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The chronology of the universe describes the history and future of the universe according to
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 ...

Big Bang
cosmology. The earliest stages of the universe's existence are estimated as taking place 13.8
billion years A billion years or giga-annum (109 year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of th ...
ago, with an
uncertainty Uncertainty refers to epistemic Epistemology (; ) is the branch of philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about reason, Metaphysics, existence, Epistemology, knowledge, ...

uncertainty
of around 21 million years at the 68% confidence level. The Planck Collaboration in 2015 published the estimate of 13.799 ± 0.021 billion years ago (68% confidence interval). See PDF: page 32, Table 4, Age/Gyr, last column.


Outline


Chronology in five stages

For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated, into five parts. It is generally considered meaningless or unclear whether
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 ...

time
existed before this chronology:


The very early universe

The first
picosecond A picosecond is an SI unit of time equal to 10−12 or (one trillionth) of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d ...
 (10−12) of
cosmic time Cosmic time, or cosmological time, is the time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the duration of ev ...
. It includes 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 ...
, during which currently established
laws of physics Scientific laws or laws of science are statements, based on repeated experiment An experiment is a procedure carried out to support, refute, or validate a hypothesis. Experiments provide insight into Causality, cause-and-effect by demonstrat ...
may not apply; the emergence in stages of the four known
fundamental interaction #REDIRECT Fundamental interaction In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physic ...
s or
force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving from a Newton's first law, state of rest), i.e., to acce ...

force
s—first
gravitation Gravity (), or gravitation, is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is t ...

gravitation
, and later the
electromagnetic Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagneti ...

electromagnetic
, weak and strong interactions; and the
expansion of space itself Expansion may refer to: Arts, entertainment and media * ''L'Expansion'', a French monthly business magazine * Expansion (album), ''Expansion'' (album), by American jazz pianist Dave Burrell, released in 2004 * Expansions (McCoy Tyner album), ''Expa ...
and
supercooling Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoch ...
of the still immensely hot universe due to
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 ...
. Tiny ripples in the universe at this stage are believed to be the basis of large-scale structures that formed much later. Different stages of the very early universe are understood to different extents. The earlier parts are beyond the grasp of practical experiments in
particle physics Particle physics (also known as high energy physics) is a branch of 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 rel ...
but can be explored through other means.


The early universe

This period lasted around 370,000 years. Initially, various kinds of
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 are formed in stages. These particles include almost equal amounts 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
antimatter In modern physics Modern physics is a branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...

antimatter
, so most of it quickly annihilates, leaving a small excess of matter in the universe. At about one second, neutrinos decouple; these
neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

neutrino
s form the
cosmic neutrino background The cosmic neutrino background (CNB or CνB) is the universe's background particle radiation composed of neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion (an elementary particle In particle physics Particle physics ...
(CνB). If
primordial black hole Primordial black hole (also abbreviated as PBH) is a hypothetical type of black hole that formed 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, ...
s exist, they are also formed at about one second of cosmic time.
Composite Composite or compositing may refer to: Materials * Composite material, a material that is made from several different substances ** Metal matrix composite, composed of metal and other parts ** Cermet, a composite of ceramic and metallic materials * ...
subatomic particles emerge—including
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 ...

proton
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 ...

neutron
s—and from about 2 minutes, conditions are suitable for
nucleosynthesis Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in ...
: around 25% of the protons and all the neutrons
fuse Fuse or FUSE may refer to: Devices * Fuse (electrical), a device used in electrical systems to protect against excessive current ** Fuse (automotive), a class of fuses for vehicles * Fuse (hydraulic), a device used in hydraulic systems to protect ...

fuse
into heavier elements, initially
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 ...

deuterium
which itself quickly fuses into mainly
helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consists ...

helium-4
. By 20 minutes, the universe is no longer hot enough for
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 ...

nuclear fusion
, but far too hot for 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 ...

atom
s to exist or
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

photon
s to travel far. It is therefore an
opaque Opacity or opaque may refer to: * Impediments to (especially, visible) light: ** Opacities, absorption coefficients ** Opacity (optics), property or degree of blocking the transmission of light * Metaphors derived from literal optics: ** Opaque con ...
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–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood plasma ...
. The recombination epoch begins at around 18,000 years, as electrons are combining with
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" ...

helium
nuclei to form . At around 47,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form,
helium hydride The helium hydride ion or hydridohelium(1+) ion or helonium is a cation An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative ...
is the first
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
. (Much later,
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 ...

hydrogen
and helium hydride react to form molecular hydrogen (H2) the fuel needed for 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 ...

star
s.) At about 370,000 years, *Notes:
Edward L. Wright Edward L. (Ned) Wright (born August 25, 1947 in Washington, D.C.) is an American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States o ...

Edward L. Wright
'
Javascript Cosmology Calculator
(last modified 23 July 2018). With a default H_0 =  (based o
''WMAP''9+SPT+ACT+6dFGS+BOSS/DR11+''H''0/Riess)
parameters, the calculated age of the universe with a redshift of ''z'' = 1100 is in agreement with Olive and Peacock (about 370,000 years). *. See PDF: page 45, Table 7, Age at decoupling, last column. Based on ''WMAP''+BAO+SN parameters, the age of decoupling occurred years after the Big Bang. *. "Without going into the details of the non-equilibrium physics, let's content ourselves by saying, in round numbers, ''z''dec ≈ 1100, corresponding to a temperature ''T''dec ≈ 3000 K, when the age of the universe was ''t''dec ≈ 350,000 yr in the Benchmark Model. (...) The relevant times of various events around the time of recombination are shown in Table 9.1. (...) Note that all these times are approximate, and are dependent on the cosmological model you choose. (I have chosen the Benchmark Model in calculating these numbers.)"
neutral hydrogen atoms finish forming ("recombination"), and as a result the universe also became
transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * Transparency (photography), a sti ...
for the first time. The newly formed atoms—mainly hydrogen and helium with 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 ...

lithium
—quickly reach their lowest energy state (
ground state The ground state of a quantum-mechanical system is its lowest-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 knowledg ...
) by releasing photons (" photon decoupling"), and these photons can still be detected today as the
cosmic microwave background The cosmic microwave background (CMB, CMBR), in 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 ...
(CMB). This is the oldest observation we currently have of the universe.


The Dark Ages and large-scale structure emergence

From 370,000 years until about 1 billion years. After recombination and decoupling, the universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and
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 ...

galaxies
, so there were no new sources of light. The only photons (electromagnetic radiation, or "light") in the universe were those released during decoupling (visible today as the cosmic microwave background) and 21 cm radio emissions occasionally emitted by hydrogen atoms. The decoupled photons would have filled the universe with a brilliant pale orange glow at first, gradually
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 ...

redshift
ing to non-visible
wavelength 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 su ...

wavelength
s after about 3 million years, leaving it without visible light. This period is known as the cosmic
Dark Ages Dark Ages or Dark Age may refer to: History and sociology *Dark Ages (historiography), the use of the term ''Dark Ages'' by historians and lay people **Byzantine Dark Ages (7th–8th centuries), period of large-scale transformation but obscure du ...
. At some point around 200 to 500 million years, the earliest generations of stars and galaxies form (exact timings are still being researched), and early large structures gradually emerge, drawn to the foam-like
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, ...

