- Induced gravity
Induced gravity (or Emergent gravity) is an idea in
quantum gravitythat space-time background emerges asa mean field approximationof underlying microscopic degrees of freedom, similar to the fluid mechanicsapproximation of BEC. The concept was originally proposed by Andrei Sakharovin 1967.
Sakharov observed that many
condensed mattersystems give rise to emergent phenomena which are identical to general relativityquantitatively. Crystal defects can look like torsion, for example. His idea was to start with an arbitrary background pseudo-Riemannian manifold(in modern treatments, possibly with torsion) and introduce quantum fields (matter) on it but not introduce any gravitational dynamics explicitly. This gives rise to an effective actionwhich to one-loop ordercontains the Einstein-Hilbert actionwith a cosmological constant. In other words, general relativity arises as an emergent property of matter fields and is not put in by hand. On the other hand, such models typically predict huge cosmological constants.
The particular models proposed by Sakharov and others have been proven impossible by the
Weinberg-Witten theorem. However, models with emergent gravity are possible as long as other things, such as spacetime dimensions, emerge together with gravity. Developments in AdS/CFT correspondenceafter 1997 suggest that the microphysical degrees of freedom in induced gravity might be radically different. The bulk space-time arises as an emergent phenomenon of the quantum degrees of freedom that live in the boundary of the space-time.
* [http://www.slac.stanford.edu/spires/find/hep/www?j=SPHDA,12,1040 A.D. Sakharov, "VACUUM QUANTUM FLUCTUATIONS IN CURVED SPACE AND THE THEORY OF GRAVITATION", 1967.]
* [http://relativity.livingreviews.org/Articles/lrr-2005-12/ Carlos Barcelo, Stefano Liberati, Matt Visser, "Living Rev.Rel." 8:12, 2005.]
* [http://online.itp.ucsb.edu/online/strings05/berenstein D. Berenstein, "Emergent Gravity from CFT", online lecture.]
* [http://arxiv.org/abs/0710.4153 C. J. Hogan "Quantum Indeterminacy of Emergent Spacetime", preprint]
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