A new proof of Birkhoff's theorem

Assuming SO(3)-spherical symmetry, the 4-dimensional Einstein equation reduces to an equation conformally related to the field equation for 2-dimensional gravity following from the Lagrangian L = R^(1/3). Solutions for 2-dimensional gravity always possess a local isometry because the traceless part of its Ricci tensor identically vanishes. Combining both facts, we get a new proof of Birkhoff's theorem; contrary to other proofs, no coordinates must be introduced. The SO(m)-spherically symmetric solutions of the (m+1)-dimensional Einstein equation can be found by considering L = R^(1/m) in two dimensions. This yields several generalizations of Birkhoff's theorem in an arbitrary number of dimensions, and to an arbitrary signature of the metric.Comment: 17 pages, LaTeX, no figures, Grav. and Cosm. in prin

The Newtonian limit of fourth-order gravity

The weak-field slow-motion limit of fourth-order gravity will be discussed.Comment: 5 pages, LaTe

On the Space of 3-dimensional Homogeneous Riemannian Manifolds

We answer the following question: Let l, m, n be arbitrary real numbers. Does there exist a 3-dimensional homogeneous Riemannian manifold whose eigenvalues of the Ricci tensor are just l, m and n ?Comment: 2 pages, LaTeX, reprinted from Proc. Conf. Brno (1995

Linear energy bounds for Heisenberg spin systems

Recently obtained results on linear energy bounds are generalized to arbitrary spin quantum numbers and coupling schemes. Thereby the class of so-called independent magnon states, for which the relative ground-state property can be rigorously established, is considerably enlarged. We still require that the matrix of exchange parameters has constant row sums, but this can be achieved by means of a suitable gauge and need not be considered as a physical restriction

Non-trivial Solutions of the Bach Equation Exist

We show that solutions of the Bach equation exist which are not conformal Einstein spaces.Comment: 3 pages, LaTeX, no figur

A new conformal duality of spherically symmetric space-times

A contribution linear in r to the gravitational potential can be created by a suitable conformal duality transformation: the conformal factor is 1/(1+r)^2 and r will be replaced by r/(1+r), where r is the Schwarzschild radial coordinate. Thus, every spherically symmetric solution of conformal Weyl gravity is conformally related to an Einstein space. This result finally resolves a long controversy about this topic. As a byproduct, we present an example of a spherically symmetric Einstein space which is a limit of a sequence of Schwarzschild-de Sitter space-times but which fails to be expressable in Schwarzschild coordinates. This example also resolves a long controversy.Comment: 11 pages, LaTeX, no figure

Motion of a thin spherically symmetric Shell of Dust in the Schwarzschild field

The equation of motion announced in the title was already deduced for the cases the inner metric being flat and the shell being negligibly small (test matter), using surface layers and geodesic trajectories resp. Here we derive the general equation of motion and solve it in closed form for the case of parabolic motion. Especially the motion near the horizon and near the singularity are examined.Comment: Reprinted from: 10th International Conference on General Relativity and Gravitation, Padova (Italy) July 4 - 9, 1983. Eds.: B. Bertotti, F. de Felice, A. Pascolini, Contributed papers Vol. 1, Roma (1983) page 339-34

Perihelion advance for orbits with large eccentricities in the Schwarzschild black hole

We deduce a new formula for the perihelion advance of a test particle in the Schwarzschild black hole by applying a newly developed non-linear transformation within the Schwarzschild space-time. By this transformation we are able to apply the well-known formula valid in the weak-field approximation near infinity also to trajectories in the strong-field regime near the horizon of the black hole.Comment: 22 pages, new results added at the end of scts. 4 and 5, accepted for Phys. Rev.

On Ellis' programme within fourth order gravity

For the non-tachyonic curvature squared action we show that the expanding Bianchi-type I models tend to the dust-filled Einstein-de Sitter model for t tending to infinity if the metric is averaged over the typical oscillation period. Applying a conformal equivalence between curvature squared action and a minimally coupled scalar field (which holds for all dimensions > 2) the problem is solved by discussing a massive scalar field in an anisotropic cosmological model.Comment: 9 pages, LaTeX, no figur

The metric in the superspace of Riemannian metrics and its relation to gravity

The space of all Riemannian metrics is infinite-dimensional. Nevertheless a great deal of usual Riemannian geometry can be carried over. The superspace of all Riemannian metrics shall be endowed with a class of Riemannian metrics; their curvature and invariance properties are discussed. Just one of this class has the property to bring the lagrangian of General Relativity into the form of a classical particle's motion. The signature of the superspace metric depends in a non-trivial manner on the signature of the original metric, we derive the corresponding formula. Our approach is a local one: the essence is a metric in the space of all symmetric rank-two tensors, and then the space becomes a warped product of the real line with an Einstein space.Comment: 10 pages, LaTeX, reprinted from Proc. Conf. Diff. Geom. Appl., Brno, Czechoslovakia 1989, WSPC Singapore, Eds. J. Janyska, D. Krupk