Two readings on Theories for Dark Energy

1. "The Cosmological Constant and Dark Energy," P. J. E. Peebles and B. Ratra, Rev. Mod. Phys. 75, 559 (2003). Introduction only. Full text available on course website.

2. "Top 10 Theories About Dark Energy," Paulina Destarac, listverse.com/2017/06/03/top-10-theories-about-dark-energy/

REVIEWS OF MODERN PHYSICS, VOLUME 75, APRIL 2003 The cosmological constant and dark energy

P. J. E. Peebles Joseph Henry Laboratories, Princeton University, Princeton, New Jersey 08544

Bharat Ratra Department of Physics, Kansas State University, Manhattan, Kansas 66506 (Published 22 April 2003)

Physics welcomes the idea that space contains energy whose gravitational effect approximates that of Einstein’s cosmological constant, ⌳; today the concept is termed dark energy or quintessence. Physics also suggests that dark energy could be dynamical, allowing for the arguably appealing picture of an evolving dark-energy density approaching its natural value, zero, and small now because the expanding universe is old. This would alleviate the classical problem of the curious energy scale of a millielectron volt associated with a constant ⌳. Dark energy may have been detected by recent cosmological tests. These tests make a good scientific case for the context, in the relativistic Friedmann-Lemaıˆtre model, in which the gravitational inverse-square law is applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one-quarter of the critical Einstein–de Sitter value. The case for detection of dark energy is not yet as convincing but still serious; we await more data, which may be derived from work in progress. Planned observations may detect the evolution of the dark-energy density; a positive result would be a considerable stimulus for attempts at understanding the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.

CONTENTS 4. The redshift-angular-size and redshift- magnitude relations 587 5. Galaxy counts 588 I. Introduction 559 6. The gravitational lensing rate 588 A. The issues for observational cosmology 560 7. Dynamics and the mean mass density 589 B. The opportunity for physics 561 8. The baryon mass fraction in clusters of C. Some explanations 562 galaxies 590 II. Basic Concepts 563 9. The cluster mass function 590 A. The Friedmann-Lemaıˆtre model 563 10. Biasing and the development of nonlinear B. The cosmological constant 565 mass density fluctuations 590 C. Inflation and dark energy 566 11. The anisotropy of the cosmic microwave III. Historical Remarks 567 background radiation 591 A. Einstein’s thoughts 567 12. The mass autocorrelation function and B. The development of ideas 569 nonbaryonic matter 593 1. Early indications of ⌳ 569 13. The gravitational inverse-square law 593 2. The coincidences argument against ⌳ 570 C. The state of the cosmological tests 594 3. Vacuum energy and ⌳ 570 V. Concluding Remarks 595 Note added in proof 596 C. Inflation 572 Acknowledgments 597 1. The scenario 572 Appendix: Recent Dark-Energy Scalar Field Research 597 2. Inflation in a low-density universe 573 References 599 D. The cold-dark-matter model 574 E. Dark energy 576 1. The XCDM parametrization 576 2. Decay by emission of matter or radiation 577 I. INTRODUCTION 3. Cosmic field defects 578 4. Dark-energy scalar field 578 There is significant observational evidence for the de- IV. The Cosmological Tests 580 tection of Einstein’s cosmological constant, ⌳, or a com- A. The theories 580 ponent of the material content of the universe that var- 1. General relativity 580 ies only slowly with time and space and so acts like ⌳. 2. The cold-dark-matter model for structure ⌳ formation 582 We shall use the term dark energy for or a component B. The tests 584 that acts like it. Detection of dark energy would be a 1. Thermal cosmic microwave background new clue to an old puzzle: the gravitational effect of the radiation 585 zero-point energies of particles and fields. The total with 2. Light-element abundances 585 other energies, that are close to homogeneous and 3. Expansion times 586 nearly independent of time, acts as dark energy. What is

