Dark energy
84867
224570758
2008-07-09T13:45:46Z
99.248.244.13
/* Supernovae */
{{Cosmology|cTopic=Components}}
In [[physical cosmology]], '''dark energy''' is a hypothetical form of [[energy]] that permeates all of space and tends to increase the [[Hubble's law|rate of expansion of the
universe]].<ref name="peebles">{{cite journal|author=P. J. E. Peebles and Bharat Ratra|title=The cosmological constant and dark energy|date=2003|journal=Reviews of Modern Physics|url=http://www.arxiv.org/abs/astro-ph/0207347|volume=75|pages=559–606 | doi = 10.1103/RevModPhys.75.559|format=subscription required}}</ref> Assuming the existence of dark energy is the most popular way to explain recent observations that the universe appears to be [[metric expansion of space|expanding]] at an [[deceleration parameter|accelerating rate]]. In the [[lambda-CDM model|standard model of cosmology]], dark energy currently accounts for 73% of the total [[Mass-energy equivalence|mass-energy]] of the universe.
Two proposed forms for dark energy are the [[cosmological constant]], a ''constant'' energy density filling space homogeneously,<ref name="carroll">{{cite journal|author=[[Sean Carroll]]|date=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|pages=1|doi=10.1038/nphys815-<span|doi_brokendate=2008-06-26}}</ref> and [[Scalar field theory|scalar field]]s such as [[quintessence (physics)|quintessence]] or [[moduli]], ''dynamic'' quantities whose energy density can vary in time and space. Contributions from [[Scalar field theory|scalar field]]s that are constant in space, are usually also included in the [[cosmological constant]]. The [[cosmological constant]] is physically 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.
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]]. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today.
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<ref name="frampton">{{cite journal|author=L.Baum and P.H. Frampton|title=Turnaround in Cyclic Cosmology|date=2007|journal=Physical Review Letters|
url=http://www.arxiv.org/abs/hep-th/0610213|volume=98|pages=071301 | doi = 10.1103/PhysRevLett.98.071301|format=subscription required
}}</ref> to formulate
a [[cyclic model]] for the universe.
==Evidence for dark energy==
===Supernovae===
In 1998, observations of [[type Ia supernova]]e ("one-A") by the [[Supernova Cosmology Project]] at the [[Lawrence Berkeley National Laboratory]] and the [[High-z Supernova Search Team]] suggested that the expansion of the [[universe]] is [[Deceleration parameter|accelerating]].<ref name="perlmutter">{{cite journal|author=[[Saul Perlmutter|S. Perlmutter]] ''et al.'' (The [[Supernova Cosmology Project]])|journal=Astrophysical J.|volume=517|pages=565–86|date=1999|title=Measurements of Omega and Lambda from 42 high redshift supernovae|url=http://www.arxiv.org/abs/astro-ph/9812133|format=subscription required}}</ref><ref name="riess">{{cite journal|author=[[Adam Riess|Adam G. Riess]] ''et al.'' ([[Supernova Search Team]])|date=1998|title=Observational evidence from supernovae for an accelerating universe and a cosmological constant|journal=Astronomical J.|volume=116|pages=1009–38|url=http://www.arxiv.org/abs/astro-ph/9805201|format=subscription required}}</ref> 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 = D. N. Spergel ''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 | year = 2006 | month = March}}</ref>
Supernovae are so 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 actually observed brightness, or [[apparent magnitude]]. Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme, and extremely consistent, brightness.
=== Cosmic Microwave Background ===
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) anisotropies, most recently by the [[Wilkinson Microwave Anisotropy Probe|WMAP]] satellite, indicate that the universe is very close to flat. For the [[shape of the universe]] to be [[flatness problem|flat]], the mass/energy density of the universe must be equal to a certain [[critical density]]. The total amount of matter in the universe (including [[baryons]] and [[dark matter]]), as measured by the CMB, 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 most recent WMAP observations are consistent with a universe made up of 74% dark energy, 22% dark matter, and 4% ordinary matter.
=== Large-Scale Structure ===
The theory of [[large-scale structure of the cosmos|large scale structure]], which governs the formation of structure in the universe ([[star]]s, [[quasar]]s, [[galaxy|galaxies]] and [[groups and clusters of galaxies|galaxy clusters]]), also suggests that the density of matter in the universe is only 30% of the critical density.