dark matter
filaments which have already begun to draw together throughout the universe. The earliest generations of stars have not yet been observed astronomically. They may have been huge (100–300
solar mass The solar mass () is a standard unit of mass in astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial obje ...
es) and non-metallic, with very short lifetimes compared to most stars we see today, so they commonly finish burning their hydrogen fuel and explode as highly energetic pair-instability
supernova A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last stellar evolution, evolutionary stages of a massive star or when a ...

supernova
e after mere millions of years. Other theories suggest that they may have included small stars, some perhaps still burning today. In either case, these early generations of supernovae created most of the everyday elements we see around us today, and seeded the universe with them.
Galaxy cluster A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous sphe ...
s 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 emerge over time. At some point, high-energy photons from the earliest stars,
dwarf galaxies Dwarf or dwarves may refer to: Common uses *Dwarf (folklore), a being from Germanic mythology and folklore * Dwarf, a person or animal with dwarfism * Dwarf to describe a small person Arts, entertainment, and media Fictional entities *Dwarf (Du ...
and perhaps
quasar A quasar (; also known as a quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus An active galactic nucleus (AGN) is a compact region at the center of a galaxy A galaxy is a gravitation Gravity () ...

quasar
s leads to a period of
reionization In the field of Big Bang The Big Bang theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated ...
that commences gradually between about 250–500 million years, is complete by about 700–900 million years, and diminishes by about 1 billion years (exact timings still being researched). The universe gradually transitioned into the universe we see around us today, and the Dark Ages only fully came to an end at about 1 billion years.


The universe as it appears today

From 1 billion years, and for about 12.8 billion years, the universe has looked much as it does today and it will continue to appear very similar for many billions of years into the future. The
thin disk The thin disk is a structural component of spiral In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and cal ...
of
our galaxy
our galaxy
began to form at about 5 billion years (8.8
Gya A billion years or giga-annum (109 year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of th ...
), and 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 ...
formed at about 9.2 billion years (4.6 Gya), with the earliest traces of
life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a ...
on Earth emerging by about 10.3 billion years (3.5 Gya). The thinning of matter over time reduces the ability of gravity to decelerate the expansion of the universe; in contrast,
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 ...

dark energy
(believed to be a constant
scalar field In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...

scalar field
throughout our universe) is a constant factor tending to accelerate the expansion of the universe. The universe's expansion passed an
inflection point In differential calculus 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 (mathem ...

inflection point
about five or six billion years ago, when the universe entered the modern "dark-energy-dominated era" where the universe's expansion is now accelerating rather than decelerating. The present-day universe is understood quite well, but beyond about 100 billion years of cosmic time (about 86 billion years in the future), uncertainties in current knowledge mean that we are less sure which path our universe will take.


The far future and ultimate fate

At some time the
Stelliferous Era Most observations suggest that the expansion 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 theor ...
will end as stars are no longer being born, and the expansion of the universe will mean that 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 ...
becomes limited to local galaxies. There are various scenarios for the far future and
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 larg ...
. More exact knowledge of our current universe will allow these to be better understood.


Tabular summary

:''Note: The radiation temperature in the table below refers to the
cosmic background radiation Image:Cobe-cosmic-background-radiation.gif, 256px, Temperature of the cosmic background radiation spectrum as determined with the COBE satellite: uncorrected (top), corrected for the dipole term due to our peculiar velocity (middle), and corrected ...
and is given by 2.725  K·(1 + ), where is the
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 ...

redshift
.''


The Big Bang

The
Standard Model The Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsi ...

Standard Model
of
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 ...
is based on a model of
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 ...
called the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. A
metric Metric or metrical may refer to: * Metric system, an internationally adopted decimal system of measurement Mathematics * Metric (mathematics), an abstraction of the notion of ''distance'' in a metric space * Metric tensor, in differential geomet ...
provides a measure of distance between objects, and the FLRW metric is the exact solution of
Einstein field equations In the general theory of relativity General relativity, also known as the general theory of relativity, is the differential geometry, geometric scientific theory, theory of gravitation published by Albert Einstein in 1915 and is the curren ...
(EFE) if some key properties of space such as
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 ...
are assumed to be true. The FLRW metric very closely matches overwhelming other evidence, showing that the universe has expanded since the Big Bang. If the FLRW metric equations are assumed to be valid all the way back to the beginning of the universe, they can be followed back in time, to a point where the equations suggest all distances between objects in the universe were zero or infinitesimally small. (This does not necessarily mean that the universe was physically small at the Big Bang, although that is one of the possibilities.) This provides a model of the universe which matches all current physical observations extremely closely. This initial period of the universe's chronology is called 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 ...

Big Bang
". The Standard Model of cosmology attempts to explain how the universe physically developed once that moment happened. The
singularity Singularity or singular point may refer to: Science, technology, and mathematics Mathematics * Mathematical singularity, a point at which a given mathematical object is not defined or not "well-behaved", for example infinite or not differentiabl ...
from the FLRW metric is interpreted to mean that current theories are inadequate to describe what actually happened at the start of the Big Bang itself. It is widely believed that a correct theory of
quantum gravity Quantum gravity (QG) is a field of theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict List of natural phenomena, ...

quantum gravity
may allow a more correct description of that event, but no such theory has yet been developed. After that moment, all distances throughout the universe began to increase from (perhaps) zero because the FLRW metric itself changed over time, affecting distances between all non-bound objects everywhere. For this reason, it is said that the Big Bang "happened everywhere".


The very early universe

During the earliest moments of cosmic time, the energies and conditions were so extreme that current knowledge can only suggest possibilities, which may turn out to be incorrect. To give one example,
eternal inflation Eternal inflation is a hypothetical inflationary universe model, which is itself an outgrowth or extension of the Big Bang The Big Bang theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or th ...
theories propose that inflation lasts forever throughout most of the universe, making the notion of "N seconds since Big Bang" ill-defined. Therefore, the earliest stages are an active area of research and based on ideas that are still speculative and subject to modification as scientific knowledge improves. Although a specific "inflationary epoch" is highlighted at around 10−32 seconds, observations and theories both suggest that distances between objects in space have been increasing at all times since the moment of the Big Bang, and are still increasing (with the exception of gravitationally bound objects such as galaxies and most clusters, once the rate of expansion had greatly slowed). The inflationary period marks a specific period when a very rapid change in scale occurred, but does not mean that it stayed the same at other times. More precisely, during inflation, the expansion accelerated. After inflation, and for about 9.8 billion years, the expansion was much slower and became slower yet over time (although it never reversed). About 4 billion years ago, it began slightly speeding up again.


Planck epoch

:''Times shorter than 10−43 seconds (
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 ...
)'' 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 ...
is an era in traditional (non-inflationary) Big Bang cosmology immediately after the event which began the known universe. During this epoch, the temperature and average energies within the universe were so high that everyday subatomic particles could not form, and even the four fundamental forces that shape the universe
gravitation Gravity (), or gravitation, is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is t ...

gravitation
,
electromagnetism Electromagnetism is a branch of 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 ...

electromagnetism
, the
weak nuclear force In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions. Other forms of nuclear matter are also studied. Nuclear physics should not be confused with atomic physics, which ...
, and the
strong nuclear force In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions. Other forms of nuclear matter are also studied. Nuclear physics should not be confused with atomic physics, which ...
were combined and formed one fundamental force. Little is understood about physics at this temperature; different hypotheses propose different scenarios. Traditional big bang cosmology predicts a
gravitational singularity A gravitational singularity, spacetime singularity or simply singularity is a condition in which gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate ...
before this time, but this theory relies on the theory 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 ...
, which is thought to break down for this epoch due to
quantum effects Quantum mechanics is a fundamental theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with s ...
. In inflationary models of cosmology, times before the end of inflation (roughly 10−32 seconds after the Big Bang) do not follow the same timeline as in traditional big bang cosmology. Models that aim to describe the universe and physics during the Planck epoch are generally speculative and fall under the umbrella of " New Physics". Examples include the Hartle–Hawking initial state,
string theory landscape The string theory landscape or landscape of vacua refers to the collection of possible false vacua in string theory In physics, string theory is a Mathematical theory, theoretical framework in which the Point particle, point-like particles ...
, string gas cosmology, and the ekpyrotic universe.


