Dark energy

Cosmology

Studied objects and processes

Universe
Observable Universe
Large-scale structure of the universe
- Galactic Superclusters
- Galactic filaments
- Voids
- Hubble Bubble
Relic radiation
Hidden mass
- Dark matter
- Dark energy

Universe History

The main stages of the Universe
Age of the Universe
Galaxy formation

Observed processes

Expansion of the Universe
- Cosmological redshift
- Hubble law
- Accelerated expansion of the universe
Nucleosynthesis

Theoretical studies

Gravitational Instability
Cosmological principle
Cosmological models
- Cosmological singularity
- Big explosion
- Model de sitter
- Hot Universe Model
- Cosmic inflation
- Friedman's Universe
  - Friedmann equation
  - Attendant distance
  - Lambda CDM Model
  - Cosmological equation of state
  - Critical density

Composition of the Universe according to WMAP

Dark energy in cosmology ( dark energy ) is a hypothetical type of energy introduced into the mathematical model of the Universe for the sake of explaining its observed expansion with acceleration ^[1] .

There are three options for explaining the essence of dark energy:

dark energy is a cosmological constant — a constant energy density that uniformly fills the space of the Universe (in other words, non-zero energy and vacuum pressure are postulated) ^[2] ;
dark energy is a certain quintessence - a dynamic field, the energy density of which can vary in space and time .
dark energy is modified gravity at distances of the order of the size of the visible part of the Universe ^[3] .

To date (2019), all known reliable observational data do not contradict the first hypothesis ^[3] , so it is accepted as standard in cosmology . The final choice between variants requires very long and highly accurate measurements of the rate of expansion of the Universe in order to understand how this speed changes with time. The rate of expansion of the universe is described by the cosmological equation of state . Solving the equation of state for dark energy is one of the most pressing problems of modern observational cosmology ^[3] .

According to the observational data from the Planck Space Observatory published in March 2013, the total mass-energy of the observed Universe consists of 68.3% of dark energy and 26.8% of dark matter ^[4] ^[5] ^[6] .

Content

Discovery of the acceleration of the universe

Based on observations made in the late 1990s of type Ia supernovae, it was concluded that the expansion of the universe accelerates with time. Then these observations were supported by other sources: measurements of the background radiation , gravitational lensing , the Big Bang nucleosynthesis . All the data obtained fits well into the lambda CDM model .

Distances to other galaxies are determined by measuring their redshift . According to the Hubble law , the amount of redshift of light from distant galaxies is directly proportional to the distance to these galaxies. The ratio between distance and redshift is called the Hubble parameter (or, not quite exactly, the Hubble constant).

However, the value of the Hubble parameter itself must first be established in some way, and for this it is necessary to measure the redshift values for galaxies, the distances to which have already been calculated by other methods . To do this, astronomy uses “standard candles”, that is, objects whose luminosity is known. The best type of “standard candle” for cosmological observations are type Ia supernovae (all flashing Ia located at the same distance should have almost the same observed brightness; it is advisable to make corrections for the rotation and composition of the original star). Comparing the observed brightness of supernovae in different galaxies, we can determine the distances to these galaxies.

At the end of the 1990s, it was discovered that in distant galaxies, the distance to which was determined by the Hubble law, type Ia supernovae have a brightness lower than it should. In other words, the distance to these galaxies, calculated by the method of "standard candles" (supernova Ia), is greater than the distance calculated on the basis of the previously established value of the Hubble parameter. It was concluded that the universe is not just expanding, it expands with acceleration.

Dark Energy and Hidden Mass Hypothesis

Previously existing cosmological models assumed that the expansion of the universe is slowing. They proceeded from the assumption that the main part of the mass of the Universe is composed of matter - both visible and invisible ( dark matter ). Based on new observations indicating acceleration of expansion, the existence of an unknown type of energy with negative pressure was postulated (see equations of state ). She was called "dark energy."

The hypothesis of the existence of dark energy (whatever it may be) solves the so-called "problem of the invisible mass ." The Big Bang nucleosynthesis theory explains the formation of light chemical elements in the young Universe, such as helium , deuterium and lithium . The theory of the large-scale structure of the universe explains the formation of the structure of the universe: the formation of stars , quasars , galaxies and clusters of galaxies. Both of these theories suggest that the density of baryonic matter and dark matter is about 30% of the critical density required for the formation of a "closed" Universe, that is, it corresponds to the density required for the shape of the Universe to be flat. The measurements of the CMB of the Universe, recently carried out by the WMAP satellite, show that space-time in the Universe does indeed have a global curvature very close to zero. Consequently, some previously unknown form of invisible energy should give the missing 70% of the density of the Universe.