=== Late-time Integrated Sachs-Wolfe Effect ===
Accelerated cosmic expansion causes [[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|ISW]] effect is a direct signal of dark energy in a flat universe<ref>[http://arxiv.org/abs/astro-ph/9510072 "Looking for Lambda with the Rees-Sciama Effect"], Crittenden R.G., & Turok N., 1996, Phys. Rev. Lett., 76, 575</ref>, and has recently been detected at high significance by Ho et al.<ref>[http://lanl.arxiv.org/abs/0801.0642 "Correlation of CMB with large-scale structure: I. ISW Tomography and Cosmological Implications"], Ho et al., 2008, Phys Rev. D, submitted</ref> and Giannantonio et al.<ref>[http://arxiv.org/abs/0801.4380 "Combined analysis of the integrated Sachs-Wolfe effect and cosmological implications"], Giannantonio et al., 2008, Phys. Rev. D, in press</ref>. In May 2008, Granett, Neyrinck & Szapudi <ref>[http://arxiv.org/abs/0805.3695 "An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect"], Granett, Neyrinck & Szapudi, 2008, ApJL, submitted</ref> found arguably the clearest evidence yet for the ISW effect, imaging the average imprint of superclusters and supervoids on the CMB.
==Nature of dark energy==
[[Image:Cosmological composition.jpg|thumb|right|375px|As this NASA chart indicates, roughly 70 percent or more of the universe consists of dark energy, about which we know next to nothing.]]
The exact nature of this dark energy is a matter of speculation. It is known to be very [[wiktionary:Homogeneity|homogeneous]], not very [[density|dense]] and is not known to interact through any of the [[fundamental forces]] other than [[gravity]]. Since it is not very dense—roughly 10<sup>−29</sup> grams per cubic centimeter—it is hard to imagine experiments to detect it in the laboratory. Dark energy can only have such a profound impact on the universe, making up 70% of all energy, because it uniformly fills otherwise empty space. The two leading models are quintessence and the cosmological constant. Both models include the common characteristic that dark energy must have negative [[pressure]].
===Negative Pressure===
Independently from its actual nature, dark energy would need to have a strong negative [[pressure]] in order to explain the observed [[Accelerating_universe|acceleration]] in the [[Metric expansion of space|expansion rate of the universe]].
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.
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>.
This [[Accelerating_universe|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.
===Cosmological constant===
{{main article|Cosmological constant}}
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 mathematically represent this quantity. Since energy and mass are related by <math>E=mc^2</math>, Einstein's theory of [[general relativity]] predicts that it 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 [[vacuum fluctuations]] that would give the vacuum exactly 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/cm³, or about 10<sup>−120</sup> in [[reduced Planck units]]. Particle physics predicts a natural value of 1 in reduced Planck units, quite a bit off.
The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to [[Deceleration parameter|accelerate]] (see ''[[equation of state (cosmology)]]''). 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 box of vacuum energy 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''.
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 [[vacuum fluctuation|vacuum]], more than 100 [[orders of magnitude]] too large.<ref name="Nature"/> 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, which does not help. 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. Philosophically, our most elegant solution may be to say that if things were different, we would not be here to observe anything — the [[anthropic principle]].<ref name="Weinberg1987">{{cite journal |last=Weinberg |first=Steven |authorlink=Steven Weinberg |coauthors= |year=1987 |month= |title=Anthropic bound on the cosmological constant |journal=[[Physical Review Letters]] |volume=59 |issue=22 |pages=2607–2610 |doi=10.1103/PhysRevLett.59.2607 |url= |accessdate= |quote= }}</ref> 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.
Another problem arises with inclusion of the cosmic constant in the standard model, which is appearance of solutions with regions of discontinuities (see ''[[classification of discontinuities]]'' for three examples) at low matter density.<ref name="Oztas">{{cite journal|author=A.M. Öztas and M.L. Smith|title=Elliptical Solutions to the Standard Cosmology Model with Realistic Values of Matter Density|journal=International Journal of Theoretical Physics|date=2006|volume=45|pages=925–936}}</ref> The discontinuity also affects the past sign of the pressure assigned to the cosmic 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 cosmic 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|date=2007|volume=474|pages=717–729|doi=10.1051/0004-6361:20077998}}</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 cosmic constant.<ref name=”Alexander”>{{cite journal|author=Stephon Alexander, Tirthabir Biswas, Alessio Notari, and Deepak Vaid|title=Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae|journal=arXiv.0712.0370v2[astro-ph]|date=2008}}</ref> These findings should be considered shortcomings of the standard model, but only when a term for vacuum energy is included.
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.
