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In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.[1][2] Dark energy
Dark energy
is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate. Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68.3% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, respectively, and other components such as neutrinos and photons contribute a very small amount.[3][4][5][6] The density of dark energy (~ 7 × 10−30 g/cm3) is very low, much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass–energy of the universe because it is uniform across space.[7][8][9] Two proposed forms for dark energy are the cosmological constant,[10][11] representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or 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 the zero-point radiation of space i.e. the vacuum energy.[12] Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

Contents

1 History of discovery and previous speculation

1.1 Einstein's cosmological constant 1.2 Inflationary dark energy 1.3 Change in expansion over time

2 Nature

2.1 Technical definition

3 Evidence of existence

3.1 Supernovae 3.2 Cosmic microwave background 3.3 Large-scale structure 3.4 Late-time integrated Sachs-Wolfe effect 3.5 Observational Hubble constant
Hubble constant
data

4 Theories of dark energy

4.1 Cosmological constant 4.2 Modified gravity

4.2.1 Quintessence 4.2.2 Interacting dark energy

4.3 Variable dark energy models 4.4 Observational skepticism

5 Implications for the fate of the universe 6 In philosophy of science 7 See also 8 Notes 9 References 10 External links

History of discovery and previous speculation[edit] Einstein's cosmological constant[edit] The "cosmological constant" is a constant term that can be added to Einstein's field equation
Einstein's field equation
of General Relativity. If considered as a "source term" in the field equation, it can be viewed as equivalent to the mass of empty space (which conceptually could be either positive or negative), or "vacuum energy". The cosmological constant was first proposed by Einstein as a mechanism to obtain a solution of the gravitational field equation that would lead to a static universe, effectively using dark energy to balance gravity.[13] Einstein gave the cosmological constant the symbol Λ (capital lambda). The mechanism was an example of fine-tuning, and it was later realized that Einstein's static universe would not be stable: local inhomogeneities would ultimately lead to either the runaway expansion or contraction of the universe. The 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. Further, observations made by Edwin Hubble
Edwin Hubble
in 1929 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.[14] Inflationary dark energy[edit] Alan Guth
Alan Guth
and Alexei Starobinsky
Alexei Starobinsky
proposed in 1980 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. Nearly all inflation models predict that the total (matter+energy) density of the universe should be very close to the critical density. During the 1980s, most cosmological research focused on models with critical density in matter only, usually 95% cold dark matter and 5% ordinary matter (baryons). These models were found to be successful at forming realistic galaxies and clusters, but some problems appeared in the late 1980s: in particular, the model required a value for the Hubble constant
Hubble constant
lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering. These difficulties became stronger after the discovery of anisotropy in the cosmic microwave background by the COBE spacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included the Lambda-CDM model
Lambda-CDM model
and a mixed cold/hot dark matter model. The first direct evidence for dark energy came from supernova observations in 1998 of accelerated expansion in Riess et al.[15] and in Perlmutter et al.,[16] and the Lambda-CDM model
Lambda-CDM model
then became the leading model. Soon after, dark energy was supported by independent observations: in 2000, the BOOMERanG and Maxima cosmic microwave background experiments observed the first acoustic peak in the CMB, showing that the total (matter+energy) density is close to 100% of critical density. Then in 2001, the 2dF Galaxy
Galaxy
Redshift
Redshift
Survey gave strong evidence that the matter density is around 30% of critical. The large difference between these two supports a smooth component of dark energy making up the difference. Much more precise measurements from WMAP
WMAP
in 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters. The term "dark energy", echoing Fritz Zwicky's "dark matter" from the 1930s, was coined by Michael Turner in 1998.[17] Change in expansion over time[edit]

Diagram representing the accelerated expansion of the universe due to dark energy.

