Event Horizons  

In General > s.a. black holes / boundaries and holography in field theory; quantum spacetime; Rigidity; Trapped Surface.
$ Of a world-line: The past (future) event horizon is the boundary of the future (past) of the world-line.
$ Of a spacetime: The boundary of the past of \(\cal I\)+, \(\cal H\) = ∂(I(\(\cal I\)+)) (the future event horizon is the boundary of the future of \(\cal I\)); It is a global concept, defining from which points one can reach (null) infinity.
* Relationships: Its existence is equivalent to \(\cal I\) (\(\cal I\)+) being spacelike (> see de sitter spacetime).
* Properties: They are always null hypersurfaces; Their area always increases; They either coincide or are outside the apparent horizon (not true?).
* Remark: Being global properties, they are in general difficult to calculate, except when they can be shown to coincide with a Killing horizon or some other locally defined surface. [Defined by retarded time u = ∞.]
@ References: Baccetti et al a1706-GRF [existence?].

Geometry and Topology > s.a. black holes; black-hole types [toroidal]; topology change [censorship]
@ Geometry: Chruściel et al AHP(01)gq/00 [area], JGP(02)gq/00 [differentiability], CQG(06)gq/05 [no degenerate components].
@ Topology: Huang & Liang PLA(95) [torus]; Siino PRD(98)gq/97, PRD(99)gq/97.
@ Related topics: Booth & Martin PRD(10)-a1007 [geometrical measure of the distance between apparent and event horizons, and Vaidya spacetime]; Chung PRD(11)-a1011 [dynamics of diffeomorphism degrees of freedom]; Brill G&C(14) [dynamic changes in a black hole horizon as it forms and settles]; Shi & Mei PRD(17)-a1611 [extended BMS-like symmetries].

Related Topics > s.a. Firewall; Fuzzball; gravitational thermodynamics.
* Thermodynamics: Thermal properties of a static horizon (like its entropy or heat content) can be obtained either from the surface term of the Einstein-Hilbert action or by evaluating the Noether charge, corresponding to the diffeomorphisms generated by the timelike Killing vector field.
* Alternatives: To resolve some puzzles arising from properties of quantum field theory in the presence of horizons, some physicists have proposed that the equivalence principle (or "no drama" scenario) does not hold near the surface of a black hole and the event horizon is replaced by a "firewall" (Almheiri, Marolf, Polchinski, and Sully) which would cause any observer who reaches the firewall to burn and not be able to cross to the other side, or a "fuzzball", motivated by string theory.
@ Thermodynamics: Gibbons & Hawking PRD(77); Fulling & Ruijsenaars PRP(87); Davies CQG(87), CQG(88), AIHP(88), comment Raychaudhuri CQG(90); Jensen PRD(95) [stability]; Massar & Parentani NPB(00)gq/99; Padmanabhan IJMPD(04)gq [and gravity as elasticity]; Zhou et al PLB(07) [second law in accelerating universe]; Majhi & Padmanabhan EPJC(13)-a1302 [from infinitesimal coordinate transformations]; Widom et al a1602 [and gravitational vacuum tension]; > s.a. horizons.
@ And particle motion: Oliveira a1107 [velocity of test particles at the event horizon, pedagogical].
@ Horizon wave functions: Casadio a1310-proc [and effective gup]; Casadio et al PLB(16)-a1509 [in various dimensions].
@ Quantum effects: Hájíček PLB(86); Padmanabhan PLB(86); Sorkin in(96)gq/97 [wrinkling below threshold scale]; Chapline et al IJMPA(03)gq/00 [phase transition]; 't Hooft gq/04-ln [and quantum information]; Bojowald et al CQG(11) [definitions in modified spacetime structures]; Susskind a1402 [and computational complexity]; Alonso-Serrano et al PRD(14)-a1410 [unitarity is not preserved in individual regions separated by horizons]; > s.a. correlations; quantum black holes.
@ Other properties: Bergamin et al CQG(06)ht/05 [physical-to-gauge-degree-of-freedom conversion]; Mathur IJMPD(13) [what happens at the horizon].
@ In modified gravity theories: Berglund et al PRD(12) ["universal horizon" mechanics].
@ Numerical methods: Libson et al PRD(96); > s.a. models in numerical general relativity.
@ Cosmological: Kaloper et al PLB(04) [observational implications].
@ Observational evidence: Barbieri & Chapline PLB(12) [experimental signature for the absence of an event horizon]; Bambi SWJ(13)-a1205 [non-observation of radiation vs ergoregion instability]; Visser PRD(14)-a1407 [(non-)observability]; Cardoso & Pani a1707, nAstr(17)-a1709 [from gravitational wave echoes]; > s.a. black-hole phenomenology.

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