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Vacuum energy is an underlying background energy that exists in space even when devoid of matter (known as free space). The vacuum energy results in the existence of most (if not all) of the fundamental forces - and thus in all effects involving these forces, too. It is observed in various experiments (like the spontaneous emission of light or gamma radiation, the Casimir effect, Van-Der Waals bonds, the Lamb shift, etc); and it is thought (but not yet demonstrated) to have consequences for the behavior of the Universe on cosmological scales.

Elementary particle theories

Quantum field theory, which describes the interactions between elementary particles in terms of fields, allows a contribution to the vacuum energy (even when no particles are present) in a form of the zero-point energy of the fields. An example is the Casimir effect, where two metal plates experience a small attractive force between them, which can be attributed to the dependence of the zero-point energy of the electromagnetic field on the distance between the plates. Since potential energy is defined up to an arbitrary constant, the absolute value of the vacuum energy might seem to be unimportant; however, it becomes important when gravity is involved, since gravity couples to the total energy of the system.

This has important consequences on cosmological scales, where the vacuum energy is expected to contribute to the cosmological constant, which affects the expansion of the universe. The calculation of the vacuum energy in quantum field theory in terms of Feynman diagrams can be pictured as accounting for virtual particles (also known as vacuum fluctuations) which are created and destroyed out of the vacuum. Additional contributions to the vacuum energy come from spontaneous symmetry breaking in quantum field theory.

Implications

Vacuum energy has a number of consequences. For one, 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 "evaporation" of black holes. The net energy of the universe remains zero so long as the particle pairs annihilate each other within Planck 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 years for large solar-mass black holes. The Grand unification theory predicts a non-zero cosmological constant from the energy of vacuum fluctuations. Examining normal physical processes with knowledge of these field phenomena can lead to an interesting insight in electrodynamics. During discussions of perpetual motion, the topic of vacuum energy usually encourages serious inquiries.

History

In 1934, Georges Lemaître used an unusual perfect-fluid equation of state to interpret the cosmological constant as due to vacuum energy. In 1973, Edward Tryon proposed that the Universe may be a large scale quantum mechanical vacuum fluctuation where positive mass-energy is balanced by negative gravitational potential energy. During the 1980s, there were many attempts to relate the fields that generate the vacuum energy to specific fields that were predicted by Grand unification theory, and to use observations of the Universe to confirm that theory. These efforts have failed so far, and 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.

See also

• Dark energy
• Cosmological constant
• Lambdavacuum solution
• Quantum electrodynamics
• Zero-point energy
• Vacuum state

External articles and references

• Saunders, S., & Brown, H. R. (1991). The Philosophy of vacuum. Oxford : Clarendon Press.
• Poincaré Seminar, Duplantier, B., & Rivasseau, V. (2003). Poincaré Seminar 2002: vacuum energy-renormalization. Progress in mathematical physics, v. 30. Basel: Birkhäuser Verlag.

• Theories of gravitation
• Quantum field theory
• Energy