The first observational evidence for dark energy's existence came from measurements ofsupernovae.Type Ia supernovae have constant luminosity, which means that they can be used as accurate distance measures. Comparing this distance to theredshift (which measures the speed at which the supernova is receding) shows that theuniverse's expansion isaccelerating.[10][11] Prior to this observation, scientists thought that the gravitational attraction ofmatter and energy in the universe would cause the universe's expansion to slow over time. Since the discovery of accelerating expansion,several independent lines of evidence have been discovered that support the existence of dark energy.
The exact nature of dark energy remains a mystery, and many possible explanations have been theorized. The main candidates are acosmological constant[12][13] (representing a constant energy density filling space homogeneously) andscalar fields (dynamic quantities having energy densities that vary in time and space) such asquintessence ormoduli. A cosmological constant would remain constant across time and space, while scalar fields can vary. Yet other possibilities are interacting dark energy (see the sectionDark energy § Theories of dark energy), an observational effect, cosmological coupling, and shockwave cosmology (see the section§ Alternatives to dark energy).
The "cosmological constant" is a constant term that can be added to theEinstein field equations ofgeneral 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 byEinstein as a mechanism to obtain a solution to the gravitationalfield equation that would lead to a static universe, effectively using dark energy to balance gravity.[14] Einstein gave the cosmological constant the symbol Λ (capital lambda). Einstein stated that the cosmological constant required that 'empty space takes the role of gravitatingnegative masses that are distributed all over the interstellar space'.[15][16]
The mechanism was an example offine-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. Theequilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise, a universe that contracts slightly will continue contracting. According to Einstein, "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear, thereby causing accelerated expansion.[17] These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout the universe. Further, observations made byEdwin Hubble in 1929 showed that the universe appears to be expanding and is not static. Einstein reportedly referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder.[18]
Alan Guth andAlexei Starobinsky proposed in 1980 that a negative pressure field, similar in concept to dark energy, could drivecosmic 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 during its earliest stages. Such expansion is an essential feature of most current models of theBig Bang. However, inflation must have occurred at a much higher energy density than the dark energy we observe today, and inflation 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 thecritical density. During the 1980s, most cosmological research focused on models with critical density in matter only, usually 95%cold dark matter (CDM) 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 theHubble constant lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering. These difficulties became stronger after the discovery ofanisotropy in the cosmic microwave background by theCOBE spacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included theLambda-CDM model and a mixed cold/hot dark matter model. The first direct evidence for dark energy came from supernova observations in 1998 ofaccelerated expansion inRiesset al.[19] and inPerlmutteret al.,[20] and the Lambda-CDM model then became the leading model. Soon after, dark energy was supported by independent observations: in 2000, theBOOMERanG andMaxima cosmic microwave background experiments observed the firstacoustic peak in the cosmic microwave background, showing that the total (matter+energy) density is close to 100% of critical density. Then in 2001, the2dF Galaxy 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 fromWMAP in 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters.
The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation.[22] Dark energy is thought to be very homogeneous and notdense, and is not known to interact through any of thefundamental forces other thangravity. Since it is rarefied and 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 dilute, is that it is believed to uniformly fill otherwise empty space.
Thevacuum energy, that is, the particle-antiparticle pairs generated and mutually annihilated within a time frame in accord with Heisenberg'suncertainty principle in the energy-time formulation, has been often invoked as the main contribution to dark energy.[23] Themass–energy equivalence postulated bygeneral relativity implies that the vacuum energy should exert agravitational force. Hence, the vacuum energy is expected to contribute to thecosmological constant, which in turn impinges on the acceleratedexpansion of the universe. However, thecosmological constant problem asserts that there is a huge disagreement between the observed values of vacuum energy density and the theoretical large value of zero-point energy obtained byquantum field theory; the problem remains unresolved.