High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time and space. In general relativity, the evolution of the expansion rate is estimated from the curvature of the universe and the cosmological 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. Adding the cosmological constant to cosmology's standard 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. As of 2013, the Lambda-CDM model
Lambda-CDM model
is consistent with a series of increasingly rigorous cosmological observations, including the Planck spacecraft and the Supernova
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%.[18] 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. Nature[edit]

Nature timeline

view • discuss • edit

-13 — – -12 — – -11 — – -10 — – -9 — – -8 — – -7 — – -6 — – -5 — – -4 — – -3 — – -2 — – -1 — – 0 —

cosmic expansion

Earliest light

cosmic speed-up

Solar System

water

Single-celled life

photosynthesis

Multicellular life

Land life

Earliest gravity

Dark energy

Dark matter

Earliest universe (−13.80)

Earliest stars

Earliest galaxy

Earliest quasar/sbh

Omega Centauri
Omega Centauri
forms

Andromeda Galaxy
Andromeda Galaxy
forms

Milky Way
Milky Way
Galaxy spiral arms form

Alpha Centauri
Alpha Centauri
forms

Earliest Earth (−4.54)

Earliest life

Earliest oxygen

Atmospheric oxygen

Earliest sexual reproduction

Cambrian explosion

Earliest humans

L i f e

P r i m o r d i a l

Axis scale: billion years Also see: Human
Human
timeline and Life timeline

The nature of dark energy is more hypothetical than that of dark matter, and many things about the nature of dark energy remain matters of speculation.[19] Dark energy
Dark energy
is thought to be very homogeneous, not very dense and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied, un-massive — roughly 10−27 kg/m3 — it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such a profound effect on the universe, making up 68% of universal density, in spite of being so rarefied is because it uniformly fills otherwise empty space. Independently of its actual nature, dark energy would need to have a strong negative pressure (acting repulsively) like radiation pressure in a metamaterial[20] to explain the observed acceleration of the expansion 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[dubious – discuss]. 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. This accelerating expansion effect is sometimes labeled "gravitational repulsion". Technical definition[edit] See also: Friedmann equations In standard cosmology, there are three components of the universe: matter, radiation and dark energy. Matter
Matter
is anything whose energy density scales with the inverse cube of the scale factor, i.e. ρ ∝ a−3, while radiation is anything which scales to the inverse fourth power of the scale factor ρ ∝ a−4. This can be understood intuitively: for an ordinary particle in a square box, doubling the length of a side of the box decreases the density (and hence energy density) by a factor of eight (23). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift.[21] The final component, dark energy, is an intrinsic property of space, and so has a constant energy density regardless of the volume under consideration (ρ ∝ a0). Thus, unlike ordinary matter, it does not get diluted with the expansion of space, leading to an accelerated expansion, instead of the deceleration expected from a universe containing only ordinary matter. Evidence of existence[edit] The evidence for dark energy is indirect but comes from three independent sources:

Distance measurements and their relation to redshift, which suggest the universe has expanded more in the last half of its life.[22] The theoretical need for a type of additional energy that is not matter or dark matter to form the observationally flat universe (absence of any detectable global curvature). It can be inferred from measures of large scale wave-patterns of mass density in the universe.

Supernovae[edit]

A Type Ia supernova
Type Ia supernova
(bright spot on the bottom-left) near a galaxy

In 1998, the High-Z Supernova
Supernova
Search Team[15] published observations of Type Ia ("one-A") supernovae. In 1999, the Supernova
Supernova
Cosmology Project[16] followed by suggesting that the expansion of the universe is accelerating.[23] The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Brian P. Schmidt
Brian P. Schmidt
and Adam G. Riess
Adam G. Riess
for their leadership in the discovery.[24][25] 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 as well as improved measurements of supernovae have been consistent with the Lambda-CDM model.[26] Some people argue that the only indications for the existence of dark energy are observations of distance measurements and the associated redshifts. Cosmic microwave background anisotropies and baryon acoustic oscillations only serve to demonstrate that distances to a given redshift are larger than would be expected from a "dusty" Friedmann–Lemaître universe and the local measured Hubble constant.[27] Supernovae are useful for cosmology because they are excellent standard candles 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 candles: 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 consistent luminosity. 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 baryonic matter.[28] Cosmic microwave background[edit]

Estimated division of total energy in the universe into matter, dark matter and dark energy based on five years of WMAP
WMAP
data.[29]

