According to the Big Bang model, the universe expanded from an extremely dense and hot state and continues to expand today. The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy A common and useful analogy explains that space itself is expanding, carrying galaxies with it, like raisins in a rising loaf of bread. Space is the extent within which Matter is physically extended and objects and Events have positions relative to one another A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter General relativistic cosmologies, however, do not actually ascribe any 'physicality' to space.

The Big Bang is a cosmological model of the universe that has become well supported by several independent observations. Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy After Edwin Hubble discovered that galactic distances were generally proportional to their redshifts in 1929, this observation was taken to indicate that the universe is expanding. Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by [1] If the universe is seen to be expanding today, then it must have been smaller, denser, and hotter in the past. This idea has been considered in detail all the way back to extreme densities and temperatures, and the resulting conclusions have been found to conform very closely to what is observed.

Ironically, the term 'Big Bang' was first coined by Fred Hoyle in a derisory statement seeking to belittle the credibility of the theory that he did not believe to be true. Sir Fred Hoyle FRS ( 24 June, 1915  &ndash 20 August, 2001) was an English Astronomer primarily [2] However, the discovery of the cosmic microwave background in 1964 was taken as almost undeniable support for the Big Bang.

Further evidence supporting the Big Bang model comes from the relative proportion of light elements in the universe. The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy The observed abundances of hydrogen and helium throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis. In Physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than those of H-1 (i

However, there are mysteries of the universe that are not explained by the Big Bang model alone. For example, a region of the universe 12 billion lightyears distant in one direction appears little different than a region 12 billion lightyears distant in the opposite direction. But since the universe is 'only' around 13. 7 billion years old, it would appear these regions could never have been causally connected. Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in How, then, can they be so similar? Alan Guth's 1981 theory of cosmic inflation, a short, sudden burst of extreme exponential expansion in the very early universe, provided an explanation for this horizon problem and several of the features unaccounted for by the original Big Bang model. Alan Harvey Guth (born February 27, 1947) is a Theoretical physicist and Cosmologist. In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that The horizon problem is a problem with the standard cosmological model of the Big Bang which was identified in the 1970s The successor to Guth's original theory has found some circumstantial support, but it is not yet nearly as well supported as the Big Bang model.

 Physical cosmology Universe · Big BangAge of the universeTimeline of the Big BangUltimate fate of the universe This box: view • talk • edit

## History

The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy The age of the Universe is the time elapsed between the theory of the Big Bang and the present day This timeline of the Big Bang describes the events according to the Scientific theory of the Big Bang, using the cosmological time parameter of Comoving coordinates The ultimate fate of the universe is a topic in Physical cosmology. The history of the Big Bang theory began with the Big Bang 's development from observations and theoretical considerations This timeline of cosmological theories and discoveries is a chronological catalog of the evolution of humankind's understanding of the Cosmos over the last two-plus Astronomy is the oldest of the Natural sciences dating back to antiquity, with its origins in the religious, Mythological, and Astrological In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. Vesto Melvin Slipher ( November 11, 1875 &ndash November 8, 1969) was an American Astronomer. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way. This is about the infamous discussion of astronomy For the Dream Theater song about Stem cell research, see The Great Debate (song. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply [3] Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein's equations of general relativity, showing that the universe might be expanding in contrast to the static universe model advocated by Einstein. Alexander Alexandrovich Friedman or Friedmann (Александр Александрович Фридман ( June 16 1888, Saint Petersburg, Imperial Russia (Россия Rossiya) or the Russian Federation ( Rossiyskaya Federatsiya) is a transcontinental Country extending Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its A mathematician is a person whose primary area of study and research is the field of Mathematics. The Friedmann equations are a set of Equations in cosmology that govern the expansion of space in homogeneous and isotropic models Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical The Einstein field equations ( EFE) or Einstein's equations are a set of ten equations in Einstein 's theory of General relativity in which the General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 The idea of a static universe or "Einstein's universe" is one which demands that space is not expanding nor contracting but rather is dynamically stable [4] In 1924, Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, predicted that the recession of the nebulae was due to the expansion of the universe. Georges Henri Joseph Édouard Lemaître ( July 17, 1894 &ndash June 20, 1966) was a Belgian Roman Catholic Priest [5]

In 1931 Lemaître went further and suggested that the evident expansion in forward time required that the universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the universe into a single point, a "primeval atom", at a point in time before which time and space did not exist. Georges Henri Joseph Édouard Lemaître ( July 17, 1894 &ndash June 20, 1966) was a Belgian Roman Catholic Priest History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny As such, at this point, the fabric of time and space had not yet come into existence. This perhaps echoed previous speculations about the cosmic egg origin of the universe. The cosmic egg is an ancient concept resurrected by modern Science in the 1930s and explored by theoreticians during the following two decades [6]

Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2,500 mm) Hooker telescope at Mount Wilson Observatory. The cosmic distance ladder (also known as the Extragalactic Distance Scale) is the succession of methods by which astronomers determine the Distances to celestial The Mount Wilson Observatory (MWO is an Astronomical observatory in Los Angeles County California. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by In 1929, Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance [1][7] Lemaître had already shown that this was expected, given the Cosmological Principle. Georges Henri Joseph Édouard Lemaître ( July 17, 1894 &ndash June 20, 1966) was a Belgian Roman Catholic Priest The cosmological principle is an assumption invoked in Cosmology that when applied severely restricts the large variety of possible cosmological theories [8]

Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang.