Independently of its actual nature, dark energy would need to have a strong negative pressure to explain the observedacceleration of theexpansion of the universe. According to general relativity, the pressure within a substance contributes to its gravitational attraction for other objects just as its mass density does. This happens because the physical quantity that causes matter to generate gravitational effects is thestress–energy tensor, which contains both the energy (or matter) density of a substance and its pressure. In theFriedmann–Lemaître–Robertson–Walker metric, it can be shown that a strong constant negative pressure (i.e., tension) in all the universe causes an acceleration in the expansion if the universe is already expanding, or a deceleration in contraction if the universe is already contracting. This accelerating expansion effect is sometimes labeled "gravitational repulsion".
In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. Matter is anything whose energy density scales with the inverse cube of the scale factor, i.e.,ρ ∝ a−3, while radiation is anything whose energy density scales to the inverse fourth power of the scale factor (ρ ∝ a−4). This can be understood intuitively: for an ordinary particle in a cube-shaped box, doubling the length of an edge 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 and hence a decrease in energy (c.f. thePlanck relation).[24]
The final component is dark energy: it is an intrinsic property of space and has a constant energy density, regardless of the dimensions of the volume under consideration (ρ ∝ a0). Thus, unlike ordinary matter, it is not diluted by the expansion of space.
Diagram representing the accelerated expansion of the universe due to dark energy
High-precision measurements of theexpansion 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 thecurvature of the universe and the cosmologicalequation 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 standardFLRW 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 is consistent with a series of increasingly rigorous cosmological observations, including thePlanck spacecraft and the Supernova Legacy Survey (SNLS). 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%.[25] 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.[citation needed]
In March 2025, theDark Energy Spectroscopic Instrument (DESI) collaboration announce that evidence for evolving dark energy has been discovered in analysis combining DESI data onbaryon acoustic oscillations (BAO) with the CMB, weak lensing and supernovae dataset, with significance ranging from 2.8 to 4.2σ.[26][27] Results suggest that the density of dark energy is slowly decreasing with time.
The evidence for dark energy is indirect but comes from three independent sources:
Distance measurements and their relation toredshift, which suggest the universe has expanded more in the latter half of its life than in the former half of its life.[28]
The theoretical need for a type of additional energy that is not matter or dark matter to form theobservationally flat universe (absence of any detectable global curvature).
Measurements of large-scale wave patterns of mass density in the universe.
Since then, these observations have been corroborated by several independent sources. Measurements of thecosmic microwave background,gravitational lensing, and thelarge-scale structure of the cosmos, as well as improved measurements of supernovae, have been consistent with theLambda-CDM model.[32] Some people argue that the only indications for the existence of dark energy are observations of distance measurements and their associated redshifts. Cosmic microwave background anisotropies and baryon acoustic oscillations serve only 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.[33]
Supernovae are useful for cosmology because they are excellentstandard candles across cosmological distances. They allow researchers to measure the expansion history of the universe by looking at the relationship between the distance to an object and itsredshift, which gives how fast it is receding from us. The relationship is roughly linear, according toHubble'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, orabsolute magnitude, is known. This allows the object's distance to be measured from its actual observed brightness, orapparent magnitude. Type Ia supernovae are the most accurate known standard candles across cosmological distances because of their extreme and consistentluminosity.
Recent observations of supernovae are consistent with a universe made up 66.6% of dark energy and 33.4% of a combination ofdark matter andbaryonic matter assuming a flatLambda-CDM model.[34]
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.[35][36] The WiggleZ survey from theAustralian Astronomical Observatory scanned the galaxies to determine their redshift. Then, by exploiting the fact thatbaryon acoustic oscillations have leftvoids 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 confirmedcosmic acceleration up to half of the age of the universe (7 billion years) and constrain its inhomogeneity to 1 part in 10.[36] This provides a confirmation to cosmic acceleration independent of supernovae.
