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| {{Distinguish|Dark flow|Dark fluid|Dark matter}}
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| {{pp-move-indef|small=yes}}{{Cosmology|cTopic=Components}}
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| In [[physical cosmology]] and [[astronomy]], '''dark energy''' is a [[hypothesis|hypothetical]] form of [[energy]] that permeates all of space and tends to [[Accelerating universe|accelerate]] the [[Hubble's law|expansion of the universe]].<ref name="peebles">{{cite journal|author=Peebles, P. J. E. and Ratra, Bharat |title=The cosmological constant and dark energy|year=2003|journal=Reviews of Modern Physics|arxiv=astro-ph/0207347|volume=75|issue=2|pages=559–606|doi = 10.1103/RevModPhys.75.559|bibcode=2003RvMP...75..559P}}</ref> Dark energy is the most accepted hypothesis to explain observations since the 1990s that indicate that the universe is [[metric expansion of space|expanding]] at an [[deceleration parameter|accelerating rate]]. According to the [[Planck (spacecraft)#2013 data release|Planck mission team]], and based on the [[lambda-CDM model|standard model of cosmology]], on a [[mass–energy equivalence]] basis the [[universe]] contains 26.8% [[dark matter]] and 68.3% dark energy (for a total of 95.1%) and 4.9% [[matter|ordinary matter]].<ref name="planck_overview">{{cite journal |url=http://arxiv.org/pdf/1303.5062v1.pdf|title=Planck 2013 results. I. Overview of products and scientific results – Table 9. |journal=[[Astronomy and Astrophysics]] ''(submitted)'' |first1=P. A. R. |last1=Ade |first2=N. |last2=Aghanim |first3=C.|last3=Armitage-Caplan |last4=''et al''. (Planck Collaboration) |date=22 March 2013 |arxiv=1303.5062|bibcode = 2013arXiv1303.5062P }}</ref><ref name="planck_overview2">{{cite journal |url=http://www.sciops.esa.int/index.php?project=PLANCK&page=Planck_Published_Papers |title=Planck 2013 Results Papers |journal=[[Astronomy and Astrophysics]] ''(submitted)'' |first1=P. A. R. |last1=Ade |first2=N. |last2=Aghanim |first3=C.|last3=Armitage-Caplan |last4=''et al''. (Planck Collaboration) |date=31 March 2013 |arxiv=1303.5062|bibcode = 2013arXiv1303.5062P }}</ref><ref name="wmap7parameters">{{cite web|title = First Planck results: the Universe is still weird and interesting|url = http://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/}} </ref><ref name = DarkMatter> Sean Carroll, Ph.D., Cal Tech, 2007, The Teaching Company, ''Dark Matter, Dark Energy: The Dark Side of the Universe'', Guidebook Part 2 page 46, Accessed Oct. 7, 2013, "...dark energy: A smooth, persistent component of invisible energy, thought to make up about 70 percent of the current energy density of the universe. Dark energy is known to be smooth because it doesn't accumulate preferentially in galaxies and clusters..."</ref> Again on a mass-energy equivalence basis, the density of dark energy (1.67 × 10<sup>-27</sup> kg/m<sup>3</sup>) is very low: in the solar system, there are believed to be only 6 tons of dark energy within the radius of Pluto's orbit. However, it comes to dominate the mass-energy of the universe because it is uniform across space.<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/astro/dareng.html</ref>
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| Two proposed forms for dark energy are the [[cosmological constant]], a ''constant'' energy density filling space homogeneously,<ref name="carroll">{{cite journal|author=[[Sean M. Carroll|Carroll, Sean ]]|year=2001|url=http://relativity.livingreviews.org/Articles/lrr-2001-1/index.html|title=The cosmological constant|journal=Living Reviews in Relativity|volume=4|accessdate=2006-09-28}}</ref> and [[Scalar field theory|scalar fields]] such as [[quintessence (physics)|quintessence]] or [[moduli (physics)|moduli]], ''dynamic'' quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to [[vacuum energy]]. Scalar fields which do change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.
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| High-precision measurements of the [[Metric expansion of space|expansion of the universe]] are required to understand how the expansion rate changes over time. In [[general relativity]], the evolution of the expansion rate is parameterized by the cosmological [[Equation of State (Cosmology)|equation of state]] (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for dark energy is one of the biggest efforts in observational cosmology today.
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| Adding the cosmological constant to cosmology's standard [[Friedmann-Robertson-Walker metric|FLRW metric]] leads to the [[Lambda-CDM model]], which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a [[cyclic model]] for the universe.<ref name="frampton">{{cite journal|author=Baum, L. and Frampton, P.H. |title=Turnaround in Cyclic Cosmology|year=2007|journal=Physical Review Letters|arxiv=hep-th/0610213|volume=98|page=071301|doi = 10.1103/PhysRevLett.98.071301 |pmid=17359014|issue=7|bibcode=2007PhRvL..98g1301B}}</ref>
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| ==Nature of dark energy==
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| <!-- Proportions are wrong (see talk page) [[File:Cosmological composition.jpg|thumb|right|375px|As this NASA chart indicates, roughly 70% or more of the universe consists of dark energy, about which we know next to nothing.]] -->
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| Many things about the nature of dark energy remain matters of speculation. The evidence for dark energy is indirect but comes from three independent sources:
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| * Distance measurements and their relation to redshift, which suggest the universe has expanded more in the last half of its life.<ref name="Durrer">{{cite journal|author=R. Durrer|title=What do we really know about Dark Energy?|arxiv=1103.5331|year=2011|bibcode = 2011arXiv1103.5331D }}</ref>
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| * The theoretical need for a type of additional energy that is not matter or dark matter to form our observationally [[Shape of the Universe#Flat universe|flat universe]] (absence of any detectable global curvature).
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| * It can be inferred from measures of large scale wave-patterns of mass density in the universe.