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 indicate that the universe is close to flat. For the shape of the universe to be flat, the mass/energy density of the universe must be equal to the 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%.[26] The Wilkinson Microwave Anisotropy
Anisotropy
Probe (WMAP) spacecraft seven-year analysis estimated a universe made up of 72.8% dark energy, 22.7% dark matter and 4.5% ordinary matter.[5] 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.[30] Large-scale structure[edit] The theory of large-scale structure, which governs the formation of structures in the universe (stars, quasars, galaxies and galaxy groups and clusters), also suggests that the density of matter in the universe is only 30% of the critical density. 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.[31][32] The WiggleZ survey from the Australian Astronomical Observatory scanned the galaxies to determine their redshift. Then, by exploiting the fact that baryon acoustic oscillations have left voids regularly of ~150 Mpc diameter, surrounded by the galaxies, the voids were used as standard rulers to estimate distances to galaxies as far as 2,000 Mpc (redshift 0.6), allowing for accurate estimate of the speeds of 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.[32] This provides a confirmation to cosmic acceleration independent of supernovae. Late-time integrated Sachs-Wolfe effect[edit] 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) is a direct signal of dark energy in a flat universe.[33] It was reported at high significance in 2008 by Ho et al.[34] and Giannantonio et al.[35] Observational Hubble constant
Hubble constant
data[edit] A new approach to test evidence of dark energy through observational Hubble constant
Hubble constant
data (OHD) has gained significant attention in recent years.[36][37][38][39] The Hubble constant, H(z), is measured as a function of cosmological redshift. OHD directly tracks the expansion history of the universe by taking passively evolving early-type galaxies as “cosmic chronometers”.[40] From this point, this approach provides standard clocks in the universe. The core of this idea is the measurement of the differential age evolution as a function of redshift of these cosmic chronometers. Thus, it provides a direct estimate of the Hubble parameter

H ( z ) = −

1

1 + z

d z

d t

≈ −

1

1 + z

Δ z

Δ t

.

displaystyle H(z)=- frac 1 1+z frac dz dt approx - frac 1 1+z frac Delta z Delta t .

The reliance on a differential quantity, Δz/Δt, can minimize many common issues and systematic effects; and as a direct measurement of the Hubble parameter instead of its integral, like supernovae and baryon acoustic oscillations (BAO), it brings more information and is appealing in computation. For these reasons, it has been widely used to examine the accelerated cosmic expansion and study properties of dark energy. Theories of dark energy[edit] Dark energy's status as a hypothetical force with unknown properties makes it a very active target of research. The problem is attacked from a great variety of angles, such as modifying the prevailing theory of gravity (general relativity), attempting to pin down the properties of dark energy, and finding alternative ways to explain the observational data.

The equation of state of Dark Energy
Energy
for 4 common models by Redshift.[41] A: CPL Model, B: Jassal Model, C: Barboza & Alcaniz Model, D: Wetterich Model

Cosmological constant[edit] Main article: Cosmological constant Further information: Equation of state
Equation of state
(cosmology)

Estimated distribution of matter and energy in the universe[42]