Grand unification epoch

:''Between 10−43 seconds and 10−36 seconds after the Big Bang'' As the universe expanded and cooled, it crossed transition temperatures at which forces separated from each other. These
phase transition In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...
s can be visualized as similar to
condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

condensation
and
freezing Freezing is a phase transition In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as fa ...

freezing
phase transitions of ordinary matter. At certain temperatures/energies, water molecules change their behavior and structure, and they will behave completely differently. Like steam turning to water, the
fields File:A NASA Delta IV Heavy rocket launches the Parker Solar Probe (29097299447).jpg, FIELDS heads into space in August 2018 as part of the ''Parker Solar Probe'' FIELDS is a science instrument on the ''Parker Solar Probe'' (PSP), designed to mea ...
which define our universe's fundamental forces and particles also completely change their behaviors and structures when the temperature/energy falls below a certain point. This is not apparent in everyday life, because it only happens at far higher temperatures than we usually see in our present universe. These phase transitions in the universe's fundamental forces are believed to be caused by a phenomenon of
quantum field In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity and quantum mechanics Quantum mechanics is a fundamental Scientific theory, theory in physics that prov ...
s called "
symmetry breaking In physics, symmetry breaking is a phenomenon in which (infinitesimally) small Quantum fluctuation, fluctuations acting on a system crossing a critical point (thermodynamics), critical point decide the system's fate, by determining which bran ...
". In everyday terms, as the universe cools, it becomes possible for the quantum fields that create the forces and particles around us, to settle at lower energy levels and with higher levels of stability. In doing so, they completely shift how they interact. Forces and interactions arise due to these fields, so the universe can behave very differently above and below a phase transition. For example, in a later epoch, a side effect of one phase transition is that suddenly, many particles that had no mass at all acquire a mass (they begin to interact differently with the
Higgs field The Higgs boson is an elementary particle in the Standard Model of particle physics produced by the excited state, quantum excitation of the Higgs field, one of the field (physics), fields in particle physics theory. In the Standard Model, t ...
), and a single force begins to manifest as two separate forces. Assuming that nature is described by a so-called
Grand Unified Theory A Grand Unified Theory (GUT) is a model in particle physics Particle physics (also known as high energy physics) is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of a ...
(GUT), the grand unification epoch began with a phase transition of this kind, when gravitation separated from the universal combined gauge force. This caused two forces to now exist:
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 ...

gravity
, and an electrostrong interaction. There is no hard evidence yet, that such a combined force existed, but many physicists believe it did. The physics of this electrostrong interaction would be described by a Grand Unified Theory. The grand unification epoch ended with a second phase transition, as the electrostrong interaction in turn separated, and began to manifest as two separate interactions, called the strong and the
electroweak In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...
interactions.


Electroweak epoch

:''Between 10−36 seconds (or the end of inflation) and 10−32 seconds after the Big Bang'' Depending on how epochs are defined, and the model being followed, the
electroweak epoch 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 universe ...
may be considered to start before or after the inflationary epoch. In some models it is described as including the inflationary epoch. In other models, the electroweak epoch is said to begin after the inflationary epoch ended, at roughly 10−32 seconds. According to traditional Big Bang cosmology, the electroweak epoch began 10−36 seconds after the Big Bang, when the temperature of the universe was low enough (1028 K) for the
electronuclear force A Grand Unified Theory (GUT) is a model in particle physics in which, at high energy, energies, the three gauge theory, gauge interactions of the Standard Model comprising the electromagnetism, electromagnetic, weak interaction, weak, and strong ...
to begin to manifest as two separate interactions, the strong and the electroweak interactions. (The electroweak interaction will also separate later, dividing into the
electromagnetic Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagneti ...

electromagnetic
and weak interactions.) The exact point where electrostrong symmetry was broken is not certain, owing to speculative and as yet incomplete theoretical knowledge.


Inflationary epoch and the rapid expansion of space

:''Before c. 10−32 seconds after the Big Bang'' At this point of the very early universe, the
metric Metric or metrical may refer to: * Metric system, an internationally adopted decimal system of measurement Mathematics * Metric (mathematics), an abstraction of the notion of ''distance'' in a metric space * Metric tensor, in differential geomet ...
that defines distance within space suddenly and very rapidly changed in scale, leaving the early universe at least 1078 times its previous volume (and possibly much more). This is equivalent to a linear increase of at least 1026 times in every spatial dimension—equivalent to an object 1
nanometre file:EM Spectrum Properties edit.svg, 330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its derived scales. The nanometre is often used to express dimensions on an atomic scale and mostly in the Mo ...
(10−9 m, about half the width of a molecule of
DNA Deoxyribonucleic acid (; DNA) is a molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical ...

DNA
) in length, expanding to one approximately long in a tiny fraction of a second. This change is known as
inflation In economics, inflation refers to a general progressive increase in prices of goods and services in an economy. When the general price level rises, each unit of currency buys fewer goods and services; consequently, inflation corresponds to a r ...
. Although light and objects within spacetime cannot travel 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 ...
, in this case it was the
metric Metric or metrical may refer to: * Metric system, an internationally adopted decimal system of measurement Mathematics * Metric (mathematics), an abstraction of the notion of ''distance'' in a metric space * Metric tensor, in differential geomet ...
governing the size and geometry of spacetime itself that changed in scale. Changes to the metric are not limited by the speed of light. There is good evidence that this happened, and it is widely accepted that it did take place. But the exact reasons ''why'' it happened are still being explored. So a range of models exist that explain why and how it took place—it is not yet clear which explanation is correct. In several of the more prominent models, it is thought to have been triggered by the separation of the strong and electroweak interactions which ended the grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field. As this field settled into its lowest energy state throughout the universe, it generated an enormous repulsive force that led to a rapid expansion of the metric that defines space itself. Inflation explains several observed properties of the current universe that are otherwise difficult to account for, including explaining how today's universe has ended up so exceedingly
homogeneous Homogeneity and heterogeneity are concepts often used in the sciences Science () is a systematic enterprise that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about th ...
(similar) on a very large scale, even though it was highly disordered in its earliest stages. It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10−33 and 10−32 seconds after the Big Bang. The rapid expansion of space meant that
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 remaining from the grand unification epoch were now distributed very thinly across the universe. However, the huge potential energy of the inflation field was released at the end of the inflationary epoch, as the inflaton field decayed into other particles, known as "reheating". This heating effect led to the universe being repopulated with a dense, hot mixture of quarks, anti-quarks and gluons. In other models, reheating is often considered to mark the start of the electroweak epoch, and some theories, such as warm inflation, avoid a reheating phase entirely. In non-traditional versions of Big Bang theory (known as "inflationary" models), inflation ended at a temperature corresponding to roughly 10−32 seconds after the Big Bang, but this does ''not'' imply that the inflationary era lasted less than 10−32 seconds. To explain the observed homogeneity of the universe, the duration in these models must be longer than 10−32 seconds. Therefore, in inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the ''end'' of inflation. After inflation ended, the universe continued to expand, but at a much slower rate. About 4 billion years ago the expansion gradually began to speed up again. This is believed to be due to dark energy becoming dominant in the universe's large-scale behavior. It is still expanding today. On 17 March 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary
gravitational wave Gravitational waves are disturbances in the curvature of spacetime In , spacetime is any which fuses the and the one of into a single . can be used to visualize effects, such as why different observers perceive differently where and wh ...
s in the
B-modes The cosmic microwave background (CMB, CMBR), in Big Bang cosmology, is electromagnetic radiation which is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