0034-6861/2003/75(2)/559(48)/$35.00 559 ©2003 The American Physical Society 560 P. J. E. Peebles and Bharat Ratra: The cosmological constant and dark energy puzzling is that the value of the dark-energy density has The reader is referred to Leibundgut’s (2001, Sec. 4) dis- to be tiny compared to what is suggested by dimensional cussion of astrophysical hazards. Astronomers have analysis; the startling new evidence is that it may be dif- checks for this and other issues of interpretation when ferent from the only other natural value, zero. considering the observations used in cosmological tests. The main question to consider now is whether to ac- But it takes nothing away from this careful and elegant cept the evidence for detection of dark energy. We out- work to note that the checks are seldom convincing, be- line the nature of the case in this section. After review- cause the astronomy is complicated and what can be ing the basic concepts of the relativistic world model in observed is sparse. What is more, we do not know ahead Sec. II, and in Sec. III reviewing the history of ideas, we of time that the physics well tested on scales ranging present in Sec. IV a more detailed assessment of the from the laboratory to the Solar System survives the enormous extrapolation to cosmology. cosmological tests and the evidence for detection of ⌳ or The situation is by no means hopeless. We now have its analog in dark energy. significant cross-checks from the consistency of results There is little new to report on the big issue for based on independent applications of the astronomy and physics—why the dark-energy density is so small—since 1 of the physics of the cosmological model. If the physics Weinberg’s (1989) review in this journal. But there have or astronomy was faulty we would not expect consis- been analyses of a simpler idea: can we imagine that the tency from independent lines of evidence—apart from present dark-energy density is evolving, perhaps ap- the occasional accident and the occasional tendency to proaching zero? Models are introduced in Secs. II.C and stop the analysis when it approaches the ‘‘right answer.’’ III.E, and recent work is summarized in more detail in We have to demand abundant evidence of consistency, the Appendix. Feasible advances in cosmological tests and that is starting to appear. could detect evolution of the dark-energy density, and The case for detection of ⌳ or dark energy com- perhaps its gravitational response to large-scale fluctua- mences with the Friedmann-Lemaıˆtre cosmological tions in the mass distribution. This would substantially model. In this model the expansion history of the uni- motivate the search for a more fundamental physics verse is determined by a set of dimensionless parameters model for dark energy. whose sum is normalized to unity,

A. The issues for observational cosmology ⍀ ϩ⍀ ϩ⍀ ϩ⍀ ϭ M0 R0 ⌳0 K0 1. (1) We will make two points. First, cosmology has a sub- stantial observational and experimental basis, which supports many aspects of the standard model as almost ⍀ The first, M0 , is a measure of the present mean mass certainly being good approximations to reality. Second, density in nonrelativistic matter, mainly baryons and ⍀ ϳ ϫ Ϫ4 the empirical basis is not nearly as strong for cosmology nonbaryonic dark matter. The second, R0 1 10 ,is as it is for the standard model of particle physics: in a measure of the present mass in the relativistic 3-K cosmology it is not yet a matter of measuring the param- thermal cosmic microwave background radiation, which eters in a well-established theory. almost homogeneously fills space, and the accompanying To explain the second point we direct our attention to low-mass neutrinos. The third is a measure of ⌳ or the those more accustomed to experiments in the laboratory present value of the dark-energy equivalent. The fourth, ⍀ than to astronomy-related observations of astronomers’ K0 , is an effect of the curvature of space. We review Tantalus principle: one can look at distant objects but some details of these parameters in the next section, and never touch them. For example, the observations of su- of their measurements in Sec. IV. pernovae in distant galaxies offer evidence of dark en- The most direct evidence for detection of dark energy ergy, under the assumption that distant and nearby su- comes from observations of supernovae of a type whose pernovae are drawn from the same statistical sample intrinsic luminosities are close to uniform (after subtle (that is, that they are statistically similar enough for the astronomical corrections, a few details of which are dis- cussed in Sec. IV.B.4). The observed brightness as a purpose of this test). There is no direct way to check function of the wavelength shift of the radiation probes this, and it is easy to imagine differences between distant the geometry of spacetime, in what has come to be and nearby supernovae of the same nominal type. More called the redshift-magnitude relation.2 The measure- distant supernovae are seen in younger galaxies, because ments agree with the relativistic cosmological model of the travel time of light, and these younger galaxies ⍀ ϭ ⍀ with K0 0, meaning no space curvature, and ⌳0 tend to have more massive rapidly evolving stars with ϳ ⌳ ⍀ ϭ 0.7, meaning nonzero . A model with ⌳0 0 is two lower heavy-element abundances. How do we know that the properties of the supernovae are not also different? 2 ϭϪ The apparent magnitude is m 2.5 log10 f plus a constant, where f is the detected energy flux density in a chosen wave- 1Sahni and Starobinsky (2000); Carroll (2001); Weinberg length band. The standard measure of the wavelength shift, (2001); Witten (2001); and Ellwanger (2002) present more re- due to the expansion of the universe, is the redshift z defined cent reviews. in Eq. (7) below.