The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space. This is the cosmological constant, usually represented by the Greek letter Λ (Lambda, hence Lambda-CDM model). Since energy and mass are related according to the equation E = mc2, 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. The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate. The reason a cosmological constant has negative pressure can be seen from classical thermodynamics. In general, energy must be lost from inside a container (the container must do work on its environment) in order for the volume to increase. Specifically, 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, because the energy is equal to ρV, where ρ is the energy density of the cosmological constant. Therefore, P is negative and, in fact, P = −ρ. There are two major advantages for the cosmological constant. The first is that it is simple. Einstein had in fact introduced this term in his original formulation of general relativity such as to get a static universe. Although he later discarded the term after Hubble found that the universe is expanding, a nonzero cosmological constant can act as dark energy, without otherwise changing the Einstein field equations. The other advantage is that there is a natural explanation for its origin. Most quantum field theories predict vacuum fluctuations 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). A major outstanding problem is that the same quantum field theories predict a huge cosmological constant, more than 100 orders of magnitude too large.[11] This would need to be almost, but not exactly, cancelled by an equally large term of the opposite sign. Some supersymmetric theories require a cosmological constant that is exactly zero,[43] which does not help because supersymmetry must be broken. Nonetheless, the cosmological constant is the most economical solution to the problem of cosmic acceleration. Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant as an essential feature. Modified gravity[edit] The evidence for dark energy is heavily dependent on the theory of general relativity. Therefore, it is conceivable that a modification to general relativity also eliminates the need for dark energy. There are very many such theories, and research is ongoing.[44][45] The measurement of the speed of gravity with the gravitational wave event GW170817
GW170817
ruled out many modified gravity theories as alternative explanation to dark energy.[46][47][48] Quintessence[edit] Main article: Quintessence (physics) In quintessence models of dark energy, the observed acceleration of the scale factor is caused by the potential energy of a dynamical 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 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 variation of the fundamental constants in space or time.[49] Scalar fields are predicted by the Standard Model
Standard Model
of particle physics and string theory, but an analogous problem to the cosmological constant problem (or the problem of constructing models of cosmological inflation) occurs: renormalization theory predicts that scalar fields should acquire large masses. The coincidence problem asks why the acceleration of the Universe began when it did. If acceleration began earlier in the universe, structures such as 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 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.[50][51] 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. 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 such as a negative kinetic energy.[52] They can have unusual properties: phantom energy, for example, can cause a Big Rip. Interacting dark energy[edit] This 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. This could for example treat dark energy and dark matter as different facets of the same unknown substance,[53] or postulate that cold dark matter decays into dark energy.[54] 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.[55] Variable dark energy models[edit] The density of dark energy might have varied in time over the history of the universe. Modern observational data allow for estimates of the present density. Using baryon acoustic oscillations, it is possible to investigate the effect of dark energy in the history of the Universe, and constrain parameters of the equation of state of dark energy. To that end, several models have been proposed. One of the most popular models is the Chevallier–Polarski–Linder model (CPL).[56][57] Some other common models are, (Barboza & Alcaniz. 2008),[58] (Jassal et al. 2005),[59] (Wetterich. 2004).[60] Observational skepticism[edit] Some alternatives to dark energy aim to explain the observational data by a more refined use of established theories. In this scenario, dark energy doesn't actually exist, and is merely a measurement artifact. For example, 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.[61][62][63][64] 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.[65][66][67] Yet other possibilities are that the accelerated expansion of the universe is an illusion caused by the relative motion of us to the rest of the universe,[68][69] or that the supernovae sample size used wasn't large enough.[70][71] Implications for the fate of the universe[edit] Cosmologists estimate that the acceleration began roughly 5 billion years ago.[72][notes 1] Before that, it is thought that the expansion was decelerating, due to the attractive influence of matter. 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). Projections into the future can differ radically for different models of dark energy. For a cosmological constant, or any other model that predicts that the acceleration will continue indefinitely, the ultimate result will be that galaxies outside the Local Group
Local Group
will have a line-of-sight velocity that continually increases with time, eventually far exceeding the speed of light.[73] 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 Uses of the proper distance for a discussion of the subtleties of defining any notion of relative velocity in cosmology). Because the 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.[74][75] 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[76] 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 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.[75] As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and more redshifted, to the point where the wavelength becomes too large to detect in practice and the galaxies appear to vanish completely[77][78] (see Future of an expanding universe). Planet Earth, the Milky Way, and the Local Group of which the Milky way is a part, would all remain virtually undisturbed as the rest of the universe recedes and disappears from view. In this scenario, the Local Group
Local Group
would ultimately suffer heat death, just as was hypothesized for the flat, matter-dominated universe before measurements of cosmic acceleration. There are other, more speculative ideas about the future of the universe. The phantom energy model of dark energy results in 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 electrical and nuclear forces to tear apart atoms themselves, ending the universe in a "Big Rip". It is also possible the universe may never have an end and continue in its present state forever[citation needed] (see The second thermodynamics law as a law of disorder). 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",[79] or that there may even be a dark energy cycle, which implies a cyclic model of the universe in which every iteration ( Big Bang
Big Bang
then eventually a Big Crunch) takes about a trillion (1012) years.[80][81] While none of these are supported by observations, they are not ruled out. In philosophy of science[edit] In philosophy of science, dark energy is an example of an "auxiliary hypothesis", an ad hoc postulate that is added to a theory in response to observations that falsify it. It has been argued that the dark energy hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.[82] See also[edit]

