Acceleration of the Cosmological Expansion Rate  

Observations > s.a. star types [supernovas].
* 1998: Contrary to standard wisdom, the expansion seems to be accelerating, from observation of 80 supernovas out to z = 0.8.
* 2002: Acceleration parameter q0 = –0.56 0.04; Many models proposed, most assuming that the current acceleration began at z 1.
* 2004: Chandra observations confirm acceleration began about 6 Gyr ago.
* 2007: SNLS 3rd year data to be released with 250 new supernovas, SNAP mission will yield about 2000 supernovas per year and will be able to exclude many proposed explanations.
* 2008: ESSENCE results for over 200 type Ia SNe in the range 0.2 < z < 0.8 are consistent with a cosmological constant.
* 2009: Data seem to favor a scenario in which the accelerated expansion may have already peaked and is slowing down again.
@ Reviews: Filippenko PASP(01)ap; Ostriker & Steinhardt SA(01)jan; Schmidt CQG(02); Leibundgut & Sollerman EPN(02)ap/02; Perlmutter PT(03)apr; Riess & Turner SA(04)feb [onset]; Shapiro & Turner ApJ(06)ap/05 [without general relativity]; Perivolaropoulos ap/06-in; news pw(07)dec, Krauss pw(07)dec, Crease pw(07)dec [X anniversary]; Krauss & Scherrer SA(08)mar; Frieman AIP(08)-a0904; Goldhaber a0907-in [history].
@ General references: Mannheim PRD(98)ap [implications], ap/98-in; Riess et al AJ(98)ap; Perlmutter et al ApJ(99)ap/98, ap/98-in, PRL(99)ap; Sidharth hp/98-in; Starkman et al PRL(99)ap; Straumann EJP(99)ap; Turner ap/99-in; Liddle NAR(01)ap/00; Updike PT(05)apr; Gong & Wang PRD(06) [constraints]; Rapetti et al MNRAS(07)ap/06 [kinematical study]; Cattoën & Visser gq/07 [case less watertight than usually assumed]; Daly & Djorgovski AIP(07)-a0710; Durrer & Maartens GRG(08)-a0711 [rev]; Albrecht AIP(07)-a0710 [case for aggressive observational program]; Bolejko & Andersson JCAP(08)-a0807 [apparent and average acceleration]; Lima et al a0905 [new independent method].
@ Supernovae: Drell et al ApJ(00)ap/99; Gott et al ApJ(01)ap/00; Riess PASP(00)ap; et al ApJ(01)ap; Filippenko ap/03-in; Jain & Ralston ApJ(06)ap/05; SNAP ap/05-rp [goals]; Gong et al JCAP(07), Seikel & Schwarz JCAP(08)-a0711, JCAP(09) [model-independent evidence]; Leibundgut GRG(08)-a0802; Cattoen & Visser PRD(08)-a0809, a0906-in [treat data with some caution]; Krisciunas a0809 [results of ESSENCE survey].
@ Galaxy redshift surveys: Guzzo et al Nat(08)-a0803.
@ Gravitational waves: Seto et al PRL(01)ap [0.1 Hz, proposal].
@ Anisotropy search: Cooke & Lynden-Bell MNRAS-a0909 [inconclusive].

Proposed Explanations > see inhomogeneities and other theories of cosmological acceleration.

Change in Acceleration > s.a. expansion history.
* 2005: There is some evidence for oscillations in the Hubble expansion history.
* 2006, 2009: There is clear statistical evidence for a late time transition from a decelerating to an accelerating phase.
@ Theory: Blandford ap/04-in [jerk and equation of state]; Ponce de León IJMPD(06)gq/05 [model]; Poplawski PLB(06)gq [in f(R) gravity]; Kim & Yoon PLB(06)gq [2D non-commutative dilaton model]; Melchiorri et al PRD(07)-a0706 [beginning of acceleration].
@ Data: Lazkoz et al JCAP(05)ap [snIa Gold data set fit]; John ApJ(05)ap, Elgarøy & Multamäki JCAP(06)ap [past deceleration, Bayesian analysis]; Ishida et al APP(08)-a0706 [beginning of acceleration]; Daly et al a0710; Shafieloo et al a0903 [slowdown? data and interpretation].

Differing Points of View
* Suggestions: 2002, Supernovas may look dimmer because photons turn into axions along the way.
@ References: Loeb ap/98/ApJL [deceleration!]; Csáki et al PRL(02) [axions]; Narlikar et al PASP(02)ap [alternative]; Mészáros ApJ(02)ap [statistical, premature]; Sawicki ap/02 [R < 0]; Vishwakarma MNRAS(03)ap; Choudhuri ap/06-in [Thomson scattering]; Middleditch ap/06 [supernovas, no acceleration]; Arp a0712 [deceleration and variable particle masses].

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