- Induced gravity
**Induced gravity**(or**Emergent gravity**) is an idea inquantum gravity that space-time background emerges asamean field approximation of underlying microscopic degrees of freedom, similar to thefluid mechanics approximation of BEC. The concept was originally proposed byAndrei Sakharov in 1967.Sakharov observed that many

condensed matter systems give rise to emergent phenomena which are identical togeneral relativity quantitatively. Crystal defects can look liketorsion , for example. His idea was to start with an arbitrary backgroundpseudo-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 aneffective action which toone-loop order contains theEinstein-Hilbert action with acosmological 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 hugecosmological constant s.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 inAdS/CFT correspondence after 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.**External links*** [

*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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