During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model,[9] the oscillatory universe (originally suggested by Friedmann, but advocated by Einstein and Richard Tolman)[10] and Fritz Zwicky's tired light hypothesis. A non-standard cosmology is any physical cosmological model of the universe that has been or still is proposed as an alternative to the Big bang model The Milne model was a special relativistic cosmological model proposed by Edward Arthur Milne. The oscillatory universe is a Cosmological model, originally derived by Alexander Friedman in 1922 investigated briefly by Einstein in 1930 and critiqued Richard Chace Tolman ( March 4 1881 &ndash September 5 1948) was an American mathematical physicist and Physical chemist Fritz Zwicky ( February 14 1898 &ndash February 8 1974) was an American-based Swiss Astronomer of Bulgarian origin Tired light is a class of hypothetical Redshift mechanisms that were proposed as an alternative explanation for the redshift-distance relationship as alternatives [11]

After World War II, two distinct possibilities emerged. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including One was Fred Hoyle's steady state model, whereby new matter would be created as the universe seemed to expand. Sir Fred Hoyle FRS ( 24 June, 1915  &ndash 20 August, 2001) was an English Astronomer primarily In cosmology, the Steady State theory (also known as the Infinite Universe theory or continuous creation) is a model developed in 1948 by Fred In this model, the universe is roughly the same at any point in time. [12] The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis[13] and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background (CMB). Georges Henri Joseph Édouard Lemaître ( July 17, 1894 &ndash June 20, 1966) was a Belgian Roman Catholic Priest George Gamow (pronounced as ˈgamof ( March 4, 1904 &ndash August 19, 1968), born Georgiy Antonovich Gamov (Георгий Антонович Ralph Asher Alpher ( February 3, 1921 - August 12, 2007) was a U Robert Herman ( August 29, 1914 - February 13, 1997) was a United States scientist best known for his work with Ralph Alpher [14] It is an irony that it was Hoyle who coined the name that would come to be applied to Lemaître's theory, referring to it as "this big bang idea" in derision during a 1950 BBC radio broadcast. [15][16] For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor the latter. The source counts distribution of radio-sources from a radio-astronomical survey is the cumulative distribution of the number of sources ( N) brighter than The discovery of the cosmic microwave background radiation in 1964[17] secured the Big Bang as the best theory of the origin and evolution of the cosmos. 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.

Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as the analysis of copious data from satellites such as COBE,[18] the Hubble Space Telescope and WMAP. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into [19] Cosmologists now have fairly precise measurement 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.

## Overview

### Timeline of the Big Bang

A graphical timeline is available here:
Graphical timeline of the Big Bang

Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This timeline of the Big Bang describes the events according to the Scientific theory of the Big Bang, using the cosmological time parameter of Comoving coordinates Main source article Timeline of the Big Bang This timeline of the Big Bang shows the sequence of events as predicted by the Big Bang theory from the General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature [20] This singularity signals the breakdown of general relativity. A gravitational singularity (sometimes spacetime singularity) is approximately a place where quantities which are used to measure the Gravitational field become How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch. In Physical cosmology, the Planck epoch (or Planck era) named after Max Planck, is the earliest period of Time in the history of the Universe The early hot, dense phase is itself referred to as "the Big Bang",[21] and is considered the "birth" of our universe. Based on measurements of the expansion using Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13. A Type Ia supernova is a sub-category of cataclysmic Variable Correlation functions contain information about the distribution of points or events or things across some space/time 73 ± 0. 12 billion years old[22]. The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the universe. &LambdaCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter.

The earliest phases of the Big Bang are subject to much speculation. In the most common models, the universe was filled homogeneously and isotropically with an incredibly high energy density, huge temperatures and pressures, and was very rapidly expanding and cooling. In Mathematics, particularly in the theories of Lie groups Algebraic groups and Topological groups a homogeneous space for a group Isotropy is uniformity in all directions Precise definitions depend on the subject area Energy density is the amount of Energy stored in a given system or region of space per unit Volume, or per unit Mass, depending on the context although Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Approximately 10−35 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially. In Thermodynamics, phase transition or phase change is the transformation of a thermodynamic system from one phase to another In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that Exponential growth (including Exponential decay) occurs when the growth rate of a mathematical function is proportional to the function's current value [23] After inflation stopped, the universe consisted of a quark-gluon plasma, as well as all other elementary particles. A quark-gluon plasma (QGP is a phase of Quantum chromodynamics (QCD which exists at extremely high Temperature and/or Density. In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made [24] Temperatures were so high that the random motions of particles were at relativistic speeds, and particle-antiparticle pairs of all kinds were being continuously created and destroyed in collisions. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial See also Electron-positron annihilation Meitner–Hupfeld effect Pair instability supernova At some point an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and anti-leptons—of the order of 1 part in 30 million. In Physical cosmology, baryogenesis is the generic term for hypothetical physical processes that produced an asymmetry between Baryons and anti-baryons in In Particle physics, the baryon number is an approximate conserved Quantum number of a system In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. Leptons are a family of fundamental Subatomic particles comprising the Electron, the Muon, and the Tauon (or tau particle as well as their This resulted in the predominance of matter over antimatter in the present universe. Matter is commonly defined as being anything that has mass and that takes up space. In Particle physics and Quantum chemistry, antimatter is the extension of the concept of the Antiparticle to Matter, where antimatter is composed [25]

The universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. In Theoretical physics, explicit symmetry breaking is the breaking of a Symmetry of a Theory by terms in its defining equations of motion (most In Physics, a fundamental interaction or fundamental force is a mechanism by which particles interact with each other and which cannot be explained in terms In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made [26] After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. Baryons are the family of Subatomic particles with a Baryon number of 1 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 1010 of the original protons and neutrons, and none of their antiparticles. 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 photons (with a minor contribution from neutrinos). In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost

A few minutes into the expansion, when the temperature was about a billion (one thousand million; 109; SI prefix giga) Kelvin and the density was about that of air, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. For other meanings see Giga (disambiguation Giga- (symbol G is a prefix in the SI system of units denoting 109 The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom In Physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than those of H-1 (i [27] Most protons remained uncombined as hydrogen nuclei. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. Gravitation is a natural Phenomenon by which objects with Mass attract one another Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 This relic radiation is known as the cosmic microwave background radiation. [28]

The Hubble Ultra Deep Field showcases galaxies from an ancient era when the universe was younger, denser, and warmer according to the Big Bang theory. The Hubble Ultra Deep Field, or HUDF, is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope

Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The details of this process depend on the amount and type of matter in the universe. The three possible types of matter are known as cold dark matter, hot dark matter and baryonic matter. Cold dark matter (or CDM) is a refinement of the Big bang theory that contains the additional assumption that most of the matter in the Universe consists Hot dark matter is a Hypothetical form of Dark matter which consists of particles that travel with Ultrarelativistic velocities Baryons are the family of Subatomic particles with a Baryon number of 1 The best measurements available (from WMAP) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 18% of the matter in the universe. [22].

Independent lines of evidence from Type Ia supernovae and the CMB imply the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. A Type Ia supernova is a sub-category of cataclysmic Variable In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe The observations suggest 72% 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 had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations 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 observationally and theoretically. In Physical cosmology, the cosmological constant (usually denoted by the Greek capital letter Lambda: Λ was proposed by Albert Einstein as a modification The Einstein field equations ( EFE) or Einstein's equations are a set of ten equations in Einstein 's theory of General relativity in which the In cosmology, the equation of state of a Perfect fluid is characterized by a Dimensionless number w, equal to the ratio of its Pressure The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles [8]

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. See also Cosmic inflation In Physical cosmology the inflationary epoch was the period in the evolution of the early universe when according to Inflation &LambdaCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. As noted above, there is no well supported model describing the action prior to 10−15 seconds or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics. This is a list of some of the major unsolved problems in Physics.

### Big bang theory assumptions

The Big Bang theory depends on two major assumptions: the universality of physical laws, and the Cosmological Principle. A physical law or scientific law is a Scientific generalization based on empirical Observations of physical behavior (i The cosmological principle is an assumption invoked in Cosmology that when applied severely restricts the large variety of possible cosmological theories The cosmological principle states that on large scales the universe is homogeneous and isotropic. In Mathematics, particularly in the theories of Lie groups Algebraic groups and Topological groups a homogeneous space for a group Isotropy is uniformity in all directions Precise definitions depend on the subject area

These ideas were initially taken as postulates, but today there are efforts 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 over much of the age of the universe is of order 10−5. The fine-structure constant or Sommerfeld fine-structure constant, usually denoted \alpha \ is the Fundamental physical constant characterizing The age of the Universe is the time elapsed between the theory of the Big Bang and the present day [29] Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 At its introduction in 1915 the general theory of relativity did not have a solid empirical foundation [30]

If the large-scale universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican Principle, which states that there is no preferred (or special) observer or vantage point. In Cosmology, the Copernican principle, named after Nicolaus Copernicus, states the Earth is not in a central specially favoured position To this end, the cosmological principle has been confirmed to a level of 10−5 via observations of the CMB. [31] The universe has been measured to be homogeneous on the largest scales at the 10% level. [32]