Conformal gravity Dark Energy
Energy
Spectroscopic Instrument De Sitter relativity Illustris project The Dark Energy
Energy
Survey Quintessence: The Search for Missing Mass in the Universe Vacuum
Vacuum
state Negative mass

Notes[edit]

^ [72] Frieman, Turner & Huterer (2008) p. 6: "The Universe
Universe
has gone through three distinct eras: radiation-dominated, z ≳ 3000; matter-dominated, 3000 ≳ z ≳ 0.5; and dark-energy-dominated, z ≲ 0.5. The evolution of the scale factor is controlled by the dominant energy form: a(t) ∝ t2/3(1 + w) (for constant w). During the radiation-dominated era, a(t) ∝ t1/2; during the matter-dominated era, a(t) ∝ t2/3; and for the dark energy-dominated era, assuming w = −1, asymptotically a(t) ∝ exp(Ht)." p. 44: "Taken together, all the current data provide strong evidence for the existence of dark energy; they constrain the fraction of critical density contributed by dark energy, 0.76 ± 0.02, and the equation-of-state parameter, w ≈ −1 ± 0.1 (stat) ± 0.1 (sys), assuming that w is constant. This implies that the Universe
Universe
began accelerating at redshift z ∼ 0.4 and age t ∼ 10 Gyr. These results are robust – data from any one method can be removed without compromising the constraints – and they are not substantially weakened by dropping the assumption of spatial flatness."

References[edit]

