TheInfoList

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 Logic (from Ancient Greek, G ...
is the prevailing
cosmological model Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fun ...
of the
observable universe The observable universe is a ball-shaped region of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...
from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from an initial state of high
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

and
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

, and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of
light element 400px, The periodic table of the chemical elements In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, s ...
s, 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)
radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and f ...

, and large-scale structure. Crucially, the theory is compatible with
Hubble–Lemaître law Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides ...
— the observation that the farther away
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 ...

are, the faster they are moving away from Earth. Extrapolating this
cosmic expansion The expansion of the universe is the increase in proper length, distance between any two given Gravitational binding energy, gravitationally unbound parts of the observable universe with time. It is an intrinsic and extrinsic properties (philosoph ...
backwards in time using the known
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 ...
, the theory describes an increasingly concentrated cosmos preceded by a
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 ...
in which
space and time Space and Time or Time and Space, or ''variation'', may refer to: * ''Space and time 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 fo ...
lose meaning (typically named "the Big Bang singularity"). Detailed measurements of the expansion rate of the universe place the Big Bang singularity at around 13.8
billion A billion is a number with two distinct definitions: *1,000,000,000 1,000,000,000 (one billion A billion is a number with two distinct definitions: *1,000,000,000 1,000,000,000 (one billion, short scale; one thousand million or milliar ...
years ago, which is thus considered the
age of the universe 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 ...
. After its initial expansion, an event that is by itself often called "the Big Bang", the universe cooled sufficiently to allow the formation 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, and later
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s. Giant clouds of these primordial elements – mostly
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 ...

, with some
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" ...

and
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 ...

– later coalesced through
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 ...

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

s and galaxies, the descendants of which are visible today. Besides these primordial building materials, astronomers observe the gravitational effects of an unknown
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, ...

surrounding galaxies. Most of the
gravitational potential In classical mechanics, the gravitational potential at a location is equal to the work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical w ...

in the universe seems to be in this form, and the Big Bang theory and various observations indicate that this excess gravitational potential is not created by
baryonic matter 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 departmen ...
, such as normal atoms. Measurements of the redshifts of
supernovae A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion An explosion is a rapid expansion in volume Volume is a expressing the of enclosed by a . For example, the sp ...
indicate that the expansion of the universe is accelerating, an observation attributed to
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 ...

's existence.
Georges Lemaître Georges Henri Joseph Édouard Lemaître ( ; ; 17 July 1894 – 20 June 1966) was a Belgian Belgian may refer to: * Something of, or related to, Belgium Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is ...
first noted in 1927 that an expanding
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

could be traced back in time to an originating single point, which he called the "primeval atom".
Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects previously ...
confirmed through analysis of galactic
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 ...

s in 1929 that galaxies are indeed drifting apart; this is important observational evidence for an expanding universe. For several decades, the scientific community was divided between supporters of the Big Bang and the rival
steady-state model In cosmology Cosmology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its pop ...
which both offered explanations for the observed expansion, but the steady-state model stipulated an eternal universe in contrast to the Big Bang's finite age. In 1964, the CMB was discovered, which convinced many cosmologists that the steady-state theory was
falsified Falsifiability is a standard of evaluation of scientific theories and hypotheses that was introduced by the philosopher of science Karl Popper in his book The Logic of Scientific Discovery, ''Logik der Forschung'' (1934). He proposed it as the ...
, since, unlike the steady-state theory, the hot Big Bang predicted a uniform background radiation throughout the universe caused by the high temperatures and densities in the distant past. A wide range of empirical evidence strongly favors the Big Bang, which is now essentially universally accepted.: "At the same time that observations tipped the balance definitely in favor of relativistic big-bang theory, ..."

# Features of the model

The Big Bang theory offers a comprehensive explanation for a broad range of observed phenomena, including the abundances of the
light element 400px, The periodic table of the chemical elements In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, s ...
s, the
CMB The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang is the prevailing of the from the through its subsequent large-scale evolution. The model describes how the from an initial state of high and , and offers a comp ...
, large-scale structure, and
Hubble's law Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides ...
. The theory depends on two major assumptions: the universality of physical laws and the
cosmological principle #REDIRECT Cosmological principle In modern 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 ...
. The universality of physical laws is one of the underlying principles of the
theory of relativity The theory of relativity usually encompasses two interrelated theories by Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born , widely acknowledged to be one of the greatest physicists of all time ...
. The cosmological principle states that on large scales the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

is
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 ...
and
isotropic 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 ...
– appearing the same in all directions regardless of location. These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the
fine-structure constant In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
over much of the age of the universe is of order 10−5. Also,
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 ...
has passed stringent
tests Test(s), testing, or TEST may refer to: * Test (assessment) A test or examination (exam or evaluation) is an educational assessment Educational assessment or educational evaluation is the systematic process of documenting and using em ...
on the scale of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

and
binary star A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called ''Star system#Hierarchical systems, multiple star systems''. These systems, especially when more distant, oft ...

s.Further information of, and references for, tests of general relativity are given in the article
tests of general relativity Tests of general relativity serve to establish observational evidence for the theory of general relativity General relativity, also known as the general theory of relativity, is the differential geometry, geometric scientific theory, theory o ...
.
The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler
Copernican principle 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 ...
, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10−5 via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound
on the order of An order of magnitude is an approximation of the logarithm In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contain ...
10% inhomogeneity, as of 1995.

