The Cosmological Constant Problem  

In General > s.a. boundaries in field theory; branes; quintessence; vacuum.
* Idea: Why is the observed value so small (e.g., after so many phase transitions)? [Before 1997–1998, the question usually was, Why is its observed value zero?]
* Second problem: The coincidence between the epoch of galaxy formation tgal and the epoch of -domination tcc.
@ Reviews: Adler et al AJP(95); Dolgov ap/97-in; Weinberg ap/00-in; Straumann ap/00-in, ap/02-in, gq/02-in; Brax CP(04); Banks PT(04)mar; Nobbenhuis gq/06-PhD [approaches].

Proposed Solutions > s.a. particle statistics [quons]; quantum gravity and cosmology [including wormhole effects].
* Idea: Many proposals have been made; 2000, recent ones include large extra dimensions (> see brane cosmology).
* Anthropic approach: There is a probability distribution of values of the cosmological constant in different regions, and we live in one that is conducive to life.
* Conformal gravity: The theory naturally quenches the amount by which the cosmological constant gravitates, rather than its value.
* Remark: Supersymmetry doesn't help, it is broken!
@ From vacuum fluctuations: Gurzadyan & Xue MPLA(03)ap/01, ap/05; Padmanabhan CQG(05)ht/04; Qi ht/05 [QED]; Samuel & Sinha PRL(06)cm [surface tension analog]; Beck & Mackey PhyA(07)ap/06 [measurability]; Arzano et al gq/06 [and holography]; Doran & Jaeckel JCAP(06)ap [and measurements].
@ Supersymmetric: Jejjala et al GRG(03)ht-GRF [holography and supersymmetry]; Froggatt et al NPB(06) [supergravity models].
@ Lattice-type approaches: Vergeles ht/04 [lattice quantum gravity]; Loll et al NPB(06) [2D causal, with topology change].
@ Quantum cosmology: Hawking PLB(84); Weiss PLB(87), Duff PLB(89) [criticism]; Wu PLB(08)-a0709.
@ Other quantum gravity: Özer NPB(91) [third quantization]; Mok ap/01 [discrete spacetime]; Singh a0805-GRG [classical-quantum duality and non-commutative gravity].
@ Other fields: Barr et al PRD(06) [cancellation by cosmon].
@ Strings/M-theory: Witten MPLA(95)ht, Kane et al PLB(05) [string vacua]; McGuigan ht/06 [seesaw]; Hawking & Hertog PRD(06) + pn(06)jun; de Alwis PLB(07)ht/06; Bousso ht/06-in [local values and the landscape]; Barvinsky PRL(07);> s.a. string phenomenology.
@ Brane world: Burgess et al PLB(00); Förste et al JHEP(00); Chen et al JHEP(00); Arkani-Hamed et al PLB(00)ht; Kakushadze PLB(00)ht, PLB(00)ht, PLB(00)ht; de Alwis NPB(01); Flanagan et al PRD(01) [and hierarchy]; Nojiri et al RMF(02)ht/01; Corradini et al MPLA(03), IJMPA(03)ht/02; Sorkin BJP(05)gq-in [ would be too small]; Ellwanger MPLA(05); Steinhard & Turok Sci(06)ap + pw(06)may [cyclic universe with decreasing cosmological constant], comment Vilenkin Sci(06)ap; Diakonos & Saridakis a0708-PRL.
@ Other higher-dimensional: Wesson & Liu IJMPD(01)gq [Kaluza-Klein theory]; Erdem gq/06-in, JPA(08)-a0712 [symmetries].
@ Anthropic: Weinberg PRL(87); Efstathiou MNRAS(95); Garriga et al PRD(00)ap/99; Donoghue JHEP(00); Förste et al PLB(00)gq; Garriga & Vilenkin PRD(00)ap/99, PRD(01)ht/00; Barrow et al PRD(02)ap/01; Banks et al JHEP(01); Vilenkin ht/01-in; Graesser et al PLB(04)ht; Garriga & Vilenkin PTPS(06)ht/05; Loeb JCAP(06)ap [observational test]; Starkman & Trotta PRL(06)ap, Trotta & Starkman ap/06-in [against]; Pogosian & Vilenkin JCAP(07)ap/06 [and WMAP-3 data]; Bousso et al PRD(07)ht [causal entropic principle]; Miralda-Escudé ap/07 [stryngbohtyk model]; Peacock a0705-MNRAS; Olum & Schwartz-Perlov a0705 [in a large toy landscape]; Maor et al PRL(08)-a0709 [criticism].
@ Other proposals: Brown & Teitelboim PLB(87) [dynamical neutralization]; Linde PLB(88) [two universes]; Duncan & Jensen NPB(90) [4-form dynamics]; Ghoroku CQG(91); Ng IJMPD(92); Squires PLA(92); Sivaram MPLA(99) [early universe phase transition]; Özer gq/00 [from gravity + electromagnetism unification]; Kim & Lee PLB(04)ht/03 [self-tuning]; Carneiro IJMPD(03)gq-GRF [holography]; Sundrum PRD(04)ht/03 [fat gravitons]; Alexander et al ht/04 [vacuum gravitational instability]; Nobbenhuis FP(06)gq/04; Jackiw et al PLA(05)ht [4 field in AdS]; Novello ap/05 [and graviton mass]; Triay IJMPD(05)gq; Nishino & Rajpoot MPLA(06) [Hodge duality]; Freese et al PLB(06) [devaluation]; Prokopec gq/06 [semiclassical general relativity + matter]; 't Hooft & Nobbenhuis CQG(06)gq [complex transformations]; McGuigan ht/06 [seesaw, quantum cosmology]; Shapiro & Sola gq/06-in; Comelli a0704 [modified Lagrangian]; O'Connell PLA(07) [causality-based]; Rodrigo CQG(07)-a0705 [void bubbles]; Andrianov et al PLB(07) [and spontaneous symmetry breaking]; > s.a. Topos Theory.

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