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[[Image:Universe_expansion.png|thumb|240px|According to the Big Bang theory, the [[universe]] emerged from an extremely dense and hot state (bottom). Since then, space itself has expanded with the passage of time, carrying the galaxies with it.]] |
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In [[physical cosmology]], the '''Big Bang''' is the [[Theory#science|theory]] of how the [[universe]] emerged from a tremendously [[density|dense]] and [[temperature|hot]] state about 13.7 billion years ago. The Big Bang theory is based on the observed [[Hubble's law]] [[redshift]] of [[Cosmic distance ladder|distant]] [[galaxy|galaxies]] that when taken together with the [[cosmological principle]] indicate that [[metric space|space]] is [[metric expansion of space|expanding]] according to the [[Robertson-Walker coordinates|Friedmann-Lemaître model]] of [[general relativity]]. [[Extrapolation|Extrapolated]] into the past, these [[observation]]s show that the universe has expanded from a state in which all the [[matter]] and [[energy]] in the universe was at an immense temperature and density. [[physics|Physicists]] do not widely agree on what happened before this, although general relativity predicts a [[gravitational singularity]] (for reporting on some of the more notable speculation on this issue, see [[cosmogony]]). |
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The term ''Big Bang'' is used both in a narrow sense to refer to a point in time when the observed expansion of the universe ([[Hubble's law]]) began — calculated to be 13.7 [[billion]] ([[1 E17 s|1.37 × 10<sup>10</sup>]]) years ago (±2%) — and in a more general sense to refer to the prevailing cosmological [[paradigm]] explaining the origin and expansion of the universe, as well as the composition of primordial matter through [[nucleosynthesis]] as predicted by the [[Alpher-Bethe-Gamow theory]].<ref>R. A. Alpher, H. A. Bethe, G. Gamow, "The Origin of Chemical Elements,"''Physical Review'' '''73''' (1948), 803. </ref> |
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From this [[Model (abstract)|model]], [[George Gamow]] in 1948 was able to predict, at least qualitatively, the existence of [[cosmic microwave background radiation]] (CMB).<ref>G. Gamow, ''Nature'' '''162''' (1948), 680. </ref> The CMB was discovered in the 1960s and further validated the Big Bang theory over its chief rival, the [[steady state theory]].<br clear="right"/> |
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{{Cosmology}} |
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==History== |
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{{main|History of the Big Bang}} |
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The Big Bang theory developed from observations and theoretical considerations. Observationally, it was determined that most spiral nebulae were receding from Earth, but those who made the observation weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own [[Milky Way]].<ref>V. Slipher, paper presented to the [[American Astronomical Society]], (1915).</ref> In 1927, [[Georges Lemaître]] independently derived the [[Friedmann equations|Friedmann-Lemaître-Robertson-Walker equations]] from [[Albert Einstein]]'s [[Einstein equation|equations]] of [[general relativity]] and proposed, on the basis of the recession of spiral [[nebula]]e, that the universe began with the "explosion" of a "primeval [[atom]]"—what was later called the Big Bang.<ref>G. Lemaître, ''Annals of the Scientific Society of Brussels'' '''47A''' (1927).</ref> Because the net energy of the Universe is zero (as the positive energy of matter in the Universe mc<sup>2</sup> is equilibrated by the same amount of negative energy of mutual gravitation of this matter), currently no "primeval atom" is considered to be necessary to start the Big Bang. |
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In 1929, [[Edwin Hubble]] provided an observational basis for Lemaître's theory. He discovered that, seen from Earth, light from other galaxies is red-shifted in direct proportion to their distance from the Earth. This fact is now known as [[Hubble's law]].<ref>E. Christianson ''Edwin Hubble: Mariner of the Nebulae''. </ref> Given the [[cosmological principle]] whereby the universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places, Hubble's law suggested that the universe was expanding, contradicting the infinite and unchanging [[static universe]] scenario developed by Einstein. |
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[[image:WMAP2.jpg|thumb|left|Artist depiction of the [[WMAP]] satellite gathering data to help scientists understand the Big Bang.]] |
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This idea allowed for two opposing possibilities. One was Lemaître's Big Bang theory, advocated and developed by [[George Gamow]]. The other possibility was [[Fred Hoyle]]'s [[steady state model]] in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time.<ref>F. Hoyle '"A New Model for the Expanding universe", ''Monthly Notices of the Royal Astronomical Society'', '''108''' (1948), 372.</ref> It was actually Hoyle who coined the name of Lemaître's theory, referring to it sarcastically as "this ''big bang'' idea" during a program broadcast on [[March 28]], [[1949]] by the [[BBC]] [[Third Programme]]. Hoyle repeated the term in further broadcasts in early 1950, as part of a series of five lectures entitled ''The Nature of Things''. The text of each lecture was published in ''[[The Listener (British magazine)|The Listener]]'' a week after the broadcast, the first time that the term "big bang" appeared in print. [http://www.nap.edu/books/0309093139/html/136.html] |
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For a number of years the support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. Since the discovery of the [[cosmic microwave background radiation]] in 1965 it has been regarded as the best theory of the origin and evolution of the cosmos. Virtually all theoretical work in cosmology now involves extensions and refinements to the basic Big Bang theory. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding what happened at the Big Bang, and reconciling observations with the basic theory. |