Estimated division of total energy in the universe into matter, dark matter and dark energy based on five years of WMAP data[37]
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 ofcosmic microwave backgroundanisotropies indicate that the universe is close toflat. For theshape of the universe to be flat, the mass–energy density of the universe must be equal to thecritical density. The total amount of matter in the universe (includingbaryons anddark matter), as measured from the cosmic microwave background 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%.[32] TheWilkinson Microwave Anisotropy Probe (WMAP) spacecraftseven-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 thePlanck spacecraft observations of the cosmic microwave background gave a more accurate estimate of 68.3% dark energy, 26.8% dark matter, and 4.9% ordinary matter.[38]
Accelerated cosmic expansion causesgravitational potential wells and hills to flatten asphotons pass through them, producing cold spots and hot spots on the cosmic microwave background aligned with vast supervoids and superclusters. This so-called late-timeIntegrated Sachs–Wolfe effect (ISW) is a direct signal of dark energy in a flat universe.[39] It was reported at high significance in 2008 by Hoet al.[40] and Giannantonioet al.[41]
A new approach to test evidence of dark energy through observationalHubble constant data (OHD), also known as cosmic chronometers, has gained significant attention in recent years.[42][43][44][45]
The Hubble constant,H(z), is measured as a function of cosmologicalredshift. OHD directly tracks the expansion history of the universe by taking passively evolving early-type galaxies as "cosmic chronometers".[46] 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
The reliance on a differential quantity,Δz/Δt, brings more information and is appealing for computation: It can minimize many common issues and systematic effects. Analyses ofsupernovae andbaryon acoustic oscillations (BAO) are based on integrals of the Hubble parameter, whereasΔz/Δt measures it directly. For these reasons, this method has been widely used to examine the accelerated cosmic expansion and study properties of dark energy.[citation needed]
Dark energy's status as a hypothetical force with unknown properties makes it an active target of research. The problem is attacked from a 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 for 4 common models by Redshift.[47] A: CPL Model, B: Jassal Model, C: Barboza & Alcaniz Model, D: Wetterich Model
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 letterΛ (Lambda, hence the nameLambda-CDM model). Since energy and mass are related according to the equationE =mc2, Einstein's theory ofgeneral relativity predicts that this energy will have a gravitational effect. It is sometimes calledvacuum energy because it is the energy density of empty space – ofvacuum.
Somesupersymmetric theories require a cosmological constant that is exactly zero.[49] It is also unknown whether a positive cosmological constant is consistent with simple interpretations ofstring theory, in which our universe is afalse vacuum with a positive cosmological constant.[50] It has been conjectured by Ulf Danielssonet al. that no such state exists.[51] However, even if string theory does not allow such a false vacuum, other models of dark energy, such as quintessence, could still be viable.[50]
Inquintessence models of dark energy, the observed acceleration of the scale factor is caused by the potential energy of a dynamicalfield, 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 formstructure like matter, the field must be very light so that it has a largeCompton wavelength. In the simplest scenarios, the quintessence field has a canonicalkinetic term, isminimally coupled to gravity, and does not feature higher order operations in its Lagrangian.
No evidence of quintessence is yet available, nor has it been ruled out. 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'sequivalence principle andvariation of the fundamental constants in space or time.[52]Scalar fields are predicted by theStandard Model of particle physics andstring theory, but an analogous problem to the cosmological constant problem (or the problem of constructing models ofcosmological inflation) occurs:renormalization theory predicts that scalar fields should acquire large masses.
The coincidence problem asks why theacceleration of the Universe began when it did. If acceleration began earlier in the universe, structures such asgalaxies 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 theanthropic 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 untilmatter–radiation equality, which triggers quintessence to start behaving as dark energy, eventually dominating the universe. This naturally sets the lowenergy scale of the dark energy.[53][54]
In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that theequation of state had possibly crossed the cosmological constant boundary from above to below. Ano-go theorem has been proved that this scenario requires models with at least two types of scalar fields. This scenario is calledQuintom, which was proposed by Xinmin Zhang's group in 2004.[55]
Some special cases of quintessence arephantom dark 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 ofkinetic energy such as anegative kinetic energy.[56] They can have unusual properties: phantom dark energy, for example, can cause aBig Rip.