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| Dark energy is thought to be very [[wikt:Homogeneous|homogeneous]], not very [[density|dense]] and is not known to interact through any of the [[fundamental forces]] other than [[gravity]]. Since it is quite rarefied—roughly 10<sup>−29</sup> g/cm<sup>3</sup>—it is unlikely to be detectable in laboratory experiments. Dark energy can have such a profound effect on the universe, making up 68% of universal density, only because it uniformly fills otherwise empty space. The two leading models are a [[cosmological constant]] and [[quintessence (physics)|quintessence]]. Both models include the common characteristic that dark energy must have negative pressure.
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| ===Effect of dark energy: a small constant negative pressure of vacuum===
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| Independently from its actual nature, dark energy would need to have a strong negative pressure (acting repulsively) in order to explain the observed [[Accelerating universe|acceleration]] in the [[Metric expansion of space|expansion rate of the universe]].
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| According to General Relativity, the pressure within a substance contributes to its gravitational attraction for other things just as its mass density does. This happens because the physical quantity that causes matter to generate gravitational effects is the [[stress–energy tensor]], which contains both the energy (or matter) density of a substance and its pressure and viscosity.
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| In the [[Friedmann-Lemaître-Robertson-Walker]] metric, it can be shown that a strong constant negative pressure in all the universe causes an acceleration in universe expansion if the universe is already expanding, or a deceleration in universe contraction if the universe is already contracting. More exactly, the second derivative of the universe scale factor, <math>\ddot{a}</math>, is positive if the [[Equation of state (cosmology)|equation of state]] of the universe is such that <math>\! w<-1/3</math>.{{citation needed|date=September 2012}}
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| This accelerating expansion effect is sometimes labeled "gravitational repulsion", which is a colorful but possibly confusing expression. In fact a negative pressure does not influence the gravitational interaction between masses—which remains attractive—but rather alters the overall evolution of the universe at the cosmological scale, typically resulting in the accelerating expansion of the universe despite the attraction among the masses present in the universe.
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| The acceleration is simply a function of dark energy density. Dark energy is persistent: its density remains constant (experimentally, within a factor of 1:10), i.e. it does not get diluted when space expands.
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| ==Evidence of existence==
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| ===Supernovae===
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| In 1998, published observations of [[Type Ia supernova]]e ("one-A") by the [[High-z Supernova Search Team]]<ref name="riess">{{cite journal|author=[[Adam Riess|Adam G. Riess]] ''et al.'' ([[Supernova Search Team]])|year=1998|title=Observational evidence from supernovae for an accelerating universe and a cosmological constant|journal=Astronomical J.|volume=116|issue=3|pages=1009–38|arxiv=astro-ph/9805201 |doi=10.1086/300499|bibcode=1998AJ....116.1009R}}</ref> followed in 1999 by the [[Supernova Cosmology Project]]<ref name="perlmutter">{{cite journal|author=[[Saul Perlmutter|Perlmutter, S.]] ''et al.'' (The [[Supernova Cosmology Project]])|journal=Astrophysical Journal|volume=517|issue=2|pages=565–86|year=1999|title=Measurements of Omega and Lambda from 42 high redshift supernovae|arxiv=astro-ph/9812133 |doi=10.1086/307221|bibcode=1999ApJ...517..565P}}</ref> suggested that the expansion of the [[universe]] is [[Deceleration parameter|accelerating]].<ref name="paalhorvathlukacs">The first paper, using observed data, which claimed a positive Lambda term was {{cite journal|author=Paal, G. ''et al.'' |year=1992|title=Inflation and compactification from galaxy redshifts?|journal=ApSS|volume=191|pages=107–24|bibcode=1992Ap&SS.191..107P |doi=10.1007/BF00644200}}</ref> The 2011 [[List of Nobel laureates in Physics|Nobel Prize in Physics]] was awarded to [[Brian P. Schmidt]] and [[Adam G. Riess]] for this work.<ref name=N11>{{cite web | title = The Nobel Prize in Physics 2011 | publisher = Nobel Foundation | url = http://nobelprize.org/nobel_prizes/physics/laureates/2011/index.html|accessdate=2011-10-04}}</ref><ref>[http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html The Nobel Prize in Physics 2011]. Perlmutter got half the prize, and the other half was shared between Schmidt and Riess.</ref>
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| Since then, these observations have been corroborated by several independent sources. Measurements of the [[cosmic microwave background]], [[gravitational lensing]], and the [[large-scale structure of the cosmos|large scale structure]] of the cosmos as well as improved measurements of supernovae have been consistent with the [[Lambda-CDM model]].<ref name="wmap">{{cite journal|author = Spergel, D. N. ''et al.'' (WMAP collaboration)|title = Wilkinson Microwave Anisotropy Probe (WMAP) three year results: implications for cosmology|url = http://lambda.gsfc.nasa.gov/product/map/current/map_bibliography.cfm|date=March 2006}}</ref> Some people argue that the only indication for the existence of dark energy is observations of distance measurements and associated redshifts. Cosmic microwave background anisotropies and baryon acoustic oscillations are only observations that redshifts are larger than expected from a "dusty" Friedmann–Lemaître universe and the local measured Hubble constant.<ref name="durrer">{{cite journal|author=Durrer, R.|year=2011|title=What do we really know about dark energy?|journal=[[Philosophical Transactions of the Royal Society A]]|volume=369|pages=5102–5114|arxiv=astro-ph/1103.5331 |doi=10.1098/rsta.2011.0285 |bibcode = 2011RSPTA.369.5102D }}</ref>
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| Supernovae are useful for cosmology because they are excellent [[standard candle]]s across cosmological distances. They allow the expansion history of the Universe to be measured by looking at the relationship between the distance to an object and its [[redshift]], which gives how fast it is receding from us. The relationship is roughly linear, according to [[Hubble's law]]. It is relatively easy to measure redshift, but finding the distance to an object is more difficult. Usually, astronomers use [[standard candle]]s: objects for which the intrinsic brightness, the [[absolute magnitude]], is known. This allows the object's distance to be measured from its actual observed brightness, or [[apparent magnitude]]. Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme and extremely consistent luminosity.