Dark Energy Nature

The essence of dark energy is the subject of controversy. It is known that it is very evenly distributed, has a low density and does not interact any noticeably with ordinary matter by means of known fundamental types of interaction - with the exception of gravity. Since the hypothetical density of dark energy is low (about 10 ⁻²⁹ g / cm ³), it can hardly be detected by laboratory experiment. Dark energy can have such a profound effect on the Universe (making up 70% of all energy) only because it fills the space uniformly (otherwise).

Cosmological constant

A diagram representing the accelerated expansion of the Universe due to dark energy.

The simplest explanation is that dark energy is simply “the cost of existence of space”: that is, any amount of space has some kind of fundamental, inherent energy to it. It is also sometimes referred to as vacuum energy, since it is the energy density of a pure vacuum . This is the cosmological constant , sometimes called the "lambda member" (from the name of the Greek letter $\Lambda$ ${\ displaystyle \ Lambda}$ used to refer to it in the equations of the general theory of relativity ) ^[7] . The introduction of the cosmological constant into the standard cosmological model, based on the Friedmann – Lemaitre – Robertson – Walker metric , led to the emergence of a modern cosmology model, known as the lambda-CDM model . This model fits well with the existing cosmological observations.

Many physical theories of elementary particles predict the existence of vacuum fluctuations , that is, they impart a vacuum with precisely this kind of energy. The value of the cosmological constant is estimated in the order of 10 ⁻²⁹ g / cm³, or about 1.03 keV / cm³ (about 10 ⁻¹²³ in Planck units ) ^[8] .

The cosmological constant has a negative pressure equal to its energy density. The reasons for which the cosmological constant has a negative pressure follow from classical thermodynamics. The amount of energy contained in the "box with vacuum" volume $V$ ${\ displaystyle V}$ equals $\rho V$ ${\ displaystyle \ rho V}$ where $\rho$ ${\ displaystyle \ rho}$ - energy density of the cosmological constant. Increase the volume of the "box" $d V$ ${\ displaystyle dV}$ positive) leads to an increase in its internal energy, and this means that it performs negative work. Since the work done by changing the volume $d V$ ${\ displaystyle dV}$ equals $p d V$ ${\ displaystyle pdV}$ where $p$ ${\ displaystyle p}$ - pressure, then $p$ ${\ displaystyle p}$ - negatively and, in fact, $p=-\rho$ ${\ displaystyle p = - \ rho}$ (coefficient $c^{2}$ ${\ displaystyle c ^ {2}}$ , connecting mass and energy, equated 1) ^[2] .

According to the general theory of relativity , gravity depends not only on mass (density), but also on pressure , and pressure has a greater coefficient than density. Negative pressure should generate repulsion, antigravity , and therefore causes acceleration of the expansion of the Universe ^[9] .

The most important unsolved problem of modern physics is that most of the quantum field theories , based on the quantum vacuum energy, predict the enormous value of the cosmological constant - by many orders of magnitude greater than the allowable cosmological ideas. The usual formula of quantum field theory for the summation of vacuum zero-point field oscillations (with cutting off the wavenumber of vibrational modes corresponding to the Planck length ) gives a huge vacuum energy density ^[10] ^[11] . This value, therefore, must be compensated by some action almost equal (but not exactly equal) in absolute value, but having the opposite sign. Some supersymmetry theories (SATHISH) require that the cosmological constant be exactly zero, which also does not contribute to solving the problem. Such is the essence of the “ problem of the cosmological constant ”, the most difficult problem of “ fine tuning ” in modern physics: no method has been found to derive from the physics of elementary particles the extremely small value of the cosmological constant defined in cosmology. Some physicists, including Stephen Weinberg , consider the so-called. The “ anthropic principle ” is the best explanation for the observed fine balance of quantum vacuum energy.

Despite these problems, the cosmological constant is in many respects the most economical solution to the problem of an accelerating Universe. A single numerical value explains many observations. Therefore, the current generally accepted cosmological model ( lambda-CDM model ) includes the cosmological constant as an essential element.