Rev. Mod. Phys., Vol. 75, No. 2, April 2003 P. J. E. Peebles and Bharat Ratra: The cosmological constant and dark energy 561 or three standard deviations off the best fit, depending geneous and isotropic way from a hotter denser state: on the data set and analysis technique. This is an impor- how else could space, which is transparent now, have tant indication, but 2 to 3 ␴ is not convincing, even when been filled with radiation that has relaxed to a thermal we can be sure that systematic errors are under reason- spectrum? The debate is when the expansion com- able control. And we have to consider that there may be menced or became a meaningful concept. Some whose opinions and research we respect question the extrap- a significant systematic error from differences between olation of the gravitational inverse-square law, in distant, high-redshift, and nearby, low-redshift, superno- its use in estimates of masses in galaxies and systems of vae. ⍀ galaxies, and of M0 . We agree that this law is one of There is a check, based on the cold-dark-matter the hypotheses to be tested. Our conclusion from the 3 (CDM) model for structure formation. The fit of the cosmological tests of Sec. IV is that the law passes model to the observations reviewed in Sec. IV.B yields significant, though not yet complete, tests, and that two key constraints. First, the angular power spectrum we already have a strong scientific case, resting on the of fluctuations in the temperature of the 3-K thermal abundance of cross-checks, that the matter density ⍀ cosmic microwave background radiation across the sky parameter M0 is about one-quarter. The case for ⍀ detection of ⍀⌳ is significant too, but not yet as com- indicates that K0 is small. Second, the power spectrum 0 ⍀ pelling. of the spatial distribution of the galaxies requires M0 ϳ ⍀ For the most part the results of the cosmological tests 0.25. Similar estimates of M0 follow from indepen- dent lines of observational evidence. The rate of gravi- agree wonderfully well with accepted theory. But the ob- ⍀ servational challenges to the tests are substantial: we are tational lensing prefers a somewhat larger value (if K0 is small), and some dynamical analyses of systems of drawing profound conclusions from very limited infor- ⍀ mation. We have to be liberal when considering ideas galaxies prefer lower M0 . But the differences could all ⍀ about what the universe is like, and conservative when result from measurement uncertainties. Since R0 in Eq. ⍀ accepting ideas into the established canon. (1) is small, the conclusion is that ⌳0 is large, in excel- lent agreement with the supernovae result. B. The opportunity for physics Caution is in order, however, because this check depends on the CDM model for structure formation. Unless there is some serious and quite unexpected We cannot see the dark matter, so we naturally assign flaw in our understanding of the principles of physics we it the simplest properties possible. Maybe it is significant can be sure the zero-point energy of the electromagnetic that the model has observational problems with galaxy field at laboratory wavelengths is real and measurable, formation, as discussed in Sec. IV.A.2, or maybe these as in the Casimir (1948) effect.5 Like all energy, this problems are only apparent, due to the complications of zero-point energy has to contribute to the source term in the astronomy. We are going to have to determine which Einstein’s gravitational field equation. If, as seems likely, is correct before we can have confidence in the role of the zero-point energy of the electromagnetic field is the CDM model in cosmological tests. We will get a close to homogeneous and independent of the velocity strong hint from current precision angular distribution of the observer, it manifests itself as a positive contribu- measurements of the 3-K thermal cosmic microwave tion to Einstein’s ⌳, or dark energy. The zero-point en- 4 background radiation. If the results match precisely the ergies of the fermions make a negative contribution. prediction of the relativistic model for cosmology and Other contributions, perhaps including the energy den- the CDM model for structure formation, with parameter sities of fields that interact only with themselves and choices that agree with the constraints from all the other cosmological tests, there will be strong evidence that we are approaching a good approximation to reality, and 5See Bordag, Mohideen, and Mostepanenko (2001) for a re- the completion of the great program of cosmological cent review. The attractive Casimir force between two parallel tests that commenced in the 1930s. But all that is in the conducting plates results from the boundary condition that future. suppresses the number of modes of oscillation of the electro- We wish to emphasize that the advances in the empiri- magnetic field between the plates, thus suppressing the energy cal basis for cosmology already are very real and sub- of the system. One can understand the effect at small separa- stantial. How firm the conclusion is depends on the is- tion without reference to the quantum behavior of the electro- sue, of course. Every competent cosmologist we know magnetic field, such as in the analysis of the van der Waals accepts as established beyond reasonable doubt that interaction in quantum mechanics, by taking account of the the universe is expanding and cooling in a near homo- term in the particle Hamiltonian for the Coulomb potential energy between the charged particles in the two separate neu- tral objects. But a more complete treatment, as discussed by Cohen-Tannoudji, Dupont-Roc, and Grynberg (1992), replaces 3The model is named after the nonbaryonic cold dark matter the Coulomb interaction with the coupling of the charged par- that is assumed to dominate the masses of galaxies in the ticles to the electromagnetic-field operator. In this picture the present universe. There are more assumptions in the CDM van der Waals interaction is mediated by the exchange of vir- model, of course; they are discussed in Secs. III.D and IV.A.2. tual photons. With either way of looking at the Casimir 4At the time of writing the Microwave Anisotropy Probe effect—the perturbation of the normal modes or the exchange (MAP) satellite is collecting data; the project is described in of virtual quanta of the unperturbed modes—the effect is the http://map.gsfc.nasa.gov/ same, the suppression of the energy of the system.