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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]]. Such data have allowed cosmologists to calculate many of the parameters of the Big Bang to a new level of precision and led to the unexpected discovery that the expansion of the universe appears to be accelerating. (See [[dark energy]].) |
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See also: '''[[Timeline of cosmology]]''' |
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== Overview == |
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{{External_Timeline|Graphical timeline of the Big Bang|Graphical timeline of the Big Bang}} |
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Based on measurements of the expansion of the universe using [[Type I supernova|Type 1a supernovae]], measurements of the lumpiness of the [[cosmic microwave background radiation|cosmic microwave background]], and measurements of the [[correlation function]] of galaxies, the universe has a calculated [[Age of the universe|age]] of [[1 E17 s|13.7 ± 0.2 billion years]]. The agreement of these three independent measurements is considered strong evidence for the so-called [[Lambda-CDM model|ΛCDM model]] that describes the detailed nature of the contents of the universe. |
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The early universe was filled homogeneously and isotropically with an incredibly high [[energy]] density and concomitantly huge [[temperature]]s and [[pressure]]s. It expanded and cooled, going through [[phase transition]]s analogous to the condensation of steam or freezing of water as it cools, but related to elementary particles. |
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Approximately 10<sup>−35</sup> seconds after the [[Planck epoch]] a phase transition caused the universe to experience [[exponential growth]] during a period called [[cosmic inflation]]. After inflation stopped, the material components of the universe were in the form of a [[quark-gluon plasma]] (also including all other particles—and perhaps experimentally produced recently as a quark-gluon liquid [http://www.aip.org/pnu/2005/split/728-1.html]) in which the constituent particles were all moving [[relativity|relativistically]]. As the universe continued growing in size, the temperature dropped. At a certain temperature, by an as-yet-unknown transition called [[baryogenesis]], the quarks and gluons combined into [[baryon]]s such as protons and neutrons, somehow producing the observed [[asymmetry]] between [[matter]] and [[antimatter]]. Still lower temperatures led to further [[symmetry breaking]] phase transitions that put the [[fundamental force|forces of physics]] and [[particle physics|elementary particles]] into their present form. Later, some protons and neutrons combined to form the universe's [[deuterium]] and [[helium]] [[atomic nucleus|nuclei]] in a process called [[Big Bang nucleosynthesis]]. As the universe cooled, matter gradually stopped moving relativistically and its [[rest mass]] energy density came to [[gravity|gravitationally]] dominate that of [[electromagnetic radiation|radiation]]. After about 300,000 years the electrons and nuclei combined into atoms (mostly [[hydrogen]]); hence the radiation [[decoupling|decoupled]] from matter and continued through space largely unimpeded. This relic radiation is the cosmic microwave background. |
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Over time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, [[star]]s, 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 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. |
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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 [[Hubble's law|expansion of the universe]] to deviate from a linear velocity-distance relationship, observed as a faster 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 equation]]s of general relativity, but its composition is unknown and, more generally, the details of its [[equation of state (cosmology)|equation of state]] and relationship with the [[standard model]] of particle physics continue to be investigated both observationally and theoretically. |
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All these observations are encapsulated in the [[Lambda-CDM model|ΛCDM model]] of cosmology, which is a [[mathematical model]] of the Big Bang with six free parameters. Mysteries appear as one looks closer to the beginning, when particle energies were higher than can yet be studied by experiment. There is no compelling physical model for the first 10<sup>−33</sup> seconds of the universe, before the phase transition that [[grand unification theory]] predicts. At the "first instant", Einstein's theory of gravitation predicts a [[gravitational singularity]] where densities become infinite. To resolve this [[physical paradox|paradox]], a theory of [[quantum gravity|quantum gravitation]] is needed. Understanding this period of the history of the universe is one of the greatest [[unsolved problems in physics]]. |
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See also: '''[[Timeline of the Big Bang]]''' |
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== Theoretical underpinnings == |
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As it stands today, the Big Bang is dependent on three assumptions: |
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# The universality of [[physical law]]s |
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# The [[cosmological principle]] |
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# The [[Copernican principle]] |
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When first developed, these ideas were simply taken as postulates, but today there are efforts underway to test each of them. Tests of the universality of physical laws have found that the largest possible deviation of the [[fine structure constant]] over the age of the universe is of order 10<sup>-5</sup>.<ref>A. V. Ivanchik, et al. "The fine-structure constant: a new observational limit on its cosmological variation and some theoretical consequences", ''Astronomy and Astrophysics'' '''343''' (1999) 439.</ref> The [[isotropy]] of the universe that defines the Cosmological Principle has been tested to a level of 10<sup>-5</sup> and the universe has been measured to be homogeneous on the largest scales to the 10% level.<ref>J. Goodman ''Physics Review D'', '''52''' (1995) 1821.</ref> There are efforts underway to test the Copernican Principle by means of looking at the interaction of [[galaxy groups and clusters]] with the CMB through the [[Sunyaev-Zeldovich effect|Sunyaev-Zel'dovich effect]] to a level of 1% accuracy.<ref>Caltech Submillimeter Observatory has a program underway for measuring detail observations of the CMB to look for Sunyaev-Zel'dovich Effect correlations. [http://www.submm.caltech.edu/cso/]</ref> |