A group of researchers argued in 2021 that observations of theHubble tension may imply that only quintessence models with a nonzerocoupling constant are viable.[57]
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,[58] or postulate that cold dark matter decays into dark energy.[59] 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 spacetime such that the modified dynamics stems to what have been assigned to the presence of dark energy and dark matter.[60] Dark energy could in principle interact not only with the rest of the dark sector, but also with ordinary matter. However, cosmology alone is not sufficient to effectively constrain the strength of the coupling between dark energy and baryons, so that other indirect techniques or laboratory searches have to be adopted.[61] It was briefly theorized in the early 2020s that excess observed in theXENON1T detector in Italy may have been caused by achameleon model of dark energy, but further experiments disproved this possibility.[62][63]
The density of dark energy might have varied in time during the history of the universe. Modern observational data allows us to estimate the present density of dark energy. Usingbaryon acoustic oscillations, it is possible to investigate the effect of dark energy in the history of the universe, and constrain parameters of theequation 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).[64][65] Some other common models are Barboza & Alcaniz (2008),[66] Jassal et al. (2005),[67] Wetterich. (2004),[68] and Oztas et al. (2018).[69][70]
There is some observational evidence that dark energy is indeed decreasing with time. Data from theDark Energy Spectroscopic Instrument (DESI), tracking the size ofbaryon acoustic oscillations over the universe's expansion history, suggests that the amount of dark energy is 10% lower than it was 4.5 billion years ago.[71][27] However, there is not yet sufficient data to rule out dark energy being the cosmological constant.
The evidence for dark energy is heavily dependent on the theory of general relativity. Therefore, it is conceivable that amodification to general relativity also eliminates the need for dark energy. There are many such theories, and research is ongoing.[72][73] The measurement of the speed of gravity in the first gravitational wave measured by non-gravitational means (GW170817) ruled out many modified gravity theories as explanations to dark energy.[74][75][76]
AstrophysicistEthan Siegel states that, while such alternatives gain mainstream press coverage, almost all professional astrophysicists are confident that dark energy exists and that none of the competing theories successfully explain observations to the same level of precision as standard dark energy.[77]
Some alternatives to dark energy, such asinhomogeneous cosmology, aim to explain the observational data by a more refined use of established theories. In this scenario, dark energy does not 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.[78][79][80][81] A different approach uses a cosmological extension of theequivalence 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 aHubble bubble.[82][83][84] 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,[85][86] or that the statistical methods employed were flawed.[87][88] A laboratory direct detection attempt failed to detect any force associated with dark energy.[89]
Observational skepticism explanations of dark energy have generally not gained much traction among cosmologists. For example, a paper that suggested the anisotropy of the local Universe has been misrepresented as dark energy[90] was quickly countered by another paper claiming errors in the original paper.[91] Another study questioning the essential assumption that the luminosity of Type Ia supernovae does not vary with stellar population age[92][93] was also swiftly rebutted by other cosmologists.[94]
As a general relativistic effect due to black holes
This theory was formulated by researchers of theUniversity of Hawaiʻi at Mānoa in February 2023. The idea is that if one requires theKerr metric (which describes rotating black holes) to asymptote to theFriedmann-Robertson-Walker metric (which describes theisotropic andhomogeneous universe that is the basic assumption of modern cosmology), then one finds that black holes gain mass as the universe expands. The rate is measured to be∝a3, wherea is thescale factor. This particular rate means that the energy density of black holes remains constant over time, mimicking dark energy (seeDark energy#Technical definition). The theory is called "cosmological coupling" because the black holes couple to a cosmological requirement.[95] Other astrophysicists are skeptical,[96] with a variety of papers claiming that the theory fails to explain other observations.[97][98]