## Expansion of space

The expansion of the Universe was inferred from early twentieth century astronomical observations and is an essential ingredient of the Big Bang theory. Mathematically, general relativity describes
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 ...
by 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 ...
, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, are specified using a
coordinate chart In topology, a branch of mathematics, a topological manifold is a topological space which locally resembles real numbers, real ''n''-dimension (mathematics), dimensional Euclidean space. Topological manifolds are an important class of topological sp ...
or "grid" that is laid down over all spacetime. The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. This metric contains a
scale factor A scale factor is usually a decimal which scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory)In the mathematical discipline of descriptive set theory, a scale is a certain kind of object defined on a set (mathemat ...
, which describes how the size of the universe changes with time. This enables a convenient choice of a
coordinate system In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position o ...

comoving coordinates In Big Bang, standard cosmology, comoving distance and proper distance are two closely related distance measures (cosmology), distance measures used by cosmologists to define distances between objects. ''Proper distance'' roughly corresponds to w ...
. In this coordinate system, the grid expands along with the universe, and objects that are moving only because of the
expansion of the universe The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby ''the scale of space itself changes''. The universe does n ...
, remain at fixed points on the grid. While their ''coordinate'' distance (
comoving distance In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects. ''Proper distance'' roughly corresponds to where a distant object would be at a spec ...
) remains constant, the ''physical'' distance between two such co-moving points expands proportionally with the scale factor of the universe. The Big Bang is not an explosion 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 ...
moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distances between comoving points. In other words, the Big Bang is not an explosion ''in space'', but rather an expansion ''of space''. Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our universe only on large scales—local concentrations of matter such as our galaxy do not necessarily expand with the same speed as the whole Universe.

## Horizons

An important feature of the Big Bang spacetime is the presence of
particle horizon The particle horizon (also called the cosmological horizon, the comoving horizon (in Dodelson's text), or the cosmic light horizon) is the maximum distance from which light from Elementary particle, particles could have traveled to the observation, ...
s. Since the universe has a finite age, and
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

travels at a finite speed, there may be events in the past whose light has not yet had time to reach us. This places a limit or a ''past horizon'' on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a ''future horizon'', which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe. Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well.

## Thermalisation

Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic An axiom, postulate or assumption is a statement that is taken to be true True most commonly refers to truth Truth is the property of being in accord with fact or reality.Merriam-Webster's Onlin ...
. Others were fast enough to reach
thermalisation In physics, thermalisation is the process of physical bodies reaching thermal equilibrium through mutual interaction. In general the natural tendency of a system is towards a state of equipartition of energy and uniform temperature that maximize ...
. The parameter usually used to find out whether a process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and 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 ...
. The larger the ratio, the more time particles had to thermalise before they were too far away from each other.

# Timeline

According to the Big Bang theory, the universe at the beginning was very hot and very compact, and since then it has been expanding and cooling down.

## Singularity

Extrapolation of the expansion of the universe backwards in time using general relativity yields an
infinite Infinite may refer to: Mathematics *Infinite set, a set that is not a finite set *Infinity, an abstract concept describing something without any limit Music *Infinite (band), a South Korean boy band *''Infinite'' (EP), debut EP of American mus ...

density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

and
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

at a finite time in the past. This irregular behavior, known as the
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 ...
, indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone can not extrapolate toward the singularity—before the end of the so-called
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 ...
. This primordial singularity is itself sometimes called "the Big Bang", but the term can also refer to a more generic early hot, dense phase of the universe. In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a
regime In politics Politics (from , ) is the set of activities that are associated with Decision-making, making decisions in Social group, groups, or other forms of Power (social and political), power relations between individuals, such as the dist ...
where the laws of physics as we understand them (specifically general relativity and 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 ...

of
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 ...
) work. Based on measurements of the expansion using
Type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller wh ...
e and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event—known as the "
age of the universe 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 ...
"—is 13.799 ± 0.021 billion years. (See Table 4, Age/Gyr, last column.) Despite being extremely dense at this time—far denser than is usually required to form a —the universe did not re-collapse into a singularity. Commonly used calculations and limits for explaining
gravitational collapse Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the centre of mass#Centre of gravity, centre of gravity. Gravitational collapse is a fundamenta ...
are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible
density gradient Density gradient is a spatial variation in density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek ...
.

## Inflation and baryogenesis

The earliest phases of the Big Bang are subject to much speculation, since astronomical data about them are not available. In the most common models the universe was filled homogeneously and isotropically with a very high
energy density 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 suc ...

and huge temperatures and
pressure Pressure (symbol: ''p'' or ''P'') is the 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 fr ...

s, and was very rapidly expanding and cooling. The period from 0 to 10−43 seconds into the expansion, 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 ...
, was a phase in which the four
fundamental force #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 — the
electromagnetic force Electromagnetism 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 time, and the related ...
, 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 ...
, the
weak nuclear force In nuclear physics Nuclear physics is the field 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 ...
, and the
gravitational force 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 ...
, were unified as one. In this stage, the characteristic scale length of the universe was the
Planck length In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
, , and consequently had a temperature of approximately 1032 degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a 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, ...