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The Big Bang theory uses [[Weyl's postulate]] to unambiguously measure [[time]] at any point as the "time since the [[Planck epoch]]". Measurements in this system rely on [[conformal]] coordinates in which so-called [[comoving distance]]s and conformal times remove the expansion of the universe, parameterized by the cosmological [[scale factor (universe)|scale factor]], from consideration of [[spacetime]] measurements. The comoving distances and conformal times are defined so that objects moving with the cosmological flow are always the same comoving distance apart and the [[particle horizon]] or observational limit of the local universe is set by the conformal time. |
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As the universe can be described by such coordinates, the Big Bang is not an explosion of matter moving outward to fill an empty universe; [[metric expansion of space|what is expanding is space itself]]. It is this expansion that causes the physical distance between any two fixed points in our universe to increase. Objects that are bound together (for example, by [[gravity]]) do not expand with spacetime's expansion because the physical laws that govern them are assumed to be uniform and independent of the [[Metric space|metric expansion]]. Moreover, the expansion of the universe on today's local scales is so small that any dependence of physical laws on the expansion is unmeasurable by current techniques. |
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==Observational evidence== |
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It is generally stated that there are three observational pillars that support the Big Bang theory of cosmology. These are the [[Hubble's law|Hubble-type expansion]] seen in the [[redshift]]s of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements. (See [[Big Bang nucleosynthesis]].) Additionally, the observed [[correlation function (astronomy)|correlation function]] of [[large-scale structure of the cosmos]] fits well with standard Big Bang theory. |
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===Hubble's law expansion=== |
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{{main|Hubble's law}} |
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[[Image:HubbleData.JPG|thumb|right|350px|Hubble's original data from his 1929 paper.<ref>Hubble, Edwin, "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1929PNAS...15..168H&db_key=AST&data_type=HTML&format=&high=42ca922c9c30954 A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae]" (1929) ''Proceedings of the National Academy of Sciences of the United States of America'', Volume 15, Issue 3, pp. 168-173 ([http://www.pnas.org/cgi/reprint/15/3/168 Full article], PDF)</ref>]] |
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Observations of distant galaxies and [[quasar]]s show that these objects are [[redshift]]ed, meaning that the [[light]] emitted from them has been shifted to longer wavelengths. This is seen by taking a [[frequency spectrum]] of the objects and then matching the [[spectroscopy|spectroscopic]] pattern of [[emission line]]s or [[absorption line]]s corresponding to [[atom]]s of the [[chemical element]]s interacting with the light. From this analysis, a [[redshift]] corresponding to a [[Doppler shift]] for the radiation can be measured which is explained by a recessional [[velocity]]. When the recessional velocities are plotted against the distances to the objects, a linear relationship, known as [[Hubble's law]], is observed: |
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::<math>v = H_0 D \,</math> |
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where |
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:<math>v</math> is the recessional [[velocity]] of the [[galaxy]] or other distant object |
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:<math>D</math> is the distance to the object and |
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:<math>H_0</math> is Hubble's constant, measured to be (70 +2.4/-3.2) [[kilometers|km]]/[[second|s]]/[[Megaparsec|Mpc]] by the [[WMAP]] probe.<ref>D. N. Spergel, et al. "First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Determination of cosmological parameters", ''Astrophysical Journal Supplement Series'', '''148''' (2003) 175.</ref> |
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The [[Hubble's law]] observation has two possible explanations. One is that we are at the center of an explosion of galaxies, a position which is untenable given the [[Copernican principle]]. The second explanation is that the universe is [[scale factor (universe)|uniformly expanding]] everywhere as a unique property of [[spacetime]]. This type of universal expansion was developed 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 [[Robertson-Walker coordinates|Friedmann-Lemaître-Robertson-Walker]]. |
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===Cosmic microwave background radiation === |
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{{main|Cosmic microwave background radiation}} |
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[[Image:WMAP.jpg|thumb|240px|[[WMAP]] image of the cosmic microwave background radiation]] |
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The Big Bang theory predicted the existence of the [[cosmic microwave background radiation]] or CMB which is composed of [[photons]] emitted during [[baryogenesis]]. Because the early universe was in [[thermal equilibrium]], the [[temperature]] of the radiation and the [[Plasma (physics)|plasma]] were equal until the plasma [[recombination|recombined]]. Before atoms formed, radiation was constantly absorbed and reemitted in a process called [[Compton scattering]]: the early universe was opaque to light. However, cooling due to the expansion of the universe allowed the temperature to eventually fall below 3,000 [[Kelvin|K]] at which point electrons and nuclei combined to form atoms and the primordial plasma turned into a neutral gas. This is known as photon [[decoupling]]. A universe with only neutral atoms allows radiation to travel largely unimpeded. |
