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Encyclopedia > Big Bang
According to the Big Bang model, the universe developed from an extremely dense and hot state. Space itself has been expanding ever since, carrying galaxies (and all other matter) with it.

The Big Bang is the cosmological model of the universe whose primary assertion is that the universe has expanded into its current state from a primordial condition of enormous density and temperature. The term is also used in a narrower sense to describe the fundamental "fireball" that erupted at or close to an initial timepoint in the history of our observed spacetime.[1] The Big Bang is the scientific theory that the universe expanded rapidly from an enormously dense state some 14 billion years ago. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... For other uses, see Universe (disambiguation). ... Space has been an interest for philosophers and scientists for much of human history. ... For other uses, see Galaxy (disambiguation). ... This article is about the physics subject. ... For other uses, see Universe (disambiguation). ... The metric expansion of space is a key part of sciences current understanding of the universe, whereby space itself is described by a metric which changes over time. ... Primordial elements are chemical elements found on the earth that have existed in their current form since before the earth was formed, according to the big bang theory. ... For other uses, see Density (disambiguation). ... For other uses, see Temperature (disambiguation). ... For other uses of this term, see Spacetime (disambiguation). ...

Theoretical support for the Big Bang comes from mathematical models, called Friedmann models. These models show that a Big Bang is consistent with general relativity and with the cosmological principle, which states that the properties of the universe should be independent of position or orientation. // The Friedmann-LemaÃ®tre-Robertson-Walker (FLRW) metric is an exact solution of the Einstein field equations of general relativity and which describes a homogeneous, isotropic expanding/contracting universe. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... The Cosmological Principle is a principle invoked in cosmology that severely restricts the large variety of possible cosmological theories: On large scales, the Universe is homogeneous and isotropic. ...

## History

Main article: History of the Big Bang theory

Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100 inch Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law.[6][7] Lemaître had already shown that this was expected, given the cosmological principle.[8] This article is in need of attention from an expert on the subject. ... The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California. ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared with that of the Sun (left). ... Hubbles law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. ... The Cosmological Principle is a principle invoked in cosmology that severely restricts the large variety of possible cosmological theories: On large scales, the Universe is homogeneous and isotropic. ...

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.[11] WMAP satellite artist depiction from NASA Believed to be in the public domain. ... WMAP satellite artist depiction from NASA Believed to be in the public domain. ... Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ... A non-standard cosmology is a cosmological idea that contradicts the standard model of cosmology. ... Milnes model follows the description from special relativity of an observable universes spacetime diagram containing past and future light cones along with elsewhere in spacetime. ... The oscillatory universe is a cosmological model, originally derived by Alexander Friedman in 1922 and developed by Richard Tolman from 1934, in which the universe undergoes a series of oscillations, each beginning with a big bang and ending with a big crunch. ... Richard C. Tolman was California Institute of Technology professor of physical chemistry and mathematical physics. ... Fritz Zwicky (February 14, 1898 â€“ February 8, 1974) was an American-based Swiss astronomer. ... Tired light is a class of hypothetical redshift mechanisms that were proposed as an alternative explanation for the redshift-distance relationship. ...

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.[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. This article does not cite any references or sources. ... The Cosmic Background Explorer (COBE), also referred to as Explorer 66, was the first satellite built dedicated to cosmology. ... The Hubble Space Telescope (HST) is a telescope in orbit around the Earth, named after astronomer Edwin Hubble. ... Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ...

## Overview

A graphical timeline is available here:

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.[20] This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch. 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 13.7 ± 0.2 billion years.[22] The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the universe. This article does not cite any references or sources. ... Image File history File links Timeline_icon. ... This timeline of the Big Bang shows the sequence of events as predicted by the Big Bang theory, from the beginning of the Planck Epoch to the end of the Epoch of Nucleosynthesis (and beginning of the Epoch of Galaxies). ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... For other uses, see Density (disambiguation). ... For other uses, see Temperature (disambiguation). ... In mathematics, a set is called finite if and only if there is a bijection between the set and some set of the form {1, 2, ..., n} where is a natural number. ... A gravitational singularity (sometimes spacetime singularity) is, approximately, a place where quantities which are used to measure the gravitational field become infinite. ... Named after Max Planck, in cosmology the Planck epoch (or Planck Era) is the earliest period of time in the history of the universe, from zero to 10-43 seconds (one Planck time), during which all four fundamental forces were unified and elementary particles did not yet exist. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... â€œCMBâ€ redirects here. ... For stochastic processes, including those that arise in statistical mechanics and Euclidean quantum field theory, a correlation function is the correlation between random variables at two different points in space or time. ... A pie chart indicating the proportional composition of different energy-density components of the universe. ...