Quintessence

An alternative approach was proposed in 1987 by the German theoretical physicist Christoph Wetterich ^[12] ^[13] . Vetterich proceeded from the assumption that dark energy is a kind of particle-like excitations of a certain dynamic scalar field , called “quintessence” ^[14] . The difference from the cosmological constant is that the density of quintessence can vary in space and time. In order for quintessence to not “gather” and form large-scale structures like ordinary matter (stars, etc.), it must be very light, that is, have a large Compton wavelength .

No evidence of the existence of quintessence has yet been discovered, but such a existence cannot be ruled out. The quintessence hypothesis predicts a slightly slower acceleration of the Universe, in comparison with the cosmological constant hypothesis. Some scientists believe that the best evidence in favor of quintessence would be violations of the Einstein equivalence principle and variations of fundamental constants in space or time. The existence of scalar fields is predicted by the standard model and string theory , but a problem arises that is similar to the variant with a cosmological constant: the theory of renormalization predicts that scalar fields should acquire significant mass.

The problem of cosmic coincidence raises the question of why the acceleration of the Universe began at a particular point in time. If the acceleration in the Universe began before this moment, the stars and galaxies simply would not have time to form, and life would have no chance of occurring, at least in a form known to us. Supporters of the " anthropic principle " consider this fact to be the best argument in favor of their constructions. However, many models of quintessence provide the so-called "tracking behavior", which solves this problem. In these models, the field of quintessence has a density that adjusts to the radiation density (not reaching it) until the moment when the Big Bang develops, when an equilibrium of matter and radiation develops. After this moment, the quintessence begins to behave like the desired “dark energy” and ultimately dominates the Universe. This development naturally sets a low value for dark energy.

The equation of state (the dependence of pressure on energy density) for quintessence: $p=w\cdot \varepsilon ,$ ${\ displaystyle p = w \ cdot \ varepsilon,}$ Where $-1<w<-1/3$ ${\ displaystyle -1 <w <-1/3}$ (for vacuum $w=-1$ ${\ displaystyle w = -1}$ ).

Other possible forms of dark energy have been proposed: phantom energy , for which the energy density increases with time (in the equation of state of this type of dark energy $w<-1$ ${\ displaystyle w <-1}$ ), and the so-called "kinetic quintessence", having the form of non-standard kinetic energy . They have unusual properties: for example, phantom energy can lead to the Big Gap ^{[15] of the} Universe.

In 2014, data from the BOSS project ( Baryon Oscillation Spectroscopic Survey ) showed that with a high degree of accuracy, the value of dark energy is constant ^[16] .

The manifestation of unknown properties of gravity

There is a hypothesis that there is no dark energy at all, and the accelerated expansion of the Universe is explained by the unknown properties of the forces of gravity , which begin to manifest themselves at distances of the order of the size of the visible part of the Universe ^[3] .

Implications for the fate of the universe

It is estimated that the accelerating expansion of the universe began about 5 billion years ago. It is assumed that before this expansion was slowed down due to the gravitational action of dark matter and baryonic matter. The density of baryonic matter in an expanding Universe decreases faster than the density of dark energy. In the end, dark energy begins to prevail. For example, when the volume of the Universe doubles, the density of baryonic matter decreases by half, and the density of dark energy remains almost unchanged (or exactly the same - in the version with a cosmological constant).

If the accelerating expansion of the Universe continues indefinitely, as a result of a galaxy outside our Superclusters, galaxies will sooner or later go beyond the horizon of events and become invisible to us, since their relative speed will exceed the speed of light . Это не является нарушением специальной теории относительности . На самом деле невозможно даже определить «относительную скорость» в искривлённом пространстве-времени. Относительная скорость имеет смысл и может быть определена только в плоском пространстве-времени, или на достаточно малом (стремящемся к нулю) участке искривлённого пространства-времени. Любая форма коммуникации далее пределов горизонта событий становится невозможной, и всякий контакт между объектами теряется. Земля , Солнечная система , наша Галактика , и наше Сверхскопление будут видны друг другу и в принципе достижимы путём космических полётов, в то время как вся остальная Вселенная исчезнет вдали. Со временем наше Сверхскопление придёт в состояние тепловой смерти , то есть осуществится сценарий, предполагавшийся для предыдущей, плоской модели Вселенной с преобладанием материи.