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Because the early universe was in thermal equilibrium, the radiation from this time had a [[blackbody]] spectrum and freely streamed through space until today, becoming redshifted because of the Hubble expansion. This reduces the high temperature of the blackbody spectrum. The radiation should be observable at every point in the universe to come from all directions of space. |
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In 1964, [[Arno Penzias]] and [[Robert Woodrow Wilson|Robert Wilson]], while conducting a series of diagnostic observations using a new [[microwave]] receiver owned by [[Bell Laboratories]], discovered the cosmic background radiation. 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 the [[Nobel Prize]] for their discovery. |
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In 1989, [[National Aeronautics and Space Administration|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 determined that the CMB was isotropic to about one part in 10<sup>5</sup>.<ref>N.W. Boggess, et al. "The COBE Mission: Its Design and Performance Two Years after the launch," ''Astrophysical Journal'', '''397''' (1992), 420. </ref> During the 1990s, CMB anisotropies were further investigated by a large number of ground-based experiments and the universe was shown to be almost geometrically flat by measuring the typical angular size (the size on the sky) of the anisotropies. (See [[shape of the universe]].) |
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In early 2003 the results of the [[WMAP|Wilkinson Microwave Anisotropy satellite]] (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. (See [[Cosmic microwave background radiation#Experiments|cosmic microwave background radiation experiments]].) This satellite also disproved several specific [[cosmic inflation]] models, but the results were consistent with the inflation theory in general. |
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===Abundance of primordial elements=== |
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{{main|Big Bang nucleosynthesis}} |
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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 [[photon]]s to [[baryon]]s. The ratios predicted (by mass, not by number) are about 0.25 for <sup>4</sup>He/H, about 10<sup>-3</sup> for <sup>2</sup>H/H, about 10<sup>-4</sup> for <sup>3</sup>He/H and about 10<sup>-9</sup> for <sup>7</sup>Li/H. |
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The measured abundances all agree with those predicted from a single value of the baryon-to-photon ratio. The agreement is relatively poor for <sup>7</sup>Li and <sup>4</sup>He, the two elements for which the [[systematic error|systematic uncertainties]] are least understood. This is considered strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements. 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 essentially free of [[stellar nucleosynthesis]] products) should have more helium than deuterium or more deuterium than <sup>3</sup>He, and in constant ratios, too. |
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===Galactic evolution and distribution=== |
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{{main|Large-scale structure of the cosmos}} |
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Detailed observations of the [[Galaxy classification|morphology]] and [[Large-scale structure of the cosmos|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 groups and clusters|galaxy clusters]] and [[supercluster]]s. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of [[star formation]], galaxy and quasar distributions, and larger structures agree well with Big Bang simulations of the formation of structure in the universe and are helping to complete details of the theory. |
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==Features, issues and problems== |
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A number of problems have arisen within the Big Bang theory throughout its history. Some of them are mainly of historical interest today, and have been avoided 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 they can be addressed through refinements of the theory. |
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There are a small number of proponents of [[non-standard cosmology|non-standard cosmologies]] who doubt that there was a Big Bang at all. They claim that solutions to standard problems in the Big Bang theory involve [[ad hoc]] modifications and addenda to the theory. Most often attacked are the parts of standard cosmology that include [[dark matter]], [[dark energy]], and [[cosmic inflation]]. However, while explanations for these features remain at the [[Unsolved problems in physics|frontiers of inquiry in physics]], together they are suggested by independent observations of [[Big Bang nucleosynthesis]], the [[cosmic microwave background]], [[Large-scale structure of the cosmos|large scale structure]] and Type Ia [[supernova]]e. The [[gravity|gravitational]] effects of these features are understood observationally and theoretically but they have not yet been successfully incorporated into the [[Standard Model]] of [[particle physics]]. Though some aspects of the theory remain inadequately explained by fundamental physics, almost all astronomers and physicists accept that the close agreement between Big Bang theory and observation have firmly established all the basic parts of the theory. |
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The following is a short list of Big Bang "problems" and puzzles: |
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===Horizon problem=== |
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{{main|horizon problem}} |
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The '''horizon problem''' results from the premise that information cannot travel [[faster than light]], and hence two regions of space which are separated by a greater distance than the speed of light multiplied by the age of the universe cannot be in [[causality (physics)|causal]] contact. The observed isotropy of the [[cosmic microwave background]] (CMB) is problematic in this regard, because the [[particle horizon|horizon]] size at that time corresponds to a size that is about 2 degrees on the sky. If the universe has had the same expansion history since the [[Planck epoch]], there is no mechanism to cause these regions to have the same temperature. |