. The Planck epoch was succeeded by the
grand unification 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 ...
beginning at 10−43 seconds, where gravitation separated from the other forces as the universe's temperature fell. At approximately 10−37 seconds into the expansion, 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 facts A fact is an occurrence in ...
caused a
cosmic inflation In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of the universe, expansion of space in the early universe. The inflationary epoch lasted from  seconds after the conj ...
, during which the universe grew
exponentially Exponential may refer to any of several mathematical topics related to exponentiation, including: *Exponential function, also: **Matrix exponential, the matrix analogue to the above *Exponential decay, decrease at a rate proportional to value *Expo ...

, unconstrained by the light speed invariance, and temperatures dropped by a factor of 100,000. Microscopic
quantum fluctuation In quantum physics Quantum mechanics is a fundamental 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 m ...
s that occurred because of
Heisenberg's uncertainty principle In quantum mechanics, the uncertainty principle (also known as Heisenberg's uncertainty principle) is any of a variety of Inequality (mathematics), mathematical inequalities asserting a fundamental limit to the accuracy with which the values fo ...

were amplified into the seeds that would later form the large-scale structure of the universe. At a time around 10−36 seconds, 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 ...
begins when the strong nuclear force separates from the other forces, with only the electromagnetic force and weak nuclear force remaining unified. Inflation stopped at around the 10−33 to 10−32 seconds mark, with the universe's volume having increased by a factor of at least 1078. Reheating occurred until the universe obtained the temperatures required for the
production Production may refer to: Economics and business * Production (economics) Production is the process of combining various material inputs and immaterial inputs (plans, know-how) in order to make something for consumption (output). It is the act of ...

of a
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 ...
as well as all other
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. Temperatures were so high that the random motions of particles were at relativistic speeds, and of all kinds were being continuously created and destroyed in collisions. At some point, an unknown reaction called
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 ...
violated the conservation of baryon number, leading to a very small excess 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 other particles. Particles currently thought to be elementary include the fundam ...

s and
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 ...

s over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.

## Cooling

The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the
fundamental force #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 of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10−12 seconds. After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies, and to contain them in well-defined particle beam, beams. Large accelerators are used for funda ...
s. At about 10−6 seconds, quarks and
gluon A gluon () is an 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 (s, s, s, and s), which generally ar ...

s combined to form
baryon 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 ...
s such as
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

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

s. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton–antiproton pairs (similarly for neutrons–antineutrons), so a mass annihilation immediately followed, leaving just one in 108 of the original matter particles and none of 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. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by
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 ...

s (with a minor contribution from
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 ...

s). A few minutes into the expansion, when the temperature was about a billion
kelvin The kelvin is the base unit of temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal en ...

and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's
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 ...

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

nuclei ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
in a process called
Big Bang nucleosynthesis In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the univer ...
(BBN). Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the
rest energy The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** ...
density of matter came to gravitationally dominate that of the photon
radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and f ...

. After about 379,000 years, the electrons and nuclei combined into
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s (mostly
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 ...

), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background.

## Structure formation

Over a long period of time, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as
cold dark matter In Physical cosmology, cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. Observations indicate that approximately 85% of the matter in the universe is dark matter, with only a small fraction being the ordinary b ...
,
warm dark matter Warm dark matter (WDM) is a hypothesized form of 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 ...
,
hot dark matter Hot dark matter (HDM) is a theoretical form of 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 o ...
, and
baryonic matter 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 departmen ...
. The best measurements available, from the
Wilkinson Microwave Anisotropy Probe The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory age ...
(WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold (warm dark matter is ruled out by early
reionization In the fields of Big Bang theory and physical cosmology, cosmology, reionization is the process that caused matter in the universe to reionize after the lapse of the "Timeline of the Big Bang#Dark Ages, dark ages". Reionization is the second of ...
), and is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. (See Table 8.) In an "extended model" which includes hot dark matter in the form of neutrinos, then if the "physical baryon density" $\Omega _\text h^2$ is estimated at about 0.023 (this is different from the 'baryon density' $\Omega _\text$ expressed as a fraction of the total matter/energy density, which is about 0.046), and the corresponding cold dark matter density $\Omega _\text h^2$ is about 0.11, the corresponding neutrino density $\Omega _\text h^2$ is estimated to be less than 0.0062.

## Cosmic acceleration

Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as
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 ...