^ Overbye, Dennis (20 February 2017). "Cosmos Controversy: The Universe
Universe
Is Expanding, but How Fast?". New York Times. Retrieved 21 February 2017.  ^ Peebles, P. J. E.; Ratra, Bharat (2003). "The cosmological constant and dark energy". Reviews of Modern Physics. 75 (2): 559–606. arXiv:astro-ph/0207347 . Bibcode:2003RvMP...75..559P. doi:10.1103/RevModPhys.75.559.  ^ Ade, P. A. R.; Aghanim, N.; Armitage-Caplan, C.; et al. (Planck Collaboration), C.; Arnaud, M.; Ashdown, M.; Atrio-Barandela, F.; Aumont, J.; Aussel, H.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Barrena, R.; Bartelmann, M.; Bartlett, J. G.; Bartolo, N.; Basak, S.; Battaner, E.; Battye, R.; Benabed, K.; Benoît, A.; Benoit-Lévy, A.; Bernard, J.-P.; Bersanelli, M.; Bertincourt, B.; Bethermin, M.; Bielewicz, P.; Bikmaev, I.; Blanchard, A.; et al. (22 March 2013). "Planck 2013 results. I. Overview of products and scientific results – Table 9". Astronomy
Astronomy
and Astrophysics. 571: A1. arXiv:1303.5062 . Bibcode:2014A&A...571A...1P. doi:10.1051/0004-6361/201321529.  ^ Ade, P. A. R.; Aghanim, N.; Armitage-Caplan, C.; et al. (Planck Collaboration), C.; Arnaud, M.; Ashdown, M.; Atrio-Barandela, F.; Aumont, J.; Aussel, H.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Barrena, R.; Bartelmann, M.; Bartlett, J. G.; Bartolo, N.; Basak, S.; Battaner, E.; Battye, R.; Benabed, K.; Benoît, A.; Benoit-Lévy, A.; Bernard, J.-P.; Bersanelli, M.; Bertincourt, B.; Bethermin, M.; Bielewicz, P.; Bikmaev, I.; Blanchard, A.; et al. (31 March 2013). "Planck 2013 Results Papers". Astronomy
Astronomy
and Astrophysics. 571: A1. arXiv:1303.5062 . Bibcode:2014A&A...571A...1P. doi:10.1051/0004-6361/201321529. Archived from the original on 23 March 2013.  ^ a b "First Planck results: the Universe
Universe
is still weird and interesting".  ^ Sean Carroll, Ph.D., Caltech, 2007, The Teaching Company, Dark Matter, Dark Energy: The Dark Side of the Universe, Guidebook Part 2 page 46. Retrieved 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
Dark energy
is known to be smooth because it doesn't accumulate preferentially in galaxies and clusters..." ^ Paul J. Steinhardt; Neil Turok
Neil Turok
(2006). "Why the cosmological constant is small and positive". Science. 312 (5777): 1180–1183. arXiv:astro-ph/0605173 . Bibcode:2006Sci...312.1180S. doi:10.1126/science.1126231. PMID 16675662.  ^ "Dark Energy". Hyperphysics. Retrieved January 4, 2014.  ^ Ferris, Timothy. "Dark Matter(Dark Energy)". Retrieved 2015-06-10.  ^ "Moon findings muddy the water".  ^ a b Carroll, Sean (2001). "The cosmological constant". Living Reviews in Relativity. 4. arXiv:astro-ph/0004075 . Bibcode:2001LRR.....4....1C. doi:10.12942/lrr-2001-1. Archived from the original on 2006-10-13. Retrieved 2006-09-28.  ^ Kragh, H. 2012. Preludes to dark energy: zero-point energy and vacuum speculations. Archive for History of Exact Sciences. Volume 66, Issue 3, pp 199–240 ^ Harvey, Alex (2012). "How Einstein Discovered Dark Energy". arXiv:1211.6338 .  ^ Gamow, George (1970) My World Line: An Informal Autobiography. p. 44: "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. ^ a b Riess, Adam G.; Filippenko; Challis; Clocchiatti; Diercks; Garnavich; Gilliland; Hogan; Jha; Kirshner; Leibundgut; Phillips; Reiss; Schmidt; Schommer; Smith; Spyromilio; Stubbs; Suntzeff; Tonry (1998). "Observational evidence from supernovae for an accelerating universe and a cosmological constant". Astronomical Journal. 116 (3): 1009–38. arXiv:astro-ph/9805201 . Bibcode:1998AJ....116.1009R. doi:10.1086/300499.  ^ a b Perlmutter, S.; Aldering; Goldhaber; Knop; Nugent; Castro; Deustua; Fabbro; Goobar; Groom; Hook; Kim; Kim; Lee; Nunes; Pain; Pennypacker; Quimby; Lidman; Ellis; Irwin; McMahon; Ruiz‐Lapuente; Walton; Schaefer; Boyle; Filippenko; Matheson; Fruchter; et al. (1999). "Measurements of Omega and Lambda from 42 high redshift supernovae". Astrophysical Journal. 517 (2): 565–86. arXiv:astro-ph/9812133 . Bibcode:1999ApJ...517..565P. doi:10.1086/307221.  ^ 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
Energy
via Supernova
Supernova
Distance Measurements", which was posted to the ArXiv.org e-print archive
ArXiv.org e-print archive
in August 1998 and published in Huterer, D.; Turner, M. (1999). "Prospects for probing the dark energy via supernova distance measurements". Physical Review D. 60 (8). arXiv:astro-ph/9808133 . Bibcode:1999PhRvD..60h1301H. doi:10.1103/PhysRevD.60.081301. , although the manner in which the term is treated there suggests it was already in general use. Cosmologist Saul Perlmutter
Saul Perlmutter
has credited Turner with coining the term in an article they wrote together with Martin White, where it is introduced in quotation marks as if it were a neologism. Perlmutter, S.; Turner, M.; White, M. (1999). "Constraining Dark Energy
Energy
with Type Ia Supernovae and Large-Scale Structure". Physical Review Letters. 83 (4): 670. arXiv:astro-ph/9901052 . Bibcode:1999PhRvL..83..670P. doi:10.1103/PhysRevLett.83.670.  ^ Astier, Pierre ( Supernova
Supernova
Legacy Survey); Guy; Regnault; Pain; Aubourg; Balam; Basa; Carlberg; Fabbro; Fouchez; Hook; Howell; Lafoux; Neill; Palanque-Delabrouille; Perrett; Pritchet; Rich; Sullivan; Taillet; Aldering; Antilogus; Arsenijevic; Balland; Baumont; Bronder; Courtois; Ellis; Filiol; et al. (2006). "The Supernova
Supernova
legacy survey: Measurement of ΩM, ΩΛ and W from the first year data set". Astronomy
Astronomy
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WMAP
collaboration); et al. (March 2006). " Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe
(WMAP) three year results: implications for cosmology".  ^ Durrer, R. (2011). "What do we really know about dark energy?". Philosophical Transactions of the Royal Society A. 369 (1957): 5102–5114. arXiv:1103.5331 . Bibcode:2011RSPTA.369.5102D. doi:10.1098/rsta.2011.0285.  ^ Kowalski, Marek; Rubin, David; Aldering, G.; Agostinho, R. J.; Amadon, A.; Amanullah, R.; Balland, C.; Barbary, K.; Blanc, G.; Challis, P. J.; Conley, A.; Connolly, N. V.; Covarrubias, R.; Dawson, K. S.; Deustua, S. E.; Ellis, R.; Fabbro, S.; Fadeyev, V.; Fan, X.; Farris, B.; Folatelli, G.; Frye, B. L.; Garavini, G.; Gates, E. L.; Germany, L.; Goldhaber, G.; Goldman, B.; Goobar, A.; Groom, D. E.; et al. (October 27, 2008). "Improved Cosmological Constraints from New, Old and Combined Supernova
Supernova
Datasets". The Astrophysical Journal. Chicago: University of Chicago Press. 686 (2): 749–778. arXiv:0804.4142 . Bibcode:2008ApJ...686..749K. doi:10.1086/589937. . They find a best fit value of the dark energy density, ΩΛ of 0.713+0.027–0.029(stat)+0.036–0.039(sys), of the total matter density, ΩM, of 0.274+0.016–0.016(stat)+0.013–0.012(sys) with an equation of state parameter w of −0.969+0.059–0.063(stat)+0.063–0.066(sys). ^ "Content of the Universe
Universe
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BBC
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Dark energy
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External links[edit]