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A resolution to this apparent inconsistency is offered by [[inflationary theory]] in which a homogeneous and isotropic scalar energy field dominates the universe at a time 10<sup>-35</sup> seconds after the Planck epoch. During inflation, the universe undergoes exponential expansion, and regions in causal contact expand so as to be beyond each other's horizons. [[Heisenberg's uncertainty principle]] predicts that during the inflationary phase there would be [[primordial fluctuations|quantum thermal fluctuations]], which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the universe. After inflation, the universe expands according to [[Hubble's law]], and regions that were out of causal contact come back into the horizon. This explains the observed isotropy of the CMB. Inflation predicts that the [[primordial fluctuations]] are nearly [[Scale invariance|scale invariant]] and [[Normal distribution|Gaussian]] which has been accurately confirmed by measurements of the CMB. |
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===Flatness problem=== |
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[[Image:End_of_universe.jpg|thumb|275px|The overall [[shape of the universe|geometry of the universe]] is determined by whether the [[Friedmann equations#The density parameter|Omega cosmological parameter]] is less than, equal to or greater than 1. From top to bottom: geometry in a [[Shape of the Universe#Closed Universe|closed universe]], an [[Shape of the Universe#Open Universe|open universe]] and a [[Shape of the Universe#Flat Universe|flat universe]].]] |
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{{main|flatness problem}} |
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The '''flatness problem''' is an observational problem that results from considerations of the [[shape of the universe|geometry]] associated with a [[Robertson-Walker coordinates|Friedmann-Lemaître-Robertson-Walker]] metric. In general, the universe can have three different kinds of geometries: [[hyperbolic geometry]], [[Euclidean geometry]], or [[elliptic geometry]]. The geometry is determined by the total energy density of the universe (as measured by means of the [[stress-energy tensor]]): hyperbolic results from a density less than the [[critical density]], elliptic from a density greater than the critical density, and Euclidean from exactly the critical density. The universe is required to be within one part in 10<sup>15</sup> of the critical density in its earliest stages. Any greater deviation would have caused either a [[Heat Death]] or a [[Big Crunch]], and the universe would not exist as it does today. |
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A possible resolution to this problem is again offered by [[inflationary theory]]. During the inflationary period, spacetime expanded to such an extent that any residual [[curvature]] associated with it would have been smoothed out to a high degree of precision. Thus, it is believed that inflation drove the universe to be very nearly spatially flat. |
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===Magnetic monopoles=== |
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The '''[[magnetic monopole]]''' objection was raised in the late 1970s. [[Grand unification theory|Grand unification theories]] predicted [[Topological defect|point defects]] in space that would manifest as [[magnetic monopole]]s with a density much higher than was consistent with observations, given that searches have never found any monopoles. This problem is also resolvable by [[cosmic inflation]], which removes all point defects from the observable universe in the same way that it drives the geometry to flatness. |
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===Baryon asymmetry=== |
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It is not yet understood why the universe has more [[matter]] than [[antimatter]]. It is generally assumed that when the universe was young and very hot, it was in statistical equilibrium and contained equal numbers of [[baryon]]s 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, which were laid out by [[Andrei Sakharov]], must be satisfied. They require that [[baryon number]] be not conserved, that [[C-symmetry]] and [[CP-symmetry]] be violated, and that the universe depart from [[thermodynamic equilibrium]]. All these conditions occur in the [[Standard Model]], but the effect is not strong enough to explain the present baryon asymmetry. Experiments taking place at [[CERN]] near Geneva seek to trap enough [[anti-hydrogen]] to compare its spectrum with hydrogen. Any difference would be evidence of a [[CPT symmetry]] violation and therefore a [[Lorentz violation]]. |
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===Globular cluster age=== |
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In the mid-1990s, observations of '''[[globular cluster]]s''' appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the [[star|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 wind]]s, indicated a much younger age for globular clusters.<ref>A. A. Navabi and N. Riazi, "Is the Age Problem Resolved?" ''Journal of Astrophysics and Astronomy'' '''24''' (2003), 3.</ref> 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. |
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===Dark matter=== |
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{{main|dark matter}} |
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[[Image:Cosmological composition.jpg|thumb|right|375px|A [[pie chart]] indicating the proportional composition of different energy-density components of the universe, according to the best [[Lambda-CDM model|ΛCDM model]] fits. Roughly ninety-five percent is in the exotic forms of [[dark matter]] and [[dark energy]].]] |
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During the 1970s and 1980s various observations (notably of [[galaxy rotation problem|galactic rotation curves]]) showed that there was 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 not normal or [[baryon]]ic matter but rather '''[[dark matter]]'''. In addition, assuming that the universe was mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe is far less lumpy and contains far less [[deuterium]] than can be accounted for without dark matter. While dark matter was initially controversial, it is now a widely accepted part of standard cosmology due to observations of the anisotropies in the CMB, [[galaxy groups and clusters|galaxy cluster]] velocity dispersions, large-scale structure distributions, [[gravitational lensing]] studies, and [[x-ray]] measurements from galaxy clusters. Dark matter has only been detected through its gravitational signature; no particles that might make it up have yet been observed in laboratories. However, there are many [[particle physics]] candidates for dark matter, and several projects to detect them are underway. |