, which apparently permeates all of space. The observations suggest 73% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together,
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 ...

predominated, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the declining density of matter relative to the density of dark energy caused the expansion of the universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in
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 ...
of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theoretically. All of this cosmic evolution after the
inflationary epoch __NOTOC__ In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of th ...
can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of
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 thinking is often associated with ...
and general relativity. There are no easily testable models that would describe the situation prior to approximately 10−15 seconds. Understanding this earliest of eras in the history of the universe is currently one of the greatest
unsolved problems in physics The following is a list of notable unsolved problems grouped into broad areas of physics. Some of the major unsolved problems in physics are theoretical, meaning that existing theory, theories seem incapable of explaining a certain observed phe ...
.

# History

## Etymology

English English usually refers to: * English language English is a West Germanic language of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication use ...
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

Fred Hoyle Sir Fred Hoyle Fellow of the Royal Society, FRS (24 June 1915 – 20 August 2001) was an English astronomer who formulated the theory of stellar nucleosynthesis. He also held controversial stances on other scientific matters—in particula ...
is credited with coining the term "Big Bang" during a talk for a March 1949 BBC Radio broadcast, saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past." It is popularly reported that Hoyle, who favored an alternative "
steady-state In systems theory Systems theory is the interdisciplinary Interdisciplinarity or interdisciplinary studies involves the combination of two or more academic disciplines into one activity (e.g., a research project). It draws knowledge from s ...
" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.: "To create a picture in the mind of the listener, Hoyle had likened the explosive theory of the universe's origin to a 'big bang'."

## Development

The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. In 1912,
Vesto Slipher Vesto Melvin Slipher (; November 11, 1875 – November 8, 1969) was an American astronomer who performed the first measurements of radial velocities for galaxies. He was the first to discover that distant galaxies are redshifts, redshifted, thus pr ...
measured the first
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer (physics), observer who is moving relative to the wave source. It is named after the Austrian physicist ...

of a "
spiral nebula Spiral galaxies form a galaxy morphological classification, class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''highly controversial whether or not these nebulae were "island universes" outside our
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

. Ten years later,
Alexander Friedmann Alexander Alexandrovich Friedmann (also spelled Friedman or Fridman ; russian: Алекса́ндр Алекса́ндрович Фри́дман) (June 16 .S. 4 1888 – September 16, 1925) was a Russians, Russian and USSR, Soviet physicist ...
, a
Russia Russia ( rus, link=no, Россия, Rossiya, ), or the Russian Federation, is a country spanning Eastern Europe Eastern Europe is the eastern region of . There is no consistent definition of the precise area it covers, partly because th ...

n
cosmologist 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 ...
and
mathematician A mathematician is someone who uses an extensive knowledge 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 ...

, derived the
Friedmann equations The Friedmann equations are a set of equation In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and qua ...
from Einstein field equations, showing that the universe might be expanding in contrast to the
static universe In cosmology Cosmology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its pop ...
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest physicists of all time. Einstein is known for developing the theory of relativity The theo ...

at that time. *Translated in: In 1924,
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 of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or A ...
astronomer
Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects previously ...
's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed a series of distance indicators, the forerunner of the
cosmic distance ladder The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field out ...

, using the
Hooker telescope The Mount Wilson Observatory (MWO) is an astronomical observatory An observatory is a location used for observing terrestrial, marine, or celestial event A celestial event is an astronomy, astronomical phenomenon of interest that involves ...
at
Mount Wilson Observatory The Mount Wilson Observatory (MWO) is an Observatory#Astronomical observatories, astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson (California), Mount Wilson, a peak in the San Gabriel ...
. This allowed him to estimate distances to galaxies whose
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 ...

s had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and
recessional velocity Recessional velocity is the rate at which an extragalactic astronomical Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies celestial objects an ...
—now known as Hubble's law. By that time, Lemaître had already shown that this was expected, given the cosmological principle. Independently deriving Friedmann's equations in 1927,
Georges Lemaître Georges Henri Joseph Édouard Lemaître ( ; ; 17 July 1894 – 20 June 1966) was a Belgian Belgian may refer to: * Something of, or related to, Belgium Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is ...
, a
Belgian Belgians ( nl, Belgen, french: Belges, german: Belgier) are people identified with the Belgium, Kingdom of Belgium, a federation, federal state in Western Europe. As Belgium is a multinational state, this connection may be residential, legal, ...

physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at leas ...

and Roman Catholic priest, proposed that the inferred recession of the nebulae was due to the expansion of the universe. *Translated in: In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence. In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady-state theory. This perception was enhanced by the fact that the originator of the Big Bang theory, Lemaître, was a Roman Catholic priest.
Arthur Eddington Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a Philosophy of science, philosopher of science and a populariser of science. The Eddington limit, the nat ...

