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Dark energy

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Not to be confused withdark matter.

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Inphysical cosmologyandastronomy,dark energyis a proposed form ofenergythat affects theuniverseon the largest scales. Its primary effect is to drive theaccelerating expansion of the universe.Assuming that thelambda-CDM modelof cosmology is correct,[1]dark energy dominates the universe, contributing 68% of the total energy in the present-dayobservable universewhiledark matterandordinary (baryonic)matter contribute 26% and 5%, respectively, and other components such asneutrinosandphotonsare nearly negligible.[2][3][4][5]Dark energy'sdensityis very low:7×10−30g/cm3(6×10−10J/m3inmass-energy), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy content because it is uniform across space.[6][7][8]

The first observational evidence for dark energy's existence came from measurements ofsupernovae.Type Ia supernovaehave 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 expansionisaccelerating.[9][10]Prior to this observation, scientists thought that the gravitational attraction ofmatterand energy in the universe would cause the universe's expansion to slow over time. Since the discovery of accelerating expansion,several independent lines of evidencehave been discovered that support the existence of dark energy.

The exact nature of dark energy remains a mystery, and possible explanations abound. The main candidates are acosmological constant[11][12](representing a constant energy density filling space homogeneously) andscalar fields(dynamic quantities having energy densities that vary in time and space) such asquintessenceormoduli.A cosmological constant would remain constant across time and space, while scalar fields can vary. Yet other possibilities are interacting dark energy, an observational effect, and cosmological coupling (see the sectionDark energy § Theories of dark energy).

History of discovery and previous speculation

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Einstein's cosmological constant

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The "cosmological constant"is a constant term that can be added toEinstein field equationsofgeneral 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 byEinsteinas a mechanism to obtain a solution to the gravitationalfield equationthat would lead to a static universe, effectively using dark energy to balance gravity.[13]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 masseswhich are distributed all over the interstellar space'.[14][15]

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. Theequilibriumis 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. 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.[16]These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout the universe. Further, observations made byEdwin Hubblein 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.[17]

Inflationary dark energy

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Alan GuthandAlexei Starobinskyproposed in 1980 that a negative pressure field, similar in concept to dark energy, could drivecosmic inflationin 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 theBig Bang.Such expansion is an essential feature of most current models of the Big Bang. However, inflation must have occurred at a much higher (negative) 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 constantlower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering. These difficulties became stronger after the discovery ofanisotropyin the cosmic microwave background by theCOBEspacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included theLambda-CDM modeland a mixed cold/hot dark matter model. The first direct evidence for dark energy came from supernova observations in 1998 ofaccelerated expansioninRiesset al.[18]and inPerlmutteret al.,[19]and the Lambda-CDM model then became the leading model. Soon after, dark energy was supported by independent observations: in 2000, theBOOMERanGandMaximacosmic microwave background experiments observed the firstacoustic peakin the cosmic microwave background, showing that the total (matter+energy) density is close to 100% of critical density. Then in 2001, the2dF Galaxy Redshift Surveygave 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 fromWMAPin 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters.

The term "dark energy", echoingFritz Zwicky's "dark matter" from the 1930s, was coined byMichael S. Turnerin 1998.[20]

Change in expansion over time

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

High-precision measurements of theexpansion of the universeare 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 universeand 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 metricleads 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 spacecraftand the 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%.[21]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]

Nature

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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 forcesother thangravity.Since it is rarefied and un-massive—roughly 10−27kg/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 principlein the energy-time formulation, has been often invoked as the main contribution to dark energy.[23]Themass–energy equivalencepostulated bygeneral relativityimplies that the vacuum energy should exert agravitationalforce. Hence, the vacuum energy is expected to contribute to thecosmological constant,which in turn impinges on the acceleratedexpansion of the universe.However, thecosmological constant problemasserts 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 observedaccelerationof 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".

Technical definition

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See also:Friedmann equations

In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. This 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.[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.

Evidence of existence

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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.[25]
  • 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.

Supernovae

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A Type Ia supernova (bright spot on the bottom-left) nearNGC 4526

In 1998, theHigh-Z Supernova Search Team[18]published observations ofType Ia( "one-A" )supernovae.In 1999, theSupernova Cosmology Project[19]followed by suggesting that the expansion of the universe isaccelerating.[26]The 2011Nobel Prize in Physicswas awarded toSaul Perlmutter,Brian P. Schmidt,andAdam G. Riessfor their leadership in the discovery.[27][28]

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.[29]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.[30]

Supernovae are useful for cosmology because they are excellentstandard candlesacross 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 best-known standard candles across cosmological distances because of their extreme and consistentluminosity.