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===Dark energy=== |
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{{main|dark energy}} |
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In the 1990s, detailed measurements of the [[density|mass density]] of the universe revealed a value that was 30% that of the [[critical density]]. Since the universe is very nearly spatially flat, as is indicated by measurements of the [[cosmic microwave background]], about 70% of the energy density of the universe was left unaccounted for. This mystery now appears to be connected to another one: Independent measurements of [[Type I supernova#Type Ia|Type Ia supernovae]] have revealed that the expansion of the universe is undergoing a non-linear [[accelerating universe|acceleration]] rather than following strictly [[Hubble's law]]. To explain this acceleration, [[general relativity]] requires that much of the universe consist of an energy component with large [[equation of state (cosmology)|negative pressure]]. This '''[[dark energy]]''' is now thought to make up the missing 70%. Its nature remains one of the great mysteries of the Big Bang. Possible candidates include a scalar [[cosmological constant]] and [[quintessence (physics)|quintessence]]. Observations to help understand this are ongoing. Results from WMAP in 2006 indicate that the universe is 74% dark energy, 22% dark matter, and 4% regular matter (see external link). |
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==The future according to the Big Bang theory== |
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{{main|ultimate fate of the universe}} |
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Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass [[density]] of the universe is above the [[critical density]], then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state that was similar to that in which it started—a [[Big Crunch]]. Alternatively, if the density in the universe is equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as the universe grows less dense. The average temperature of the universe would asymptotically approach [[absolute zero]]—a [[Big Freeze]]. [[Black holes]] would [[Hawking radiation|evaporate]]. 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]]. Moreover, if [[proton decay]] exists, then hydrogen, the predominant form of baryonic matter in the universe today, would disappear, leaving only radiation. |
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Modern observations of [[accelerating universe|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 [[Lambda-CDM model|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 cools and expands. Other explanations of dark energy — so-called [[phantom energy]] theories — suggest that ultimately [[galaxy groups and clusters|galaxy clusters]] and eventually [[galaxies]] themselves will be torn apart by the ever-increasing expansion in a so-called [[Big Rip]]...... |
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==Speculative physics beyond the Big Bang== |
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[[Image:CMB_Timeline300.jpg|right|300px|thumb|A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the [[metric tensor|metric]] seen on the left. Image from [[WMAP]] press release, 2006. ([[:Image:CMB_Timeline300.jpg|Detail]])]] |
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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 universe, when [[cosmic inflation|inflation]] is hypothesized to have occurred. There may also be parts of the universe well beyond what can be observed in principle. In the case of inflation this is required: exponential expansion has pushed large regions of space beyond our observable horizon. It may be possible to deduce what happened when we better understand physics at very high energy scales. Speculations about this often involve theories of [[quantum gravity|quantum gravitation]]. |
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Some proposals are: |
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* models including the [[Hartle-Hawking state|Hartle-Hawking boundary condition]] in which the whole of space-time is finite; |
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* [[brane cosmology]] models, including [[brane inflation]], in which inflation is due to the movement of branes in [[string theory]]; the [[pre-big bang model]]; and the [[ekpyrotic]] model, in which the Big Bang is the result of a collision between branes; |
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* an [[oscillatory universe]] in which the early universe's hot, dense state resulted from the Big Crunch of a universe similar to ours. The universe could have gone through an infinite number of big bangs and big crunches. The [[cyclic model|cyclic]] extension of the ekpyrotic model is a modern version of such a scenario. |
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* [[chaotic inflation]], in which inflation starts from random initial conditions for the universe. |
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Some of these scenarios are qualitatively compatible with one another. Each entails untested hypotheses. |
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==Philosophical and religious interpretations== |
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There are a number of interpretations of the Big Bang theory that are extra-scientific. Some of these ideas purport to explain the cause of the Big Bang itself ([[first cause]]), although science cannot possibly show a first cause, so they have been criticized by some [[philosophical naturalism|naturalist]] philosophers as being modern [[creation myth]]s. Some people believe that the Big Bang theory lends support to traditional views of creation as given in [[Genesis]], for example, while others believe that the Big Bang theory is inconsistent with such views. |
