Algebraic isometric embeddings of charged spherically symmetric space-times

Recent results concerning isometric embeddings of charged spherically symmetric space-times as algebraic 4-surfaces of a pseudo-Euclidean ⁿ are given. Several problems arising in algebraic isometric embeddings of space-times with horizons are analyzed by studying the Reissner-Nordström solution. The connection between the images of radial null geodesics and the possibility of extending the algebraic isometric embedding through a horizon is studied in detail.     

Empty space-times of embedding class two

Using the canonical forms for symmetric tensors in Minkowski space and the Petrov forms of the Weyl tensor, algebraic relationships between the two kinds of classification are developed for empty space-times of embedding class two.     

Three-dimensional space-times

A real version of the Newman-Penrose formalism is developed for (2+1)-dimensional space-times. The complete algebraic classification of the (Ricci) curvature is given. The field equations of Deser, Jackiw, and Templeton, expressing balance between the Einstein and Bach tensors, are reformulated in triad terms. Two exact solutions are obtained, one characterized by a null geodesic eigencongruence of the Ricci tensor, and a second for which all the polynomial curvature invariants are constant.     

Quasi-Spherical Gravitational Collapse in Any Dimension

We study the occurrence and nature of naked singularities for a dust model with non-zero cosmological constant in (n+2)-dimensional Szekeres space-times (which possess no Killing vectors) for n 2. We find that central shell-focusing singularities may be locally naked in higher dimensions but depend sensitively on the choice of initial data. In fact, the nature of the initial density determines the possibility of naked singularity in space-times with more than five dimensions. The results are similar to the collapse in spherically symmetric Tolman-Bondi-Lemaître space-times.     

Segre types of symmetric two-tensors inn-dimensional spacetimes

Three propositions about Jordan matrices are proved and applied to algebraically classify the Ricci tensor inn-dimensional Kaluza-Klein-type spacetimes. We show that the possible Segre types are [1, 1... 1], [21 ... 1], [31 ... 1], [z¯z1 ... 1] and degeneracies thereof. A set of canonical forms for the Segre types is obtained in terms of semi-null bases of vectors.     

Hidden symmetries and killing tensors on curved spaces

Higher-order symmetries corresponding to Killing tensors are investigated. The intimate relation between Killing-Yano tensors and nonstandard supersymmetries is pointed out. In the Dirac theory on curved spaces, Killing-Yano tensors generate Dirac-type operators involved in interesting algebraic structures as dynamical algebras or even infinite dimensional algebras or superalgebras. The general results are applied to space-times which appear in modern studies. One presents the infinite dimensional superalgebra of Dirac type operators on the 4-dimensional Euclidean Taub-NUT space that can be seen as a twisted loop algebra. The existence of the conformal Killing-Yano tensors is investigated for some spaces with mixed 3-Sasakian structures.     

Classification of Teleparallel Homothetic Vector Fields in Cylindrically Symmetric Static Space-Times in Teleparallel Theory ofGravitation

In this paper we classify cylindrically symmetric static space-timesaccording to their teleparallel homothetic vector fields using directintegration technique. It turns out that the dimensions of the teleparallelhomothetic vector fields are 4, 5, 7 or 11, which are the same in numbers asin general relativity. In case of 4, 5 or 7 proper teleparallel homotheticvector fields exist for the special choice to the space-times. In the caseof 11 teleparallel homothetic vector fields the space-time becomes Minkowskiwith all the zero torsion components. Teleparallel homothetic vector fieldsin this case are exactly the same as in general relativity. It is importantto note that this classification also covers the plane symmetric static space-times.     

Classification of Static, Spherically Symmetric Solutions of the Einstein-Yang-Mills Theory with Positive Cosmological Constant

We give a complete classification of all static, spherically symmetric solutions of the SU(2) Einstein-Yang-Mills theory with a positive cosmological constant. Our classification proceeds in two steps. We first extend solutions of the radial field equations to their maximal interval of existence. In a second step we determine the Carter-Penrose diagrams of all 4-dimensional space-times constructible from such radial pieces. Based on numerical studies we sketch a complete phase space picture of all solutions with a regular origin.     

Stationary vacuum fields with a conformally flat three-space. II. Proof of axial symmetry

The conjecture is proved that stationary vacuum space-times having a conformally flat three-space are axially symmetric. The proof uses the Ernst potential and the complex conjugate potential as independent coordinates. Two field equations: a combination of the Einstein equations and an integrability condition are algebraic in one of the field variables. Their coefficients, computed by employing a REDUCE program, separately vanish unless axial symmetry holds. Solution of the coefficient equations yields the proof of axial symmetry. Certain special classes of metrics must be excluded from the discussion. The axial symmetry of these exceptional classes has been proved in I.     