Recent observations of supernovae are consistent with a universe made up 71.3% of dark energy and 27.4% of a combination ofdark matterandbaryonic matter.[31]

Large-scale structure

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The theory oflarge-scale structure,which governs the formation of structures in the universe (stars,quasars,galaxiesandgalaxy 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.[32][33]The WiggleZ survey from theAustralian Astronomical Observatoryscanned the galaxies to determine their redshift. Then, by exploiting the fact thatbaryon acoustic oscillationshave leftvoidsregularly 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 accelerationup to half of the age of the universe (7 billion years) and constrain its inhomogeneity to 1 part in 10.[33]This provides a confirmation to cosmic acceleration independent of supernovae.

Cosmic microwave background

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Estimated division of total energy in the universe into matter, dark matter and dark energy based on five years of WMAP data.[34]

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 backgroundanisotropiesindicate that the universe is close toflat.For theshape of the universeto be flat, the mass–energy density of the universe must be equal to thecritical density.The total amount of matter in the universe (includingbaryonsanddark 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%.[29]TheWilkinson Microwave Anisotropy Probe(WMAP) spacecraftseven-year analysisestimated a universe made up of 72.8% dark energy, 22.7% dark matter, and 4.5% ordinary matter.[4]Work done in 2013 based on thePlanck spacecraftobservations of the cosmic microwave background gave a more accurate estimate of 68.3% dark energy, 26.8% dark matter, and 4.9% ordinary matter.[35]

Late-time integrated Sachs–Wolfe effect

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Accelerated cosmic expansion causesgravitational potential wellsand hills to flatten asphotonspass 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.[36]It was reported at high significance in 2008 by Hoet al.[37]and Giannantonioet al.[38]

Observational Hubble constant data

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A new approach to test evidence of dark energy through observationalHubble constantdata (OHD), also known as cosmic chronometers, has gained significant attention in recent years.[39][40][41][42]

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".[43]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 ofsupernovaeandbaryon acoustic oscillations(BAO) are based on integrals of the Hubble parameter, whereas⁠Δz/Δt⁠measures 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]

Theories of dark energy

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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.[44]
A: CPL Model,
B: Jassal Model,
C: Barboza & Alcaniz Model,
D: Wetterich Model

Cosmological constant

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Main article:Cosmological constant
Further information:Equation of state (cosmology)
Estimated distribution ofmatterandenergyin the universe[45]

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 relativitypredicts that this energy will have a gravitational effect. It is sometimes calledvacuum energybecause it is the energy density of empty space – ofvacuum.

A major outstandingproblemis that the samequantum field theoriespredict a hugecosmological constant,about 120orders of magnitudetoo large. This would need to be almost, but not exactly, cancelled by an equally large term of the opposite sign.[12]

Somesupersymmetrictheories require a cosmological constant that is exactly zero.[46]Also, it is unknown whether there is a metastable vacuum state instring theorywith a positive cosmological constant,[47]and it has been conjectured by Ulf Danielssonet al.that no such state exists.[48]This conjecture would not rule out other models of dark energy, such as quintessence, that could be compatible with string theory.[47]

Quintessence

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Main article:Quintessence (physics)

Inquintessencemodels 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 formstructurelike matter, the field must be very light so that it has a largeCompton wavelength.In the simplest scenarios, the quintessence field has a canonical kinetic term, is minimally 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 principleandvariation of the fundamental constantsin space or time.[49]Scalar fieldsare predicted by theStandard Modelof particle physics andstring theory,but an analogous problem to the cosmological constant problem (or the problem of constructing models ofcosmological inflation) occurs:renormalizationtheory predicts that scalar fields should acquire large masses.

The coincidence problem asks why theaccelerationof the Universe began when it did. If acceleration began earlier in the universe, structures such asgalaxieswould 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 principleview 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 scaleof the dark energy.[50][51]

In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that theequation of statehad possibly crossed the cosmological constant boundary (w = −1) from above to below. Ano-go theoremhas been proved that this scenario requires models with at least two types of quintessence. This scenario is the so-calledQuintom scenario.[52]

Some special cases of quintessence arephantom 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 energysuch as anegative kinetic energy.[53]They can have unusual properties:phantom energy,for example, can cause aBig Rip.

A group of researchers argued in 2021 that observations of theHubble tensionmay imply that only quintessence models with a nonzerocoupling constantare viable.[54]

Interacting dark energy

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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,[55]or postulate that cold dark matter decays into dark energy.[56]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.[57]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.[58]It was briefly theorized in the early 2020s that excess observed in theXENON1Tdetector in Italy may have been caused by achameleonmodel of dark energy, but further experiments disproved this possibility.[59][60]

Variable dark energy models

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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 stateof dark energy. To that end, several models have been proposed. One of the most popular models is the Chevallier–Polarski–Linder model (CPL).[61][62]Some other common models are (Barboza & Alcaniz. 2008),[63](Jassal et al. 2005),[64](Wetterich. 2004),[65]and (Oztas et al. 2018).[66][67]

