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{{Short description|Background energy existing in space}}
{{For|articles related to vacuum energy|Quantum vacuum (disambiguation)}}
{{More citations needed|date=February 2023}}
{{Quantum mechanics}}

'''Vacuum energy''' is an underlying background [[energy]] that exists in [[space]] throughout the entire [[universe]].<ref>{{Cite web|last=Battersby|first=Stephen|title=It's confirmed: Matter is merely vacuum fluctuations|url=https://www.newscientist.com/article/dn16095-its-confirmed-matter-is-merely-vacuum-fluctuations/|access-date=2020-06-18|website=New Scientist|language=en-US}}</ref> The vacuum energy is a special case of [[zero-point energy]] that relates to the [[quantum vacuum]].<ref>Scientific American. 1997. FOLLOW-UP: What is the 'zero-point energy' (or 'vacuum energy') in quantum physics? Is it really possible that we could harness this energy? – Scientific American. [ONLINE] Available at: http://www.scientificamerican.com/article/follow-up-what-is-the-zer/. [Accessed 27 September 2016].</ref>

{{unsolved|physics|Why does the [[zero-point energy]] of the vacuum not cause a large [[cosmological constant]]? What cancels it out?}}
The effects of vacuum energy can be experimentally observed in various phenomena such as [[spontaneous emission]], the [[Casimir effect]], and the [[Lamb shift]], and are thought to influence the behavior of the Universe on [[Physical cosmology|cosmological scales]]. Using the upper limit of the [[cosmological constant]], the vacuum energy of free space has been estimated to be 10<sup>−9</sup> [[joule]]s (10<sup>−2</sup> [[erg]]s), or ~5 [[Electronvolt|GeV]] per cubic meter.<ref>Sean Carroll, Senior Research Associate – Physics, [[California Institute of Technology]], June 22, 2006. [[C-SPAN]] broadcast of Cosmology at Yearly Kos Science Panel, Part 1.</ref> However, in [[quantum electrodynamics]], consistency with the principle of [[Lorentz covariance]] and with the magnitude of the [[Planck constant]] suggests a much larger value of 10<sup>113</sup> joules per cubic meter. This huge discrepancy is known as the [[cosmological constant problem]] or, colloquially, the "vacuum catastrophe."<ref name="AdlerCasey1995">{{cite journal|last1=Adler|first1=Ronald J.|last2=Casey|first2=Brendan|last3=Jacob|first3=Ovid C.|title=Vacuum catastrophe: An elementary exposition of the cosmological constant problem|journal=American Journal of Physics|volume=63|issue=7|year=1995|pages=620–626|issn=0002-9505|doi=10.1119/1.17850|bibcode=1995AmJPh..63..620A|doi-access=free}}</ref>

==Origin==
[[Quantum field theory]] states that all fundamental [[field (physics)|fields]], such as the [[electromagnetic field]], must be [[Quantization (physics)|quantized]] at every point in space. A field in physics may be envisioned as if space were filled with interconnected vibrating balls and springs, and the strength of the field is like the displacement of a ball from its rest position. The theory requires "vibrations" in, or more accurately changes in the strength of, such a field to propagate as per the appropriate [[wave equation]] for the particular field in question. The [[second quantization]] of quantum field theory requires that each such ball–spring combination be quantized, that is, that the strength of the field be quantized at each point in space. Canonically, if the field at each point in space is a [[Harmonic oscillator|simple harmonic oscillator]], its quantization places a [[quantum harmonic oscillator]] at each point. Excitations of the field correspond to the [[elementary particle]]s of [[particle physics]]. Thus, according to the theory, even the [[vacuum]] has a vastly complex structure and all calculations of quantum field theory must be made in relation to this model of the vacuum.

The theory considers vacuum to implicitly have the same properties as a particle, such as [[Spin (physics)|spin]] or [[Polarization (waves)|polarization]] in the case of [[light]], energy, and so on. According to the theory, most of these properties cancel out on average leaving the vacuum empty in the literal sense of the word. One important exception, however, is the vacuum energy or the [[vacuum expectation value]] of the energy. The quantization of a simple harmonic oscillator requires the lowest possible energy, or [[zero-point energy]] of such an oscillator to be

<math display="block">{E}=\tfrac12 \hbar \omega \ </math><ref>{{cite web |title=3.4. The Cosmological Constant Problem(s) |url=https://ned.ipac.caltech.edu/level5/Sept03/Trodden/Trodden3_4.html}}</ref>

Summing over all possible oscillators at all points in space gives an infinite quantity. To remove this infinity, one may argue that only differences in energy are physically measurable, much as the concept of [[potential energy]] has been treated in [[classical mechanics]] for centuries. This argument is the underpinning of the theory of [[renormalization]]. In all practical calculations, this is how the infinity is handled.{{Citation needed|date=February 2023}}