Существуют и более экзотические гипотезы о будущем Вселенной. Одна из них предполагает, что фантомная энергия приведёт к т. н. «расходящемуся» расширению. Это подразумевает, что расширяющая сила действия тёмной энергии продолжит неограниченно увеличиваться, пока не превзойдёт все остальные силы во Вселенной. По этому сценарию, тёмная энергия со временем разорвёт все гравитационно связанные структуры Вселенной, затем превзойдёт силы электростатических и внутриядерных взаимодействий , разорвёт атомы, ядра и нуклоны и уничтожит Вселенную в Большом Разрыве .

С другой стороны, тёмная энергия может со временем рассеяться или даже сменить отталкивающее действие на притягивающее. В этом случае гравитация возобладает и приведёт Вселенную к « Большому Сжатию ». Некоторые сценарии предполагают «циклическую модель» Вселенной. Хотя эти гипотезы пока не подтверждаются наблюдениями, они и не отвергаются полностью. Решающую роль в установлении конечной судьбы Вселенной (развивающейся по теории Большого Взрыва ) должны сыграть точные измерения темпа ускорения.

Ускоренное расширение Вселенной было открыто в 1998 году при наблюдениях за сверхновыми типа Ia ^[17] ^[18] . За это открытие Сол Перлмуттер , Брайан П. Шмидт и Адам Рисс получили премию Шао по астрономии за 2006 год и Нобелевскую премию по физике за 2011 год.

Notes

↑ Тёмная энергия вблизи нас // Астронет
↑ ¹ ² Астронет > Тёмная энергия вблизи нас
↑ ¹ ² ³ ⁴ Марио Ливио , Адам Рисс. Ребус тёмной энергии // В мире науки . — 2016. — № 5—6 . — С. 50—57 .
↑ Ade, PAR; Aghanim, N.; Armitage-Caplan, C.; et al . (Planck Collaboration). Planck 2013 results. I. Overview of products and scientific results – Table 9. (англ.) // Astronomy and Astrophysics (submitted) : journal. — 2013. — 22 March. — . — arXiv : 1303.5062 . Архивировано 23 марта 2013 года.
↑ Francis, Matthew. First Planck results: the Universe is still weird and interesting (неопр.) . Arstechnica (22 марта 2013).
↑ Planck captures portrait of the young Universe, revealing earliest light (неопр.) . University of Cambridge (21 марта 2013). The appeal date is March 21, 2013.
↑ Игнатьев, 2016 , с. ten.
↑ Игнатьев, 2016 , с. 48.
↑ Астронет > Тёмная энергия вблизи нас
↑ С. Вайнберг «Проблема космологической постоянной», Успехи физических наук , август 1989 г., т. 158, вып. 4, стр. 640—678
↑ Я. Б. Зельдович «Теория вакуума, быть может, решает загадку космологии» , Успехи физических наук , март 1981 г., т. 133, вып. 3, стр. 479—503
↑ Cosmology and the Fate of Dilatation Symmetry , C. Wetterich, Nucl. Phys. B 302 , 668 (1988)
↑ The Cosmon Model for an Asymptotically Vanishing Time Dependent Cosmological «Constant» , C. Wetterich, Astron. Astrophys. 301 , 321 (1995), arXiv: hep-th/9408025v1
↑ Caldwell RR, Steinhardt PJ Phys.Rev. D 57, 6057 (1998).
↑ Разрыв Вселенной
↑ Точная карта космоса подтверждает идею бесконечной плоской Вселенной
↑ Riess, A. et al. 1998, Astronomical Journal , 116, 1009
↑ Perlmutter, S. et al. 1999, Astrophysical Journal , 517, 565

Literature

Books

Игнатьев Ю.Г. Классическая космология и тёмная энергия . — Казань: Изд-во Казанского ун-та , 2016. — 248 с. — ISBN 978-5-00019-692-2 .
Amendola L., Tsujikawa S. Dark Energy: Theory and Observations. — Cambridge University Press, 2010. — 491 p.
Dark Energy: Observational and Theoretical Approaches / ed. P. Ruiz-Lapuente. — Cambridge University Press, 2010. — 339 p.
Kragh HS, Overduin JM The Weight of the Vacuum: A Scientiﬁc History of Dark Energy. — Springer, 2014. — 113 p.
Li M., Li X., Wang S., Wang Y. Dark Energy. — Singapore: World Scientific, 2015. — 254 p.
Einasto J. , Chernin A.D. Dark matter and dark energy. - M: Century-2, 2018. - 176 p. - ISBN 978-5-85099-197-5 .