### FLRW metric

General relativity describes spacetime by a metric, which determines the distances that separate nearby points. The metric expansion of space is the averaged increase of metric (i In the mathematical field of Differential geometry, a metric tensor is a type of function defined on a Manifold (such as a Surface in space The points, which can be galaxies, stars, or other objects, themselves are specified using a coordinate chart or "grid" that is laid down over all spacetime. For other uses of "atlas" see Atlas (disambiguation. In Mathematics, particularly topology an atlas describes how SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann-Lemaître-Robertson-Walker metric (FLRW metric). In Mathematics, particularly in the theories of Lie groups Algebraic groups and Topological groups a homogeneous space for a group Isotropy is uniformity in all directions Precise definitions depend on the subject area This metric contains a scale factor, which describes how the size of the universe changes with time. A scale factor is a number which scales, or multiplies some quantity This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In Mathematics and its applications a coordinate system is a system for assigning an n - Tuple of Numbers or scalars to each point In standard cosmology, ' comoving' distance and ' proper distance' are two closely related distance measures used by cosmologists to define distances between In this coordinate system, the grid expands along with the universe, and objects that are moving only due to the expansion of the universe remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such comoving points expands proportionally with the scale factor of the universe. In standard cosmology, ' comoving' distance and ' proper distance' are two closely related distance measures used by cosmologists to define distances between A scale factor is a number which scales, or multiplies some quantity [33]

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distance between two comoving points. The metric expansion of space is the averaged increase of metric (i 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 are gravitationally bound and as such do not experience the large-scale expansion of space.

### Horizons

Main article: Cosmological horizon

An important feature of the Big Bang spacetime is the presence of horizons. In Physical cosmology, a cosmological horizon marks a limit to observability and marks the boundary of a region that an observer cannot see into directly In Physical cosmology, a cosmological horizon marks a limit to observability and marks the boundary of a region that an observer cannot see into directly Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not 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 was a past horizon, though in practice our view is limited by the opacity of the universe at early times. The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. If the expansion of the universe continues to accelerate, there is a future horizon as well. The accelerating universe is the observation that the universe appears to be expanding at an accelerated rate [34]

## Observational evidence

The earliest and most direct kinds of observational evidence are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements (see Big Bang nucleosynthesis). Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by In Physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than those of H-1 (i These are sometimes called the three pillars of the big bang theory. Many other lines of evidence now support the picture, notably various properties of the large-scale structure of the cosmos[35] which are predicted to occur due to gravitational growth of structure in the standard Big Bang theory. In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light

### Hubble's law and the expansion of space

$v = H_0 D \,$

where

v is the recessional velocity of the galaxy or other distant object
D is the proper distance to the object and
H0 is Hubble's constant, measured to be 70. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance In Physics, velocity is defined as the rate of change of Position. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter In relativistic Physics, proper Length is an invariant quantity which is the rod Distance between Spacelike Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance 1 ± 1. 3 km/s/Mpc by the WMAP probe. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 [22]

Hubble's law has two possible explanations. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance Either we are at the center of an explosion of galaxies—which is untenable given the Copernican Principle—or the universe is uniformly expanding everywhere. In Cosmology, the Copernican principle, named after Nicolaus Copernicus, states the Earth is not in a central specially favoured position The metric expansion of space is the averaged increase of metric (i This universal expansion was predicted from general relativity by Alexander Friedman in 1922[4] and Georges Lemaître in 1927,[5] 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. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Alexander Alexandrovich Friedman or Friedmann (Александр Александрович Фридман ( June 16 1888, Saint Petersburg, Imperial Georges Henri Joseph Édouard Lemaître ( July 17, 1894 &ndash June 20, 1966) was a Belgian Roman Catholic Priest

The theory requires the relation v = HD to hold at all times, where D is the proper distance, v = dD / dt, and v, H, and D all vary as the universe expands (hence we write H0 to denote the present-day Hubble "constant"). In relativistic Physics, proper Length is an invariant quantity which is the rod Distance between Spacelike For distances much smaller than the size of the observable universe, 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. [36]

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. The metric expansion of space is the averaged increase of metric (i The cosmological principle is an assumption invoked in Cosmology that when applied severely restricts the large variety of possible cosmological theories Astronomical redshifts are extremely isotropic and homogenous,[1] supporting the Cosmological Principle that the universe looks the same in all directions, along with much other evidence. In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by Isotropy is uniformity in all directions Precise definitions depend on the subject area 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 the Earth is not in a central position, on a cosmological scale. In Cosmology, the Copernican principle, named after Nicolaus Copernicus, states the Earth is not in a central specially favoured position [37] Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the cosmic microwave background 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.

WMAP image of the cosmic microwave background radiation

During the first few days of the universe, the universe was in full thermal equilibrium, with photons being continually emitted and absorbed, giving the radiation a blackbody spectrum. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and In Physics, a black body is an object that absorbs all light that falls on it As the universe expanded, it cooled to a temperature at which photons could no longer be created or destroyed. The temperature was still high enough for electrons and nuclei to remain unbound, however, and photons were constantly "reflected" from these free electrons through a process called Thomson scattering. In Physics, Thomson scattering is the scattering of Electromagnetic radiation by acharged particle Because of this repeated scattering, the early universe was opaque to light.