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.[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. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. 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). Promotional picture Symmetry Breaking is a rock band from Northern New Jersey, in the United States. ... A fundamental interaction is a mechanism by which particles interact with each other, and which cannot be explained by another more fundamental interaction. ... In particle physics, an elementary particle is a particle of which other, larger particles are composed. ... Thousands of particles explode from the collision point of two relativistic (100 GeV per nucleon) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... Combinations of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet. ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... Neutrinos are elementary particles denoted by the symbol Î½. Travelling close to the speed of light, lacking electric charge and able to pass through ordinary matter almost undisturbed, they are extremely difficult to detect. ...

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.[27] Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 380,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation.[28] Look up giga- in Wiktionary, the free dictionary. ... For other uses, see Kelvin (disambiguation). ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... For other uses, see Helium (disambiguation). ... The nucleus of an atom is the very small dense region, of positive charge, in its centre consisting of nucleons (protons and neutrons). ... In cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... The term mass in special relativity is used in a couple of different ways, occasionally leading to a great deal of confusion. ... Gravity is a force of attraction that acts between bodies that have mass. ... Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... In physics, decoupling is the general phenomenon in which the interactions between some physical objects (such as elementary particles) disappear. ...

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. 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. 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 20% of the matter in the universe.[19] STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... Cold dark matter (or CDM) is a refinement of the big bang theory, as well as being one possible variation of the more generic Dark Matter theory. ... Hot dark matter is a form of dark matter, which consists of particles that travel with relativistic velocities. ... In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called Greek barys, meaning heavy, as they are heavier than the other main groups of particles. ... Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ...

The universe today appears to be dominated by a mysterious form of energy known as dark energy. Approximately 70% of the total energy density of today's universe is in this form. This dark energy causes the expansion of the universe to accelerate, observed as a slower than expected expansion at very large distances. Dark energy in its simplest formulation takes the form of a cosmological constant term in Einstein's field equations of general relativity, but its composition is 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.[8] In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. ... Hubbles law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. ... In physical cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: Î›) was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe. ... In physics, the Einstein field equation or Einstein equation is a differential equation in Einsteins theory of general relativity. ... In cosmology, the equation of state of a perfect fluid is characterized by a dimensionless number w, equal to the ratio of its pressure p to its energy density &#961;: . It is closely related to the thermodynamic equation of state and ideal gas law. ... The Standard Model of Fundamental Particles and Interactions For the Standard Model in Cryptography, see Standard Model (cryptography). ...

All these observations can be explained by the ΛCDM model of cosmology, which is a mathematical model of the Big Bang with six free parameters. As noted above, there is no compelling physical model for the first 10−11 seconds of the universe. To resolve the paradox of the initial singularity, a theory of quantum gravitation is needed. Understanding this period of the history of the universe is one of the greatest unsolved problems in physics. A pie chart indicating the proportional composition of different energy-density components of the universe. ... A mathematical model is an abstract model that uses mathematical language to describe the behaviour of a system. ... A physical paradox is an apparent contradiction relating to physical descriptions of the universe. ... Singularity has several different meanings: mathematical singularity - a point where a mathematical function goes to infinity or is in certain other ways ill-behaved. ... This article does not cite any references or sources. ... This is a list of some of the unsolved problems in physics. ...

The Big Bang theory depends on two major assumptions: the universality of physical laws, and the cosmological principle. The cosmological principle states that on large scales the universe is homogeneous and isotropic. For a list of set rules, see Laws of science. ... The Cosmological Principle is a principle invoked in cosmology that severely restricts the large variety of possible cosmological theories: On large scales, the Universe is homogeneous and isotropic. ... In mathematics, in particular in the theory of Lie groups, algebraic groups and topological groups, a homogeneous space for a group G is a manifold or topological space X on which G acts by symmetry in a transitive way; it is not assumed that the action of G is faithful. ... Isotropy (the opposite of anisotropy) is the property of being independent of direction. ...

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.[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.[30] The fine-structure constant or Sommerfeld fine-structure constant, usually denoted , is the fundamental physical constant characterizing the strength of the electromagnetic interaction. ... The age of the universe, in Big Bang cosmology, refers to the time elapsed between the Big Bang and the present day. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... Tests of Einsteins general theory of relativity did not provide an experimental foundation for the theory until well after it was introduced in 1915. ...

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. 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] In cosmology, the Copernican principle, named after Nicolaus Copernicus, states [1] More recently, the principle is generalised to the relativistic concept that humans are not privileged observers of the universe. ...