Articles

Chernin A.D. Dark energy and worldwide anti-busting // UFN . - 2008. - T. 178 . - p . 267-300 . - DOI : 10.3367 / UFNr.0178.200803c.0267 .
Lukash V.N. , Rubakov V.A. Dark energy: myths and reality // UFN. - 2008. - T. 178 . - p . 301-308 . - DOI : 10.3367 / UFNr.0178.200803d.0301 .
Carroll SM The Cosmological Constant // Living Reviews in Relativity. - 2001. - Vol. 4. - P. 1-56. - DOI : 10.12942 / lrr-2001-1 .
Calder L., Lahav O. Dark Energy: back to Newton? // Astronomy & Geophysics. - 2008. - Vol. 49. - P. 1.13-1.18. - DOI : 10.1111 / j.1468-4004.2008.49113.x . - arXiv : 0712.2196 .
Huterer D., Shafer DL Dark energy two decades after: observables, probes, consistency tests // Reports on Progress in Physics. - 2018. - Vol. 81. - P. 016901. - DOI : 10.1088 / 1361-6633 / aa997e . - arXiv : 1709.01091 .
Brax P. What makes the Universe accelerate? A review of what it could have been. // Reports on Progress in Physics. - 2018. - Vol. 81. - P. 016902. - DOI : 10.1088 / 1361-6633 / aa8e64 .

Links

Articles and reviews about dark energy on the Modern Cosmology website
Chernin A.D. Dark energy near us
Chernin A.D. Physical vacuum and cosmic anti-gravity
Rincon P. New method 'confirms dark energy' // BBC News. - 19 May 2011.
d / f "Dark matter, dark energy" (2008)

[http://www.astronet.ru/db/msg/1210535-1] Тёмная энергия вблизи нас // Астронет

[http://www.astronet.ru/db/msg/1210535/node5.html-2] ¹ ² Астронет > Тёмная энергия вблизи нас

[WMN201606-3] ¹ ² ³ ⁴ Марио Ливио , Адам Рисс. Ребус тёмной энергии // В мире науки . — 2016. — № 5—6 . — С. 50—57 .

[planck_overview-4] Ade, PAR; Aghanim, N.; Armitage-Caplan, C.; et al . (Planck Collaboration). Planck 2013 results. I. Overview of products and scientific results – Table 9. (англ.) // Astronomy and Astrophysics (submitted) : journal. — 2013. — 22 March. — . — arXiv : 1303.5062 . Архивировано 23 марта 2013 года.

[wmap7parameters-5] Francis, Matthew. First Planck results: the Universe is still weird and interesting (неопр.) . Arstechnica (22 марта 2013).

[planckcam-6] Planck captures portrait of the young Universe, revealing earliest light (неопр.) . University of Cambridge (21 марта 2013). The appeal date is March 21, 2013.

[_2daa74d433acdb58-7] Игнатьев, 2016 , с. ten.

[_2daa79d433ace3df-8] Игнатьев, 2016 , с. 48.

[http://www.astronet.ru/db/msg/1210535/node7.html-9] Астронет > Тёмная энергия вблизи нас

[10] С. Вайнберг «Проблема космологической постоянной», Успехи физических наук , август 1989 г., т. 158, вып. 4, стр. 640—678

[11] Я. Б. Зельдович «Теория вакуума, быть может, решает загадку космологии» , Успехи физических наук , март 1981 г., т. 133, вып. 3, стр. 479—503

[12] Cosmology and the Fate of Dilatation Symmetry , C. Wetterich, Nucl. Phys. B 302 , 668 (1988)

[13] The Cosmon Model for an Asymptotically Vanishing Time Dependent Cosmological «Constant» , C. Wetterich, Astron. Astrophys. 301 , 321 (1995), arXiv: hep-th/9408025v1

[qe-14] Caldwell RR, Steinhardt PJ Phys.Rev. D 57, 6057 (1998).

[15] Разрыв Вселенной

[16] Точная карта космоса подтверждает идею бесконечной плоской Вселенной

[17] Riess, A. et al. 1998, Astronomical Journal , 116, 1009

[18] Perlmutter, S. et al. 1999, Astrophysical Journal , 517, 565