When the temperature fell to a few thousand Kelvin, electrons and nuclei began to combine to form atoms, a process known as recombination. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic This timeline of the Big Bang describes the events according to the Scientific theory of the Big Bang, using the cosmological time parameter of Comoving coordinates Since photons scatter infrequently from neutral atoms, radiation decoupled from matter when nearly all the electrons had recombined, at the epoch of last scattering, 379,000 years after the Big Bang. These photons make up the CMB that is observed today, and the observed pattern of fluctuations in the CMB is a direct picture of the universe at this early epoch. The energy of photons was subsequently redshifted by the expansion of the universe, which preserved the blackbody spectrum but caused its temperature to fall, meaning that the photons now fall into the microwave region of the electromagnetic spectrum. Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies The radiation is thought to be observable at every point in the universe, and comes from all directions with (almost) the same intensity.

In 1964, Arno Penzias and Robert Wilson accidentally discovered the cosmic background radiation while conducting diagnostic observations using a new microwave receiver owned by Bell Laboratories. Arno Allan Penzias (born April 26, 1933) is an American Physicist and Nobel laureate in physics. Robert Woodrow Wilson (born January 10, 1936) is an American Astronomer, Nobel laureate in physics, who with Arno Allan Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 Bell Laboratories (also known as Bell Labs and formerly known as AT&T Bell Laboratories and Bell Telephone Laboratories) is the Research organization [17] Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K—and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded a Nobel Prize for their discovery. The Nobel Prize (Nobelpriset (Nobelprisen is a Swedish prize established in the 1895 will of Swedish chemist Alfred Nobel; it was first awarded in Peace, Literature

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program COBE found a residual temperature of 2. 726 K and in 1992 detected for the first time the fluctuations (anisotropies) in the CMB, at a level of about one part in 105. [18] John C. Mather and George Smoot were awarded Nobels for their leadership in this work. John Cromwell Mather (b August 7, 1946, Roanoke, Virginia) is an American astrophysicist, cosmologist and George Fitzgerald Smoot III (born February 20 1945) is an American astrophysicist, cosmologist and Nobel Prize in Physics 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, found the universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies. The BOOMERanG experiment ( B alloon O bservations O f M illimetric E xtragalactic R adiation an d G eophysics (See shape of the universe. The shape of the Universe is an informal name for a subject of investigation within Physical cosmology which describes the Geometry of the Universe )

In early 2003, the first results of the Wilkinson Microwave Anisotropy satellite (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. This satellite also disproved several specific cosmic inflation models, but the results were consistent with the inflation theory in general[19], it confirms too that a sea of cosmic neutrinos permeates the universe, a clear evidence that the first stars took more than a half-billion years to create a cosmic fog. In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that The cosmic neutrino background (CνB is the universe's background particle radiation composed of Neutrinos Like the Cosmic microwave background radiation Another satellite like it will be launched within the next few years, the Planck Surveyor, which will provide even more accurate measurements of the CMB anisotropies. The Planck satellite is a Spacecraft built in the Cannes Mandelieu Space Center, that is designed to observe the anisotropies of the cosmic microwave Many other ground- and balloon-based experiments are also currently running; see Cosmic microwave background experiments. See also Discovery of the cosmic microwave background There have been a variety of Experiments to measure the Cosmic microwave background radiation anisotropies

The background radiation is exceptionally smooth, which presented a problem in that conventional expansion would mean that photons coming from opposite directions in the sky were coming from regions that had never been in contact with each other. The leading explanation for this far reaching equilibrium is that the universe had a brief period of rapid exponential expansion, called inflation. In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that This would have the effect of driving apart regions that had been in equilibrium, so that all the observable universe was from the same equilibrated region. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and

### Abundance of primordial elements

Using the Big Bang model it is possible to calculate the concentration of helium-4, helium-3, deuterium and lithium-7 in the universe as ratios to the amount of ordinary hydrogen, H. In Physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than those of H-1 (i Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 [27] All the abundances depend on a single parameter, the ratio of photons to baryons, which itself can be calculated independently from the detailed structure of CMB fluctuations. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Baryons are the family of Subatomic particles with a Baryon number of 1 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. [27]

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 a factor of two off for 7Li; in the latter two cases there are substantial systematic uncertainties. Systematic errors are Biases in Measurement which lead the situation where the Mean of many separate measurements differs Significantly 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. [38] Indeed there is no obvious reason outside of the Big Bang that, for example, the young universe (i. e. , before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than ³He, and in constant ratios, too. Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in Stars to build the nuclei of the heavier elements.