agreed with
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

that the universe did not have a beginning in time, ''viz''., that matter is eternal. A beginning in time was "repugnant" to him. Lemaître, however, disagreed: During the 1930s, other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the
Milne model The Milne model was a Special relativity, special-relativistic Physical cosmology, cosmological Scientific modeling, model proposed by Edward Arthur Milne in 1935.Edward Arthur Milne''Relativity, Gravitation and World Structure'' Oxford Univer ...
, the
oscillatory universe A cyclic model (or oscillating model) is any of several cosmological model 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 ...
(originally suggested by Friedmann, but advocated by Albert Einstein and ) and
Fritz Zwicky Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) was a Swiss Swiss may refer to: * the adjectival form of Switzerland , french: Suisse(sse), it, svizzero/svizzera or , rm, Svizzer/Svizra , government_type = Federalism, ...
's
tired light Tired light is a class of hypothetical redshift mechanisms that was proposed as an alternative explanation for the Hubble's law, redshift-distance relationship. These models have been proposed as alternatives to the models that require metric exp ...
hypothesis. After
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
, two distinct possibilities emerged. One was Fred Hoyle's steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time. The other was Lemaître's Big Bang theory, advocated and developed by
George Gamow George Gamow (March 4, 1904 – August 19, 1968), born Georgiy Antonovich Gamov (russian: Георгий Антонович Гамов), was a Ukrainian-Russian born American American(s) may refer to: * American, something of, from, or related ...
, who introduced BBN and whose associates,
Ralph Alpher Ralph Asher Alpher (February 3, 1921 – August 12, 2007) was an American cosmologist Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the ...

and
Robert Herman Robert Herman (August 29, 1914 – February 13, 1997) was an American scientist, best known for his work with Ralph Alpher Ralph Asher Alpher (February 3, 1921 – August 12, 2007) was an American cosmologist Cosmology (from Ancient Gree ...
, predicted the CMB. Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this ''big bang'' idea" during a BBC Radio broadcast in March 1949. For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio
source counts The source counts distribution of radio-sources from a radio-astronomical survey is the cumulative distribution of the number of sources (''N'') brighter than a given flux density (''S''). As it is usually plotted on a log-log scale its distributi ...
, began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the universe at earlier and earlier times, and reconciling observations with the basic theory. In 1968 and 1970,
Roger Penrose Sir Roger Penrose (born 8 August 1931) is a British mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas an ...
,
Stephen Hawking Stephen William Hawking (8 January 1942 – 14 March 2018) was an English theoretical physicist Theoretical physics is a branch of physics Physics is the that studies , its , its and behavior through , and the related enti ...

, and George F. R. Ellis published papers where they showed that
mathematical singularities In mathematics, a singularity is a point at which a given mathematical object is not defined, or a point where the mathematical object ceases to be well-behaved in some particular way, such as by lacking derivative, differentiability or Analyticity ...
were an inevitable initial condition of relativistic models of the Big Bang. Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981,
Alan Guth Alan Harvey Guth (; born February 27, 1947) is an American theoretical physicist and cosmologist. Guth has researched elementary particle theory (and how particle theory is applicable to the early universe). He is Victor Weisskopf Professor of ...
made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang theory with the introduction of an epoch of rapid expansion in the early universe he called "inflation". Meanwhile, during these decades, two questions in
observational cosmology Observational cosmology is the study of the structure, the evolution and the origin 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 ...
that generated much discussion and disagreement were over the precise values of the Hubble Constant and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe). In the mid-1990s, observations of certain
globular cluster A globular cluster is a sphere, spherical collection of stars. wiktionary:globular, Globular clusters are very tightly bound by gravity, with a high concentration of stars towards their centers. Their name is derived from Latin —a small sphere. ...
s appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with the age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a 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 d ...
s, indicated a much younger age for globular clusters. While there still remain some questions as to how accurately the ages of the clusters are measured, globular clusters are of interest to cosmology as some of the oldest objects in the universe. Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in
telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...

technology as well as the analysis of data from satellites such as the
Cosmic Background Explorer The Cosmic Background Explorer (COBE ), also referred to as Explorer 66, was a NASA satellite dedicated to physical cosmology, cosmology, which operated from 1989 to 1993. Its goals were to investigate the Cosmic microwave background, cosmic mi ...
(COBE), the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first ...

and WMAP. Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.

# Observational evidence

The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by
Big Bang nucleosynthesis In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the univer ...
(BBN). More recent evidence includes observations of
galaxy formation and evolution The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences Science (from the Latin Latin (, or , ) is a classical lan ...
, and the distribution of large-scale cosmic structures, These are sometimes called the "four pillars" of the Big Bang theory. Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features,
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, ...

is currently the subject of most active laboratory investigations. Remaining issues include the
cuspy halo problem The cuspy halo problem (also known as the core-cusp problem) refers to a discrepancy between the inferred dark matter Dark matter is a hypothetical form of matter In classical physics and general chemistry, matter is any substance that h ...
and the
dwarf galaxy problem The dwarf galaxy problem, also known as the missing satellites problem, arises from a mismatch between observed dwarf galaxy numbers and Cross section (physics), collisionless numerical physical cosmology, cosmological simulations that predict the ...
of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible. Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics.