B-modes
power spectrum The power spectrum S_(f) of a time series x(t) describes the distribution of power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System ...

power spectrum
which was interpreted as clear experimental evidence for the theory of inflation. "A version of this article appears in print on March 18, 2014, Section A, Page 1 of the New York edition with the headline: Space Ripples Reveal Big Bang’s Smoking Gun." The online version of this article was originally titled "Detection of Waves in Space Buttresses Landmark Theory of Big Bang". However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported "A version of this article appears in print on June 20, 2014, Section A, Page 16 of the New York edition with the headline: Astronomers Stand by Their Big Bang Finding, but Leave Room for Debate." and finally, on 2 February 2015, a joint analysis of data from BICEP2/Keck and the
European Space Agency , owner = , headquarters = Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estimated population of 2,175,601 residents , in an area ...

European Space Agency
's''
Planck Max Karl Ernst Ludwig Planck, (; ; 23 April 1858 – 4 October 1947) was a 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 cit ...
'' microwave space telescope concluded that the statistical "significance
f the data F, or f, is the sixth Letter (alphabet), letter in the English alphabet, modern English alphabet and the ISO basic Latin alphabet. Its name in English is English alphabet#Letter names, ''ef'' (pronounced ), plural ''efs''. History The origin ...
is too low to be interpreted as a detection of primordial B-modes" and can be attributed mainly to polarized dust in the Milky Way. "A version of this article appears in print on Jan. 31, 2015, Section A, Page 11 of the New York edition with the headline: Speck of Interstellar Dust Obscures Glimpse of Big Bang."


Supersymmetry breaking (speculative)

If
supersymmetry In a supersymmetric theory the equations for force and the equations for matter are identical. In theoretical A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason ...
is a property of our universe, then it must be broken at an energy that is no lower than 1
TeV TEV may refer to: * Transient Earth Voltage Echelon may refer to: * A level formation ** A level or rank (disambiguation), rank in an organization, profession, or society ** Echelon formation, a step-like arrangement of units * ECHELON, a worldw ...
, the electroweak scale. The masses of particles and their
superpartner In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of ...
s would then no longer be equal. This very high energy could explain why no superpartners of known particles have ever been observed.


Electroweak symmetry breaking

:''10−12 seconds after the Big Bang'' As the universe's temperature continued to fall below 159.5±1.5 
GeV GEV may refer to: * ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games * Ashe County Airport, in North Carolina, United States * Gällivare Lapland Airport, in Sweden * Generalized extreme value distribution In probability theory ...
,
electroweak symmetry breaking In the Standard Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces (the Electromagnetism, electromagnetic, Weak interaction, weak, and Strong interaction, strong interactions, and ...
happened. So far as we know, it was the penultimate symmetry breaking event in the formation of our universe, the final one being
chiral symmetry breaking In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the ...
in the quark sector. This has two related effects: # Via the
Higgs mechanism In the Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' ...
, all elementary particles interacting with the Higgs field become massive, having been massless at higher energy levels. # As a side-effect, the weak nuclear force and electromagnetic force, and their respective
boson In quantum mechanics Quantum mechanics is a fundamental theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinkin ...

boson
s (the
W and Z bosons 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 ...
and photon) now begin to manifest differently in the present universe. Before electroweak symmetry breaking these bosons were all massless particles and interacted over long distances, but at this point the W and Z bosons abruptly become massive particles only interacting over distances smaller than the size of an atom, while the photon remains massless and remains a long-distance interaction. After electroweak symmetry breaking, the fundamental interactions we know of—gravitation, electromagnetic, weak and strong interactions—have all taken their present forms, and fundamental particles have their expected masses, but the temperature of the universe is still too high to allow the stable formation of many particles we now see in the universe, so there are no protons or neutrons, and therefore no atoms,
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 ...
, or molecules. (More exactly, any composite particles that form by chance, almost immediately break up again due to the extreme energies.)


The early universe

After cosmic inflation ends, the universe is filled with a hot
quark–gluon plasma Quark–gluon plasma or QGP is an interacting localized assembly of quark A quark () is a type of elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of othe ...
, the remains of reheating. From this point onwards the physics of the early universe is much better understood, and the energies involved in the
Quark epoch 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 chro ...
are directly accessible in particle physics experiments and other detectors.


Electroweak epoch and early thermalization

:''Starting anywhere between 10−22 and 10−15 seconds after the Big Bang, until 10−12 seconds after the Big Bang'' Some time after inflation, the created particles went through
thermalization 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 ...
, where mutual interactions lead to
thermal equilibrium Two physical system A physical system is a collection of physical objects. In physics, it is a portion of the physical universe chosen for analysis. Everything outside the system is known as the environment (systems), environment. The enviro ...

thermal equilibrium
. The earliest stage of which we are quite confident about is some time before the
electroweak symmetry breaking In the Standard Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces (the Electromagnetism, electromagnetic, Weak interaction, weak, and Strong interaction, strong interactions, and ...
, at a temperature of around 1015 K, approximately 10−15 seconds after the Big Bang. The electromagnetic and weak interaction have not yet separated, and as far as we know all particles were massless, as the
Higgs mechanism In the Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' ...
had not operated yet. However exotic massive particle-like entities,
sphaleron A sphaleron ( el, σφαλερός "slippery") is a static (time-independent) solution to the electroweak interaction, electroweak field equations of the Standard Model of particle physics, and is involved in certain hypothetical processes that vi ...
s, are thought to have existed. This epoch ended with electroweak symmetry breaking; according to the
standard model of particle physics The Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis ...
,
baryogenesis 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 univers ...
also happened at this stage, creating an imbalance between matter and anti-matter (though in extensions to this model this may have happened earlier). Little is known about the details of these processes.


Thermalization

The number density of each particle species was, by a similar analysis to
Stefan–Boltzmann law The Stefan–Boltzmann law describes the power radiated from a black body A black body or blackbody is an idealized physical object, physical body that absorption (electromagnetic radiation), absorbs all incident electromagnetic radiation, r ...
: :n = 2 \sigma_B T^3 / c k_B \approx 10^ m^, which is roughly just (k_B T/\hbar c)^3. Since the interaction was strong, the cross section \sigma was approximately the particle wavelength squared, which is roughly n^. The rate of collisions per particle species can thus be calculated from the
mean free path 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 ...

mean free path
, giving approximately: :\sigma \cdot n \cdot c \approx n^\cdot c \approx 10^ s^. For comparison, since the cosmological constant was negligible at this stage, the
Hubble parameter Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are the faster they are moving aw ...
was: :H \approx \sqrt \approx \sqrt\approx ~ 3\cdot 10^ s^ , where ''x'' ~ 102 was the number of available particle species.12 gauge bosons, 2 Higgs-sector scalars, 3 left-handed quarks x 2 SU(2) states x 3 SU(3) states and 3 left-handed leptons x 2 SU(2) states, 6 right-handed quarks x 3 SU(3) states and 6 right-handed leptons, all but the scalar having 2 spin states Thus ''H'' is orders of magnitude lower than the rate of collisions per particle species. This means there was plenty of time for thermalization at this stage. At this epoch, the collision rate is proportional to the third root of the number density, and thus to a^, where a is the
scale parameter In probability theory Probability theory is the branch of mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in ...
. The Hubble parameter, however, is proportional to a^. Going back in time and higher in energy, and assuming no new physics at these energies, a careful estimate gives that thermalization was first possible when the temperature was: :T_ \approx 2.5\cdot 10^ GeV \approx 10^ K , approximately 10−22 seconds after the Big Bang.