Rev. Mod. Phys., Vol. 75, No. 2, April 2003 562 P. J. E. Peebles and Bharat Ratra: The cosmological constant and dark energy

⍀ gravity, might have either sign. The value of the sum things—baryonic and nonbaryonic matter—and ⌳0 , suggested by dimensional analysis is much larger than which is thought to represent something completely dif- what is allowed by the relativistic cosmological model. ferent, is not much larger. Also, if the parameters were The only other natural value is ⌳ϭ0. If ⌳ really is tiny measured when the universe was one-tenth its present but not zero, this introduces a most stimulating though size the time-independent ⌳ parameter would contrib- enigmatic clue to the physics yet to be discovered. ute ⍀⌳ϳ0.003. That is, we seem to have come on the ⌳ To illustrate the problem we outline an example of a scene just as has become an important factor in the contribution to ⌳. The energy density in the 3-K thermal expansion rate. These curiosities surely are in part acci- cosmic microwave background radiation, which amounts dental, but maybe in part physically significant. In par- to ⍀ ϳ5ϫ10Ϫ5 in Eq. (1) (ignoring the neutrinos), ticular, one might imagine that the dark-energy density R0 ⌳ peaks at wavelength ␭ϳ2 mm. At this Wien peak the represented by is rolling to its natural value, zero, but photon occupation number is about one-fifteenth. The is very small now because we measure it when the uni- zero-point energy amounts to half the energy of a pho- verse is very old. We shall discuss efforts along this line ton at the given frequency. This means the zero-point to at least partially rationalize the situation. energy in the electromagnetic field at wavelengths ␭ ϳ ␦⍀ ϳ ϫ Ϫ4 2 mm amounts to a contribution ⌳0 4 10 to the density parameter in ⌳ or the dark energy. The sum Ϫ over the modes scales as ␭ 4 [as illustrated in Eq. (37)]. C. Some explanations Thus a naive extrapolation to visible wavelengths deter- ␦⍀ ϳ ϫ 10 mines that the contribution amounts to ⌳0 5 10 , We have to explain our choice of nomenclature. Basic already a ridiculous number. concepts of physics say that space contains homoge- The situation can be compared to the development of neous zero-point energy, and perhaps also energy that is the theory of weak interactions. The Fermi pointlike in- homogeneous or nearly so in other forms, real or effec- teraction model is strikingly successful for a consider- tive (such as from counter terms in gravity physics, able range of energies, but it was clear from the start which make the net energy density cosmologically ac- that the model fails at high energy. A fix was discussed— ceptable). In the literature this near homogeneous en- mediate the interaction by an intermediate boson—and ergy has been termed vacuum energy, the sum of eventually incorporated into the even more successful vacuum energy and quintessence (Caldwell, Dave´, and electroweak theory. General relativity and quantum me- Steinhardt, 1998), and dark energy (Turner, 1999). We chanics are extremely successful over a considerable have adopted the last term, and we shall refer to the range of