### FLRW metric

Main articles: Friedmann-Lemaître-Robertson-Walker metric and Metric expansion of space

General relativity describes spacetime by a metric, which determines the distances that separate nearby points. 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. 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). This metric contains a scale factor, which describes how the size of the universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. 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.[33] // The Friedmann-LemaÃ®tre-Robertson-Walker (FLRW) metric is an exact solution of the Einstein field equations of general relativity and which describes a homogeneous, isotropic expanding/contracting universe. ... The metric expansion of space is a key part of sciences current understanding of the universe, whereby space itself is described by a metric which changes over time. ... In mathematics, in Riemannian geometry, the metric tensor is a tensor of rank 2 that is used to measure distance and angle in a space. ... In topology, an atlas describes how a complicated space is glued together from simpler pieces. ... For other uses of this term, see Spacetime (disambiguation). ... In mathematics, in particular in the theory of Lie groups, algebraic groups and topological groups, a homogeneous space for a group G is a manifold or topological space X on which G acts by symmetry in a transitive way; it is not assumed that the action of G is faithful. ... Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ... // The Friedmann-LemaÃ®tre-Robertson-Walker (FLRW) metric is an exact solution of the Einstein field equations of general relativity and which describes a homogeneous, isotropic expanding/contracting universe. ... lol rofl taco hahaThere is also a nscale factor for the expansion of the Universe Scale factors are used in computer science when certain real world numbers need to be represented on a different scale in order to fit a required number format. ... In mathematics as applied to geometry, physics or engineering, a coordinate system is a system for assigning a tuple of numbers to each point in an n-dimensional space. ... The comoving distance or conformal distance of two objects in the universe is the distance divided by a time-varying scale factor representing the expansion of the universe. ... The comoving distance or conformal distance of two objects in the universe is the distance divided by a time-varying scale factor representing the expansion of the universe. ... lol rofl taco hahaThere is also a nscale factor for the expansion of the Universe Scale factors are used in computer science when certain real world numbers need to be represented on a different scale in order to fit a required number format. ...

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. 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. The metric expansion of space is a key part of sciences current understanding of the universe, whereby space itself is described by a metric which changes over time. ...

### Horizons

Main article: Cosmological horizon

An important feature of the Big Bang spacetime is the presence of horizons. 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 FRW 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. If the expansion of the universe continues to accelerate, there is a future horizon as well.[34] In cosmology, a cosmological horizon marks a limit to observability, and marks the boundary of a region that an observer cannot see into directly due to cosmological effects. ... In cosmology, a cosmological horizon marks a limit to observability, and marks the boundary of a region that an observer cannot see into directly due to cosmological effects. ... For other uses, see Big Bang (disambiguation). ... The Accelerating universe is the idea that our universe is undergoing accelerated expansion: distant objects are receding from our galaxy with speeds that increase over time. ...

## 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). 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 which are predicted to occur due to gravitational growth of structure in the standard Big Bang theory. Hubbles law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared with that of the Sun (left). ... In cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. ... Astronomy and cosmology examine the universe to understand the large-scale structure of the cosmos. ...

### Hubble's law expansion

Main article: Hubble's law

$v = H_0 D ,$

where

v is the recessional velocity of the galaxy or other distant object
D is the distance to the object and
H0 is Hubble's constant, measured to be (70 +2.4/-3.2) (km/s)/Mpc by the WMAP probe.[22]

Hubble's law has two possible explanations. 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. This universal expansion was considered mathematically in the context of general relativity well before Hubble made his analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson and Walker. This article is about velocity in physics. ... For other uses, see Galaxy (disambiguation). ... A kilometre (American spelling: kilometer) (symbol: km) is a unit of length equal to 1000 metres (from the Greek words khilia = thousand and metro = count/measure). ... Look up second in Wiktionary, the free dictionary. ... The megaparsec (abbreviated Mpc) is a unit of distance used in astronomy, equal to one million parsecs. ... Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ... Hubbles law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. ... In cosmology, the Copernican principle, named after Nicolaus Copernicus, states [1] More recently, the principle is generalised to the relativistic concept that humans are not privileged observers of the universe. ... The scale factor, parameter of Friedmann-LemaÃ®tre-Robertson-Walker model, is a function of time which represents the relative expansion of the universe. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... // The Friedmann-LemaÃ®tre-Robertson-Walker (FLRW) metric is an exact solution of the Einstein field equations of general relativity and which describes a homogeneous, isotropic expanding/contracting universe. ...

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"). 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.[35] In physics, proper length is the length of an object or a contour as measured in the reference frame of the object itself in the context of special relativity. ...

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. 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. Because of this repeated scattering, the early universe was opaque to light. â€œCMBâ€ redirects here. ... Image File history File links Download high resolution version (2198x1274, 1278 KB)WMAP map of CMB anisotropy, from NASA.gov File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Download high resolution version (2198x1274, 1278 KB)WMAP map of CMB anisotropy, from NASA.gov File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ... In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium. ... As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ... Thomson scattering is the scattering of electromagnetic radiation by a charged particle. ...