The quark epoch

:''Between 10−12 seconds and 10−5 seconds after the Big Bang'' The
quark epoch 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 chro ...
began approximately 10−12 seconds after the Big Bang. This was the period in the evolution of the early universe immediately after electroweak symmetry breaking, when the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction had taken their present forms, but the temperature of the universe was still too high to allow
quark A quark () is a type 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 fundam ...

quark
s to bind together to form
hadron In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...
s. During the quark epoch the universe was filled with a dense, hot quark–gluon plasma, containing quarks,
lepton In particle physics, a lepton is an elementary particle of half-integer spin (spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-lik ...

lepton
s and their
antiparticle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that departm ...
s. Collisions between particles were too energetic to allow quarks to combine into
meson 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 partic ...

meson
s or baryons. The quark epoch ended when the universe was about 10−5 seconds old, when the average energy of particle interactions had fallen below the mass of lightest hadron, the pion.


Baryogenesis

:''Perhaps by 10−11 seconds'' Baryons are subatomic particles such as protons and neutrons, that are composed of three
quark A quark () is a type 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 fundam ...

quark
s. It would be expected that both baryons, and particles known as antimatter, antibaryons would have formed in equal numbers. However, this does not seem to be what happened—as far as we know, the universe was left with far more baryons than antibaryons. In fact, almost no antibaryons are observed in nature. It is not clear how this came about. Any explanation for this phenomenon must allow the Baryogenesis#GUT Baryogenesis under Sakharov conditions, Sakharov conditions related to baryogenesis to have been satisfied at some time after the end of cosmological inflation. Current particle physics suggests asymmetries under which these conditions would be met, but these asymmetries appear to be too small to account for the observed baryon-antibaryon asymmetry of the universe.


Hadron epoch

:''Between 10−5 second and 1 second after the Big Bang'' The quark–gluon plasma that composes the universe cools until hadrons, including baryons such as protons and neutrons, can form. Initially, hadron/anti-hadron pairs could form, so matter and antimatter were in
thermal equilibrium Two physical system A physical system is a collection of physical objects. In physics, it is a portion of the physical universe chosen for analysis. Everything outside the system is known as the environment (systems), environment. The enviro ...

thermal equilibrium
. However, as the temperature of the universe continued to fall, new hadron/anti-hadron pairs were no longer produced, and most of the newly formed hadrons and anti-hadrons annihilation, annihilated each other, giving rise to pairs of high-energy photons. A comparatively small residue of hadrons remained at about 1 second of cosmic time, when this epoch ended. Theory predicts that about 1 neutron remained for every 6 protons, with the ratio falling to 1:7 over time due to neutron decay. This is believed to be correct because, at a later stage, the neutrons and some of the protons nuclear fusion, fused, leaving hydrogen, a hydrogen isotope called deuterium, helium and other elements, which can be measured. A 1:7 ratio of hadrons would indeed produce the observed element ratios in the early and current universe.


Neutrino decoupling and cosmic neutrino background (CνB)

:''Around 1 second after the Big Bang'' At approximately 1 second after the Big Bang neutrinos decouple and begin travelling freely through space. As neutrinos rarely interact with matter, these neutrinos still exist today, analogous to the much later cosmic microwave background emitted during recombination, around 370,000 years after the Big Bang. The neutrinos from this event have a very low energy, around 10−10 times smaller than is possible with present-day direct detection. *Coverage of original paper: Even high-energy neutrinos are neutrino detector, notoriously difficult to detect, so this cosmic neutrino background (CνB) may not be directly observed in detail for many years, if at all. However, Big Bang cosmology makes many predictions about the CνB, and there is very strong indirect evidence that the CνB exists, both from Big Bang nucleosynthesis predictions of the helium abundance, and from anisotropies in the cosmic microwave background (CMB). One of these predictions is that neutrinos will have left a subtle imprint on the CMB. It is well known that the CMB has irregularities. Some of the CMB fluctuations were roughly regularly spaced, because of the effect of baryon acoustic oscillations, baryonic acoustic oscillations. In theory, the decoupled neutrinos should have had a very slight effect on the Phase (waves), phase of the various CMB fluctuations. In 2015, it was reported that such shifts had been detected in the CMB. Moreover, the fluctuations corresponded to neutrinos of almost exactly the temperature predicted by Big Bang theory ( compared to a prediction of 1.95K), and exactly three types of neutrino, the same number of Neutrino#Neutrino flavor, neutrino flavors predicted by the Standard Model.


Possible formation of primordial black holes

: ''May have occurred within about 1 second after the Big Bang'' Primordial black holes are a hypothetical type of black hole proposed in 1966, that may have formed during the so-called Scale factor (cosmology)#Radiation-dominated era, radiation-dominated era, due to the high densities and inhomogeneous conditions within the first second of cosmic time. Random fluctuations could lead to some regions becoming dense enough to undergo gravitational collapse, forming black holes. Current understandings and theories place tight limits on the abundance and mass of these objects. Typically, primordial black hole formation requires density contrasts (regional variations in the universe's density) of around \delta \rho / \rho \sim 0.1  (10%), where \rho is the average density of the universe. Several mechanisms could produce dense regions meeting this criterion during the early universe, including reheating, cosmological phase transitions and (in so-called "hybrid inflation models") axion inflation. Since primordial black holes didn't form from stellar gravitational collapse, their masses can be far below stellar mass (~2×1033 g). Stephen Hawking calculated in 1971 that primordial black holes could have a mass as low as 10−5 g. But they can have any size, so they could also be large, and may have contributed to the Galaxy formation and evolution, formation of galaxies.


Lepton epoch

:''Between 1 second and 10 seconds after the Big Bang'' The majority of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving
lepton In particle physics, a lepton is an elementary particle of half-integer spin (spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-lik ...

lepton
s (such as the electron, muons and certain neutrinos) and antileptons, dominating the mass of the universe. The lepton epoch follows a similar path to the earlier hadron epoch. Initially leptons and antileptons are produced in pairs. About 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton–antilepton pairs are no longer created and most remaining leptons and antileptons quickly annihilated each other, giving rise to pairs of high-energy photons, and leaving a small residue of non-annihilated leptons.


Photon epoch

:''Between 10 seconds and 370,000 years after the Big Bang'' After most leptons and antileptons are annihilated at the end of the lepton epoch, most of the mass-energy in the universe is left in the form of photons. (Much of the rest of its mass-energy is in the form of neutrinos and other special relativity, relativistic particles.) Therefore, the energy of the universe, and its overall behavior, is dominated by its photons. These photons continue to interact frequently with charged particles, i.e., electrons, protons and (eventually) nuclei. They continue to do so for about the next 370,000 years.