length scales, provided we agree not to use the dark-energy density ␳⌳ that manifests itself as an effec- rules of quantum mechanics to count the zero-point en- tive version of Einstein’s cosmological constant, but one ergy density in the vacuum, even though we know we that may vary slowly with time and position.6 have to count the zero-point energies in all other situa- Our subject involves two quite different traditions, in tions. There are thoughts on improving the situation, physics and astronomy. Each has familiar notation, and though they seem to be less focused than was the case familiar ideas that may be ‘‘in the air’’ but not in recent for the Fermi model. Perhaps a new energy component literature. Our attempt to take account of these tradi- spontaneously cancels the vacuum energy density or the tions commences with the summary in Sec. II of the ba- new component varies slowly with position and here and sic notation with brief explanations. We expect that there happens to cancel the vacuum energy density well readers will find some of these concepts trivial and oth- enough to allow observers like us to exist. Whatever the ers of some use, and that the useful parts will be differ- nature of the more perfect theory, it must reproduce the ent for different readers. successes of general relativity and quantum mechanics. We offer in Sec. III our reading of the history of ideas That includes the method of representing the material on ⌳ and its generalization to dark energy. This is a content of the observable universe—all forms of mass fascinating and we think edifying illustration of how sci- and energy—by the stress-energy tensor, and the rela- ence may advance in unexpected directions. It is rel- tion between the stress-energy tensor and the curvature evant to an understanding of the present state of re- of macroscopic spacetime. One part has to be adjusted. search in cosmology, because traditions inform opinions, The numerical values of the parameters in Eq. (1) also and people have had mixed feelings about ⌳ ever since are enigmatic, and possibly trying to tell us something. Einstein (1917) introduced it 85 years ago. The concept The evidence is that the parameters have the approxi- never entirely disappeared in cosmology because a se- mate values ries of observations hinted at its presence, and because to some cosmologists ⌳ fits the formalism too well to be ⍀ ϳ ⍀ ϳ ⍀ ϳ ⌳0 0.7, DM0 0.2, B0 0.05. (2) ignored. The search for the physics of the vacuum, and its possible relation to ⌳, has a long history too. Despite ⍀ ⍀ We have written M0 in two parts: B0 measures the ⍀ density of the baryons we know exist and DM0 mea- sures the hypothetical nonbaryonic cold dark matter we 6The dark energy should of course be distinguished from a ⍀ need to fit the cosmological tests. The parameters B0 hypothetical gas of particles with velocity dispersion large ⍀ and DM0 have similar values but represent different enough that the distribution is close to homogeneous.

Rev. Mod. Phys., Vol. 75, No. 2, April 2003 P. J. E. Peebles and Bharat Ratra: The cosmological constant and dark energy 563