===Quintessence===
{{main article|Quintessence (physics)}}
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]].
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#Some tests of the Einstein equivalence principle|variation of the fundamental constants]] in space or time. [[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.
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.
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]].
===Alternative ideas===
Some theorists think that dark energy and [[cosmic acceleration]] are a failure of [[general relativity]] on very large scales, larger than [[supercluster]]s. It is a tremendous extrapolation to think that our law of gravity, which works so well in the [[solar system]], should work without correction on the scale of the universe. Most attempts at modifying general relativity, however, have turned out to be either equivalent to theories of [[quintessence (physics)|quintessence]], or inconsistent with observations. It is of interest to note that if the equation for gravity were to approach r instead of r<sup>2</sup> at large, intergalactic distances, then the acceleration of the expansion of the universe becomes a mathematical artifact,{{huh}} negating the need for the existence of Dark Energy.
Alternative ideas for dark energy have come from [[string theory]], [[brane cosmology]] and the [[holographic principle]], but have not yet proved as compelling as quintessence and the cosmological constant. On string theory, an article in the journal [[nature (journal)|''Nature'']] described:
<blockquote>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"/></blockquote>
[[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 |authorlink= |coauthors= |year=2007 |month= |title=Unseen Universe: Welcome to the dark side |journal=Nature |volume=448 |issue=7151 |pages=240–245 |doi=10.1038/448240a |url= |accessdate= |quote= }}</ref>
Yet another, "radically conservative" class of proposals aims to explain the observational data by a more refined use of established theories rather than through the introduction of dark energy, focusing, for example, on the gravitational effects of density inhomogeneities <ref>{{cite journal |last=Wiltshire |first=David L. |authorlink= |coauthors= |year=2007 |month= |title= Exact Solution to the Averaging Problem in Cosmology |journal=Phys. Rev. Lett. |volume=99 |issue= |pages=251101 |doi=10.1103/PhysRevLett.99.251101 |url= |accessdate= |quote= }}</ref><ref>[http://arxiv.org/abs/0708.2943v1 ] arXiv:0708.2943v1 [http://arxiv.org/abs/0711.4264 Dark energy as a mirage] HIP-2007-64/TH</ref> or on consequences of [[electroweak symmetry breaking]] in the early universe.
==Implications for the fate of the universe==
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).
If the acceleration continues indefinitely, the ultimate result will be that galaxies outside the [[local supercluster]] will move beyond the [[event horizon|cosmic horizon]]: they will no longer be visible, because their [[radial velocity|line-of-sight velocity]] becomes greater than the speed of light. This is not a violation of [[special relativity]], and the effect cannot be used to send a signal between them. (Actually there is no way to even define "relative speed" in a curved spacetime. Relative speed and velocity can only be meaningfully defined in flat spacetime or in sufficiently small (infinitesimal) regions of curved spacetime). Rather, it prevents any communication between them and the objects pass out of contact. The [[Earth]], the [[Milky Way]] and the [[Virgo supercluster]], however, would remain virtually undisturbed while the rest of the universe recedes. 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]].
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. While these ideas are not supported by observations, they are not ruled out. Measurements of acceleration are crucial to determining the ultimate fate of the universe in big bang theory.
==History==
The cosmological constant was first proposed by [[Albert Einstein|Einstein]] as a mechanism to obtain a stable solution of the [[Einstein's field equation|gravitational field equation]] that would lead to a static universe, effectively using dark energy to balance gravity. Not only was the mechanism an inelegant example of [[fine-tuning]], it was 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 famously referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder. Following this realization, the cosmological constant was largely ignored as a historical curiosity.
[[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.
The term "dark energy" 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 per cent.<ref name="snls">{{cite journal|author=Pierre Astier ''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|date=2006|url=http://www.arxiv.org/abs/astro-ph/0510447|doi=10.1051/0004-6361:20054185|format=subscription required}}</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.
==See also==
*[[Vacuum energy]]
*[[Lambda-CDM model]]
*[[Cosmic inflation]]
==References==
{{reflist|colwidth=40em}}
==Bibliography==
* HubbleSite press release: [http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/12/text/ New Clues About the Nature of Dark Energy: Einstein May Have Been Right After All].
* 1998 paper announcing the dark energy discovery: [http://xxx.lanl.gov/abs/astro-ph/9805201 Riess et al]
* 1999 paper confirming dark energy discovery [http://xxx.lanl.gov/abs/astro-ph/9812133 Perlmutter et al].