Nucleosynthesis of light elements

:''Between 2 minutes and 20 minutes after the Big Bang'' Between about 2 and 20 minutes after the Big Bang, the temperature and pressure of the universe allowed nuclear fusion to occur, giving rise to nuclei of a few light elements beyond hydrogen ("Big Bang nucleosynthesis"). About 25% of the protons, and all the neutrons fuse to form deuterium, a hydrogen isotope, and most of the deuterium quickly fuses to form helium-4. Atomic nuclei will easily unbind (break apart) above a certain temperature, related to their binding energy. From about 2 minutes, the falling temperature means that deuterium no longer unbinds, and is stable, and starting from about 3 minutes, helium and other elements formed by the fusion of deuterium also no longer unbind and are stable. The short duration and falling temperature means that only the simplest and fastest fusion processes can occur. Only tiny amounts of nuclei beyond helium are formed, because nucleosynthesis of heavier elements is difficult and triple-alpha process, requires thousands of years even in stars. Small amounts of tritium (another hydrogen isotope) and Isotopes of beryllium, beryllium-7 and -8 are formed, but these are unstable and are quickly lost again. A small amount of deuterium is left unfused because of the very short duration. Therefore, the only stable nuclides created by the end of Big Bang nucleosynthesis are protium (single proton/hydrogen nucleus), deuterium, helium-3, helium-4, and Isotopes of lithium#Lithium-7, lithium-7. By mass, the resulting matter is about 75% hydrogen nuclei, 25% helium nuclei, and perhaps 10−10 by mass of lithium-7. The next most common stable isotopes produced are Isotopes of lithium#Lithium-6, lithium-6, beryllium-9, Boron, boron-11, carbon, nitrogen and oxygen ("CNO"), but these have predicted abundances of between 5 and 30 parts in 1015 by mass, making them essentially undetectable and negligible. Conference: "Nuclear Physics in Astrophysics VI (NPA6) 19–24 May 2013, Lisbon, Portugal". The amounts of each light element in the early universe can be estimated from old galaxies, and is strong evidence for the Big Bang. For example, the Big Bang should produce about 1 neutron for every 7 protons, allowing for 25% of all nucleons to be fused into helium-4 (2 protons and 2 neutrons out of every 16 nucleons), and this is the amount we find today, and far more than can be easily explained by other processes. Similarly, deuterium fuses extremely easily; any alternative explanation must also explain how conditions existed for deuterium to form, but also left some of that deuterium unfused and not immediately fused again into helium. Any alternative must also explain the proportions of the various light elements and their isotopes. A few isotopes, such as lithium-7, were found to be present in amounts that differed from theory, but over time, these differences have been resolved by better observations.


Matter domination

:''47,000 years after the Big Bang'' Until now, the universe's large-scale dynamics and behavior have been determined mainly by radiation—meaning, those constituents that move relativistically (at or near the speed of light), such as photons and neutrinos. As the universe cools, from around 47,000 years (redshift ''z'' = 3600), the universe's large-scale behavior becomes dominated by matter instead. This occurs because the energy density of matter begins to exceed both the energy density of radiation and the vacuum energy density. Around or shortly after 47,000 years, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) become equal, the Jeans instability#Jeans' length, Jeans length, which determines the smallest structures that can form (due to competition between gravitational attraction and pressure effects), begins to fall and perturbations, instead of being wiped out by free streaming radiation, can begin to grow in amplitude. According to the Lambda-CDM model, by this stage, the matter in the universe is around 84.5% cold dark matter and 15.5% "ordinary" matter. There is overwhelming evidence that
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, ...

dark matter
exists and dominates our universe, but since the exact nature of dark matter is still not understood, the Big Bang theory does not presently cover any stages in its formation. From this point on, and for several billion years to come, the presence of dark matter accelerates the structure formation, formation of structure in our universe. In the early universe, dark matter gradually gathers in huge filaments under the effects of gravity, collapsing faster than ordinary (baryonic) matter because its collapse is not slowed by radiation pressure. This amplifies the tiny inhomogeneities (irregularities) in the density of the universe which was left by cosmic inflation. Over time, slightly denser regions become denser and slightly rarefied (emptier) regions become more rarefied. Ordinary matter eventually gathers together faster than it would otherwise do, because of the presence of these concentrations of dark matter. The properties of dark matter that allow it to collapse quickly without radiation pressure, also mean that it cannot ''lose'' energy by radiation either. Losing energy is necessary for particles to collapse into dense structures beyond a certain point. Therefore, dark matter collapses into huge but diffuse filaments and haloes, and not into stars or planets. Ordinary matter, which ''can'' lose energy by radiation, forms dense objects and also Interstellar cloud, gas clouds when it collapses.


Recombination, photon decoupling, and the cosmic microwave background (CMB)

About 370,000 years after the Big Bang, two connected events occurred: the ending of recombination and photon decoupling. Recombination describes the ionized particles combining to form the first neutral atoms, and decoupling refers to the photons released ("decoupled") as the newly formed atoms settle into more stable energy states. Just before recombination, the baryonic matter in the universe was at a temperature where it formed a hot ionized plasma. Most of the photons in the universe interacted with electrons and protons, and could not travel significant distances without interacting with ionized particles. As a result, the universe was opaque or "foggy". Although there was light, it was not possible to see, nor can we observe that light through telescopes. Starting around 18,000 years, the universe has cooled to a point where free electrons can combine with helium atomic nucleus, nuclei to form atoms. Neutral helium nuclei then start to form at around 100,000 years, with neutral hydrogen formation peaking around 260,000 years. This process is known as recombination. The name is slightly inaccurate and is given for historical reasons: in fact the electrons and atomic nuclei were combining for the first time. At around 100,000 years, the universe had cooled enough for
helium hydride The helium hydride ion or hydridohelium(1+) ion or helonium is a cation An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative ...
, the first molecule, to form. In April 2019, this molecule was first announced to have been observed in interstellar space, in NGC 7027, a planetary nebula within our galaxy. (Much later, atomic hydrogen reacted with helium hydride to create molecular hydrogen, the fuel required for star formation.) Directly combining in a low energy state (ground state) is less efficient, so these hydrogen atoms generally form with the electrons still in a high-energy state, and once combined, the electrons quickly release energy in the form of one or more photons as they transition to a low energy state. This release of photons is known as photon decoupling. Some of these decoupled photons are captured by other hydrogen atoms, the remainder remain free. By the end of recombination, most of the protons in the universe have formed neutral atoms. This change from charged to neutral particles means that the
mean free path 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 ...

mean free path
photons can travel before capture in effect becomes infinite, so any decoupled photons that have not been captured can travel freely over long distances (see Thomson scattering). The universe has become transparent to visible light, radio waves and other electromagnetic radiation for the first time in its history. The photons released by these newly formed hydrogen atoms initially had a Color temperature, temperature/energy of around ~ 4000 K. This would have been visible to the eye as a pale yellow/orange tinted, or "soft", white color. Over billions of years since decoupling, as the universe has expanded, the photons have been red-shifted from visible light to radio waves (microwave radiation corresponding to a temperature of about 2.7 K). Red shifting describes the photons acquiring longer wavelengths and lower frequency, frequencies as the universe expanded over billions of years, so that they gradually changed from visible light to radio waves. These same photons can still be detected as radio waves today. They form the cosmic microwave background, and they provide crucial evidence of the early universe and how it developed. Around the same time as recombination, existing Longitudinal wave, pressure waves within the electron-baryon plasma—known as baryon acoustic oscillations—became embedded in the distribution of matter as it condensed, giving rise to a very slight preference in distribution of large-scale objects. Therefore, the cosmic microwave background is a picture of the universe at the end of this epoch including the tiny fluctuations generated during inflation (see #9-year WMAP image, 9-year WMAP image), and the spread of objects such as galaxies in the universe is an indication of the scale and size of the universe as it developed over time.