Vorticity- and shear-free space-time in general relativity

The paper studies the vorticity- and shear-free nonstatic space-times with perfect fluid source in genera] relativity and finds that such space-times are either spherically symmetric or pseudospherical or plane symmetric.     

Topics on space-time topology

The orientability properties of space-times are analysed in detail using elementary algebraic methods. Time, space and charge orientability are discussed and various possible generalisations of charge orientability suggested. There is also a bundle-theoretic analysis of the first two topological properties together with a discussion of spinor-structures from the point of view of the Lorentz bundle of bases over a space-time. A section is devoted to some comments on the topologisation of certain space-times with topologies derived from their causal relations.     

Spherically symmetric space-times transparent to scalar multipole waves

Using nonscattering potentials of Chang and Janis, a large class of spherically symmetric space-times is constructed on which all multipole solutions to the minimally coupled scalar wave equation are expressible in terms of characteristic data functions in essentially as simple a fashion as for flat space-time. The space-times are transparent to multipole waves in the same sense that flat space-time is. Both conformally flat and not conformally flat space-times are obtained. Some examples are discussed which show that the variety of transparent space-times is large even within the class of Robertson-Walker spaces.     

Stationary vacuum fields with a conformally flat three-space I. General theory

A generalized notion of conformastat space-times is introduced in relativity theory. In this sense, the conformastat space-time is stationary with the three-space of timelike Killing trajectories being conformally flat. A 3+1 decomposition of the field equations is given, and two classes of nonstatic conformastat vacuum fields are exhaustively investigated. The resulting three metrics form a NUT-type extension of the solution of the static conformastat vacuum problem. We conjecture that all conformastat vacuum space-times are axially symmetric.     

Characterization of locally rotationally symmetric space-times

We give a simple characterization of locally rotationally symmetric space-times in terms of the existence of a canonical null tetrad or canonical orthonormal tetrad. The result is applied to space-times which satisfy the Einstein field equations with a perfect fluid or electromagnetic field as source.     

On the algebraic types of the Bel–Robinson tensor

The Bel–Robinson tensor is analyzed as a linear map on the space of the traceless symmetric tensors. This study leads to an algebraic classification that refines the usual Petrov–Bel classification of the Weyl tensor. The new classes correspond to degenerate type I space-times which have already been introduced in literature from another point of view. The Petrov–Bel types and the additional ones are intrinsically characterized in terms of the sole Bel–Robinson tensor, and an algorithm is proposed that enables the different classes to be distinguished. Results are presented that solve the problem of obtaining the Weyl tensor from the Bel–Robinson tensor in regular cases.     

Ghost neutrinos as test fields in curved space-time

Without restricting to empty space-times, it is shown that ghost neutrinos (their energy-momentum tensor vanishes) can only be found in algebraically special space-times with a neutrino flux vector parallel to one of the principal null vectors of the conformal tensor. The optical properties are studied. There are no ghost neutrinos in the Kerr-Newman and in spherically symmetric space-times. The example of a non-vacuum gravitational pp-wave accompagnied by a ghost neutrino pp-wave is discussed.     

Intrinsic curvature of the 3-surfaces of homothety in spatially homothetic space-times

It is shown how the sign of the intrinsic scalar curvature of spatially homothetic space-times depends on their algebraic type, thus generalizing a corresponding result for the homogeneous case.     

Equivalence of the Regge and Einstein equations for almost flat simplicial space-times

The theory of distributions is applied to almost flat simplicial space-times. Explicit expressions are given for the first-order defects. It is shown explicitly that the Riemann tensor for an almost flat simplicial space-time contains delta-functions on the bones and derivatives of delta-functions on the 3-dimensional faces of the boundary of the space-time. The latter terms have not previously been seen in the Regge calculus. It is shown that the Regge and Hilbert actions have equal values on almost fiat simplicial space-times and that the Einstein equations lead directly to the Regge field equations.     

Cauchy's problem and Huygens' principle for the linearized Einstein field equations

The linearized Einstein field equations with Einstein space-times as background are studied by use of the harmonic gauge. By means of Riesz' integration method a representation theorem for the solution of Cauchy's problem, using the constraints of the Cauchy data and the calculus of symmetric differential forms, is proved. We introduce some linear differential operators, which map the set of symmetric differential forms into the subset with vanishing divergence and trace and use these operators to derive necessary conditions for the validity of Huygens' principle from which it follows that the linearized field equations satisfy Huygens' principle only in flat space-times.Dedicated to Gudrun Schmidt on the occasion of her 50th birthday.