- Vacuum energy
**Vacuum energy**is an underlying backgroundenergy that exists inspace even when devoid ofmatter (known asfree space ). The vacuum energy is deduced from the concept of virtual particles, which is itself derived from the energy-time uncertainty principle. Its effects can be observed in various phenomena (such asspontaneous emission , theCasimir effect , the Van-Der Waals bonds, or theLamb shift ), and it is thought to have consequences for the behavior of theUniverse on cosmological scales.**Origin**Quantum field theory states that all of the various fundamental fields, such as theelectromagnetic field , must bequantized at each and every point in space. In a naive sense, a field in physics may be envisioned as if space were filled with interconnected vibrating balls and springs, and the strength of the field can be visualized as the displacement of a ball from its rest position. Vibrations in this field propagate and are governed by the appropriatewave equation for the particular field in question. Thesecond 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, the field at each point in space is a simple harmonic oscillator, and its quantization places aquantum harmonic oscillator at each point. Excitations of the field correspond to theelementary particle s ofparticle physics . Thus, even thevacuum has a vastly complex structure. All calculations of quantum field theory must be made in relation to this model of the vacuum.The vacuum implicitly has the same properties as a particle, which are spin, or

polarization in the case oflight , energy, and so on. On average, all of these properties cancel out: the vacuum is, after all, "empty" in this sense. One important exception is the vacuum energy or thevacuum expectation value of the energy. The quantization of a simple harmonic oscillator states that the lowest possible energy, orzero-point energy , that such an oscillator may have is:$\{E\}\; =\; egin\{matrix\}\; frac\{1\}\{2\}\; end\{matrix\}\; hbar\; omega\; .$

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 inclassical mechanics for centuries. This argument is the underpinning of the theory ofrenormalization . In all practical calculations, this is how the infinity is handled.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 shortly annihilate each other and disappear. However, these particles and antiparticles may interact with others before disappearing, a process which can be mapped usingFeynman diagrams . Note that this method of computing vacuum energy is mathematically equivalent to having aquantum harmonic oscillator at each point and, therefore, suffers the same renormalization problems.Additional contributions to the vacuum energy come from

spontaneous symmetry breaking inquantum field theory .**Implications**Vacuum energy has a number of consequences. In 1948, Dutch

physicist s Hendrik B. G. Casimir andDirk Polder predicted the existence of a tiny attractive force between closely placed metal plates due toresonance s in the vacuum energy in the space between them. This is now known as theCasimir effect and has since been extensively experimentally verified. 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.Other predictions are more esoteric and harder to verify. Vacuum fluctuations are always created as particle/antiparticle pairs. The creation of these virtual particles near the

event horizon of ablack hole has been hypothesized by physicistStephen Hawking to be a mechanism for the eventual "evaporation" of black holes. The net energy of the Universe remains zero so long as the particle pairs annihilate each other withinPlanck time . 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 but could be on the order of 10^{100}years for large solar-mass black holes.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 gravitational force. Essentially, a non-zero vacuum energy is expected to contribute to thecosmological constant , which affects theexpansion of the universe . However, the vacuum energy is mathematically infinite withoutrenormalization , which is based on the assumption that we can only measure energy in a relative sense, which is not true if we can observe it indirectly via thecosmological constant .The existence of vacuum energy is also sometimes used, outside of mainstream physics, as controversial theoretical justification for the possibility of

free energy machines. It has been argued that due to the broken symmetry (in QED), free energy does not violate conservation of energy, since the laws of thermodynamics only apply to equilibrium systems. However, consensus among particle physicists is that this is incorrect and that vacuum energy cannot be harnessed to do usable work. In particular, thesecond law of thermodynamics is unaffected by the existence of vacuum energy.**History**In 1934,

Georges Lemaître used an unusual perfect-fluidequation of state to interpret the cosmological constant as due to vacuum energy. In 1948, theCasimir effect provided the first experimental verification of the existence of vacuum energy. In 1957, Lee and Yang proved the concepts of broken symmetry and parity violation, for which they won the Nobel prize. In 1973,Edward Tryon proposed that the Universe may be a large-scale quantum-mechanical vacuum fluctuation where positivemass -energy is balanced by negative gravitationalpotential energy . During the 1980s, there were many attempts to relate the fields that generate the vacuum energy to specific fields that were predicted byGrand unification theory and to use observations of the Universe to confirm that theory. 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.**ee also***

Dark energy

*Casimir effect

*Cosmological constant

* Heisenberg's Uncertainty principle

*Lambdavacuum solution

*Quantum electrodynamics

*Vacuum state

*Virtual particles

*Zero-point energy

*Zero-point field **External articles and references*** 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.

* [*http://arxiv.org/abs/gr-qc/0011083v1 Futamase & Yoshida "Possible measurement of vacuum energy"*]

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