The Dark Ages and large-scale structure emergence

:'' 370 thousand to about 1 billion years after the Big Bang''


Dark Ages

After recombination and decoupling, the universe was transparent and had cooled enough to allow light to travel long distances, but there were no light-producing structures such as stars and galaxies. Stars and galaxies are formed when dense regions of gas form due to the action of gravity, and this takes a long time within a near-uniform density of gas and on the scale required, so it is estimated that stars did not exist for perhaps hundreds of millions of years after recombination. This period, known as the Dark Ages, began around 370,000 years after the Big Bang. During the Dark Ages, the temperature of the universe cooled from some 4000 K to about 60 K (3727 °C to about −213 °C), and only two sources of photons existed: the photons released during recombination/decoupling (as neutral hydrogen atoms formed), which we can still detect today as the cosmic microwave background (CMB), and photons occasionally released by neutral hydrogen atoms, known as the hydrogen line, 21 cm spin line of neutral hydrogen. The hydrogen spin line is in the microwave range of frequencies, and within 3 million years, the CMB photons had redshifted out of visible light to infrared; from that time until the first stars, there were no visible light photons. Other than perhaps some rare statistical anomalies, the universe was truly dark. The first generation of stars, known as Stellar population#Population III stars, Population III stars, formed within a few hundred million years after the Big Bang. These stars were the first source of visible light in the universe after recombination. Structures may have begun to emerge from around 150 million years, and early galaxies emerged from around 380 to 700 million years. (We do not have separate observations of very early individual stars; the earliest observed stars are discovered as participants in very early galaxies.) As they emerged, the Dark Ages gradually ended. Because this process was gradual, the Dark Ages only fully ended around 1 billion years, as the universe took its present appearance. There is also an Low-Frequency Array (LOFAR), observational effort underway to detect the faint 21 cm spin line radiation, as it is in principle an even more powerful tool than the cosmic microwave background for studying the early universe.


Speculative "habitable epoch"

:''c. 10–17 million years after the Big Bang'' For about 6.6 million years, between about 10 to 17 million years after the Big Bang (redshift 137–100), the background temperature was between , a temperature compatible with liquid water and common biological chemical reactions. Avi Loeb, Abraham Loeb (2014) speculated that Abiogenesis, primitive life might in principle have appeared during this window, which he called the "habitable epoch of the early Universe". "A version of this article appears in print on Dec. 2, 2014, Section D, Page 2 of the New York edition with the headline: Much-Discussed Views That Go Way Back." Loeb argues that carbon-based life might have evolved in a hypothetical pocket of the early universe that was dense enough both to generate at least one massive star that subsequently releases carbon in a supernova, and that was also dense enough to generate a planet. (Such dense pockets, if they existed, would have been extremely rare.) Life would also have required a heat differential, rather than just uniform background radiation; this could be provided by naturally occurring geothermal energy. Such life would likely have remained primitive; it is highly unlikely that intelligent life would have had sufficient time to evolve before the hypothetical oceans freeze over at the end of the habitable epoch.


Earliest structures and stars emerge

:''Around 150 million to 1 billion years after the Big Bang'' The matter in the universe is around 84.5% cold dark matter and 15.5% "ordinary" matter. Since the start of the matter-dominated era, dark matter has gradually been gathering in huge spread-out (diffuse) filaments under the effects of gravity. Ordinary matter eventually gathers together faster than it would otherwise do, because of the presence of these concentrations of dark matter. It is also slightly more dense at regular distances due to early baryon acoustic oscillations (BAO) which became embedded into the distribution of matter when photons decoupled. Unlike dark matter, ordinary matter can lose energy by many routes, which means that as it collapses, it can lose the energy which would otherwise hold it apart, and collapse more quickly, and into denser forms. Ordinary matter gathers where dark matter is denser, and in those places it collapses into clouds of mainly hydrogen gas. The first stars and galaxies form from these clouds. Where numerous galaxies have formed, galaxy clusters and superclusters will eventually arise. Large void (cosmology), voids with few stars will develop between them, marking where dark matter became less common. The exact timings of the first stars, galaxies, supermassive black holes, and quasars, and the start and end timings and progression of the period known as
reionization In the field of Big Bang The Big Bang theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated ...
, are still being actively researched, with new findings published periodically. As of 2019, the earliest confirmed galaxies date from around 380–400 million years (for example GN-z11), suggesting surprisingly fast gas cloud condensation and stellar birth rates, and observations of the Lyman-alpha forest and other changes to the light from ancient objects allows the timing for reionization, and its eventual end, to be narrowed down. But these are all still areas of active research. Structure formation in the Big Bang model proceeds hierarchically, due to gravitational collapse, with smaller structures forming before larger ones. The earliest structures to form are the first stars (known as Population III stars), dwarf galaxies, and quasars (which are thought to be bright, early Active galactic nucleus, active galaxies containing a supermassive black hole surrounded by an inward-spiralling accretion disk of gas). Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulation, ''N''-body simulations with billions of particles. The Bolshoi Cosmological Simulation is a high precision simulation of this era. These Population III stars are also responsible for turning the few light elements that were formed in the Big Bang (hydrogen, helium and small amounts of lithium) into many heavier elements. They can be huge as well as perhaps small—and non-metallic (no elements except hydrogen and helium). The larger stars have very short lifetimes compared to most Main Sequence stars we see today, so they commonly finish burning their hydrogen fuel and explode as
supernova A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last stellar evolution, evolutionary stages of a massive star or when a ...

supernova
e after mere millions of years, seeding the universe with heavier elements over repeated generations. They mark the start of the Stelliferous Era. As yet, no Population III stars have been found, so our understanding of them is based on computational models of their formation and evolution. Fortunately, observations of the cosmic microwave background radiation can be used to date when star formation began in earnest. Analysis of such observations made by the ''Planck'' microwave space telescope in 2016 concluded that the first generation of stars may have formed from around 300 million years after the Big Bang. The October 2010 discovery of UDFy-38135539, the first observed galaxy to have existed during the following
reionization In the field of Big Bang The Big Bang theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated ...
epoch, gives us a window into these times. Subsequently, Leiden University's Rychard Bouwens, Rychard J. Bouwens and Garth D. Illingworth from UC Observatories/Lick Observatory found the galaxy UDFj-39546284 to be even older, at a time some 480 million years after the Big Bang or about halfway through the Dark Ages 13.2 billion years ago. In December 2012 the first candidate galaxies dating to before reionization were discovered, when UDFy-38135539, EGSY8p7 and GN-z11 galaxies were found to be around 380–550 million years after the Big Bang, 13.4 billion years ago and at a distance of around . Quasars provide some additional evidence of early structure formation. Their light shows evidence of elements such as carbon, magnesium, iron and oxygen. This is evidence that by the time quasars formed, a massive phase of star formation had already taken place, including sufficient generations of Population III stars to give rise to these elements.