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. 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. 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 geometrically flat by measuring the typical angular size (the size on the sky) of the anisotropies. (See shape of the universe.) NASA Logo Listen to this article Â· (info) This audio file was created from the revision dated 2005-09-01, and does not reflect subsequent edits to the article. ... The Cosmic Background Explorer (COBE), also referred to as Explorer 66, was the first satellite built dedicated to cosmology. ... John Cromwell Mather (b. ... George Fitzgerald Smoot III (born February 20, 1945) is an American astrophysicist and cosmologist awarded the 2006 Nobel Prize in Physics with John C. Mather for their discovery of the black body form and anisotropy of the cosmic microwave background radiation. This work helped cement the big-bang theory of... The Telescope being readied for launch The BOOMERanG experiment (Balloon Observations Of Millimetric Extragalactic Radiation and Geophysics) measured the cosmic microwave background radiation of a part of the sky during three sub-orbital (high altitude) balloon flights. ... The shape of the Universe is an informal name for a subject of investigation within physical cosmology. ...

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.[22] This satellite is still gathering data. Another satellite will be launched within the next few years, the Planck Surveyor, which will provide even more accurate measurements of the CMB anisotropies. Many other ground- and balloon-based experiments are also currently running; see Cosmic microwave background experiments. Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ... In physical cosmology, cosmic inflation is the idea that the nascent universe passed through a phase of exponential expansion that was driven by a negative-pressure vacuum energy density. ... WMAP image, unrelated to Planck The Planck Surveyor is the third Medium-Sized Mission (M3) of ESAs Horizon 2000 Scientific Programme. ... Cosmic background radiation spectrum as determined with the COBE satellite, (top) uncorrected, (middle) corrected for the dipole term due to our peculiar velocity, (bottom) corrected for contributions from the dipole term and from our galaxy. ...

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. 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 chemical equilibrium. ...

### 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.[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. 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] In cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. ... For other uses, see Helium (disambiguation). ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... This article is about the chemical element named Lithium. ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... Combinations of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet. ...

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. 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.[36] 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. Italic textSystematic errorsBold text are biases in measurement which lead to measured values being systematically too high or too low. ... Cross section of a red giant showing nucleosynthesis and elements formed 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

Detailed observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. 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. 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.[37] Astronomy and cosmology examine the universe to understand the large-scale structure of the cosmos. ... It has been suggested that this article or section be merged into Large-scale structure of the cosmos. ... In astrophysics, the questions of galaxy formation and evolution are: How, from a homogeneous universe, did we obtain the very heterogeneous one we live in? How did galaxies form? How do galaxies change over time? A spectacular head-on collision between two galaxies is seen in this NASA Hubble Space... Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). ... Astronomy and cosmology examine the universe to understand the large-scale structure of the cosmos. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ... The galaxies of HCG 87, about four hundred million light-years distant. ... Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. ... Media:Example. ...

### 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. In astronomy, stellar evolution is the sequence of radical changes that a star undergoes during its lifetime (the time in which it emits light and heat). ... A globular cluster is a spherical bundle of stars (star cluster) that orbits a galaxy as a satellite. ... Radiometric dating (often called radioactive dating) is a technique used to date materials, based on a comparison between the observed abundance of particular naturally occurring radioactive isotopes and their known decay rates. ... Stars can be grouped into two general types called Population I and Population II. The criteria for classification include space velocity, location in the galaxy, age, chemical composition, and differences in distribution on the Hertzsprung-Russell diagram. ...

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-Zeldovich effect (SZ effect or Sunyaev-Zeldovich theory) is due to high energy electrons distorting the cosmic microwave background radiation (CMB) through the inverse Compton effect, in which some of the high energy of the electrons is transferred to the low energy photons. ... Galaxy groups and clusters are super-structures in the spread of galaxies of the cosmos. ...

## 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. 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). 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. 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. A non-standard cosmology is a cosmological idea that contradicts the standard model of cosmology. ... Observational cosmology is the study of the structure, the evolution and the origin of the universe through observation, using instruments such as telescopes and cosmic ray detectors. ... Scientific consensus is the collective judgment, position, and opinion of the community of scientists in a particular field of science at a particular time. ... The cuspy halo problem is one that arises from cosmological simulations that seem to indicate cold dark matter would form cuspy distributions in the most dense areas of the universe. ... The dwarf galaxy problem is one that arises from numerical cosmological simulations that predict the evolution of the distribution of matter in the universe. ... Cold dark matter (or CDM) is a refinement of the big bang theory, as well as being one possible variation of the more generic Dark Matter theory. ...

Some researchers still doubt the Big Bang Theory because recent observations led to the postulation of dark matter and dark energy as described below. The current model postulates 22% dark matter and 74% dark energy[19]. Neither dark matter nor dark energy have been observed directly and it is not (yet) understood what exactly dark matter or dark energy is. This means that the Big Bang theory currently satisfactorily explains at most 4% of what is thought to make up the universe.

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 cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. ... WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003) The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ... Astronomy and cosmology examine the universe to understand the large-scale structure of the cosmos. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ...

Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not yet been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark energy and dark matter are 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.[38] Explanations for such phenomena remain at the frontiers of inquiry in physics. The Standard Model of Fundamental Particles and Interactions For the Standard Model in Cryptography, see Standard Model (cryptography). ... Thousands of particles explode from the collision point of two relativistic (100 GeV per nucleon) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. ... In astrophysics and cosmology, dark matter refers to hypothetical matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. ... Baryogenesis is the generic designation for the physical processes that generate matter (more specifically, a class of fundamental particle called baryon) from an otherwise matter-empty state (such as it is generally believed to be the state of the Universe at its onset, the so-called Big Bang). ... This is a list of some of the unsolved problems in physics. ...

### Horizon problem

Main article: Horizon problem

The horizon problem results from the premise that information cannot travel 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 causal contact.[39] 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. This article does not cite any references or sources. ... For other uses, see Faster than the speed of light (disambiguation). ... It has been suggested that this article or section be merged into Observable universe. ... Causality describes the relationship between causes and effects, and is fundamental to all natural science, especially physics. ...

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). 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. Inflation is the idea&#8212;first proposed by Alan Guth (1981)&#8212;that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. ...

Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. 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. In quantum physics, the Heisenberg uncertainty principle, sometimes called the Heisenberg indeterminacy principle, expresses a limitation on accuracy of (nearly) simultaneous measurement of observables such as the position and the momentum of a particle. ... Primordial fluctuations are density variations in the early universe which are considered the seeds of all structure in the universe. ... Primordial fluctuations are density variations in the early universe which are considered the seeds of all structure in the universe. ... In physics, scale invariance is the feature of physical objects of laws that do not change if the space is magnified, i. ... 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. From top to bottom: a closed universe with positive curvature, a hyperbolic universe with negative curvature and a flat universe with zero curvature.

The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann-Lemaître-Robertson-Walker metric.[39] 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. The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.[40] 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. 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.[41] The flatness problem is a cosmological problem with the Big Bang theory, which is solved by hypothesising an inflationary universe. ... Image File history File links End_of_universe. ... Image File history File links End_of_universe. ... The shape of the Universe is an informal name for a subject of investigation within physical cosmology. ... The Friedmann equations relate various cosmological parameters within the context of general relativity. ... The shape of the Universe is an informal name for a subject of investigation within physical cosmology. ... The shape of the Universe is an informal name for a subject of investigation within physical cosmology. ... The shape of the Universe is an informal name for a subject of investigation within physical cosmology. ... // The Friedmann-LemaÃ®tre-Robertson-Walker (FLRW) metric is an exact solution of the Einstein field equations of general relativity and which describes a homogeneous, isotropic expanding/contracting universe. ... In mathematics, curvature refers to a number of loosely related concepts in different areas of geometry. ... In cosmology, the Big Crunch is a hypothesis that states the universe will stop expanding and start to collapse upon itself; a counterpart to the Big Bang. ... 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 reached maximum entropy. ... This article is about the cosmological theory. ...

A resolution to this problem is offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that its curvature would have been smoothed out. Thus, it is believed that inflation drove the universe to a very nearly spatially flat state, with almost exactly the critical density. Inflation is the idea&#8212;first proposed by Alan Guth (1981)&#8212;that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. ... In mathematics, curvature refers to a number of loosely related concepts in different areas of geometry. ...

### Magnetic monopoles

Main article: Magnetic monopole

The magnetic monopole objection was raised in the late 1970s. Grand unification theories 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 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.[39] In physics, a magnetic monopole is a hypothetical particle that may be loosely described as a magnet with only one pole (see electromagnetic theory for more on magnetic poles). ... It has been suggested that this article or section be merged into Unified field theory. ... In cosmology, a topological defect is a (often) stable configuration of matter predicted by some theories to form at phase transitions in the very early universe. ... In physics, a magnetic monopole is a hypothetical particle that may be loosely described as a magnet with only one pole (see electromagnetic theory for more on magnetic poles). ... In physical cosmology, cosmic inflation is the idea that the nascent universe passed through a phase of exponential expansion that was driven by a negative-pressure vacuum energy density. ...

### Baryon asymmetry

Main article: Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter.[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. 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. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the universe depart from thermodynamic equilibrium.[42] All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry. The baryon asymmetry problem in astrophysics refers to the apparent fact that the baryons in the universe which have been observed are overwhelmingly matter as opposed to anti-matter. ... This article is about matter in physics and chemistry. ... For other senses of this term, see antimatter (disambiguation). ... Combinations of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet. ... Baryogenesis is the generic designation for the physical processes that generate matter (more specifically, a class of fundamental particle called baryon) from an otherwise matter-empty state (such as it is generally believed to be the state of the Universe at its onset, the so-called Big Bang). ... Baryogenesis is the generic designation for the hypothetical physical processes that generated an asymmetry between baryons and anti-baryons in the very early universe. ... In particle physics, the baryon number is an approximate conserved quantum number. ... C-symmetry means the symmetry of physical laws over a charge-inversion transformation. ... CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ... In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium. ... The Standard Model of Fundamental Particles and Interactions For the Standard Model in Cryptography, see Standard Model (cryptography). ...

