Lorentzian Geometry |

**In General** > s.a. differential geometry; riemann
tensor / metric types and matching;
types of lorentzian geometries [including scalar invariants, generalizations].

* __Idea__: A Lorentzian
metric is one with signature (–, +, ..., +).

* __Remark__: This signature
convention gives normal signs to spatial components, while the opposite ones
gives *p*^{m}* p*_{m}
= *m*^{2} for a relativistic particle.

$ __Lorentzian structure__:
A reduction of the bundle of frames *F*(*M*) to the
Lorentz group, as a subgroup of GL(*n*, \(\mathbb R\)).

* __Conditions__: The necessary
and sufficient condition for the existence of a Lorentzian structure on a manifold
*M* is that *M* be non-compact, or that the Euler number
*χ*(*M*) = 0.

@ __Books__: Steenrod 51;
O'Neill 83;
Beem et al 96;
Hall 04.

@ __General references__: Iliev JGP(00)gq/98 [relation with Riemannian geometry];
Müller & Sánchez JDMV-math/06;
Chen 11 [submanifolds, *δ*-invariants and applications];
news ea(11)apr [visualization through tendex lines and vortex lines];
Gilkey et al IJGMP(13) [with boundary, universal curvature identities].

@ __Global aspects__: Nawarajan & Visser IJMPD(16)-a1601
[physical considerations on orientability, the use of tetrads as variables, and other issues].

@ __Emergent geometry__: Wilczek PRL(98)ht [metrics from volumes and gauge symmetries]; Brown a0911 [metric
as spacetime property or emergent field]; Mukohyama & Uzan PRD(13)-a1301, Kehayias et al PRD(14)-a1403 [Lorentzian signature as emergent from Riemannian one and classical fields]; Majid EPJwc(14)-a1401; Cirilo-Lombardo & Prudêncio IJGMP(14) [from supergeometries]; > s.a. emergent gravity [analog models of spacetime metrics].

@ __Related topics__: Bugajska JMP(89)
[open spin 4-manifolds].

> __Related topics__:
see affine connection; fluids; jacobi metric; lines [space of timelike lines]; types of distances [Lorentzian length spaces].

> __Online resources__:
see Technische Universität Berlin group [2012].

**Specific Concepts and Results** > s.a. causality; holonomy;
minkowski space; spectral geometry; simplex
(Lorentzian geometry case).

* __Flat deformation theorem__:
Given a semi-Riemannian analytic metric *g* on a manifold, there always
exists a two-form *F*, a scalar function *c*, and an arbitrarily
prescribed scalar constraint depending on the point *x* of the manifold
and on *F* and *c*, say Ψ(*c*, *F*, *x*)
= 0, such that the deformed metric *η* = *c g* –
*ε* *F*^{2} is
semi-Riemannian and flat; It implies that every (Lorentzian analytic) metric
*g* can be written in the extended Kerr-Schild form *η*_{ab}:=
*a* *g*_{ab} –
2 *b k*_{(a} *l*_{b)},
where *η* is flat and *k*_{a}, *l*_{a}
are two null covectors such that *k*_{a} *l*^{ a}
= –1.

* __Result__: Given a metric
*g* with scalar curvature *R*, there is another *g'* with
*R*' = 0 iff for all *φ* ∈ C_{0}^{∞},

–(1/8) ∫ *Rφ* d*μ* < ∫ |∇*φ*|^{2} d*μ* (sufficient
condition: ||* R *||_{L3/2} < some known *c*) .

@ __General references__: Kim BAusMS(90);
Tod CQG(92) [diagonalizability];
Pezzaglia & Adams gq/97-conf [(–,+,+,+) vs (+,–,–,–)];
Gerhardt GRG(03)m.DG/02 [volume estimates];
Milson et al IJGMP(05)gq/04 [alignment];
Sánchez DG&A(06)
[compact, causality]; Llosa & Carot CQG(09)-a0809 [flat
deformation theorem and symmetries]; Kim JGP(09), JGP(11)
[volume comparison between hypersurfaces]; Pugliese et al a0910-proc
[deformations]; de Siqueira a1006 [every *n*-dimensional
pseudo-Riemannian manifold is conformal to one of constant curvature?]; Kim JMP(11) [covering spaces and homotopy classes of causal curves]; Impera JGP(12) [Hessian and Laplacian comparison theorems]; Hintz & Uhlmann a1705 [reconstruction of Lorentzian manifolds from boundary light observation sets].

@ __Extensions__: Chruściel JDG(10)
[conformal boundary extensions]; Low CQG(12) [maximal extensions and Zorn's lemma]; > s.a. spacetime
boundaries and completions.

> __Related topics__:
see distance; Extremal Surface; geodesics; Hypersurface; Osserman Manifolds; Pythagorean
Theorem; Splitting Theorem; world
function.

**Space of Lorentzian Geometries** > s.a. distance between geometries;
spacetime singularities; solutions
of general relativity.

* C^{k} __open
topology__: (a.k.a. Whitney fine or uniform convergence topology)
A neighborhood basis for *g* is

*B*_{f}(*g*):=
{*g'* | for all *p* ∈ *M*
||*g *– *g'*||(*p*) < *f*(*p*),
..., ||∂^{k}*g* – ∂^{k}*g'*||(*p*) < *f*(*p*)}
,

where *f* : *M* → \(\mathbb R\) is continuous and strictly positive and the norms are
calculated using some positive-definite (inverse) metric; Intuitively, for C^{0}
the light cones are close, for C^{1} the geodesic
systems are close, for C^{2}
the curvature tensors are also close; This is an extremely fine topology; For example, on a non-compact *M* a sequence *g*_{(n)} of metrics cannot converge unless there exists a compact subset in *M* such that for sufficiently large *n* all metrics coincide outside it [@ in Golubitsky & Guillemin 73].

* W^{k} __compact-open topology__:
A neighborhood subbasis is

*B*_{U, δ}(*g*):=
{*g'* | ||(*g* –
*g'*)|_{U} ||_{Wk} < *δ*}
,

where *U* is an open set of compact closure in *M* and *δ* a positive constant.

* __Partial order__: For
each *A* ⊂ *M*, *g* <_{A} *g*'
iff for all *p* ∈ *A*,
all non-spacelike vectors with respect to *g* are non-spacelike with
respect to *g*' (the light cones of *g* are narrower).

@ __General references__: Geroch JMP(70),
in(70);
Hawking GRG(71);
in Hawking & Ellis 73;
Lerner CMP(73);
in Beem et al 96.

@ __Structures__: Beem 81 [Lorentzian distance function];
Bombelli JMP(00)gq [pseudodistance];
Noldus CQG(02)-a1104 [topology];
García-Parrado & Senovilla mp/02-proc,
CQG(03)gq/02 [causal equivalence];
Noldus CQG(04)gq/03,
CQG(04)gq/03 [distance];
Bombelli & Noldus CQG(04)gq.

@ __Limits of spacetimes__: Geroch CMP(69);
Bampi & Cianci IJTP(80);
Paiva et al CQG(93)gq;
Sormani a1006-fs [convergence].

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