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| Recent observations of supernovae are consistent with a universe made up 71.3% of dark energy and 27.4% of a combination of [[dark matter]] and [[Baryon|baryonic matter]].<ref name="Kowalski2008">{{cite journal|last=Kowalski|first=Marek|coauthors=Rubin, David|date=October 27, 2008|title=Improved Cosmological Constraints from New, Old and Combined Supernova Datasets|journal=[[The Astrophysical Journal]]|publisher=[[University of Chicago Press]]|location=[[Chicago]], [[Illinois]]|volume=686|issue=2|pages=749–778|doi=10.1086/589937 |arxiv=0804.4142 |bibcode=2008ApJ...686..749K}}. They find a best fit value of the [[Lambda-CDM model#Parameters|dark energy density]], <math>\Omega_{\Lambda}</math> of 0.713+0.027–0.029([[Random error|stat]])+0.036–0.039([[Systematic error|sys]]), of the [[Lambda-CDM model#Parameters|total matter density]], <math>\Omega_{M}</math>, of 0.274+0.016–0.016(stat)+0.013–0.012(sys) with an [[Equation of state (cosmology)|equation of state parameter]] w of −0.969+0.059–0.063(stat)+0.063–0.066(sys).</ref>
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| ===Cosmic microwave background===
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| [[File:DMPie 2013.svg|thumb|right|250px|Estimated distribution of [[matter]] and [[energy]] in the [[universe]]]]
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| The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of [[cosmic microwave background]] (CMB) [[anisotropy|anisotropies]] indicate that the universe is close to [[flatness problem|flat]]. For the [[shape of the universe]] to be flat, the mass/energy density of the universe must be equal to the [[Friedmann equations#Density parameter|critical density]]. The total amount of matter in the universe (including [[baryons]] and [[dark matter]]), as measured from the CMB spectrum, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%.<ref name="wmap" /> The [[Wilkinson Microwave Anisotropy Probe]] (WMAP) spacecraft [[Wilkinson Microwave Anisotropy Probe#Seven-year data release|seven-year analysis]] estimated a universe made up of 72.8% dark energy, 22.7% dark matter and 4.5% ordinary matter.<ref name="wmap7parameters" />
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| Work done in 2013 based on the [[Planck spacecraft]] observations of the CMB gave a more accurate estimate of 68.3% of dark energy, 26.8% of dark matter and 4.9% of ordinary matter.<ref name="Washington Post">{{cite web|title=Big Bang’s afterglow shows universe is 80 million years older than scientists first thought|url=http://www.washingtonpost.com/world/europe/telescope-that-sees-big-bangs-afterglow-sees-older-universe-in-glimpse-of-first-split-second/2013/03/21/ada16076-920e-11e2-9173-7f87cda73b49_story_1.html|publisher=Washington Post|accessdate=22 March 2013}}</ref>
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| ===Large-scale structure===
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| The theory of [[Observable universe#Large-scale structure|large-scale structure]], which governs the formation of structures in the universe ([[star]]s, [[quasar]]s, [[galaxy|galaxies]] and [[galaxy groups and clusters]]), also suggests that the density of matter in the universe is only 30% of the critical density.
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| A 2011 survey, the WiggleZ galaxy survey of more than 200,000 galaxies, provided further evidence towards the existence of dark energy, although the exact physics behind it remains unknown.<ref>{{cite news| url=http://www.bbc.co.uk/news/science-environment-13462926|work=BBC News|title=New method 'confirms dark energy'|date=2011-05-19}}</ref><ref name=real/> The WiggleZ survey from [[Australian Astronomical Observatory]] scanned the galaxies to determine their redshift. Then, by exploiting the fact that [[baryon acoustic oscillations]] have left [[Void (astronomy)|voids]] regularly of ~150 Mpc diameter, surrounded by the galaxies, the voids were used as standard rulers to determine distances to galaxies as far as 2,000 Mpc (redshift 0.6), which allowed astronomers to determine more accurately the speeds of the galaxies from their redshift and distance. The data confirmed [[cosmic acceleration]] up to half of the age of the universe (7 billion years) and constrain its inhomogeneity to 1 part in 10.<ref name=real>[http://wigglez.swin.edu.au/site/prmay2011a.html Dark energy is real], Swinburne University of Technology, 19 May 2011</ref> This provides a confirmation to cosmic acceleration independent of supernovae.