### Globular cluster age

In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. 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 13.7-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.[43] 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. The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earths Sun, it may be called a stellar wind instead). ...

### 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. 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. 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 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.[44] In astrophysics and cosmology, dark matter refers to hypothetical matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. ... Image File history File links Cosmological_composition. ... Image File history File links Cosmological_composition. ... A pie chart is a circular chart divided into sectors, illustrating relative magnitudes or frequencies. ... A pie chart indicating the proportional composition of different energy-density components of the universe. ... In astrophysics and cosmology, dark matter refers to hypothetical matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. ... In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. ... In astrophysics and cosmology, dark matter refers to hypothetical matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. ... Combinations of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... The galaxies of HCG 87, about four hundred million light-years distant. ... 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 massive galaxy) between the source object and the observer. ... ROSAT image of X-ray fluorescence of, and occultation of the X-ray background by, the Moon. ...

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.[45] Thousands of particles explode from the collision point of two relativistic (100 GeV per nucleon) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ...

### 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. 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 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. 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.[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. In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared with that of the Sun (left). ... The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... The Accelerating universe is the idea that our universe is undergoing accelerated expansion: distant objects are receding from our galaxy with speeds that increase over time. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... In cosmology, the equation of state of a perfect fluid is characterized by a dimensionless number w, equal to the ratio of its pressure p to its energy density &#961;: . It is closely related to the thermodynamic equation of state and ideal gas law. ... In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. ... WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003) The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ... In cosmology, the Big Crunch is a hypothesis that states the universe will stop expanding and start to collapse upon itself; a counterpart to the Big Bang. ... For other uses, see Density (disambiguation). ... This article or section is in need of attention from an expert on the subject. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

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. Possible candidates include a cosmological constant and quintessence. Results from the WMAP team in 2006, which combined data from the CMB and other sources, indicate that the universe today is 74% dark energy, 22% dark matter, and 4% regular matter.[19] 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. Vacuum energy is an underlying background energy that exists in space even when devoid of matter (known as free space). ... In physical cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: Î›) was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe. ... In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. ...

In the Lambda-CDM model, the best current model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the theory of General Relativity. However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity. Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research. A pie chart indicating the proportional composition of different energy-density components of the universe. ... In physical cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: Î›) was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... This article does not cite any references or sources. ...

## The future according to the Big Bang theory

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 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, would remain together, and they too would be subject to heat death, as the universe expands and cools. 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.[46] The Accelerating universe is the idea that our universe is undergoing accelerated expansion: distant objects are receding from our galaxy with speeds that increase over time. ... For the science fiction film, see Event Horizon (film). ... A pie chart indicating the proportional composition of different energy-density components of the universe. ... In physical cosmology, dark energy is a hypothetical 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: Î›) was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe. ... The heat death is a possible final state of the universe, in which it has reached maximum entropy. ... Phantom energy is a hypothetical form of dark energy with equation of state . ... The galaxies of HCG 87, about four hundred million light-years distant. ... The Big Rip is a cosmological hypothesis about the Ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, are progressively torn apart by the expansion of the universe at a certain time in the future. ...

## 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. Image from WMAP press release, 2006. ( Detail)

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 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.[48]
• brane cosmology models[49] 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.[50][51][52]
• 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.[53]

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. In theoretical physics, the Hartle-Hawking state, named after James Hartle and Stephen Hawking, is a hypothetical vector in the Hilbert space of a theory of quantum gravity that describes the wave function of the Universe. ... Brane cosmology is a protoscience motivated by, but not rigorously derived from, superstring theory and M-theory. ... Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory String theory is a model of fundamental physics whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point... The ekpyrotic universe or ekpyrotic scenario is a cosmological theory of the origin of the universe. ... The cyclic model is a brane cosmology model of the creation of the universe, derived from the earlier ekpyrotic model. ... The theory created by Andrei Linde that states that our universe could have come from nothing more than hundred-thousandth of a gram of matter. ... For other uses, see Multiverse (disambiguation). ...

## Philosophical and religious interpretations

Main article: Philosophical and religious interpretations of the Big Bang theory

The Big Bang is a scientific theory, and as such stands or falls by its agreement with observations. 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.[54] This perception was enhanced by the fact that Georges Lemaître, who put the theory forth, was a Roman Catholic priest. In the imagination of the general public and scholars, many philosophical and religious interpretations of the Big Bang theory of physical cosmology have been offered. ... The term origin belief refers to stories and explanations which describe the beginnings of humanity, earth, life, and the universe. ... In cosmology, the steady state theory (also known as the Infinite Universe Theory or continuous creation) is a model developed in 1948 by Fred Hoyle, Thomas Gold, Hermann Bondi and others as an alternative to the Big Bang theory (known, usually, as the standard cosmological model). ... Monsignor Georges LemaÃ®tre, priest and scientist. ... This article does not cite any references or sources. ...

