|  Spacetime Singularities | 
In General > s.a. singularity theorems.
  * Idea: A spacetime
    is said to be non-singular if it is timelike and null geodesically
    complete, by analogy with Riemannian geometry, where geodesic completeness
    is equivalent to the usual metric completeness.
  * Remark: When this
    condition is violated we may not have what we would like to call a
    singularity physically; And, when satisfied, we might have, e.g.,
    timelike lines of finite acceleration which are incomplete.
  * Consequences:
    Naked singularities would be a problem for predictive physics.
 Avoiding Singularities > s.a. early-universe
  models; singularities in quantum gravity.
  * History: An
    attempt was made by the Soviets with the mixmaster universe; Other
    possibilities include cosmic censorship, violation of energy conditions,
    "gravastars", perhaps quantum gravity (non-commutative? > see
    Kasner Solutions), and varying physical constants.
  @ General references: Einstein & Rosen BB(31),
    PR(35);
    Einstein AM(39), AM(45);
    Einstein & Straus AM(46);
    in Misner et al 73.
  @ By violating energy conditions:
    Fulling & Parker PRD(73) [quantum];
    Bekenstein PRD(75) [classical];
    Fakir gq/98.
  @ By going to a different metric: Quirós PRD(00)gq/99,
    et al PRD(00)
      [geometric duality in general relativity and Brans-Dicke theory];
    Quirós gq/00,
    et al gq/00/PRD [conformal rescaling];
    Wetterich a2004 [requires tuning of parameters];
    Casadio et al a2008 [no singularities in pure gravity].
  @ By extending the spacetime:
    Śniatycki in(91) [using the Jacobi metric];
    Deruelle & Sasaki PTPS(11)-a1012-proc [conformal transformations in Nordström's scalar theory];
    Stoica CTP(12)-a1203,
    CEJP(14)-a1203,
    PhD(13)-a1301 [new field equation applicable in wider situations];
    Heller & Król a1711
      [beyond the boundary, using Synthetic Differential Geometry];
    Nomura & Yoshida a2105 [FLRW and Bianchi I spacetimes];
    > s.a. lorentzian geometry; metric
      matching [junction conditions]; FLRW geometry;
    schwarzschild spacetime.
  @ In different theories: Mac Conamhna CMP(08)-a0708 [M-theory];
    Dąbrowski & Marosek JCAP(13)-a1207,
    Dąbrowski et al a1308-proc [varying constants];
    Garattini & Majumder NPB(14)-a1311
      [Gravity's Rainbow and non-commutative geometry];
    Bambi et al PLB(14)-a1402 [from four-fermion interaction];
    Tahamtan & Svítek EPJC(14)-a1312 [and quantum gravity];
    Bazeia et al PRD(15)-a1507 [higher-dimensional metric-affine theories];
    Koslowski et al PLB(18)-a1607 [relational degrees of freedom];
    Chamseddine & Mukhanov JCAP(17)-a1612 [modified longitudinal mode];
    Edholm & Conroy PRD(17)-a1710 [infinite-derivative gravity];
    > s.a. Relational Theories.
   @ Related topics:
    Heller & Sasin IJTP(95),
    GRG(99)gq/98 [algebraically];
    Raptis IJTP(06)gq/04 [Schwarzschild, finitary-algebraic];
    Goswami & Joshi gq/05 [by not forming trapped surface];
    Gershtein et al TMP(05)gq [in field theory of gravitation?];
    Qiu CQG(10)-a1007 [by coupling gravity to a scalar field];
    > s.a. modified electromagnetic theory; non-commutative
      gravity; types of singularities [evolving through the cosmological singularity].
Other References
  > s.a. collapse [including Hoop conjecture]; cosmic
  censorship; cosmology and models;
  types of singularities; spacetime boundary.
  @ Reviews: Canarutto RNC(88);
    Clarke in(88);
    Rendall in(05)gq;
    Cotsakis & Klaoudatou JPCS(05);
    Natário m.DG/06 [introduction for mathematicians];
    Cotsakis gq/07-MGXI;
    Joshi & Malafarina IJMPD(11)-a1201 [collapse and phenomenology];
    Joshi a1311-ch;
    Dąbrowski a1407-in
      [rev, different types, avoidance];
    Hawking EPJH(14) [intro];
    Ong IJMPA-a2005 [and censorship].
  @ History: Khalatnikov & Kamenshchik PU(08)-a0803,
    Belinski IJMPD(14)-a1404 [cosmological];
    Senovilla & Garfinkle CQG(15)-a1410 [Penrose's 1965 theorem].
  @ Philosophical: Earman 95;
    Lam PhSc(07)dec.
  @ General references: Geroch JMP(68),
    in(68);
    Hájíček GRG(70);
    Newman GRG(71);
    Penrose in(78);
    Barrow & Tipler PRP(79),
    PLA(81);
    Fuchs et al FdP(88);
    Joshi SA(09)feb [naked singularities];
    Stoica a1207-talk;
    Romero FS-a1210
      [ontology, against the physical existence of singularities];
    Cotsakis IJMPD(13)-a1212 [and asymptotics];
    Uggla a1304-conf,
    IJMPD(13)-a1306-MG13 [spacelike singularities];
    Tavakoli PhD(13)-a1405;
    Stoica a1504 [and causal structure].
  @ And initial surfaces:
    Wojtkiewicz PRD(90).
  @ Data at singularities:
    Eardley et al JMP(72);
    Tod CQG(90);
    & Goode & Wainwright.
  @ Matter at singularities:
    Stoica a1408-conf [gauge fields].
  @ Strength and physical properties:
    Kánnár & Rácz JMP(92);
    Kánnár GRG(95) [in Einstein-Cartan theory];
    Kriele & Lim CQG(95);
    Ori PRD(00);
    > s.a. wormhole solutions
      [curvature divergences and physical observers].
  @ Role, uses of singularities: Earman FP(96);
    Lopez CQG(93);
    Horowitz & Myers GRG(95)gq;
    Azhar & Namjoo a2101 [and indeterminism].
  @ Probing singularities: Horowitz & Marolf PRD(95)gq;
    Ishibashi & Hosoya PRD(99)gq;
    Piechocki PLB(02);
    Konkowski et al in(03)gq/04 [quantum particles];
    Blau et al JHEP(06)ht [with scalar fields];
    Pitelli & Letelier IJMPD(11)-a1010 [with quantum wave packets, static spacetimes];
    Hofmann & Schneider PRD(17)-a1611 [Schwarzschild black holes].
  @ In f(R) gravity: Lee et al PTP(12)-a1201;
    Tahamtan & Gurtug EPJC(12)-a1205 [with quantum test fields as probes].
  @ In other theories of gravity: Eguchi MPLA(92) [topological field theories];
    Novello et al CQG(00) [general relativity + non-linear electrodynamics];
    Holdom PRD(02) [spherical, and horizons];
    Ferraz Figueiro & Saa PRD(09)-a0906 [modified-gravity models];
    Konkowski & Helliwell IJMPA(11)-a1112 [quantum singularities];
    > s.a. Bakry-Emery Tensor;
      cosmology in higher-order gravity;
      hořava-lifshitz gravity phenomenology;
      massive gravity; singularity theorems;
      types of singularities [naked singularities].
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