet infinitely dense universe could be created in a single instant at the 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 could behave like a normal universe with periodic boundary conditions. The
ultimate fate of the universe The ultimate fate of the universe is a topic in physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest ...
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, 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 Future of an expanding universe, Big Freeze and the heat death of the universe. Modern data suggests that the rate of expansion of the universe is not decreasing, as originally expected, but increasing; if this continues indefinitely, 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.

## Multiverse hypothesis

Some speculative theories have proposed that our universe is but one of a set (mathematics), set of disconnected universes, collectively denoted as the
multiverse The multiverse is a hypothetical A hypothesis (plural hypotheses) is a proposed explanation An explanation is a set of statements usually constructed to describe a set of facts which clarifies the causes, context Context may refer ...

, challenging or enhancing more limited definitions of the universe. Scientific multiverse models are distinct from concepts such as plane (esotericism), alternate planes of consciousness and simulated reality. Max Tegmark developed a four-part Multiverse#Max Tegmark's four levels, classification scheme for the different types of multiverses that scientists have suggested in response to various Physics problems. An example of such multiverses is the one resulting from the bubble universe theory, chaotic inflation model of the early universe.
Another is the multiverse resulting from the
many-worlds interpretation The many-worlds interpretation (MWI) is an that asserts that the is , and that there is no . This implies that all outcomes of quantum measurements are physically realized in some "world" or universe. In contrast to some other interpretations ...
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. The least controversial, but still highly disputed, category of multiverse in Tegmark's scheme is Multiverse#Level I: An extension of our universe, Level I. The multiverses of this level are composed by distant spacetime events "in our own universe". Tegmark and others 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). However, the arguments used are of speculative nature. Additionally, it would be impossible to scientifically verify the existence of an identical Hubble volume. It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another. 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. According to one common terminology, each "soap bubble" of spacetime is denoted as a ''universe'', whereas our particular spacetime is denoted as ''the universe'', just as we call our moon ''the Moon''. The entire collection of these separate spacetimes is denoted as the multiverse. With this terminology, different ''universes'' are not causality, causally connected to each other. In principle, the other unconnected ''universes'' may have different dimensionalities and Topology, topologies of spacetime, different forms of
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
and
energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

, and different
physical law Scientific laws or laws of science are statements, based on repeated experiment An experiment is a procedure carried out to support or refute a , or determine the or of something previously untried. Experiments provide insight into by d ...
s and physical constants, although such possibilities are purely speculative. 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.

# Historical conceptions

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. Ancient Chinese philosophy encompassed the notion of the universe including both all of space and all of time. 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 relativity, 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 The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

.

## Mythologies

Many cultures have List of creation myths, 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). Ethnologists and anthropologists who study myths have developed various classification schemes for the various themes that appear in creation stories. For example, in one type of story, the world is born from a world egg; such stories include the Finnish people, Finnish epic poetry, epic poem ''Kalevala'', the China, Chinese story of Pangu or the History of India, 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 Greece, ancient Greek story of Gaia (mythology), Gaia (Mother Earth), the Aztec mythology, Aztec goddess Coatlicue myth, the ancient Egyptian religion, ancient Egyptian Ennead, god Atum story, and the Judeo-Christian Genesis creation narrative in which the God in Abrahamic religions, 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 mythology, 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, the Serer creation myth, creation myth of the Serer people, Serers, or the yin and yang of the Tao.

## Philosophical models

The pre-Socratic philosophy, pre-Socratic Greek philosophers and Indian philosophers developed some of the earliest philosophical concepts of the universe. 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 (classical element), water. Thales' student, Anaximander, proposed that everything came from the limitless ''Apeiron (cosmology), apeiron''. Anaximenes of Miletus, Anaximenes proposed the primordial material to be Air (classical element), 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 (classical element), 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 Pythagoras of Samos, or simply ; in Ionian Greek () was an ancient Ionians, Ionian Ancient Greek philosophy, Greek philosopher and the eponymous founder of Pythagoreanism. His political and religious teachings were well known in Magna Graec ...