* The group that first detected [[cosmic acceleration]]: [http://cfa-www.harvard.edu/cfa/oir/Research/supernova/HighZ.html High-Z supernova search team] and the group that confirmed it [http://panisse.lbl.gov/ Supernova Cosmology Project].
* [[Sean M. Carroll|Sean Carroll's]] technical reviews: [http://xxx.lanl.gov/abs/astro-ph/0310342 Why is the universe accelerating?], [http://xxx.lanl.gov/abs/astro-ph/0004075 The Cosmological Constant], and [http://xxx.lanl.gov/abs/astro-ph/0107571 Dark Energy and the Preposterous Universe].
* Jim Peebles, [http://xxx.lanl.gov/abs/astro-ph/0410284 Testing General Relativity on the Scales of Cosmology].
* "The World's Most Successful Nearby Supernova Search Engine", [http://astron.berkeley.edu/~bait/kait.html The Katzman Automatic Imaging Telescope].
* [http://snap.lbl.gov/ Supernova Acceleration Probe (SNAP)], a proposed satellite experiment.
* A reanalysis ([http://xxx.arxiv.org/abs/astro-ph/0406504], [http://www.cnd.mcgill.ca/bios/mackey/pdf_pub/darkenergy_2004.pdf]) of an experiment [R.H. Koch, D. van Harlingen, J. Clarke, Phys. Rev. B 26 (1982) 74] to find the broad-band spectrum of [[Josephson junction]] noise current claims to connect it to the spectral frequency upper limit predicted by matching estimates of the dark energy density to the measured vacuum energy density. This claim is not yet accepted. For disputes, see [http://arxiv.org/abs/astro-ph/0411034], [http://www.bath.ac.uk/pr/mrsa-nature.pdf], [http://www.nature.com/news/2004/040705/pf/430126b_pf.html].
* Christopher J. Conselice, "The Universe's Invisible Hand," Scientific American. February, 2007.
== External links ==
* {{cite news|work=Physics World|title=Dark energy|date=May 2004|author=Robert R Caldwell|url=http://physicsweb.org/articles/world/17/5/7}}
* {{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}}
* [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
* [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
* [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.
* [http://arxiv.org/abs/astro-ph/0609591 "Report of the Dark Energy Task Force"]
*[http://www.bbc.co.uk/science/space/deepspace/darkmatter/darkenergy.shtml "Dark energy"] BBC Science & Nature (2006)
*[http://snap.lbl.gov/brochure/ "Dark energy in the accelerating universe"] Supernova Acceleration Probe (SNAP) Satellite Observatory home page.
* {{cite news|work=Astroparticle physics|title=When did cosmic acceleration start |author=Emille Ishida|url=http://arxiv.org/abs/0706.0546v5}}
*[http://www.qc.fraunhofer.de/qg/lambda "Calculation of the Cosmological Constant by Unifying Matter and Dark Energy"] A geometric model of dark energy as [[Poincaré sphere]] - calculated: '''<math>\Omega_D = 0.734</math>''', observed: <math>\Omega_D = 0.65 ... 0.85</math> (see also [http://quantumgeometry.blogspot.com blog]).
*[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.
*[http://arxiv.org/abs/astro-ph/0607066 "Surveying the dark side"]
*[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
[[Category:Physical cosmology]]
[[Category:Energy]]
[[ar:طاقة مظلمة]]
[[bn:কৃষ্ণ শক্তি]]
[[ca:Energia fosca]]
[[cs:Temná energie]]
[[da:Mørk energi]]
[[de:Dunkle Energie]]
[[el:Σκοτεινή ενέργεια]]
[[es:Energía oscura]]
[[eo:Malluma energio]]
[[fr:Énergie sombre]]
[[gl:Enerxía escura]]
[[id:Energi gelap]]
[[it:Energia oscura]]
[[he:אנרגיה אפלה]]
[[lt:Tamsioji energija]]
[[hu:Sötét energia]]
[[nl:Donkere energie]]
[[ja:ダークエネルギー]]
[[no:Mørk energi]]
[[nn:Mørk energi]]
[[pl:Ciemna energia]]
[[pt:Energia escura]]
[[ro:Energie întunecată]]
[[ru:Тёмная энергия]]
[[sk:Tmavá energia]]
[[fi:Pimeä energia]]
[[sv:Mörk energi]]
[[vi:Năng lượng tối]]
[[uk:Темна енергія]]
[[ur:تاریک توانائی]]
[[zh:暗能量]]