the common and strong suspicion that ⌳ must be negli- those who have not already thought to do so, to check gibly small, because any other acceptable value is ab- that Eq. (4) is required to preserve homogeneity and surd, all this history has made the community well pre- isotropy.8 pared for the recent observational developments that The rate of change of the distance in Eq. (4) is the argue for the detection of ⌳. speed Our approach in Sec. IV to the discussion of the evi- dence for detection of ⌳, from the cosmological tests, vϭdl/dtϭHl, Hϭa˙ /a, (5) also requires explanation. One occasionally reads that the tests will show us how the world will end. That cer- where the overdot means the derivative with respect to tainly seems interesting, but it is not the main point: why world time t and H is the time-dependent Hubble pa- should we trust an extrapolation into the indefinite fu- rameter. When v is small compared to the speed of light ture of a theory that we can at best show is a good this is Hubble’s law. The present value of H is Hubble’s approximation to reality?7 As we remarked in Sec. I.A, constant, H . When needed we will use9 the purpose of the tests is to check the approximation to 0 reality, by checking the physics and astronomy of the Ϫ Ϫ Ϫ Ϫ H ϭ100h km s 1 Mpc 1ϭ67Ϯ7kms1 Mpc 1 standard relativistic cosmological model, along with any 0 viable alternatives that may be discovered. We take our ϭ͑15Ϯ2 Gyr͒Ϫ1, (6) task to be the identification of the aspects of the stan- dard theory that enter the interpretation of the measure- at two standard deviations. The first equation defines the ments and thus are or may be empirically checked or dimensionless parameter h. measured. Another measure of the expansion follows by consid- ering the stretching of the wavelength of light received ␭ from a distant galaxy. The observed wavelength obs of a ␭ II. BASIC CONCEPTS feature in the spectrum that had wavelength em at emis- sion satisfies A. The Friedmann-Lemaıˆtre model ϩ ϭ␭ ␭ ϭ ͑ ͒ ͑ ͒ 1 z obs / em a tobs /a tem , (7) The standard world model is close to homogeneous and isotropic on large scales, and lumpy on small scales—the effect of mass concentrations in galaxies, stars, people, etc. The length scale at the transition from 8We feel we have to comment on a few details about Eq. (4) nearly smooth to strongly clumpy is about 10 Mpc. We to avoid contributing to debates that are more intense than use here and throughout the standard astronomers’ seem warranted. Think of the world time t as the proper time length unit, kept by each of a dense set of observers, each moving so that all the others are isotropically moving away, and with the times 24 6 1Mpcϭ3.1ϫ10 cmϭ3.3ϫ10 light years. (3) synchronized to a common energy density, ␳(t), in the near homogeneous expanding universe. The distance l(t) is the sum To be more definite, imagine that many spheres of ra- of the proper distances between neighboring observers, all dius 10 Mpc are placed at random, and the mass within measured at time t, and along the shortest distance between each is measured. At this radius the rms fluctuation in the two observers. The rate of increase of the distance, dl/dt, the set of values of masses is about equal to the mean may exceed the velocity of light. This is no more problematic value. On smaller scales the departures from homogene- in relativity theory than is the large speed at which the beam of ity are progressively more nonlinear; on larger scales the a flashlight on Earth may swing across the face of the Moon (assuming an adequately tight beam). Space sections at fixed t density fluctuations are perturbations to the homoge- may be noncompact, and the total mass of a homogeneous neous model. From now on we mention these perturba- universe formally infinite. As far as is known this is not mean- tions only when relevant for the cosmological tests. ingful: we can only assert that the universe is close to homo- The expansion of the universe means the distance l(t) geneous and isotropic over observable scales, and that what between two well-separated galaxies varies with world can be observed is a finite number of baryons and photons. time t as 9The numerical values in Eq. (6) are determined from an analysis of all available measurements of H0 prior to mid-1999 l͑t͒ϰa͑t͒, (4) (Gott et al., 2001). They are a very reasonable summary of the current situation. For instance, the Hubble Space Tele- ϭ where the expansion or scale factor a(t) is independent scope Key Project summary measurement value H0 72 Ϯ Ϫ1 Ϫ1 ␴ of the choice of galaxies. It is an interesting exercise, for 8kms Mpc (1 uncertainty; Freedman et al., 2001) is in very good agreement with Eq. (6), as is the recent Tammann ϭ Ϯ Ϫ1 Ϫ1 et al. (2001) summary value H0 60 6kms Mpc (ap- proximate 1␴ systematic uncertainty). This is an example of 7Observations may now have detected ⌳, at a characteristic the striking change in the observational situation over the pre- energy scale of a millielectron volt [Eq. (47)]. We have no vious five years: the uncertainty in H0 has decreased by more guarantee that an even lower-energy scale does not exist; such than a factor of 3, making it one of the better-measured cos- a scale could first become apparent through cosmological tests. mological parameters.