## Notes and references

1. ^ "Even though the Universe has been expanding and cooling ever since, the sound waves have left their imprint as temperature variations on the afterglow of the big bang fireball..." Chown, Marcus (30 October 2003). "Big Bang sounded like a deep hum". New Scientist.
2. ^ Slipher, V. M.. "The radial velocity of the Andromeda nebula". Lowell Observatory Bulletin 1: 56–57.
Slipher, V. M.. "Spectrographic observations of nebulae". Popular Astronomy 23: 21–24.
3. ^ Friedman, A (1922). "Über die Krümmung des Raumes". Z. Phys. 10: 377–386.  (German) (English translation in: Friedman, A (1999). "On the Curvature of Space". General Relativity and Gravitation 31: 1991–2000. DOI:10.1023/A:1026751225741. )
4. ^ Lemaître, G. (1927). "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques". Annals of the Scientific Society of Brussels 47A: 41.  (French) Translated in: (1931) "Expansion of the universe, A homogeneous universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae". Monthly Notices of the Royal Astronomical Society 91: 483–490.
5. ^ Lemaître, G. (1931). "The evolution of the universe: discussion". Nature 128: suppl.: 704.
6. ^ a b Edwin Hubble (1929). "A relation between distance and radial velocity among extra-galactic nebulae". Proceedings of the National Academy of Sciences 15: 168–173.
7. ^ E. Christianson (1995). Edwin Hubble: Mariner of the Nebulae. Farrar Straus & Giroux. ISBN 0374146608.
8. ^ a b c P. J. E. Peebles and Bharat Ratra (2003). "The cosmological constant and dark energy". Reviews of Modern Physics 75: 559–606. DOI:10.1103/RevModPhys.75.559. arXiv:astro-ph/0207347.
9. ^ E. A. Milne (1935). Relativity, Gravitation and World Structure. Oxford University Press.
10. ^ R. C. Tolman (1934). Relativity, Thermodynamics, and Cosmology. Oxford: Clarendon Press. LCCN 340-32023.  Reissued (1987) New York: Dover ISBN 0-486-65383-8.
11. ^ Zwicky, F (1929). "On the Red Shift of Spectral Lines through Interstellar Space". Proceedings of the National Academy of Sciences 15: 773–779.  Full articlePDF (672 KiB).
12. ^ Hoyle, Fred (1948). "A New Model for the Expanding universe". Monthly Notices of the Royal Astronomical Society 108: 372.
13. ^ R. A. Alpher, H. Bethe, G. Gamow (1948). "The Origin of Chemical Elements". Physical Review 73: 803.
14. ^ R. A. Alpher and R. Herman (1948). "Evolution of the Universe". Nature 162: 774.
15. ^ Simon Singh. Big Bang. Retrieved on 2007-05-28.
16. ^ It is popularly reported that Hoyle intended this to be pejorative. However, Hoyle denied that and said it was just a striking image meant to emphasize the difference between the two theories for radio listeners. See chapter 9 of The Alchemy of the Heavens by Ken Croswell, Anchor Books, 1995.
17. ^ a b A. A. Penzias and R. W. Wilson (1965). "A Measurement of Excess Antenna Temperature at 4080 Mc/s". Astrophysical Journal 142: 419.
18. ^ a b Boggess, N.W., et al. (COBE collaboration) (1992). "The COBE Mission: Its Design and Performance Two Years after the launch". Astrophysical Journal 397: 420, Preprint No. 92-02. DOI:10.1086/171797.
19. ^ a b c d D. N. Spergel et al. (WMAP collaboration) (2006). "Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology". Retrieved on 2007-05-27.
20. ^ S. W. Hawking and G. F. R. Ellis (1973). The large-scale structure of space-time. Cambridge: Cambridge University Press. ISBN 0-521-20016-4.
21. ^ There is no consensus about how long the Big Bang phase lasted: for some writers this denotes only the initial singularity, for others the whole history of the universe. Usually at least the first few minutes, during which helium is synthesised, are said to occur "during the Big Bang".
22. ^ a b c Spergel, D. N.; et al. (2003). "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters". The Astrophysical Journal Supplement Series 148: 175—194. DOI:10.1086/377226.
23. ^ Guth, Alan H. (1998). The Inflationary Universe: Quest for a New Theory of Cosmic Origins. Vintage. ISBN 978-0099959502.
24. ^ Schewe, Phil, and Ben Stein (2005). "An Ocean of Quarks". Physics News Update, American Institute of Physics 728 (#1). Retrieved on 2007-05-27.
25. ^ a b Kolb and Turner (1988), chapter 6
26. ^ Kolb and Turner (1988), chapter 7
27. ^ a b c Kolb and Turner (1988), chapter 4
28. ^ Peacock (1999), chapter 9
29. ^ Ivanchik, A. V.; A. Y. Potekhin and D. A. Varshalovich (1999). "The fine-structure constant: a new observational limit on its cosmological variation and some theoretical consequences". Astronomy and Astrophysics 343: 459.
30. ^ Detailed information of and references for tests of general relativity are given at Tests of general relativity.