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| ===Late-time integrated Sachs-Wolfe effect===
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| Accelerated cosmic expansion causes [[gravitational well|gravitational potential wells]] and hills to flatten as [[photons]] pass through them, producing cold spots and hot spots on the CMB aligned with vast supervoids and superclusters. This so-called late-time [[Integrated Sachs-Wolfe effect|Integrated Sachs-Wolfe effect (ISW)]] is a direct signal of dark energy in a flat universe.<ref>{{cite journal |author1=Crittenden |author2=Neil Turok |doi=10.1103/PhysRevLett.76.575 |journal=Phys. Rev. Lett. |volume=76 |title=Looking for $\Lambda$ with the Rees-Sciama Effect |issue=4 |pages=575–578 |year=1995 |arxiv=astro-ph/9510072 |bibcode=1996PhRvL..76..575C |pmid=10061494}}</ref> It was reported at high significance in 2008 by Ho ''et al.''<ref>{{cite journal |author1=Shirley Ho |author2=Hirata |author3=Nikhil Padmanabhan |author4=Uros Seljak |author5=Neta Bahcall |doi=10.1103/PhysRevD.78.043519 |journal=Phys. Rev. D |title=Correlation of CMB with large-scale structure: I. ISW Tomography and Cosmological Implications |volume=78 |issue=4 |year=2008 |arxiv=0801.0642|bibcode = 2008PhRvD..78d3519H}}</ref> and Giannantonio ''et al.''<ref>{{cite journal |author1=Tommaso Giannantonio |author2=Ryan Scranton |author3=Crittenden |author4=Nichol |author5=Boughn |author6=Myers |author7=Richards |doi=10.1103/PhysRevD.77.123520 |journal=Phys. Rev. D |title=Combined analysis of the integrated Sachs-Wolfe effect and cosmological implications |volume=77 |issue=12 |year=2008 |arxiv=0801.4380|bibcode = 2008PhRvD..77l3520G}}</ref>
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| == Theories of explanation ==
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| ===Cosmological constant===
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| {{main|Cosmological constant}}
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| {{Details|Equation of state (cosmology)}}
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| The simplest explanation for dark energy is that it is simply the "cost of having space": that is, a volume of space has some intrinsic, fundamental energy. This is the cosmological constant, sometimes called Lambda (hence [[Lambda-CDM model]]) after the Greek letter Λ, the symbol used to represent this quantity mathematically. Since energy and mass are related by ''E'' = ''mc''<sup>2</sup>, Einstein's theory of [[general relativity]] predicts that this energy will have a gravitational effect. It is sometimes called a [[vacuum energy]] because it is the energy density of empty [[vacuum]]. In fact, most theories of [[particle physics]] predict [[quantum fluctuation|vacuum fluctuation]]s that would give the vacuum this sort of energy. This is related to the [[Casimir Effect]], in which there is a small suction into regions where virtual particles are geometrically inhibited from forming (e.g. between plates with tiny separation). The cosmological constant is estimated by cosmologists to be on the order of 10<sup>−29</sup> g/cm<sup>3</sup>, or about 10<sup>−120</sup> in [[reduced Planck units]]{{Citation needed|date=November 2012}}. Particle physics predicts a natural value of 1 in reduced Planck units, leading to a large discrepancy.
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| The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to [[Deceleration parameter|accelerate]]. The reason why a cosmological constant has negative pressure can be seen from classical thermodynamics; Energy must be lost from inside a container to do work on the container. A change in volume ''dV'' requires work done equal to a change of energy −''P dV'', where ''P'' is the pressure. But the amount of energy in a container full of vacuum actually increases when the volume increases (''dV'' is positive), because the energy is equal to ''ρV'', where ''ρ'' (rho) is the energy density of the cosmological constant. Therefore, ''P'' is negative and, in fact, ''P'' = −''ρ''. | |
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| A major outstanding [[Unsolved problems in physics|problem]] is that most [[quantum field theory|quantum field theories]] predict a huge cosmological constant from the energy of the [[quantum fluctuation|quantum vacuum]], more than 100 [[orders of magnitude]] too large.<ref name="carroll"/> This would need to be cancelled almost, but not exactly, by an equally large term of the opposite sign. Some [[supersymmetry|supersymmetric]] theories require a cosmological constant that is exactly zero,{{Citation needed|date=May 2013}} which does not help because supersymmetry must be broken. The present scientific consensus amounts to [[extrapolating]] the [[empirical]] evidence where it is relevant to predictions, and [[fine-tuning]] theories until a more elegant solution is found. Technically, this amounts to checking theories against macroscopic observations. Unfortunately, as the known error-margin in the constant predicts the [[fate of the universe]] more than its present state, many such "deeper" questions remain unknown.
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| Another problem arises with inclusion of the cosmological constant in the standard model: i.e., the appearance of solutions with regions of discontinuities (see ''[[classification of discontinuities]]'' for three examples) at low matter density.<ref name="Oztas">{{cite journal|author=Öztas, A.M. and Smith, M.L. |title=Elliptical Solutions to the Standard Cosmology Model with Realistic Values of Matter Density|journal=International Journal of Theoretical Physics|year=2006|volume=45|issue=5|pages=925–936|doi=10.1007/s10773-006-9082-7|bibcode = 2006IJTP...45..896O}}</ref> Discontinuity also affects the past sign of the pressure assigned to the cosmological constant, changing from the current negative pressure to attractive, as one looks back towards the early Universe. A systematic, model-independent evaluation of the supernovae data supporting inclusion of the cosmological constant in the standard model indicates these data suffer systematic error. The supernovae data are not overwhelming evidence for an accelerating universe expansion which may be simply gliding.<ref name="Schwarz">{{cite journal|author=D.J. Schwarz and B. Weinhorst|title=(An)isotropy of the Hubble diagram: comparing hemispheres|journal=Astronomy & Astrophysics|year=2007|volume=474|issue=3|pages=717–729|doi=10.1051/0004-6361:20077998|bibcode=2007A&A...474..717S|arxiv = 0706.0165 }}</ref> A numerical evaluation of WMAP and supernovae data for evidence that our local group exists in a local void with poor matter density compared to other locations, uncovered possible conflict in the analysis used to support the cosmological constant.<ref name="Alexander">{{cite journal|author=Alexander, Stephon; Biswas, Tirthabir; Notari, Alessio and Vaid, Deepak |title=Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae|doi=10.1088/1475-7516/2009/09/025|arxiv=0712.0370|year=2008|journal=Journal of Cosmology and Astroparticle Physics|volume=2009|issue=09|pages=025–025|bibcode = 2009JCAP...09..025A }}</ref> A recent theoretical investigation found the cosmological time, dt, diverges for any finite interval, ds, associated with an observer approaching the cosmological horizon, representing a physical limit to observation. This is a key component required for a complete interpretation of astronomical observations, particularly pertaining to the nature of dark energy.<ref name="Melia">{{cite journal|author=Melia, F. and Abdelqader, M. |title=The Cosmological Spacetime|journal=International Journal of Modern Physics D|year=2009|volume=18|issue=12|pages=1889–1901|doi=10.1142/S0218271809015746|bibcode = 2009IJMPD..18.1889M|arxiv = 0907.5394 }}</ref> The identification of dark energy as a cosmological constant does not appear to be consistent with the data. These findings should be considered shortcomings of the standard model, but only when a term for vacuum energy is included.
