Asymptotically Flat Spacetimes  

In General > s.a. initial-value formulation.
* Idea: To define a convenient notion (if there is one) of isolated system in general relativity, without gauge ambiguities or awkward limiting procedures; There may be several inequivalent definitions; At a quantum level, one expects gravitons to be surrounded by clouds of soft gravitons, so infrared problems may affect the notion of asymptotic flatness; It turns out that the effect is to enlarge the Poincaré group to the BMS group.
* Motivation: The theory of isolated systems is used in the proof of positive energy theorems, the definition of gravitational radiation in exact general relativity, the setup of computational simulations to extract information on the energy-momentum emitted during binary mergers and the analysis of the evaporation of black holes, through the use of appropriate Hilbert spaces of asymptotic states.
* Questions: (i) Is there a general algorithm for finding out whether a given spacetime is asymptotically flat? (ii) Is the property of being asymptotically flat stable under small perturbations? (iii) Structure of the space of solutions; (iv) Structure near spi; (v) Include matter.
* Idea, in 4D general relativity:
(1) Complete the spacetime by an appropriate conformal rescaling of the metric (there is gauge ambiguity);
(2) Detach the extra points attached and get an abstract manifold representing infinity;
(3) Divide the fields here into universal, geometrical structure, and physical or dynamical structure;
(4) Combine tensors of the two types to get physical information and interpret it.
> Online resources: see Wikipedia page.

Unification of Null and Spatial Infinity > s.a. Penrose Diagram.
* 1982: It is not clear yet whether the framework is compatible with the presence of radiation and non-zero mass.
* Remark: In an asymptotically empty and flat at null and spatial infinity (AEFANSI) spacetime, \(\cal I\)± arises as ∂J ±(i0) \ i0.
@ References: Ashtekar & Hansen JMP(78); Ashtekar in(80); Newman CMP(89); Schmidt CQG(91); Friedrich JGP(98), gq/98-GR15; Friedrich & Kánnár AdP(00)gq/99-conf; Hayward PRD(03)gq, JKPS(04)gq/03-conf [advanced/retarded conformal factors].

Related Concepts > s.a. H-Space; Peeling Property; types of spacetimes [strongly asymptotically predictable].
* Supertranslations: Angle-dependent translations at infinity, an Abelian subgroup of the BMS group corresponding to

xa \(\mapsto\) xa + f a(θ, φ, χ) ,

where θ, φ, χ are hyperbolic angles on the surfaces r = constant; At scri: They form an infinite-dimensional Lie ideal ST of the BMS group, such that BMS/ST is the Lie algebra of the Lorentz group; At spi: They correspond to the additive group of functions on the hyperboloid \(\cal D\); More physically, they are a global aspect of the gravitational memory effect; > s.a. Gravitational Memory.
@ Supertranslations: Carlip a1608 [in 2+1 dimensions].

References > s.a. at null infinity and spatial infinity; canonical general relativity; DSR.
@ General: Bergmann in(64); Penrose in(68) [asymptotic simplicity]; Geroch in(77) [intro]; Geroch & Xanthopoulos JMP(78) [stability]; Beig in(88); Frauendiener LRR(00), LRR(04); Krtouš & Podolský CQG(06)gq [asymptotic structure in higher dimensions]; Müller a1411 [spacetimes admitting conformal compactifications]; Grant & Tod GRG(15)-a1502 [obstructions to smoothness]; Bousso et al a1709 [asymptotic charges cannot be observed in finite time]; focus issue CQG(18) [BMS asymptotic symmetries].
@ Solutions: Friedrich CMP(86), JDG(91) [with Yang-Mills fields]; Roberts gq/98 [against asymptotic flatness]; Dain & Friedrich CMP(01)gq [data with prescribed regularity]; Anderson CQG(01) [vacuum 4+1, non-vacuum 3+1]; Anderson & Chruściel CMP(05)gq/04 [vacuum, all even D]; Tod a0902-wd, Bičák et al CQG(10)-a1003 [periodic in time]; Lübbe & Valiente Kroon CQG(09)-a0903 [existence]; Reiris CQG(14), CQG(14) [stationary isolated systems are asymptotically flat].
@ Timelike infinity: Moreschi CQG(87); Friedrich CMP(88) [radiativity condition]; Cutler CQG(89); Gen & Shiromizu JMP(98)gq/97; Friedrich CMP(13)-a1306 [initial-value problem around past timelike infinity].
@ Quantum asymptotic flatness: summary CQG+(14) [in lqg].

Modified Types of Spacetimes > see de sitter spacetime; hořava-lifshitz gravity.

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