### Galactic evolution and distribution

This panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light Structure formation refers to a fundamental problem in Physical cosmology. The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning the formation of the first galaxies the way Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply The galaxies are color coded by redshift. In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by

Detailed observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. 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 clusters and superclusters. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the Cosmos. 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. Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. [39]

### Other lines of evidence

After some controversy, the age of universe as estimated from the Hubble expansion and the CMB is now in good agreement with (i. e. , slightly larger than) the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. Radiometric dating (often called radioactive dating) is a technique used to date materials usually based on a comparison between the observed abundance of a naturally occurring In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of temperature-sensitive emission 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; this seems to be roughly true, but unfortunately the amplitude does depend on cluster properties which do change substantially over cosmic time, so a precise test is impossible. The Sunyaev-Zel'dovich effect (often abbreviated as the SZ effect) is the result of high energy Electrons distorting the Cosmic microwave background radiation Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation

## Features, issues and problems

While very few researchers now doubt the Big Bang occurred, the scientific community was once divided between supporters of the Big Bang and those of alternative cosmological models. A non-standard cosmology is any physical cosmological model of the universe that has been or still is proposed as an alternative to the Big bang model Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the cosmological observations (see the history section above). Observational cosmology is the study of the structure the evolution and the origin of the Universe through Observation, using instruments such as Telescopes With the overwhelming consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations. Scientific consensus is the collective judgement position and Opinion of the community of Scientists in a particular field of Science at a particular Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as it is anticipated that they can be solved through further refinements of the theory. The cuspy halo problem arises from cosmological simulations that seem to indicate Cold dark matter would form Cuspy distributions &mdash that is increasing sharply The dwarf galaxy problem is one that arises from numerical cosmological Simulations that predict the evolution of the distribution of Matter in the Cold dark matter (or CDM) is a refinement of the Big bang theory that contains the additional assumption that most of the matter in the Universe consists

The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, the formation of galaxies—are derived from many independent observations including Big Bang nucleosynthesis, the cosmic microwave background, large scale structure and Type Ia supernovae, and can hardly be doubted as important and real features of our universe. In Physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than those of H-1 (i In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light A Type Ia supernova is a sub-category of cataclysmic Variable

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. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them Of these features, dark energy and dark matter are considered the most secure, while inflation and baryogenesis remain speculative: they provide satisfying explanations for important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the theory. In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that In Physical cosmology, baryogenesis is the generic term for hypothetical physical processes that produced an asymmetry between Baryons and anti-baryons in [40] Explanations for such phenomena remain at the frontiers of inquiry in physics. This is a list of some of the major unsolved problems in Physics.

### Horizon problem

Main article: Horizon problem

The horizon problem results from the premise that information cannot travel faster than light. The horizon problem is a problem with the standard cosmological model of the Big Bang which was identified in the 1970s 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 causal contact. In Physical cosmology, particle horizon is the maximum distance from which particles could have traveled to the observer in the Age of the universe Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in [41] 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 these regions to have the same temperature.

A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period (before baryogenesis). In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that During inflation, the universe undergoes 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 quantum thermal fluctuations, which would be magnified to cosmic scale. In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain Primordial fluctuations are density variations in the early universe which are considered the seeds of all structure in the universe These fluctuations serve as the seeds of all current structure in the universe. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB. Primordial fluctuations are density variations in the early universe which are considered the seeds of all structure in the universe In Physics and Mathematics, scale invariance is a feature of objects or laws that do not change if length scales (or energy scales are multiplied by a common factor The normal distribution, also called the Gaussian distribution, is an important family of Continuous probability distributions applicable in many fields

### Flatness/oldness problem

Main article: Flatness problem
The overall geometry of the universe is determined by whether the Omega cosmological parameter is less than, equal to or greater than 1. The flatness problem is a cosmological Fine-tuning problem within the Big Bang model i The shape of the Universe is an informal name for a subject of investigation within Physical cosmology which describes the Geometry of the Universe The Friedmann equations are a set of Equations in cosmology that govern the expansion of space in homogeneous and isotropic models From top to bottom: a closed universe with positive curvature, a hyperbolic universe with negative curvature and a flat universe with zero curvature. The shape of the Universe is an informal name for a subject of investigation within Physical cosmology which describes the Geometry of the Universe The shape of the Universe is an informal name for a subject of investigation within Physical cosmology which describes the Geometry of the Universe The shape of the Universe is an informal name for a subject of investigation within Physical cosmology which describes the Geometry of the Universe

The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann-Lemaître-Robertson-Walker metric. [41] The universe may have positive, negative or zero spatial curvature depending on its total energy density. In Mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry 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. The Friedmann equations are a set of Equations in cosmology that govern the expansion of space in homogeneous and isotropic models The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat. [42] 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 nor a Big Crunch after billions of years requires some explanation. In Physics, the Planck time ( tP) is the unit of Time in the system of Natural units known as Planck units. The heat death is a possible final state of the universe, in which it has " run down " to a state of no Thermodynamic free energy to sustain In Physical cosmology, the Big Crunch is one possible scenario for the Ultimate fate of the universe, in which the Metric expansion of space eventually For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the universe must have been within one part in 1014 of the critical density, or it would not exist as it does today. [43]

A resolution to this problem is offered by inflationary theory. In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that During the inflationary period, spacetime expanded to such an extent that its curvature would have been smoothed out. In Mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry Thus, it is believed that inflation drove the universe to a very nearly spatially flat state, with almost exactly the critical density.