## Hubble's law and the expansion of space

Observations of distant galaxies and
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 () ...

s show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a
frequency spectrum The power spectrum S_(f) of a x(t) describes the distribution of into frequency components composing that signal. According to , any physical signal can be decomposed into a number of discrete frequencies, or a spectrum of frequencies over ...
of an object and matching the
spectroscopic Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way ...

pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are
uniformly Uniform distribution may refer to: * Continuous uniform distribution * Discrete uniform distribution * Uniform distribution (ecology) * Equidistributed sequence See also

* * Homogeneous distribution {{disambiguation ...
isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed: $v = H_0D$ where * $v$ is the recessional velocity of the galaxy or other distant object, * $D$ is the comoving distance to the object, and * $H_0$ is
Hubble's constant 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 ...
, measured to be km/ s/ Mpc by the WMAP. Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable under the assumption of the Copernican principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Friedmann in 1922 and Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson, and Walker. The theory requires the relation $v = HD$ to hold at all times, where $D$ is the comoving distance, ''v'' is the recessional velocity, and $v$, $H$, and $D$ vary as the universe expands (hence we write $H_0$ to denote the present-day Hubble "constant"). For distances much smaller than the size of the
observable universe The observable universe is a ball-shaped region of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...
, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity $v$. However, the redshift is not a true Doppler shift, but rather the result of the expansion of the universe between the time the light was emitted and the time that it was detected. That space is undergoing metric expansion is shown by direct observational evidence of the cosmological principle and the Copernican principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and
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 ...
, supporting the cosmological principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions. Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position. Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the CMB over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

In 1964,
Arno Penzias Arno Allan Penzias (; born April 26, 1933) 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 of America (USA), comm ...
and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the
microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...

band. Their discovery provided substantial confirmation of the big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a
blackbody A black body or blackbody is an idealized physical body 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 defined contiguous boundary in three-dimen ...

spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded the 1978
Nobel Prize in Physics The Nobel Prize in Physics is a yearly award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions for mankind in the field of physics. It is one of the five Nobel Prizes established by the will ...
. The ''surface of last scattering'' corresponding to emission of the CMB occurs shortly after '' recombination'', the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly
scattered Scattered may refer to: Music * Scattered (album), ''Scattered'' (album), a 2010 album by The Handsome Family * Scattered (The Kinks song), "Scattered" (The Kinks song), 1993 * "Scattered", a song by Ace Young * "Scattered", a song by Green Day fr ...
from free charged particles. Peaking at around , the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent. In 1989,
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

launched COBE, which made two major advances: in 1990, high-precision spectrum measurements showed that the CMB frequency spectrum is an almost perfect blackbody with no deviations at a level of 1 part in 104, and measured a residual temperature of 2.726 K (more recent measurements have revised this figure down slightly to 2.7255 K); then in 1992, further COBE measurements discovered tiny fluctuations ( anisotropies) in the CMB temperature across the sky, at a level of about one part in 105. John C. Mather and
George Smoot George Fitzgerald Smoot III (born February 20, 1945) is an American astrophysics, astrophysicist, cosmology, cosmologist, Nobel laureate, and one of two contestants to win the 1 million prize on ''Are You Smarter than a 5th Grader? (U.S. game s ...

were awarded the 2006 Nobel Prize in Physics for their leadership in these results. During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably
BOOMERanG A boomerang is a thrown tool, typically constructed as a flat airfoil, that is designed to spin about an axis perpendicular to the direction of its flight. A returning boomerang is designed to return to the thrower. It is well known as a weap ...
, found the
shape of the universe The shape of the universe, 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 structure ...
to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies. In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe were released, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistent with the inflation theory in general. The ''
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 ...
'' space probe was launched in May 2009. Other ground and balloon-based
cosmic microwave background experiments This list is a compilation of experiments measuring the cosmic microwave background radiation, cosmic microwave background (CMB) radiation anisotropies and polarization since the first detection of the CMB by Arno Allan Penzias, Penzias and Robe ...
are ongoing.

## Abundance of primordial elements

Using the Big Bang model, it is possible to calculate the concentration of
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-3 Helium-3 (3He see also helion) is a light, stable isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons A proton is a subatomic particle, symbol or , with a positive electric charge Ele ...

, deuterium, and
lithium-7 Naturally occurring lithium (3Li) is composed of two stable isotopes, lithium-6 and lithium-7, with the latter being far more abundant on Earth. Both of the natural isotopes have an unexpectedly low nuclear binding energy per nucleon ( for lithi ...

in the universe as ratios to the amount of ordinary hydrogen. The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for ^4He/H, about 10−3 for ^2H/H, about 10−4 for ^3He/H and about 10−9 for ^7Li/H. The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for ^4He, and off by a factor of two for ^7Li (this anomaly is known as the cosmological lithium problem); in the latter two cases, there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium. Indeed, there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e., before
star formation Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", Jeans instability, collapse and form stars. As a branch of astronomy, star fo ...

, as determined by studying matter supposedly free of
stellar nucleosynthesis Stellar nucleosynthesis is the creation (nucleosynthesis) of chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nu ...
products) should have more helium than deuterium or more deuterium than ^3He, and in constant ratios, too.

## Galactic evolution and distribution

Detailed observations of the
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology) In archaeology, morphology is the study of the shape of Artifact (archaeology), artefacts and ecofacts. Morphology is a major consid ...
and distribution of galaxies and quasars are in agreement with the current state of the Big Bang theory. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then, larger structures have been forming, such as
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 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 ...
s. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently, appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures, agree well with Big Bang simulations of the formation of structure in the universe, and are helping to complete details of the theory.