Reionization

As the first stars, dwarf galaxies and quasars gradually form, the intense radiation they emit reionizes much of the surrounding universe; splitting the neutral hydrogen atoms back into a plasma of free electrons and protons for the first time since recombination and decoupling. Reionization is evidenced from observations of quasars. Quasars are a form of active galaxy, and the most luminous objects observed in the universe. Electrons in neutral hydrogen have specific patterns of absorbing photons, related to electron energy levels and called the Lyman series. Ionized hydrogen does not have electron energy levels of this kind. Therefore, light travelling through ionized hydrogen and neutral hydrogen shows different absorption lines. In addition, the light will have travelled for billions of years to reach us, so any absorption by neutral hydrogen will have been redshifted by varying amounts, rather than by one specific amount, indicating when it happened. These features make it possible to study the state of ionization at many different times in the past. They show that reionization began as "bubbles" of ionized hydrogen which became larger over time. They also show that the absorption was due to the general state of the universe (the intergalactic medium) and not due to passing through galaxies or other dense areas. Reionization might have started to happen as early as ''z'' = 16 (250 million years of cosmic time) and was complete by around ''z'' = 9 or 10 (500 million years)before gradually diminishing and probably coming to an end by around ''z'' = 5 or 6 (1 billion years) as the era of Population III stars and quasars—and their intense radiation—came to an end, and the ionized hydrogen gradually reverted to neutral atoms. These observations have narrowed down the period of time during which reionization took place, but the source of the photons that caused reionization is still not completely certain. To ionize neutral hydrogen, an energy larger than 13.6 electronvolt, eV is required, which corresponds to ultraviolet photons with a wavelength of 91.2 nanometre, nm or shorter, implying that the sources must have produced significant amount of ultraviolet and higher energy. Protons and electrons will recombine if energy is not continuously provided to keep them apart, which also sets limits on how numerous the sources were and their longevity. With these constraints, it is expected that quasars and first generation stars and galaxies were the main sources of energy. The current leading candidates from most to least significant are currently believed to be Population III stars (the earliest stars) (possibly 70%), dwarf galaxies (very early small high-energy galaxies) (possibly 30%), and a contribution from quasars (a class of Active galactic nucleus, active galactic nuclei). However, by this time, matter had become far more spread out due to the ongoing expansion of the universe. Although the neutral hydrogen atoms were again ionized, the plasma was much more thin and diffuse, and photons were much less likely to be scattered. Despite being reionized, the universe remained largely transparent during reionization. As the universe continued to cool and expand, reionization gradually ended.


Galaxies, clusters and superclusters

Matter continues to draw together under the influence of gravity, to form galaxies. The stars from this time period, known as Stellar population#Population II stars, Population II stars, are formed early on in this process, with more recent Stellar population#Population I stars, Population I stars formed later. Gravitational attraction also gradually pulls galaxies towards each other to form groups, Galaxy cluster, 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. Hubble Ultra-Deep Field, Hubble Ultra Deep Field observations has identified a number of small galaxies merging to form larger ones, at 800 million years of cosmic time (13 billion years ago). (This age estimate is now believed to be slightly overstated). Using the 10-metre W. M. Keck Observatory, Keck II telescope on Mauna Kea, Richard Ellis (astronomer), Richard Ellis of the California Institute of Technology at Pasadena and his team found six star forming galaxies about 13.2 billion light-years away and therefore created when the universe was only 500 million years old. Only about 10 of these extremely early objects are currently known. More recent observations have shown these ages to be shorter than previously indicated. The most distant galaxy observed as of October 2016, GN-z11, has been reported to be 32 billion light-years away, a vast distance made possible through spacetime expansion (''z'' = 11.1; Comoving and proper distances, comoving distance of 32 billion light-years; Cosmic time, lookback time of 13.4 billion years).


The universe as it appears today

The universe has appeared much the same as it does now, for many billions of years. It will continue to look similar for many more billions of years into the future. Based upon the emerging science of nucleocosmochronology, the Galactic thin disk of the Milky Way is estimated to have been formed 8.8 ± 1.7 billion years ago.


Dark energy dominated era

:''From about 9.8 billion years after the Big bang'' From about 9.8 billion years of cosmic time, the universe's large-scale behavior is believed to have gradually changed for the third time in its history. Its behavior had originally been dominated by radiation (relativistic constituents such as photons and neutrinos) for the first 47,000 years, and since about 370,000 years of cosmic time, its behavior had been dominated by matter. During its matter-dominated era, the expansion of the universe had begun to slow down, as gravity reined in the initial outward expansion. But from about 9.8 billion years of cosmic time, observations show that the expansion of the universe slowly stops decelerating, and gradually begins to accelerate again, instead. While the precise cause is not known, the observation is accepted as correct by the cosmologist community. By far the most accepted understanding is that this is due to an unknown form of energy which has been given the name "dark energy". "A version of this article appears in print on Feb. 21, 2017, Section D, Page 1 of the New York edition with the headline: A Runaway Universe." "Dark" in this context means that it is not directly observed, but can currently only be studied by examining the effect it has on the universe. Research is ongoing to understand this dark energy. Dark energy is now believed to be the single largest component of the universe, as it constitutes about 68.3% of the entire mass-energy of the physical universe. Dark energy is believed to act like a cosmological constant—a scalar field that exists throughout space. Unlike gravity, the effects of such a field do not diminish (or only diminish slowly) as the universe grows. While matter and gravity have a greater effect initially, their effect quickly diminishes as the universe continues to expand. Objects in the universe, which are initially seen to be moving apart as the universe expands, continue to move apart, but their outward motion gradually slows down. This slowing effect becomes smaller as the universe becomes more spread out. Eventually, the outward and repulsive effect of dark energy begins to dominate over the inward pull of gravity. Instead of slowing down and perhaps beginning to move inward under the influence of gravity, from about 9.8 billion years of cosmic time, the expansion of space starts to slowly accelerate ''outward'' at a gradually ''increasing'' rate.


The far future and ultimate fate

There are several competing scenarios for the long-term evolution of the universe. Which of them will happen, if any, depends on the precise values of physical constants such as the cosmological constant, the possibility of proton decay, the False vacuum decay, energy of the vacuum (meaning, the energy of Quantum vacuum state, "empty" space itself), and the natural laws Physics beyond the Standard Model, beyond the Standard Model. If the expansion of the universe continues and it stays in its present form, eventually all but the nearest galaxies will be carried away from us by the expansion of space at such a velocity that our observable universe will be limited to Laniakea Supercluster, our own gravitationally bound local galaxy cluster. In the very long term (after many trillions—thousands of billions—of years, cosmic time), the Stelliferous Era will end, as stars cease to be born and even the Red dwarf, longest-lived stars gradually die. Beyond this, all objects in the universe will cool and (with the proton decay, possible exception of protons) gradually decompose back to their constituent particles and then into subatomic particles and very low-level photons and other Elementary particle, fundamental particles, by a variety of possible processes. Ultimately, in the extreme future, the following scenarios have been proposed for the ultimate fate of the universe: In this kind of extreme timescale, extremely rare quantum mechanics, quantum phenomena may also occur that are extremely unlikely to be seen on a timescale smaller than trillions of years. These may also lead to unpredictable changes to the state of the universe which would not be likely to be significant on any smaller timescale. For example, on a timescale of millions of trillions of years, black holes might appear to evaporate almost instantly, uncommon quantum tunnelling phenomena would appear to be common, and quantum (or other) phenomena so unlikely that they might occur just once in a trillion years may occur many times.


See also

* * – Age of the universe scaled to a single year * * * * * * * * * * * * *


Notes


References


Bibliography

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External links

* * * * * * * * * * (See: "Energy time line from the Big Bang to the present" (1984) and "History of the Universe Poster" (1989).) * * * {{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Articles containing video clips Astronomy timelines Physical cosmology Big Bang Physics timelines Chronology by event, Universe