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| In spite of its problems, the cosmological constant is in many respects the most [[Occam's razor|economical solution]] to the problem of [[cosmic acceleration]]. One number successfully explains a multitude of observations. Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant as an essential feature.
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| ===Quintessence===
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| {{main|Quintessence (physics)}}
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| In [[quintessence (physics)|quintessence]] models of dark energy, the observed acceleration of the scale factor is caused by the potential energy of a dynamical [[scalar field|field]], referred to as quintessence field. Quintessence differs from the cosmological constant in that it can vary in space and time. In order for it not to clump and form [[large-scale structure of the cosmos|structure]] like matter, the field must be very light so that it has a large [[Compton wavelength]].
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| No evidence of quintessence is yet available, but it has not been ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some scientists think that the best evidence for quintessence would come from violations of Einstein's [[equivalence principle]] and [[equivalence principle#Tests of the Einstein equivalence principle|variation of the fundamental constants]] in space or time.{{Citation needed|date=March 2011}} [[Scalar field]]s are predicted by the [[standard model]] and [[string theory]], but an analogous problem to the cosmological constant problem (or the problem of constructing models of [[cosmic inflation]]) occurs: [[renormalization]] theory predicts that scalar fields should acquire large masses.
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| <cite id=cosmiccoincidence>The '''cosmic coincidence problem''' asks why the [[cosmic acceleration]] began when it did. If [[cosmic acceleration]] began earlier in the universe, structures such as [[galaxy|galaxies]] would never have had time to form and life, at least as we know it, would never have had a chance to exist. Proponents of the [[anthropic principle]] view this as support for their arguments. However, many models of quintessence have a so-called '''tracker''' behavior, which solves this problem. In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until [[big bang|matter-radiation equality]], which triggers quintessence to start behaving as dark energy, eventually dominating the universe. This naturally sets the low [[energy scale]] of the dark energy.</cite>{{cn|date=October 2013}}
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| In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary (w=−1) from above to below. A No-Go theorem has been proved that gives this scenario at least two degrees of freedom as required for dark energy models. This scenario is so-called [[Quintom scenario]].
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| Some special cases of quintessence are [[phantom energy]], in which the energy density of quintessence actually increases with time, and k-essence (short for kinetic quintessence) which has a non-standard form of [[kinetic energy]]. They can have unusual properties: [[phantom energy]], for example, can cause a [[Big Rip]].
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| ==Alternative ideas==
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| Some alternatives to dark energy aim to explain the observational data by a more refined use of established theories, focusing, for example, on the gravitational effects of density inhomogeneities, or on consequences of [[electroweak symmetry breaking]] in the early universe. If we are located in an emptier-than-average region of space, the observed cosmic expansion rate could be mistaken for a variation in time, or acceleration.<ref>{{cite journal |last=Wiltshire |first=David L. |year=2007 |title= Exact Solution to the Averaging Problem in Cosmology |journal=Phys. Rev. Lett. |volume=99 |issue= 25|page=251101 |doi=10.1103/PhysRevLett.99.251101 |pmid=18233512 |bibcode=2007PhRvL..99y1101W|arxiv = 0709.0732 }}</ref><ref>{{cite journal |author1=Ishak, Mustapha |author2=Richardson, James |author3=Garred, David |author4=Whittington, Delilah |author5=Nwankwo, Anthony |author6=Sussman, Roberto |doi=10.1103/PhysRevD.78.123531 |journal=Phys. Rev. D|title=Dark Energy or Apparent Acceleration Due to a Relativistic Cosmological Model More Complex than FLRW? |volume=78 |issue=12 |year=2007 |arxiv=0708.2943|bibcode = 2008PhRvD..78l3531I}}</ref><ref>{{cite journal|author1=Mattsson, Teppo |doi=10.1007/s10714-009-0873-z|journal=Gen. Rel. Grav.|volume=42|title=Dark energy as a mirage|issue=3|pages=567–599|year=2007|arxiv=0711.4264|bibcode = 2010GReGr..42..567M}}</ref><ref>{{cite journal |last=Clifton |first=Timothy |coauthors=Ferreira, Pedro |date=April 2009 |title= Does Dark Energy Really Exist? |journal=Scientific American |volume=300 |issue=4 |pages=48–55 |doi= 10.1038/scientificamerican0409-48|url=http://www.sciam.com/article.cfm?id=does-dark-energy-exist |accessdate=April 30, 2009 |pmid=19363920}}</ref> A different approach uses a cosmological extension of the [[equivalence principle]] to show how space might appear to be expanding more rapidly in the voids surrounding our local cluster. While weak, such effects considered cumulatively over billions of years could become significant, creating the illusion of cosmic acceleration, and making it appear as if we live in a [[Hubble Bubble (astronomy)|Hubble bubble]].<ref>{{cite web|last=Wiltshire|first=David|title=Title: Cosmological equivalence principle and the weak-field limit|url=http://arxiv.org/abs/0809.1183|accessdate=27 January 2013}}</ref> <ref>{{cite web|last=Gray|first=Stuart|title=Dark questions remain over dark energy|url=http://www.abc.net.au/science/articles/2009/12/09/2765371.htm|publisher=ABC Science Australia|accessdate=27 January 2013}}</ref><ref>{{cite news|last=Merali|first=Zeeya|title=Is Einstein's Greatest Work All Wrong—Because He Didn't Go Far Enough?|url=http://discovermagazine.com/2012/mar/09-is-einsteins-greatest-work-wrong-didnt-go-far|accessdate=27 January 2013|newspaper=Discover magazine|date=March 2012}}</ref>