## Primordial gas clouds

In 2011, astronomers found what they believe to be pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. These two clouds of gas contain no elements heavier than hydrogen and deuterium. Since the clouds of gas have no heavy elements, they likely formed in the first few minutes after the Big Bang, during BBN.

## Other lines of evidence

The age of the universe as estimated from the Hubble expansion and the CMB is now in good agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of
stellar evolution and giant In folklore Folklore is the expressive body of culture shared by a particular group of people; it encompasses the traditions common to that culture, subculture or group. These include oral traditions such as Narrative, tales, p ...

to globular clusters and through
radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "di ...
of individual
Population II During 1944, Walter Baade Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the s ...
stars. It is also in good agreement with age estimates based on measurements of the expansion using
Type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller wh ...
e and measurements of temperature fluctuations in the cosmic microwave background. (See Table 4, Age/Gyr, last column.) The agreement of independent measurements of this age supports the
Lambda-CDM The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a Parameter#Modelization, parameterization of the Big Bang physical cosmology, cosmological model in which the universe contains three major components: first, a cosmological constant den ...
(ΛCDM) model, since the model is used to relate some of the measurements to an age estimate, and all estimates turn out to agree. Still, some observations of objects from the relatively early universe (in particular quasar
APM 08279+5255 APM 08279+5255 is a very distant, broad absorption line quasar located in the constellation Lynx (constellation), Lynx. It is magnified and split into multiple images by the gravitational lensing effect of a foreground galaxy through which its lig ...
) raise concern as to whether these objects had enough time to form so early in the ΛCDM model. The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev–Zel'dovich effect in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult.

## Future observations

Future gravitational-wave observatories might be able to detect primordial
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, relics of the early universe, up to less than a second after the Big Bang.

# Problems and related issues in physics

As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang theory. Some of these mysteries and problems have been resolved while others are still outstanding. Proposed solutions to some of the problems in the Big Bang model have revealed new mysteries of their own. For example, the
horizon problem The horizon problem (also known as the homogeneity problem) is a cosmological Cosmology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the He ...

, the magnetic monopole problem, and the flatness problem are most commonly resolved with inflationary theory, but the details of the inflationary universe are still left unresolved and many, including some founders of the theory, say it has been disproven. What follows are a list of the mysterious aspects of the Big Bang theory still under intense investigation by cosmologists and Astrophysics, astrophysicists.

## Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter. It is generally assumed that when the universe was young and very hot it was in statistical equilibrium and contained equal numbers of baryons and antibaryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP violation, CP-symmetry are violated and that the universe depart from
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic An axiom, postulate or assumption is a statement that is taken to be true True most commonly refers to truth Truth is the property of being in accord with fact or reality.Merriam-Webster's Onlin ...
. * Translated in: ** Reprinted in: All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.

## Dark energy

Measurements of the redshift–apparent magnitude, magnitude relation for type Ia supernovae indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy". Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the Friedmann equations#Density parameter, critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler. Negative pressure is believed to be a property of vacuum energy, but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos. According to theory, the energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore, matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant. The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein's cosmological constant but also extending to more exotic forms of Quintessence (physics), quintessence or other modified gravity schemes. A cosmological constant problem, sometimes called the "most embarrassing problem in physics", results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from Planck units.

## Dark matter

During the 1970s and the 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter has always been controversial, it is inferred by various observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray astronomy, X-ray measurements of galaxy clusters. Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway. * Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the dwarf galaxy problem and the cuspy halo problem. Alternative theories have been proposed that do not require a large amount of undetected matter, but instead modify the laws of gravity established by Newton and Einstein; yet no alternative theory has been as successful as the cold dark matter proposal in explaining all extant observations.

## Horizon problem

The horizon problem results from the premise that information cannot travel Faster-than-light, faster than light. In a universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in causality (physics), causal contact. The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature. A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic Scalar field, scalar energy field dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes Exponential growth, exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation. Heisenberg's uncertainty principle predicts that during the inflationary phase there would be primordial fluctuations, quantum thermal fluctuations, which would be magnified to a cosmic scale. These fluctuations served as the seeds for all the current structures in the universe. Inflation predicts that the primordial fluctuations are nearly Scale invariance, scale invariant and Normal distribution, Gaussian, which has been accurately confirmed by measurements of the CMB. If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon. A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before Higgs mechanism, electroweak symmetry breaking occurs, so inflation should not be able to prevent large-scale discontinuities in the False vacuum, electroweak vacuum since distant parts of the observable universe were causally separate when 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 ...
ended.

## Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand Unified Theory, Grand Unified theories (GUTs) predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that no monopoles have been found. This problem is resolved by cosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness.