Vacuum energy can also be thought of in terms of [[virtual particles]] (also known as vacuum fluctuations) which are created and destroyed out of the vacuum. These particles are always created out of the vacuum in particle–[[antiparticle]] pairs, which in most cases shortly annihilate each other and disappear. However, these particles and antiparticles may interact with others before disappearing, a process which can be mapped using [[Feynman diagrams]]. Note that this method of computing vacuum energy is mathematically equivalent to having a [[quantum harmonic oscillator]] at each point and, therefore, suffers the same renormalization problems.{{Citation needed|date=February 2023}}

Additional contributions to the vacuum energy come from [[spontaneous symmetry breaking]] in [[quantum field theory]].{{Citation needed|date=February 2023}}

==Implications==
[[File:Casimir plates with color coded virtual photons.jpg|thumb|upright=1|The Casimir effect is a physical force affecting macro-size objects and arises from vacuum energy, which are quantized oscillations in the electromagnetic field permeating every microscopic crevice of the Universe that give that field a non-zero energy. When two conductive flat plates of the same material are less than about 1000 nanometers apart (about twice the width of a common bacterium), they begin to form an electromagnetic cavity that excludes larger-wavelength components of vacuum energy. This reduces the energy between the plates, creating a pressure imbalance that pushes them together. Here, the gap permits only excitations with wavelengths no greater those shown in green color to pop into existence between the plates, excluding yellow, orange, red wavelengths.]]
Vacuum energy has a number of consequences. In 1948, [[Netherlands|Dutch]] [[physicist]]s [[Hendrik Casimir|Hendrik B. G. Casimir]] and [[Dirk Polder]] predicted the existence of a tiny attractive force between closely placed metal plates due to [[resonance]]s in the vacuum energy in the space between them. This is now known as the [[Casimir effect]] and has since been extensively experimentally verified.{{Page needed|date=July 2023}} It is therefore believed that the vacuum energy is "real" in the same sense that more familiar conceptual objects such as electrons, magnetic fields, etc., are real. However, alternative explanations for the Casimir effect have since been proposed.<ref>R. L. Jaffe: ''The Casimir Effect and the Quantum Vacuum''. In: ''Physical Review D''. Band 72, 2005 [https://arxiv.org/abs/hep-th/0503158].</ref>

Other predictions are harder to verify. Vacuum fluctuations are always created as particle–antiparticle pairs. The creation of these virtual particles near the [[event horizon]] of a [[black hole]] has been hypothesized by physicist [[Stephen Hawking]] to be a mechanism for the eventual [[Hawking radiation|"evaporation" of black holes]].<ref name="Page">{{cite journal |last=Page |first=Don N. |date=1976 |title=Particle emission rates from a black hole: Massless particles from an uncharged, nonrotating hole |journal=Physical Review D |volume=13 |issue=2 |pages=198–206 |doi=10.1103/PhysRevD.13.198 |bibcode = 1976PhRvD..13..198P }}</ref> If one of the pair is pulled into the black hole before this, then the other particle becomes "real" and energy/mass is essentially radiated into space from the black hole. This loss is cumulative and could result in the black hole's disappearance over time. The time required is dependent on the mass of the black hole (the equations indicate that the smaller the black hole, the more rapidly it evaporates) but could be on the order of [[Googol|10<sup>60</sup>]] years for large solar-mass black holes.<ref name="Page" />

The vacuum energy also has important consequences for [[physical cosmology]]. [[General relativity]] predicts that energy is equivalent to mass, and therefore, if the vacuum energy is "really there", it should exert a [[gravity|gravitational]] force. Essentially, a non-zero vacuum energy is expected to contribute to the [[cosmological constant]], which affects the [[expansion of the universe]].

== Field strength of vacuum energy ==
The field strength of vacuum energy is a concept proposed in a theoretical study that explores the nature of the vacuum and its relationship to gravitational interactions. The study derived a mathematical framework that uses the field strength of vacuum energy as an indicator of the bulk (spacetime) resistance to localized curvature. It illustrates the association of the field strength of vacuum energy to the curvature of the background, where this concept challenges the traditional understanding of gravity and suggests that the gravitational constant, G, may not be a universal constant, but rather a parameter dependent on the field strength of vacuum energy.<ref>[https://www.mdpi.com/2673-9984/7/1/50 MDPI, Physical Science Forum, 2023, 7(1), p. 50].</ref>

Determination of the value of G has been a topic of extensive research, with numerous experiments conducted over the years in an attempt to measure its precise value. These experiments, often employing high-precision techniques, have aimed to provide accurate measurements of G and establish a consensus on its exact value. However, the outcomes of these experiments have shown significant inconsistencies, making it difficult to reach a definitive conclusion regarding the value of G. This lack of consensus has puzzled scientists and called for alternative explanations.<ref>[https://academic.oup.com/nsr/article-pdf/7/12/1803/38880653/nwaa165.pdf National Science Review, 2020, 7, pp. 1803–1817].</ref>

To test the theoretical predictions regarding the field strength of vacuum energy, specific experimental conditions involving the position of the moon are recommended in the theoretical study. These conditions aim to achieve consistent outcomes in precision measurements of G. The ultimate goal of such experiments is to either falsify or provide confirmations to the proposed theoretical framework. The significance of exploring the field strength of vacuum energy lies in its potential to revolutionize our understanding of gravity and its interactions.