### Magnetic monopoles

Main article: Magnetic monopole

The magnetic monopole objection was raised in the late 1970s. In Physics, a magnetic monopole is a hypothetical particle that is a Magnet with only one pole (see Maxwell's equations for more on magnetic Grand unification theories predicted topological defects in space that would manifest as magnetic monopoles. Grand Unification, grand unified theory, or GUT refers to any of several very similar unified field theories or models in Physics that Also see base concepts Topology, Differential equations Quantum theory & Condensed matter physics. In Physics, a magnetic monopole is a hypothetical particle that is a Magnet with only one pole (see Maxwell's equations for more on magnetic These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that searches have never found any monopoles. This problem is also resolved by cosmic inflation, which removes all point defects from the observable universe in the same way that it drives the geometry to flatness. In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that [41]

A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic inflation is offered by the Weyl curvature hypothesis. The Weyl curvature hypothesis, which arises in the application of Albert Einstein 's General theory of relativity to Physical cosmology, was introduced [44][45]

### Baryon asymmetry

Main article: Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter. See also Baryogenesis The baryon asymmetry problem in Physics refers to the apparent fact that the Baryons in the universe which have been Matter is commonly defined as being anything that has mass and that takes up space. In Particle physics and Quantum chemistry, antimatter is the extension of the concept of the Antiparticle to Matter, where antimatter is composed [25] 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 anti-baryons. Baryons are the family of Subatomic particles with a Baryon number of 1 However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. An unknown process called "baryogenesis" created the asymmetry. In Physical cosmology, baryogenesis is the generic term for hypothetical physical processes that produced an asymmetry between Baryons and anti-baryons in For baryogenesis to occur, the Sakharov conditions must be satisfied. In Physical cosmology, baryogenesis is the generic term for hypothetical physical processes that produced an asymmetry between Baryons and anti-baryons in These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the universe depart from thermodynamic equilibrium. In Particle physics, the baryon number is an approximate conserved Quantum number of a system In Physics, C-symmetry means the symmetry of physical laws under a charge -conjugation transformation. In Particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and [46] All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles

### Globular cluster age

In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth 7-billion-year age of the universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. [47] There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the universe.

### Dark matter

Main article: Dark matter
A pie chart indicating the proportional composition of different energy-density components of the universe, according to the best ΛCDM model fits. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from A pie chart (or a circle graph) is a circular Chart divided into sectors illustrating relative magnitudes or frequencies or percents &LambdaCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. Roughly ninety-five percent is in the exotic forms of dark matter and dark energy

During the 1970s and 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. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe 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 Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from Baryons are the family of Subatomic particles with a Baryon number of 1 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. Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth While dark matter was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of X-ray astronomy is an observational branch of Astronomy, which deals with the study of X-ray emission from celestial objects [48]

The evidence for dark matter comes from its gravitational influence on other matter, and 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. Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them [49]

### Dark energy

Main article: Dark energy

Measurements of the redshiftmagnitude relation for type Ia supernovae have revealed that the expansion of the universe has been accelerating since the universe was about half its present age. In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value A Type Ia supernova is a sub-category of cataclysmic Variable The accelerating universe is the observation that the universe appears to be expanding at an accelerated rate 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". General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 In cosmology, the equation of state of a Perfect fluid is characterized by a Dimensionless number w, equal to the ratio of its Pressure In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe Dark energy is indicated by several other lines of evidence. 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 critical density of mass/energy. The Friedmann equations are a set of Equations in cosmology that govern the expansion of space in homogeneous and isotropic models 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. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different [8] Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by 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. A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light

Negative pressure is a property of vacuum energy, but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Vacuum energy is an underlying background Energy that exists in Space even when devoid of Matter (known as Free space) Possible candidates include a cosmological constant and quintessence. In Physical cosmology, the cosmological constant (usually denoted by the Greek capital letter Lambda: Λ was proposed by Albert Einstein as a modification In Physics, quintessence is a hypothetical form of Dark energy postulated as an explanation of observations of an Accelerating universe. Results from the WMAP team in 2008, which combined data from the CMB and other sources, indicate that the universe today is 72% dark energy, 23% dark matter, 4. 6% regular matter and less then 1% of neutrinos. [22] 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.

In the ΛCDM, the best current model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the general theory of relativity. &LambdaCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. In Physical cosmology, the cosmological constant (usually denoted by the Greek capital letter Lambda: Λ was proposed by Albert Einstein as a modification General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.