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The Big Bang, as a scientific theory, is not based on any [[religion]]. While some religious interpretations conflict with the Big Bang story of the universe, there are many other interpretations that do not. |
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The following is a list of various religious interpretations of the Big Bang theory: |
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* A number of [[Christianity|Christian]] churches, the [[Roman Catholic Church]] in particular, have accepted the Big Bang as a possible description of the origin of the universe, interpreting it to allow for a philosophical first cause. [[Pope Pius XII]] was an enthusiastic proponent of the Big Bang even before the theory was scientifically well established. This view is shared by many religious Jews in all branches of rabbinic [[Judaism]]. |
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* Traditional [[Jewish]] [[Origin belief#Judaism|sources]] describe a [[Creation according to Genesis|creation ''ex nihilo'']] that can be interpreted as consistent with the Big Bang. Adherents of [[Kabbalah]], esoteric Jewish mysticism, accept the Big Bang theory as factual, and relate it to the theory of "divine retraction" ([[tzimtzum]]) as explained in Jewish mystical texts, such as the [[Zohar]]. |
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* Some modern [[Islam]]ic scholars believe that the [[Qur'an]] parallels the Big Bang in its account of creation, described as follows: "Do not the unbelievers see that the heavens and the earth were joined together as one unit of creation, before We clove them asunder?" (Ch:21,Ver:30). The claim has also been made that the Qur'an describes an expanding universe: "The heaven, We have built it with power. And verily, We are expanding it." (Ch:51,Ver:47). Parallels with the [[Big Crunch]] and an [[oscillating universe]] have also been suggested: "On the day when We will roll up the heavens like the rolling up of the scroll for writings, as We originated the first creation, (so) We shall reproduce it; a promise (binding on Us); surely We will bring it about." (Ch:21,Ver:104). |
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* Certain [[theism|theistic]] branches of [[Hinduism]], such as in [[Vaishnavism]], conceive of a theory of creation with similarities to the theory of the Big Bang. The Hindu mythos, narrated for example in the third book of the [[Bhagavata Purana]] (primarily, chapters 10 and 26), describes a primordial state which bursts forth as the Great [[Vishnu]] glances over it, transforming into the active state of the sum-total of matter ("[[prakriti]]"). Other forms of Hinduism assert a universe without beginning or end. |
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*[[Buddhism]] has a concept of a universe that has no creation event, but instead goes through infinitely repeated cycles of expansion, stability, contraction, and quiescence. The Big Bang, however, is not seen to be in conflict with this since there are ways to conceive an eternal universe within the paradigm. A number of popular [[Zen]] philosophers were intrigued, in particular, by the concept of the [[oscillating universe]]. |
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==Notes== |
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<div class="references-small"> |
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<references /> |
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</div> |
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==External links and references== |
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===Big Bang overviews=== |
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<div class="references-small"> |
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*[[Open Directory Project]]: [http://www.dmoz.org/Science/Astronomy/Cosmology/ Cosmology] |
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*[[PBS]].org, [http://www.pbs.org/deepspace/timeline/ "From the Big Bang to the End of the universe. The Mysteries of Deep Space Timeline"] |
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*[http://www.historyoftheuniverse.com/ "Welcome to the History of the universe"]. Penny Press Ltd. |
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*[[University of Cambridge|Cambridge University]] Cosmology, "[http://www.damtp.cam.ac.uk/user/gr/public/bb_home.html The Hot Big Bang Model]". Includes a discussion of the problems with the Big Bang. |
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*[[Smithsonian Institution]], "[http://cfa-www.harvard.edu/seuforum/bigbanglanding.htm UNIVERSE! - The Big Bang and what came before]". |
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*D'Agnese, Joseph, "[http://www.findarticles.com/p/articles/mi_m1511/is_7_20/ai_55030837 The last Big Bang man left standing, physicist Ralph Alpher devised Big Bang Theory of universe]". ''Discover'', July 1999. |
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*Felder, Gary, "[http://www.ncsu.edu/felder-public/kenny/papers/cosmo.html The Expanding universe]". |
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*LaRocco, Chris and Blair Rothstein, [http://www.umich.edu/~gs265/bigbang.htm "THE BIG BANG: It sure was BIG!!"]. |
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*Mather, John C., and John Boslough 1996, ''The very first light: the true inside story of the scientific journey back to the dawn of the universe''. ISBN 0-465-01575-1 p.300 |
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*Shestople, Paul, "[http://cosmology.berkeley.edu/Education/IUP/Big_Bang_Primer.html "Big Bang Primer"]. |
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*Singh, Simon, [[Big Bang (book)|''Big Bang: The most important scientific discovery of all time and why you need to know about it'']], Fourth Estate (2004). A historical review of the Big Bang. Sample text and reviews can be found at [http://www.321books.co.uk/reviews/big-bang-simon-singh.htm]. |
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*Wright, Edward L., [http://www.astro.ucla.edu/~wright/BBhistory.html "Brief History of the universe"]. |
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*Feuerbacher, Björn and Ryan Scranton (2006). "[http://www.talkorigins.org/faqs/astronomy/bigbang.html Evidence for the Big Bang]", FAQ at [http://www.talkorigins.org/ talkorigins.org]. |