31. ^ This ignores the dipole anisotropy at a level of 0.1% due to the peculiar velocity of the solar system through the radiation field.
32. ^ Goodman, J. (1995). "Geocentrism reexamined". Physical Review D 52: 1821. DOI:10.1103/PhysRevD.52.1821.
33. ^ d'Inverno, Ray (1992). Introducing Einstein's Relativity. Oxford: Oxford University Press. ISBN 0-19-859686-3.  Chapter 23
34. ^ a b Kolb and Turner (1988), chapter 3
35. ^ Peacock (1999), chapter 3
36. ^ Steigman, Gary. "Primordial Nucleosynthesis: Successes And Challenges". arXiv:astro-ph/0511534.
37. ^ E. Bertschinger (2001). "Cosmological perturbation theory and structure formation". arXiv:astro-ph/0101009.
Edmund Bertschinger (1998). "Simulations of structure formation in the universe". Annual Review of Astronomy and Astrophysics 36: 599–654.
38. ^ If inflation is true, baryogenesis must have occurred, but not vice versa.
39. ^ a b c Kolb and Turner (1988), chapter 8
40. ^ Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; but our universe remained close to flat for several billion years, before the dark energy density became significant.
41. ^ R. H. Dicke and P. J. E. Peebles. "The big bang cosmology — enigmas and nostrums". S. W. Hawking and W. Israel (eds) General Relativity: an Einstein centenary survey: 504–517, Cambridge University Press.
42. ^ A. D., Sakharov (1967). "Violation of CP invariance, C asymmetry and baryon asymmetry of the universe". Pisma Zh. Eksp. Teor. Fiz. 5: 32.  (Russian) Translated in JETP Lett. 5, 24 (1967).
43. ^ Navabi, A. A.; N. Riazi (2003). "Is the Age Problem Resolved?". Journal of Astrophysics and Astronomy 24: 3.
44. ^ Keel, Bill. Galaxies and the Universe lecture notes - Dark Matter. University of Alabama Astronomy. Retrieved on 2007-05-28.
45. ^ Yao, W. M.; et al. (2006). "Review of Particle Physics". J. Phys. G: Nucl. Part. Phys. 33: 1–1232. DOI:10.1088/0954-3899/33/1/001.  Chapter 22: Dark matterPDF (152 KiB).
46. ^ (2003) "Phantom Energy and Cosmic Doomsday". Phys. Rev. Lett. 91: 071301. arXiv:astro-ph/0302506.
47. ^ Hawking, Stephen; and Ellis, G. F. R. (1973). The Large Scale Structure of Space-Time. Cambridge: Cambridge University Press. ISBN 0-521-09906-4.
48. ^ J. Hartle and S. W. Hawking (1983). "Wave function of the universe". Phys. Rev. D 28: 2960.
49. ^ Langlois, David (2002). "Brane cosmology: an introduction". arXiv:hep-th/0209261.
50. ^ Linde, Andre (2002). "Inflationary Theory versus Ekpyrotic/Cyclic Scenario". arXiv:hep-th/0205259.
51. ^ "Recycled Universe: Theory Could Solve Cosmic Mystery", Space.com, 8 May 2006. Retrieved on 2007-07-03.
52. ^ What Happened Before the Big Bang?. Retrieved on 2007-07-03.
53. ^ A. Linde (1986). "Eternal chaotic inflation". Mod. Phys. Lett. A1.
A. Linde (1986). "Eternally existing self-reproducing chaotic inflationary universe". Phys. Lett. B175.
54. ^ Kragh, Helge (1996). Cosmology and Controversy. Princeton University Press. ISBN 069100546X.

### 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.

For an annotated list of textbooks and monographs, see physical cosmology.

This article is about the physics subject. ... Physics Today magazine, created in 1948, is the flagship publication of The American Institute of Physics. ... Big Bang: The most important scientific discovery of all time and why you need to know about it is a book written by Simon Singh and published in 2004 by Fourth Estate. ... The American Institute of Physics (AIP) is a professional body representing American physicists and publishing physics related journals. ... Scientific American is a popular-science magazine, published (first weekly and later monthly) since August 28, 1845, making it the oldest continuously published magazine in the United States. ... Scientific American is a popular-science magazine, published (first weekly and later monthly) since August 28, 1845, making it the oldest continuously published magazine in the United States. ...

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 Big Bang - Wikipedia, the free encyclopedia (6045 words) Huge advances in Big Bang cosmology were made in the late 1990s and the early 21st century as a result of major advances in telescope technology in combination with large amounts of satellite data such as that from COBE, the Hubble Space Telescope and WMAP. The Big Bang theory predicted the existence of the cosmic microwave background radiation or CMB which is composed of photons emitted during baryogenesis. 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. All the abundances depend on a single parameter, the ratio of photons to baryons.
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