## The future according to the Big Bang theory

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. The ultimate fate of the universe is a topic in Physical cosmology. In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion 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. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different The Friedmann equations are a set of Equations in cosmology that govern the expansion of space in homogeneous and isotropic models It would become denser and hotter again, ending with a state that was similar to that in which it started—a Big Crunch. In Physical cosmology, the Big Crunch is one possible scenario for the Ultimate fate of the universe, in which the Metric expansion of space eventually [34] 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 as all the interstellar gas in each galaxy is consumed; stars would burn out leaving white dwarfs, neutron stars, and black holes. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would asymptotically approach absolute zero—a Big Freeze. Absolute zero is the point at which molecules do not move (relative to the rest of the body more than they are required to by a quantum mechanical effect called Zero-point The heat death is a possible final state of the universe, in which it has " run down " to a state of no Thermodynamic free energy to sustain Moreover, if the proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. In Particle physics, proton decay is a hypothetical form of Radioactive decay in which the Proton decays into lighter Subatomic particles Eventually, black holes would evaporate by emitting Hawking radiation. Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes 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. In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy The heat death is a possible final state of the universe, in which it has " run down " to a state of no Thermodynamic free energy to sustain

Modern observations of accelerated expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The accelerating universe is the observation that the universe appears to be expanding at an accelerated rate In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. &LambdaCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe In Physical cosmology, the cosmological constant (usually denoted by the Greek capital letter Lambda: Λ was proposed by Albert Einstein as a modification This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the universe expands and cools. The heat death is a possible final state of the universe, in which it has " run down " to a state of no Thermodynamic free energy to sustain Other explanations of dark energy—so-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. Phantom energy is a hypothetical form of Dark energy with equation of state w. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation The Big Rip is a cosmological Hypothesis first published in 2003 about the Ultimate fate of the universe, in which the matter of the universe [50]

## Speculative physics beyond the Big Bang

A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metric seen on the left. In the mathematical field of Differential geometry, a metric tensor is a type of function defined on a Manifold (such as a Surface in space
Image from WMAP press release, 2006.

While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the universe's history. The Penrose-Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time. The Penrose-Hawking singularity theorems are a set of results in General relativity which attempt to answer the question of whether gravity is necessarily singular However, these theorems assume that general relativity is correct, but general relativity must break down before the universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the singularity. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 The Planck temperature, named after German Physicist Max Planck, is the unit of Temperature, denoted by TP in the system of Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature [51]

There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.

Some proposals, each of which entails untested hypotheses, are:

• models including the Hartle-Hawking no-boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity. In Theoretical physics, the Hartle-Hawking state, named after James Hartle and Stephen Hawking, is the Wave function of the Universe [52]
• brane cosmology models[53] in which inflation is due to the movement of branes in string theory; the pre-big bang model; the 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. Brane cosmology refers to several theories in Particle physics and cosmology motivated by but not exclusively derived from Superstring theory and String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings The ekpyrotic universe, or ekpyrotic scenario, is a cosmological model about the origin and shape of the Universe. Cyclic model refers to several Cosmological models in which the Universe follows infinite self-sustaining cycles (for example an Eternity of Big [54][55][56]
• chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, each leading to a bubble universe expanding from its own big bang. The Chaotic Inflation theory is a variety of the Inflationary universe model which is itself an outgrowth (or extension of the Big bang theory [57]

Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse, and not the literal beginning. The multiverse (or meta-universe) is the hypothetical set of multiple possible Universes (including our universe that together comprise all of Reality.

## Philosophical and religious interpretations

The Big Bang is a scientific theory, and as such stands or falls by its agreement with observations. Since the creation of the Big Bang theory many religious interpretations of the Big Bang theory of Physical cosmology have been offered But as a theory which addresses, or at least seems to address, the origins of reality, it has always been entangled with theological and philosophical implications. In the 1920s and '30s almost every major cosmologist preferred an eternal 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. In cosmology, the Steady State theory (also known as the Infinite Universe theory or continuous creation) is a model developed in 1948 by Fred [58] This perception was enhanced by the fact that Georges Lemaître, who put the theory forth, was a Roman Catholic priest. Georges Henri Joseph Édouard Lemaître ( July 17, 1894 &ndash June 20, 1966) was a Belgian Roman Catholic Priest A priest or priestess is a person having the authority or power to administer religious rites in particular rites of sacrifice to and propitiation of a deity or deities

## Arguments

Some arguments for the Big Bang model include the fact that the universe is continually expanding, as well as evidence that the universe was in a very hot state long ago. The metric expansion of space is the averaged increase of metric (i The model is often used in conjunction with the theory of evolution, inferring that not only life, but all of the universe, has evolved. Although widely supported by atheists, many theists also endorse the theory, as long as it can co-exist along with their Creator theory. Atheism Theism, in its most inclusive usage is the belief in at least one Deity. A creator deity is a Deity in a Creation myth responsible for the creation of the World (or Universe)

Those questioning the Big Bang theory often try to disprove it through a series of questions, each asking what caused something, leading to "what caused the Big Bang", with the rhetorical answer being the "uncaused cause", a God. The cosmological argument is an Argument for the Existence of God or a " First Cause " God is the principal or sole Deity in Religions and other belief systems that worship one deity. Claimed scientific chinks in the Bang theory's armor are that of the above-mentioned flat/oldness problem and the horizon problem, although they deal more with miscalculations of the model rather than there not being one at all.

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### Books

• Kolb, Edward; Michael Turner (1988). The Early Universe. Addison-Wesley. ISBN 0-201-11604-9.
• Peacock, John (1999). Cosmological Physics. Cambridge University Press. ISBN 0521422701.