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*[http://news.nationalgeographic.com/news/2006/03/0317_060317_big_bang.html "Proof of Big Bang Seen by Space Probe, Scientists Say" — ''National Geographic News''] |
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*[http://www.sabanciuniv.edu/do/eng/?PodCast/PodCasts.php The Sabanci University School of Languages Podcasts: Origin of Elements by Alpay Taralp]. |
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*[http://www.sciam.com/article.cfm?chanID=sa006&articleID=0009F0CA-C523-1213-852383414B7F0147 Scientific American Magazine (March 2005 Issue) Misconceptions about the Big Bang] |
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*[http://www.sciam.com/article.cfm?chanID=sa006&articleID=0009A312-037F-1448-837F83414B7F014D Scientific American Magazine (May 2006 Issue) The First Few Microseconds] |
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For an annotated list of textbooks and monographs, see [[physical cosmology#Textbooks|physical cosmology]]. |
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</div> |
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===Some primary sources=== |
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<div class="references-small"> |
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*G. Lemaître, "''Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques''" (A homogeneous universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae), ''Annals of the Scientific Society of Brussels'' '''47A''' (1927):41—[[General relativity]] implies the universe has to be expanding. Einstein brushed him off in the same year. Lemaître's note was translated in ''Monthly Notices of the Royal Astronomical Society'' '''91''' (1931): 483–490. |
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*G. Lemaître, ''Nature'' '''128''' (1931) suppl.: 704, with a reference to the primeval atom. |
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*R. A. Alpher, H. A. Bethe, G. Gamow, "The Origin of Chemical Elements,"''Physical Review'' '''73''' (1948), 803. The so-called αβγ paper, in which Alpher and Gamow suggested that the light elements were created by protons capturing neutrons in the hot, dense early universe. Bethe's name was added for symmetry. |
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*G. Gamow, "The Origin of Elements and the Separation of Galaxies," ''Physical Review'' '''74''' (1948), 505. These two 1948 papers of Gamow laid the foundation for our present understanding of big-bang nucleosynthesis. |
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*G. Gamow, ''Nature'' '''162''' (1948), 680. |
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*R. A. Alpher, "A Neutron-Capture Theory of the Formation and Relative Abundance of the Elements," ''Physical Review'' '''74''' (1948), 1737. |
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*R. A. Alpher and R. Herman, "On the Relative Abundance of the Elements," ''Physical Review'' '''74''' (1948), 1577. This paper contains the first estimate of the present temperature of the universe. |
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*R. A. Alpher, R. Herman, and G. Gamow ''Nature'' '''162''' (1948), 774. |
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*A. A. Penzias and R. W. Wilson, "A Measurement of Excess Antenna Temperature at 4080 Mc/s," ''Astrophysical Journal'' '''142''' (1965), 419. The paper describing the discovery of the cosmic microwave background. |
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*[[R. H. Dicke]], P. J. E. Peebles, P. G. Roll and [[D. T. Wilkinson]], "Cosmic Black-Body Radiation," ''Astrophysical Journal'' '''142''' (1965), 414. The theoretical interpretation of Penzias and Wilson's discovery. |
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*A. D. Sakharov, "Violation of CP invariance, C asymmetry and baryon asymmetry of the universe," ''Pisma Zh. Eksp. Teor. Fiz.'' '''5''', 32 (1967), translated in ''JETP Lett.'' '''5''', 24 (1967). |
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*R. A. Alpher and R. Herman, "Reflections on early work on 'big bang' cosmology" ''Physics Today'' '''Aug 1988''' 24–34. A review article. |
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</div> |
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===Religion and philosophy=== |
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<div class="references-small"> |
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* Leeming, David Adams, and Margaret Adams Leeming, ''A Dictionary of Creation Myths''. Oxford University Press (1995), ISBN 0195102754. |
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* Pius XII (1952), "Modern Science and the Existence of God," ''The Catholic Mind'' 49:182–192. |
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</div> |
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===WMAP results=== |
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<div class="references-small"> |
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* [http://skyandtelescope.com/news/article_1697_1.asp WMAP Bolsters Case for Cosmic Inflation] Sky & Telescope, March 17, 2006 |
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</div> |
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=== Research articles === |
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<div class="references-small"> |
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Most scientific papers about cosmology are initially released as preprints on [http://arxiv.org arxiv.org]. They are generally technical, but sometimes have introductions in plain English. The most relevant archives, which cover experiment and theory, are the [http://arxiv.org/archive/astro-ph astrophysics] archive, where papers closely grounded in observations are released, and the [http://arxiv.org/archive/gr-qc general relativity and quantum cosmology] archive, which covers more speculative ground. Papers of interest to cosmologists also frequently appear on the [http://arxiv.org/archive/hep-ph high energy phenomenology] and [http://arxiv.org/archive/hep-th high energy theory] archives. |
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{{featured article}} |
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[[Category:Astrophysics]] |
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[[Category:Physical cosmology]] |
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Revision as of 21:27, 7 August 2006
outoftuneviolin wuz here