, 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
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s moving through a void (astronomy), void (vacuum), although Aristotle did not believe that to be feasible because air, like water, offers Drag (physics), resistance to motion. Air will immediately rush in to fill a void, and moreover, without resistance, it would do so indefinitely fast. Although Heraclitus argued for eternal change, his contemporary 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 denoted this reality as (The One). Parmenides' idea seemed implausible to many Greeks, but his student Zeno of Elea challenged them with several famous Zeno's paradoxes, paradoxes. 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 philosophy, Indian philosopher Kanada (philosopher), Kanada, founder of the Vaisheshika school, developed a notion of atomism and proposed that light and heat were varieties of the same substance. In the 5th century AD, the Buddhist atomism, Buddhist atomist philosopher Dignāga proposed
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s 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. The notion of temporal finitism was inspired by the doctrine of creation shared by the three Abrahamic religions: Judaism, Christianity and Islam. The Christian philosophy, 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 Islamic philosophy, early Muslim philosopher, Al-Kindi (Alkindus); the Jewish philosophy, Jewish philosopher, Saadia Gaon (Saadia ben Joseph); and the Kalam, Muslim theologian, Al-Ghazali (Algazel).

## Astronomical concepts

Astronomical models of the universe were proposed soon after astronomy began with the Babylonian astronomy, Babylonian astronomers, who viewed the universe as a Flat Earth, flat disk floating in the ocean, and this forms the premise for early Greek maps like those of Anaximander and Hecataeus of Miletus. Later Ancient Greece, 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 uniform circular motion, rotate with uniform motion around a stationary Earth. Normal classical elements, matter is entirely contained within the terrestrial sphere. ''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". 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 series, Fourier modes). Other Greek scientists, such as the Pythagoreans, Pythagorean philosopher Philolaus, postulated (according to Stobaeus account) that at the center of the universe was a "central fire" around which the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

,
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

, Moon and
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

s revolved in uniform circular motion. The Greek astronomy, Greek astronomer Aristarchus of Samos was the first known individual to propose a Heliocentrism, 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
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 [i.e. the Earth], ... supposing the heaven to remain at rest and the Earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis
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. 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. 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. Alternatively, he may have proved heliocentricity by determining the constants of a Geometry, geometric model for it, and by developing methods to compute planetary positions using this model, like what
Nicolaus Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, link=no, Niclas Koppernigk, modern: ''Nikolaus Kopernikus''; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic Church, C ...

later did in the 16th century. During the Middle Ages, Heliocentrism, heliocentric models were also proposed by the Indian astronomy, Indian astronomer Aryabhata, and by the Islamic astronomy, Persian astronomers Ja'far ibn Muhammad Abu Ma'shar al-Balkhi, Albumasar and Al-Sijzi. 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 Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

rotated on its axis and if the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

were placed at the center of the universe. As noted by Copernicus himself, the notion that the Earth's rotation, 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). Al-Sijzi also proposed that the Earth rotates on its axis. Empirical research, Empirical evidence for the Earth's rotation on its axis, using the phenomenon of comets, was given by Nasīr al-Dīn al-Tūsī, Tusi (1201–1274) and Ali Qushji (1403–1474). This cosmology was accepted by
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

, Christiaan Huygens and later scientists.#Misner, Misner, Thorne and Wheeler, pp. 755–56. Edmund Halley (1720)#Misner, Misner, Thorne and Wheeler, p. 756. and Jean-Philippe de Chéseaux (1744) 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. 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. This instability was clarified in 1902 by the Jeans instability criterion. One solution to these paradoxes is the Carl Charlier, 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.#Misner, Misner, Thorne and Wheeler, p. 757. A significant astronomical advance of the 18th century was the realization by Thomas Wright (astronomer), Thomas Wright, Immanuel Kant and others of nebulae. In 1919, when the Hooker Telescope was completed, the prevailing view still 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 Galaxy, Andromeda Nebula and Triangulum Nebula, Triangulum among others, were entire galaxies outside our own, thus proving that universe consists of a multitude of galaxies. 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.

* Chronology of the universe * Cosmic Calendar (scaled down timeline) * Cosmic latte * Cosmos * 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 * Multiverse (set theory) (''Hyperverse'', ''Megaverse'' or ''Omniverse'') * Non-standard cosmology * Nucleocosmochronology * Panspermia * Rare Earth hypothesis * Religious cosmology * 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 Citations

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