Shockwave cosmology, proposed by Joel Smoller and Blake Temple in 2003, has the "big bang" as an explosion inside a black hole, producing the expanding volume of space and matter that includes the observable universe.[99] A related theory by Smoller, Temple, and Vogler proposes that this shockwave may have resulted in our part of the universe having a lower density than that surrounding it, causing the accelerated expansion normally attributed to dark energy.[100][101] They also propose that this related theory could be tested: a universe with dark energy should give a figure for the cubic correction to redshift versus luminosity C = −0.180 at a =a whereas for Smoller, Temple, and Vogler's alternative C should be positive rather than negative. They give a more precise calculation for their shockwave model alternative as: the cubic correction to redshift versus luminosity at a =a is C = 0.359.[100]
Although shockwave cosmology produces a universe that "looks essentially identical to the aftermath of the big bang",[102] cosmologists consider that it needs further development before it could be considered as a more advantageous model than the big bang theory (or standard model) in explaining the universe. In particular, and especially for the proposed alternative to dark energy, it would need to explain big bang nucleosynthesis, the quantitative details of the microwave background anisotropies, the Lyman-alpha forest, and galaxy surveys.[101]
Cosmologists estimate that theacceleration began roughly 5 billion years ago.[103][a] 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 ofdark 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 theLocal Group will have aline-of-sight velocity that continually increases with time, eventually far exceeding the speed of light.[104] This is not a violation ofspecial relativity because the notion of "velocity" used here is different from that of velocity in a localinertial frame of reference, which is still constrained to be less than the speed of light for any massive object (seeUses of the proper distance for a discussion of the subtleties of defining any notion of relative velocity in cosmology). Because theHubble 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.[105][106]
However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmologicalevent horizon where any light they emit past that point will never be able to reach us at any time in the infinite future[107] 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 inUses of the proper distance). Assuming the dark energy is constant (acosmological constant), the current distance to this cosmological event horizon is about 16 billion light years, meaning that a signal from an event happeningat 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.[106]
As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and moreredshifted, to the point where the wavelength becomes too large to detect in practice and the galaxies appear to vanish completely[108][109] (seeFuture of an expanding universe). Planet Earth, theMilky Way, and theLocal Group of galaxies 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 would ultimately sufferheat death, just as was hypothesized for the flat, matter-dominated universe before measurements ofcosmic acceleration.[citation needed]
There are other, more speculative ideas about the future of the universe. Thephantom dark energy model of dark energy results indivergent 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 theelectrical andnuclear forces 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 of gravity eventually prevailing and lead to a universe that contracts in on itself in a "Big Crunch",[110] or that there may even be a dark energy cycle, which implies acyclic model of the universe in which every iteration (Big Bang then eventually aBig Crunch) takes about atrillion (1012) years.[111][112] While none of these are supported by observations, they are not ruled out.[citation needed]
The astrophysicistDavid Merritt identifies dark energy as an example of an "auxiliary hypothesis", anad hoc postulate that is added to a theory in response to observations thatfalsify it. He argues that the dark energy hypothesis is aconventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence isunfalsifiable in the sense defined byKarl Popper.[113] However, his opinion is not shared by all scientists.[114]
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 began accelerating at redshiftz ~ 0.4 and aget ~ 10Ga . 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.[103]: 44
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^Barnes, Luke A.; Lewis, Geraint F. (2020).The cosmic revolutionary's handbook: or: how to beat the big bang. Cambridge, United Kingdom ; New York, NY, USA: Cambridge University Press.ISBN978-1-108-76209-0.
^Krauss, Lawrence M.; Scherrer, Robert J. (March 2008)."The End of Cosmology?".Scientific American.82.Archived from the original on 19 March 2011. Retrieved6 January 2011.
^Using Tiny Particles To Answer Giant QuestionsArchived 6 May 2018 at theWayback Machine. Science Friday, 3 April 2009. According to thetranscriptArchived 6 May 2018 at theWayback Machine,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."
^How the Universe Works 3. Vol. End of the Universe. Discovery Channel. 2014.
^Helbig, Phillip (2020). "Sonne und Mond, or, the good, the bad, and the ugly: comments on the debate between MOND and LambdaCDM".The Observatory.140:225–247.Bibcode:2020Obs...140..225H.