Rev. Mod. Phys., Vol. 75, No. 2, April 2003 Top 10 Theories About Dark Energy

Paulina Destarac June 3, 2017 http://listverse.com/2017/06/03/top-10-theories-about-dark-energy/

Humanity has amassed an enormous amount of information about our universe and how it works. We pride ourselves in being the most intelligent species on Earth, and so far, the most capable kind in all of the universe.

However, the information we have about the structure of our universe is derived from the mere 4 percent that we can observe, measure, and analyze—regular, ordinary matter. The remaining 96 percent is “dark” stuff. It is dark because we know nothing of it (and because physicists tend to lack creativity in the naming department).

Of that 96 percent, roughly 68 percent is dark energy. This makes it the largest component of the universe, and right now, the most mysterious. Thousands of scientists around the world are working to decipher this mysterious energy that seems to guide how our universe creates large structures.

Without dark energy, our universe would eventually end up in a “Big Crunch”—a gravity- dominated fate where the universe rapidly collapses into itself. So, although we don’t know what dark energy is, we should be thankful that it’s there.

Here are the top 10 theories of what dark energy could be and what each scenario can tell us about the fate of our universe. 10 It Is A Property Of Space

Photo credit: NASA

This theory is derived from Einstein’s theory of gravitation, specifically from the fact that “empty space” can have its own energy—dubbed “the cosmological constant.” Einstein also believed that space could come into existence from nothing, and as more space is created, more energy can consequently be held within it.[1]

This would explain the rapid expansion of the universe we observe. This sort of universe would continue expanding forever until every object in the universe was so far away from every other object that everything would end in cold darkness. 9 Theory Of Everything

Photo credit: hubblesite.org

A lot of astronomers believe the search for dark energy is futile. Instead, they lecture that finding the elusive “theory of everything” (not the Stephen Hawking movie) would naturally solve the problem of dark energy.

This theory should be able to explain the behavior of all objects in the universe—from very big to incredibly small. For now, our theories of how the universe works are divided into large-scale theories (like the theory of gravity) and small-scale theories (like quantum mechanics).

Although solving the problem of dark energy this way is logically sound, finding this theory has proven impossible for even the brightest minds in physics. Normal laws of physics seem to “break down” when one reaches the quantum level.[2] But alas, the search goes on.

8 It Creates A New Fundamental Force

Photo credit: novan.info

The fundamental forces we know of (gravity, electromagnetism, weak force, and strong force) all act within different ranges. Some affect only atomic-sized objects, while others cause the motions of planets and instigate the formation of galaxies.

This theory of dark energy states that that there is a fundamental force we still have not found that acts on enormous scales and can only be observed when the universe reaches a certain size. It would work to oppose gravity and thus pull objects in the universe away from each other.[3]

Scientists argue that because this force acts on such a large scale, we have not yet encountered it in our everyday lives and measurements made on Earth cannot be affected by it. No one really knows if this force would be temporary or permanent. Yet, depending on this, the universe would either expand forever and turn cold or expand and contract periodically for the rest of time. 7 Einstein’s Theory of Gravity Is Wrong

Photo credit: NASA

Try telling one of the smartest physicists who ever lived that his (arguably) most famous theory is wrong . . . yikes. Einstein’s theory of relativity states that every body in the universe is attracted to every other body, with the strength of that attraction depending solely on the masses of the objects and the distance between their centers.

However, some physicists have argued that this theory might be incorrect and have been developing new theories of gravity to explain dark energy. In these theories, they reverse the effects of gravity on large scales so that objects repel each other.[4]

Although these theories do not have much experimental backing (since Einstein’s model of gravity has worked pretty well for us this far), they would explain why the universe is expanding. With these new models of gravitation, our universe would again reach a state of cold darkness after a state of rapid expansion. 6 Time Dilation

If you have ever seen the movie Interstellar, you have probably heard of time dilation. It is a phenomenon that occurs when objects moving close to the speed of light experience a slowing of time.

This is the same idea presented in the twin paradox, where one twin boards a spaceship that moves close to the speed of light while his brother remains on Earth. When they meet again after years of separation, the twin on Earth is significantly older than his astronaut brother.