Dark Energy
Energy
on In Our Time at the BBC. Dark energy
Dark energy
Eric Linder Scholarpedia
Scholarpedia
3(2):4900. doi:10.4249/scholarpedia.4900 Dark energy: how the paradigm shifted Physicsworld.com Dennis Overbye (November 2006). "9 Billion-Year-Old 'Dark Energy' Reported". The New York Times.  "Mysterious force's long presence" BBC
BBC
News online (2006) More evidence for dark energy being the cosmological constant " Astronomy
Astronomy
Picture of the Day" one of the images of the Cosmic Microwave Background which confirmed the presence of dark energy and dark matter SuperNova Legacy Survey home page The Canada-France-Hawaii Telescope Legacy Survey Supernova
Supernova
Program aims primarily at measuring the equation of state of Dark Energy. It is designed to precisely measure several hundred high-redshift supernovae. "Report of the Dark Energy
Energy
Task Force" "HubbleSite.org – Dark Energy
Energy
Website" Multimedia presentation explores the science of dark energy and Hubble's role in its discovery. "Surveying the dark side" " Dark energy
Dark energy
and 3-manifold topology" Acta Physica Polonica 38 (2007), p. 3633–3639 The Dark Energy
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Survey The Joint Dark Energy
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Mission Harvard: Dark Energy
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Found Stifling Growth in Universe, primary source April 2010 Smithsonian Magazine Article HETDEX Dark energy
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experiment Dark Energy
Energy
FAQ "The Hunt for Dark Energy" George FR Ellis, Peter Cameron and David Tong discuss the presence of dark energy in the Universe Euclid ESA
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