## Flatness problem

The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW. The universe may have positive, negative, or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density; positive if greater; and zero at the critical density, in which case space is said to be ''flat''. Observations indicate the universe is consistent with being flat. The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; however, our universe remained close to flat for several billion years before the dark energy density became significant. Given that a natural timescale for departure from flatness might be the Planck time, 10−43 seconds, the fact that the universe has reached neither a heat death of the universe, heat death nor a Big Crunch after billions of years requires an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 1014 of its critical value, or it would not exist as it does today.

# Ultimate fate of the universe

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a Big Crunch. Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out, leaving white dwarfs, neutron stars, and black holes. Collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would very gradually asymptotically approach absolute zero—a Future of an expanding universe, Big Freeze. Moreover, if protons are proton decay, unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.. Modern observations of accelerating expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the universe expands and cools. Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.

# Misconceptions

One of the common misconceptions about the Big Bang model is that it fully explains the Cosmogony, origin of the universe. However, the Big Bang model does not describe how energy, time, and space were caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state. It is misleading to visualize the Big Bang by comparing its size to everyday objects. When the size of the universe at Big Bang is described, it refers to the size of the observable universe, and not the entire universe. Hubble's law predicts that galaxies that are beyond Hubble distance recede faster than the speed of light. However, special relativity does not apply beyond motion through space. Hubble's law describes velocity that results from expansion ''of'' space, rather than ''through'' space. Astronomers often refer to the cosmological redshift as a Doppler shift which can lead to a misconception. Although similar, the cosmological redshift is not identical to the classically derived Doppler redshift because most elementary derivations of the Doppler redshift do not accommodate the expansion of space. Accurate derivation of the cosmological redshift requires the use of general relativity, and while a treatment using simpler Doppler effect arguments gives nearly identical results for nearby galaxies, interpreting the redshift of more distant galaxies as due to the simplest Doppler redshift treatments can cause confusion.

# Pre–Big Bang cosmology

The Big Bang explains the evolution of the universe from a starting density and temperature that is well beyond humanity's capability to replicate, so extrapolations to the most extreme conditions and earliest times are necessarily more speculative. Lemaître called this initial state the "''primeval atom''" while Gamow called the material "''ylem''". How the initial state of the universe originated is still an open question, but the Big Bang model does constrain some of its characteristics. For example, specific scientific law, laws of nature most likely came to existence in a random way, but as inflation models show, some combinations of these are far more probable. A topologically flat universe implies a balance between Gravitational energy, gravitational potential energy and other energy forms, requiring no additional energy to be created. The Big Bang theory, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time, and such an infinite energy density may be a physical impossibility. However, the physical theories of general relativity and quantum mechanics as currently realized are not applicable before the Planck epoch, and correcting this will require the development of a correct treatment of quantum gravity. Certain quantum gravity treatments, such as the Wheeler–DeWitt equation, imply that time itself could be an Emergence#Nonliving, physical systems, emergent property. As such, physics may conclude that time did not exist before the Big Bang. While it is not known what could have preceded the hot dense state of the early universe or how and why it originated, or even whether such questions are sensible, speculation abounds on the subject of "cosmogony". Some speculative proposals in this regard, each of which entails untested hypotheses, are: * The simplest models, in which the Big Bang was caused by
quantum fluctuation In quantum physics Quantum mechanics is a fundamental 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 m ...
s. That scenario had very little chance of happening, but, according to the totalitarian principle, even the most improbable event will eventually happen. It took place instantly, in our perspective, due to the absence of perceived time before the Big Bang. *Models including the Hartle–Hawking state, Hartle–Hawking no-boundary condition, in which the whole of spacetime is finite; the Big Bang does represent the limit of time but without any singularity. In such case, the universe is self-sufficient. * Brane cosmology models, in which inflation is due to the movement of branes in string theory; the pre-Big Bang model; the Ekpyrotic universe, ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model the Big Bang was preceded by a Big Crunch and the universe cycles from one process to the other. * * Eternal inflation, in which universal inflation ends locally here and there in a random fashion, each end-point leading to a ''bubble universe'', expanding from its own big bang. Proposals in the last two categories see the Big Bang as an event in either a much larger and Roger Penrose#An earlier universe, older universe or in a multiverse.

# Religious and philosophical interpretations

As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy. As a result, it has become one of the liveliest areas in the discourse between Relationship between religion and science, science and religion. Some believe the Big Bang implies a creator, * * while others argue that Big Bang cosmology makes the notion of a creator superfluous.

* * * * * * * , a Big Bang speculation * * . Also known as the Big Chill and the Big Freeze * * , a discredited theory that denied the Big Bang and posited that the universe always existed.

# References

## Bibliography

* * "Reprinted from ''Astrophysics and Space Science'' Volumes 269–270, Nos. 1–4, 1999". * * * * * * * * * * * * * * * * * * * * * * "Lectures presented at the XX Canary Islands Winter School of Astrophysics, held in Tenerife, Spain, November 17–18, 2008." * * * * * * "Symposium held in Dallas, Tex., Dec. 11-16, 1988." * The 2004 edition of the book is available from th
Internet Archive
Retrieved 20 December 2019. * * * * * * * * *

* * * * * * * 1st edition is available from th
Internet Archive
Retrieved 23 December 2019.