A recent paper by Edward Kipreos, a professor at the University of Georgia, argues that only the moving object itself undergoes time dilation.[5] (Usually, the person observing the fast-moving object also experiences the effects.)

This would mean that the passage of time was faster in the past. This eliminates the need to have a repulsive force or substance since the apparent expansion of the universe would be a mere miscalculation of distances that have been affected by time dilation.

If this theory is true, it would not only contradict another one of Einstein’s famous theories (his theory of special relativity) but it would also imply that our universe would continue to expand due to the effects of the Hubble Constant.

5 An Exotic New Particle

Photo credit: PBS

The idea of particles and fields has been around for centuries now. We know that an electron creates an electric field, and recently, the gravitational field has been associated with the “graviton”—gravity’s “force particle.” Particle physicists and theorists are comfortable with the idea that the energy of a particular field must be transmitted by its force particle instead of by the field itself.

This concept can be translated to dark energy, with dark matter (the other 27 percent of the universe) being its force particle.[6] This idea seems plausible, especially because some force particles are unobservable, like the graviton. However, there is little evidence supporting this theory because we have not found a way to measure any of the properties associated with dark energy or dark matter thus far. 4 f(R) Theories

Photo credit: sciencemag.org f(R) theories are models of the current curvature of the universe (with curvature denoted as R). In 2007, researchers from the University of Chicago showed that, with a specific value of R, a model of the universe is created in which dark energy is not needed to explain the expansion of the universe.

This type of universe contorts itself in a way that minimizes its overall curvature while producing an extra gravity-like force that can either attract or repel objects, depending on a set of conditions.[7]

The University of Chicago theorists agree that for this theory to hold, the extra force has to disappear where gravity is relatively strong (for example, on the scale of planets and galaxies) and appear only on the largest of scales. A team of astronomers at Peking University have begun to take measurements of clusters to see if this f(R) theory could be a correct description of our universe. 3 Multiverses And The Anthropic Principle

Photo credit: phys.org

One of the greatest failures of modern physics is the prediction of the actual value of dark energy. Quantum theory predicts a very small number, but physicists calculated a number over 10120 times larger! (This measured value of dark energy is the cosmological constant—the same one from item 10 on this list.)

This is where the anthropic principle[8] comes in—the idea that fundamental constants of physics and chemistry (such as the speed of light, the gravitational constant, etc.) are “just right” to support life in our particular universe but may have different values in other universes. In an infinite set of parallel universes, it does not seem improbable that our universe just happens to be the one with the correct value of dark energy to allow for the formation of life. 2 Virtual Particles

Photo credit: NASA/WMAP Science Team

Quantum mechanics is weird. It allows for things to pop in and out of existence, shattering every concept we learn in high school physics. (“Matter cannot be created or destroyed,” we mumbled.)

This theory uses the idea of virtual particles—small bits of matter that appear only for small instances and then disappear. This constant appearance and disappearance of particles releases energy because matter is converted into energy when these particles disappear.

Physicists believe this is how space itself can attain enough continuous energy to create a “negative pressure” that causes the expansion of the universe. If this theory is true, the energy space gains from these virtual particles could be the mysterious dark energy and our universe would continue to expand for as long as this process occurs.[9] 1 Quintessence

Photo credit: NASA

The number of theories on this list shows how far we are from understanding two-thirds of our universe. So far, each theory has had huge implications for the fate of the universe or for poor Einstein.

Deciphering dark energy could open doors for an entirely new branch of physics or could radically change existing ones. That is why so many physicists and astronomers today are questioning this large, mysterious, “dark stuff” that guides the evolution of our universe.

This last theory of dark energy is, by far, the strangest. A universe dominated by “quintessence” is one full of an “energy fluid.”[10] Other physicists like calling this energy “phantom energy.”

The idea behind it is that quintessence varies with time and location and that its energy density increases over time. This universe would meet a violent fate that physicists call a “Big Rip” (again, with the very literal naming), in which the universe would literally explode as atoms cannot keep up with how fast they are being pulled apart and stretched.

Everything in and out of sight would be obliterated. The universe would go out with a bang.