Possibly decreasing levels

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Researchers using theDark Energy Spectroscopic Instrument(DESI) to make the largest 3-D map of the universe as of 2024,[68]have obtained an expansion history that has greater than 1% precision. From this level of detail, DESI Director Michael Levi stated:

We're also seeing some potentially interesting differences that could indicate that dark energy is evolving over time. Those may or may not go away with more data, so we're excited to start analyzing our three-year dataset soon.[69]

Observational skepticism

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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.[70][71][72][73]A different approach uses a cosmological extension of theequivalence principleto 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.[74][75][76]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,[77][78]or that the statistical methods employed were flawed.[79][80]A laboratory direct detection attempt failed to detect any force associated with dark energy.[81]

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[82]was quickly countered by another paper claiming errors in the original paper.[83]Another study questioning the essential assumption that the luminosity of Type Ia supernovae does not vary with stellar population age[84][85]was also swiftly rebutted by other cosmologists.[86]

As a general relativistic effect due to black holes

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This theory was formulated by researchers of theUniversity of Hawaiʻi at Mānoain 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 theisotropicandhomogeneousuniverse 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,whereais 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.[87]Other astrophysicists are skeptical,[88]with a variety of papers claiming that the theory fails to explain other observations.[89][90]

Other mechanism driving acceleration

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Modified gravity

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See also:Massive gravity

The evidence for dark energy is heavily dependent on the theory of general relativity. Therefore, it is conceivable that amodification to general relativityalso eliminates the need for dark energy. There are many such theories, and research is ongoing.[91][92]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.[93][94][95]

AstrophysicistEthan Siegelstates 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.[96]

Non-linearities of General Relativity equations

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TheGRSI modelexplains theaccelerating expansion of the universea suppression of gravity as large distance.[97]Such suppression is a consequence of an increasedbinding energywithin a galaxy due to General Relativity's field self-interaction. The increased binding requires, byenergy conservation,a suppression of gravitational attraction outside said galaxy. The suppression is in lieu of dark energy. This is analogous to the central phenomenology ofStrong Nuclear Forcewhere thegluonsfield self-interaction dramatically strengthens the binding of quarks, ultimately leading to theirconfinement.This in turnsuppresses the Strong Nuclear Force outside hadrons.

Implications for the fate of the universe

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Cosmologists estimate that theaccelerationbegan roughly 5 billion years ago.[98][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 matteris 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 Groupwill have aline-of-sight velocitythat continually increases with time, eventually far exceeding the speed of light.[99]This is not a violation ofspecial relativitybecause 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 distancefor a discussion of the subtleties of defining any notion of relative velocity in cosmology). Because theHubble parameteris 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.[100][101]

However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmologicalevent horizonwhere any light they emit past that point will never be able to reach us at any time in the infinite future[102]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 presentwould 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.[101]

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[103][104](seeFuture of an expanding universe). Planet Earth, theMilky Way,and theLocal Groupof 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 energymodel of dark energy results indivergentexpansion, 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 theelectricalandnuclear forcesto 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",[105]or that there may even be a dark energy cycle, which implies acyclic model of the universein which every iteration (Big Bangthen eventually aBig Crunch) takes about atrillion(1012) years.[106][107]While none of these are supported by observations, they are not ruled out.[citation needed]

In philosophy of science

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The astrophysicistDavid Merrittidentifies dark energy as an example of an "auxiliary hypothesis", anad hocpostulate that is added to a theory in response to observations thatfalsifyit. He argues that the dark energy hypothesis is aconventionalisthypothesis, that is, a hypothesis that adds no empirical content and hence isunfalsifiablein the sense defined byKarl Popper.[108]However, his opinion is not accepted by a majority of physicists.[109]

See also

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Notes

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  1. ^Taken from Frieman, Turner, & Huterer (2008):[98]: 6, 44 

    The Universe has gone through three distinct eras:

    Radiation-dominated,z≳ 3000;
    Matter-dominated,3000 ≳z≳ 0.5;and
    Dark-energy-dominated,0.5 ≳z.

    The evolution of the scale factor is controlled by the dominant energy form:

    (for constantw). During the radiation-dominated era,

    during the matter-dominated era,

    and for the dark energy-dominated era, assumingw≃ −1asymptotically

    [98]: 6 

    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 thatwis constant. This implies that the Universe began accelerating at redshiftz~0.4and 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.[98]: 44 

References

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  3. ^Ade, P. A. R.;Aghanim, N.;Alves, M. I. R.; et al. (Planck Collaboration) (31 March 2013)."Planck 2013 Results Papers".Astronomy and Astrophysics.571:A1.arXiv:1303.5062.Bibcode:2014A&A...571A...1P.doi:10.1051/0004-6361/201321529.S2CID218716838.Archived fromthe originalon 23 March 2013.
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