==History==
In 1934, [[Georges Lemaître]] used an unusual [[ideal gas|perfect-fluid]] [[equation of state]] to interpret the cosmological constant as due to vacuum energy. In 1948, the [[Casimir effect]] provided an experimental method for a verification of the existence of vacuum energy; in 1955, however, [[Evgeny Lifshitz]] offered a different origin for the Casimir effect. In 1957, [[Tsung-Dao Lee|Lee]] and [[Chen Ning Yang|Yang]] proved the concepts of broken symmetry and [[parity violation]], for which they won the Nobel prize. In 1973, [[Edward Tryon]] proposed the [[zero-energy universe]] hypothesis: that the Universe may be a large-scale quantum-mechanical vacuum fluctuation where positive [[mass]]–energy is balanced by negative [[gravitational energy|gravitational potential energy]].<ref name=Tryon1973>{{cite journal |last=Tryon |first=E. P. |title=Is the Universe a Vacuum Fluctuation? |journal=Nature |volume=246 |pages=396–397 |date=1973 |issue=5433 |doi=10.1038/246396a0|bibcode=1973Natur.246..396T }}</ref> During the 1980s, there were many attempts to relate the fields that generate the vacuum energy to specific fields that were predicted by attempts at a [[Grand Unified Theory]] and to use observations of the Universe to confirm one or another version. However, the exact nature of the particles (or fields) that generate vacuum energy, with a density such as that required by inflation theory, remains a mystery.<ref>{{cite journal |last=Morikawa |first=M. |title=Quantum Fluctuations in Vacuum Energy: Cosmic Inflation as a Dynamical Phase Transition |journal=Universe |date=2022 |volume=8 |issue=6 |page=295 |doi=10.3390/universe8060295 |doi-access=free|bibcode=2022Univ....8..295M }}</ref>

== Vacuum energy in fiction ==
* Arthur C. Clarke's novel ''[[The Songs of Distant Earth]]'' features a starship powered by a "quantum drive" based on aspects of this theory.
* In the sci-fi television/film franchise ''[[Stargate]]'', a Zero Point Module (ZPM) is a power source that extracts [[zero-point energy]] from a [[Multiverse|micro parallel universe]].<ref>[[Rising (Stargate Atlantis)]].</ref>
* The book ''[[Star Trek: Deep Space Nine Technical Manual]]'' describes the operating principle of the so-called [[quantum torpedo]]. In this fictional weapon, an antimatter reaction is used to create a multi-dimensional membrane in a vacuum that releases at its decomposition more energy than was needed to produce it. The missing energy is removed from the vacuum. Usually about twice as much energy is released in the explosion as would correspond to the initial antimatter matter annihilation.<ref>{{cite book |last=Zimmerman |first=Herman |title=Star Trek: Deep Space Nine Technical Manual |title-link=Star Trek: Deep Space Nine Technical Manual |author2=Sternbach |first2=Rick |author3=Drexler |first3=Doug}}</ref>
* In the video game ''[[Half-Life 2]]'', the item generally known as the "gravity gun" is referred to as both the "zero point field energy manipulator" and the "zero point energy field manipulator."<ref>{{cite web |author=Laidlaw |first=Marc |title=Half-Life 2 Transcript |url=https://gamefaqs.gamespot.com/pc/914642-half-life-2/faqs/38075}}</ref>

==See also==
* [[Cosmic microwave background]]
* [[Dark energy]]
* [[False vacuum]]
* [[Normal ordering]]
* [[Quantum fluctuation]]
* [[Sunyaev–Zeldovich effect]]
* [[Vacuum state]]

== References ==
{{Reflist}}

==External articles and references==
* Free PDF copy of ''[http://www.physics.arizona.edu/~rafelski/Books/StructVacuumE.pdf The Structured Vacuum – thinking about nothing]'' by [[Johann Rafelski]] and Berndt Muller (1985); {{ISBN|3-87144-889-3}}.
* Saunders, S., & Brown, H. R. (1991). ''The Philosophy of Vacuum''. Oxford [England]: Clarendon Press.
* Poincaré Seminar, Duplantier, B., & Rivasseau, V. (2003). "Poincaré Seminar 2002: vacuum energy-renormalization". ''Progress in mathematical physics'', v. 30. Basel: Birkhäuser Verlag.
* [https://web.archive.org/web/20180104132548/https://arxiv.org/abs/gr-qc/0011083v1 Futamase & Yoshida ''Possible measurement of vacuum energy''].
* [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040171927_2004178265.pdf Study of Vacuum Energy Physics for Breakthrough Propulsion] 2004, NASA Glenn Technical Reports Server (PDF, 57 pages, Retrieved 2013-09-18).

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[[Category:Dark energy]]
[[Category:Energy (physics)]]
[[Category:Physical cosmological concepts]]
[[Category:Quantum field theory]]
[[Category:Vacuum]]