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| Another class of theories attempts to come up with an all-encompassing theory of both dark matter and dark energy as a single phenomenon that modifies the laws of gravity at various scales. An example of this type of theory is the theory of [[dark fluid]]. Another class of theories that unifies dark matter and dark energy are suggested to be covariant theories of modified gravities. These theories alter the dynamics of the space-time such that the modified dynamic stems what have been assigned to the presence of dark energy and dark matter.<ref>{{cite journal |last=Exirifard |first=Q. |year=2010 |pages=93–106 |volume=43|title=Phenomenological covariant approach to gravity |journal=General Relativity and Gravitation |doi=10.1007/s10714-010-1073-6|bibcode = 2011GReGr..43...93E|arxiv = 0808.1962 }}</ref>
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| A 2011 paper in the journal [[Physical Review|Physical Review D]] by Christos Tsagas, a cosmologist at Aristotle University of Thessaloniki in Greece, argued that it is likely that the accelerated expansion of the universe is an illusion caused by the relative motion of us to the rest of the universe. The paper cites data showing that the 2.5 billion ly wide region of space we are inside of is moving very quickly relative to everything around it. If the theory is confirmed, then dark energy would not exist (but the "[[dark flow]]" still might).<ref>Wolchover, Natalie (27 September 2011) [http://www.msnbc.msn.com/id/44690771/ns/technology_and_science-science/#.ToNr_6h4Cdd 'Accelerating universe' could be just an illusion], msnbc.msn.com</ref><ref>{{cite journal|last=Tsagas|first=Christos G.|title=Peculiar motions, accelerated expansion, and the cosmological axis|journal=Physical Review D|year=2011|volume=84|pages=063503|doi=10.1103/PhysRevD.84.063503|bibcode = 2011PhRvD..84f3503T |arxiv = 1107.4045 }}</ref>
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| Some theorists think that dark energy and [[cosmic acceleration]] are a failure of [[general relativity]] on very large scales, larger than [[supercluster]]s.{{citation needed|date=November 2010}} However most attempts at modifying general relativity have turned out to be either equivalent to theories of [[quintessence (physics)|quintessence]], or inconsistent with observations.{{citation needed|date=November 2010}} Other ideas for dark energy have come from [[string theory]], [[brane cosmology]] and the [[holographic principle]], but have not yet proved{{citation needed|date=November 2010}} as compellingly as quintessence and the cosmological constant.
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| On string theory, an article in the journal [[nature (journal)|''Nature'']] described:
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| {{quote|String theories, popular with many particle physicists, make it possible, even desirable, to think that the observable universe is just one of 10<sup>500</sup> universes in a grander [[multiverse]], says <nowiki></nowiki>[[Leonard Susskind]], a cosmologist at Stanford University in California<nowiki></nowiki>. The vacuum energy will have different values in different universes, and in many or most it might indeed be vast. But it must be small in ours because it is only in such a universe that observers such as ourselves can evolve.|<ref name="Nature"/>}}
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| [[Paul Steinhardt]] in the same article criticizes string theory's explanation of dark energy stating "...Anthropics and randomness don't explain anything... I am disappointed with what most theorists are willing to accept".<ref name="Nature">{{cite journal |last=Hogan |first=Jenny |year=2007 |title=Unseen Universe: Welcome to the dark side |journal=Nature |volume=448 |issue=7151 |pages=240–245 |doi=10.1038/448240a |pmid=17637630 |bibcode = 2007Natur.448..240H}}</ref>
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| Another set of proposals is based on the possibility of a double [[metric tensor]] for space-time.<ref>{{cite journal |last=Hossenfelder |first=S. |year=2008|title=A Bi-Metric Theory with Exchange Symmetry |journal=Physical Review D |volume=78 |issue=4 |page=044015 |doi=10.1103/PhysRevD.78.044015|bibcode = 2008PhRvD..78d4015H|arxiv = 0807.2838 }}</ref><ref>{{cite journal |doi=10.1142/S0217751X05024602 |last=Henry-Couannier |first=F. |year=2005 |journal=International Journal of Modern Physics A|volume=20 |issue=11 |page=2341|arxiv = gr-qc/0410055 |bibcode = 2005IJMPA..20.2341H}}</ref> It has been argued that time reversed solutions in [[general relativity]] require such double metric for consistency, and that both [[dark matter]] and dark energy can be understood in terms of time reversed solutions of general relativity.<ref>{{cite journal|author1 = Ripalda, Jose M.|title = Time reversal and negative energies in general relativity |arxiv = gr-qc/9906012|journal = Eprint arXiv|page = 6012|year = 1999|bibcode = 1999gr.qc.....6012R}}</ref>
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| ==Implications for the fate of the universe==
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| Cosmologists estimate that the [[Deceleration parameter|acceleration]] began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of [[dark matter]] and [[baryon]]s. The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates. Specifically, when the volume of the universe doubles, the density of [[dark matter]] is halved, but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).
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| If the acceleration continues indefinitely, the ultimate result will be that galaxies outside the [[local supercluster]] will have a [[radial velocity|line-of-sight velocity]] that continually increases with time, eventually far exceeding the speed of light.<ref>{{cite journal |url=http://www.scientificamerican.com/article.cfm?id=the-end-of-cosmology |title=The End of Cosmology?|journal= Scientific American |date=March 2008|author=Krauss, Lawrence M. and Scherrer, Robert J. | volume= 82|accessdate=2011-01-06}}</ref> This is not a violation of [[special relativity]] because the notion of "velocity" used here is different from that of velocity in a local [[inertial frame of reference]], which is still constrained to be less than the speed of light for any massive object (see [[Comoving distance#Uses of the proper distance|Uses of the proper distance]] for a discussion of the subtleties of defining any notion of relative velocity in cosmology). Because the [[Hubble's law#Interpretation|Hubble parameter]] is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.<ref>[http://curious.astro.cornell.edu/question.php?number=575 Is the universe expanding faster than the speed of light?] (see the last two paragraphs)</ref><ref name=ly93>{{cite web|last = Lineweaver|first = Charles|coauthors = Tamara M. Davis|year = 2005|url = http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf|title = Misconceptions about the Big Bang|publisher = Scientific American|accessdate = 2008-11-06}}</ref> However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmological [[event horizon]] where any light they emit past that point will never be able to reach us at any time in the infinite future<ref>{{cite journal|last = Loeb|first = Abraham|title = The Long-Term Future of Extragalactic Astronomy|journal = Physical Review D|volume = 65|issue = 4|year = 2002|doi = 10.1103/PhysRevD.65.047301|arxiv=astro-ph /0107568|bibcode = 2002PhRvD..65d7301L}}</ref> because the light never reaches a point where its "peculiar velocity" toward us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in [[Comoving distance#Uses of the proper distance|Uses of the proper distance]]). Assuming the dark energy is constant (a [[cosmological constant]]), the current distance to this cosmological event horizon is about 16 billion light years, meaning that a signal from an event happening ''at present'' would eventually be able to reach us in the future if the event were less than 16 billion light years away, but the signal would never reach us if the event were more than 16 billion light years away.<ref name=ly93 />
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| As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and more [[redshift]]ed, to the point where the wavelength becomes too large to detect in practice and the galaxies appear to disappear completely<ref>{{cite journal|last = Krauss|first = Lawrence M.|coauthors = Robert J. Scherrer|title = The Return of a Static Universe and the End of Cosmology|journal = General Relativity and Gravitation|volume = 39|issue = 10|pages = 1545–1550|year = 2007|doi = 10.1007/s10714-007-0472-9|arxiv=0704.0221|bibcode = 2007GReGr..39.1545K}}</ref><ref>[http://www.npr.org/templates/story/story.php?storyId=102715275 Using Tiny Particles To Answer Giant Questions]. Science Friday, 3 Apr 2009. According to the [http://www.npr.org/templates/transcript/transcript.php?storyId=102715275 transcript], [[Brian Greene]] makes the comment "And actually, in the far future, everything we now see, except for our local galaxy and a region of galaxies will have disappeared. The entire universe will disappear before our very eyes, and it's one of my arguments for actually funding cosmology. We've got to do it while we have a chance."</ref> (''see'' [[Future of an expanding universe#Galaxies outside the Local Supercluster are no longer detectable|Future of an expanding universe]]). The [[Earth]], the [[Milky Way]], and the [[Virgo supercluster]], however, would remain virtually undisturbed while the rest of the universe recedes and disappears from view. In this scenario, the local supercluster would ultimately suffer [[Heat death of the universe|heat death]], just as was thought for the flat, matter-dominated universe before measurements of [[cosmic acceleration]].
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| There are some very speculative ideas about the future of the universe. One suggests that phantom energy causes ''divergent'' expansion, which would imply that the effective force of dark energy continues growing until it dominates all other forces in the universe. Under this scenario, dark energy would ultimately tear apart all gravitationally bound structures, including galaxies and solar systems, and eventually overcome the [[electric force|electrical]] and [[nuclear force]]s to tear apart atoms themselves, ending the universe in a "[[Big Rip]]". On the other hand, dark energy might dissipate with time or even become attractive. Such uncertainties leave open the possibility that gravity might yet rule the day and lead to a universe that contracts in on itself in a "[[Big Crunch]]". Some scenarios, such as the [[cyclic model]], suggest this could be the case. It is also possible the universe may never have an end and continue in its present state forever (see [[Ludwig Boltzmann#The Second Law as a law of disorder|The Second Law as a law of disorder]]). While these ideas are not supported by observations, they are not ruled out.
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| ==History of discovery and previous speculation==
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| {{citations missing|section|date=November 2010}}
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| The cosmological constant was first proposed by [[Albert Einstein|Einstein]] as a mechanism to obtain a solution of the [[Einstein's field equation|gravitational field equation]] that would lead to a static universe, effectively using dark energy to balance gravity.<ref name="Einstein">{{cite journal|author=Harvey, Alex |title=How Einstein Discovered Dark Energy|journal=arXiv|year=2012|volume=|issue=|pages=|arxiv = 1211.6338 |bibcode = 2012arXiv1211.6338H }}</ref> Not only was the mechanism an inelegant example of [[fine-tuning]] but it was also soon realized that Einstein's static universe would actually be unstable because local inhomogeneities would ultimately lead to either the runaway expansion or contraction of the universe. The [[dynamic equilibrium|equilibrium]] is unstable: If the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise, a universe which contracts slightly will continue contracting. These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout the universe. More importantly, observations made by [[Edwin Hubble]] showed that the universe appears to be expanding and not static at all. Einstein reportedly referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder.<ref>George Gamow, in his autobiography "My World Line: An Informal Autobiography" (1970) on page 44 writes "Much later, when I was discussing cosmological problems with Einstein, he remarked that the introduction of the cosmological term was the biggest blunder he ever made in his life." – Here the "cosmological term" refers to the cosmological constant in the equations of general relativity, whose value Einstein initially picked to ensure that his model of the universe would neither expand nor contract; if he hadn't done this he might have theoretically predicted the universal expansion that was first observed by Edwin Hubble.</ref>
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| [[Alan Guth]] proposed in the 1970s that a negative pressure field, similar in concept to dark energy, could drive [[cosmic inflation]] in the very early universe. Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the [[Big Bang]]. Such expansion is an essential feature of most current models of the Big Bang. However, inflation must have occurred at a much higher energy density than the dark energy we observe today and is thought to have completely ended when the universe was just a fraction of a second old. It is unclear what relation, if any, exists between dark energy and inflation. Even after inflationary models became accepted, the cosmological constant was thought to be irrelevant to the current universe.
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| The term "dark energy", echoing [[Fritz Zwicky]]'s "dark matter" from the 1930s, was coined by [[Michael Turner (cosmologist)|Michael Turner]] in 1998.<ref>The first appearance of the term "dark energy" is in the article with another cosmologist and Turner's student at the time, Dragan Huterer, "Prospects for Probing the Dark Energy via Supernova Distance Measurements", which was posted to the [[ArXiv.org e-print archive]] in [http://arxiv.org/abs/astro-ph/9808133 August 1998] and published in Physical Review D in 1999 (Huterer and Turner, Phys. Rev. D 60, 081301 (1999)), although the manner in which the term is treated there suggests it was already in general use. Cosmologist Saul Perlmutter has credited Turner with coining the term [http://www.lbl.gov/Science-Articles/Archive/dark-energy.html in an article] they wrote together with Martin White of the University of Illinois for [http://arxiv.org/abs/astro-ph/9901052v2 Physical Review Letters], where it is introduced in quotation marks as if it were a neologism.</ref> By that time, the missing mass problem of [[big bang nucleosynthesis]] and [[Large-scale structure of the cosmos|large scale structure]] was established, and some cosmologists had started to theorize that there was an additional component to our universe. The first direct evidence for dark energy came from supernova observations of [[deceleration parameter|accelerated expansion]] in [[Adam Riess|Riess]] ''et al.''<ref name="riess" /> and later confirmed in [[Saul Perlmutter|Perlmutter]] ''et al.''<ref name="perlmutter" /> This resulted in the [[Lambda-CDM model]], which as of 2006 is consistent with a series of increasingly rigorous cosmological observations, the latest being the 2005 Supernova Legacy Survey. First results from the SNLS reveal that the average behavior (i.e., equation of state) of dark energy behaves like Einstein's cosmological constant to a precision of 10%.<ref name="snls">{{cite journal|author=Astier, Pierre ''et al.'' ([[Supernova Legacy Survey]])|title=The Supernova legacy survey: Measurement of omega(m), omega(lambda) and W from the first year data set|journal=Astronomy and Astrophysics|volume=447|pages=31–48|year=2006|arxiv=astro-ph/0510447|doi=10.1051/0004-6361:20054185|bibcode=2006A&A...447...31A}}</ref> Recent results from the Hubble Space Telescope Higher-Z Team indicate that dark energy has been present for at least 9 billion years and during the period preceding cosmic acceleration.
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| ==See also==
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| * [[De Sitter relativity]]
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| * [[The Dark Energy Survey]]
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| * [[Vacuum state]]
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| ==References==
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| {{reflist|30em}}
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| ==External links==
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| * {{In Our Time|Dark Energy|p003k9g5|Dark_Energy}}
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| *[http://physicsworld.com/cws/article/indepth/2010/jun/02/dark-energy-how-the-paradigm-shifted Dark energy: how the paradigm shifted] Physicsworld.com
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| *{{cite news|work=The New York Times| title=9 Billion-Year-Old 'Dark Energy' Reported|date=November 2006|author=Dennis Overbye| url=http://www.nytimes.com/2006/11/17/science/space/17dark.html?em&ex=1163998800&en=f02de71136ca5dd5&ei=5087%0A}}
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| *[http://news.bbc.co.uk/2/hi/science/nature/6156110.stm "Mysterious force's long presence"] BBC News online (2006) More evidence for dark energy being the cosmological constant
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| *[http://antwrp.gsfc.nasa.gov/apod/ap020529.html "Astronomy Picture of the Day"] one of the images of the [[Cosmic Microwave Background]] which confirmed the presence of dark energy and dark matter
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| *[http://www.cfht.hawaii.edu/SNLS SuperNova Legacy Survey home page] The Canada-France-Hawaii Telescope Legacy Survey Supernova Program aims primarily at measuring the equation of state of Dark Energy. It is designed to precisely measure several hundred high-redshift supernovae.
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| *[http://arxiv.org/abs/astro-ph/0609591 "Report of the Dark Energy Task Force"]
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| *[http://hubblesite.org/hubble_discoveries/dark_energy/ "HubbleSite.org – Dark Energy Website"] Multimedia presentation explores the science of dark energy and Hubble's role in its discovery.
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| *[http://arxiv.org/abs/astro-ph/0607066 "Surveying the dark side"]
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| *[http://th-www.if.uj.edu.pl/acta/vol38/pdf/v38p3633.pdf "Dark energy and 3-manifold topology"] [[Acta Physica Polonica]] 38 (2007), p. 3633–3639
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| *[https://www.darkenergysurvey.org/ The Dark Energy Survey]
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| *[http://jdem.gsfc.nasa.gov/ The Joint Dark Energy Mission]
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| *[http://chandra.harvard.edu/press/08_releases/press_121608.html Harvard: Dark Energy Found Stifling Growth in Universe], primary source
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| *[http://www.smithsonianmag.com/science-nature/Dark-Energy-The-Biggest-Mystery-in-the-Universe.html April 2010 Smithsonian Magazine Article]
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| *[http://hetdex.org/ HETDEX Dark energy experiment]
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| *[http://blogs.discovermagazine.com/cosmicvariance/2011/10/04/dark-energy-faq Dark Energy FAQ]
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| {{Breakthrough of the Year}}
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| {{DEFAULTSORT:Dark Energy}}
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| [[Category:Energy (physics)]]
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| [[Category:Physical cosmology]]
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| [[Category:Unsolved problems in astronomy]]
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| [[Category:Unsolved problems in physics]]
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