Hans-Jürgen Schmidt | Searcharxiv

arXiv2012

The tetralogy of Birkhoff theorems

We classify the existent Birkhoff-type theorems into four classes: First, in field theory, the theorem states the absence of helicity 0- and spin 0-parts of the gravitational field. Second, in relativistic astrophysics, it is the statement that the gravitational far-field of a spherically symmetric star carries, apart from its mass, no information about the star; therefore, a radially oscillating star has a static gravitational far-field. Third, in mathematical physics, Birkhoff's theorem reads: up to singular exceptions of measure zero, the spherically symmetric solutions of Einstein's vacuum field equation with Lambda = 0 can be expressed by the Schwarzschild metric; for Lambda unequal 0, it is the Schwarzschild-de Sitter metric instead. Fourth, in differential geometry, any statement of the type: every member of a family of pseudo-Riemannian space-times has more isometries than expected from the original metric ansatz, carries the name Birkhoff-type theorem. Within the fourth of these classes we present some new results with further values of dimension and signature of the related spaces; including them are some counterexamples: families of space-times where no Birkhoff-type theorem is valid. These counterexamples further confirm the conjecture, that the Birkhoff-type theorems have their origin in the property, that the two eigenvalues of the Ricci tensor of two-dimensional pseudo-Riemannian spaces always coincide, a property not having an analogy in higher dimensions. Hence, Birkhoff-type theorems exist only for those physical situations which are reducible to two dimensions.

gr-qc↗

arXiv2000

Cosmology with Curvature-Saturated Gravitational Lagrangian R/\sqrt{1 + l^4 R^2}

We argue that the Lagrangian for gravity should remain bounded at large curvature, and interpolate between the weak-field tested Einstein-Hilbert Lagrangian L_EH = R /16 pi G and a pure cosmological constant for large R with the curvature-saturated ansatz L_cs=L_EH/ \sqrt{1+l^4 R^2}, where l is a length parameter expected to be a few orders of magnitude above the Planck length. The curvature-dependent effective gravitational constant defined by dL/dR = 1/16 pi G_eff is G_eff = G \sqrt{1+l^4 R^2}^{3}, and tends to infinity for large $R$, in contrast to most other approaches where G_eff-> 0. The theory possesses neither ghosts nor tachyons. In a curvature-saturated cosmology, the coordinates with ds^2 = a^2 [da^2/B(a) - dx^2 - dy^2- dz^2] are most convenient since the curvature scalar becomes a linear function of $B(a)$. Solutions with a big-bang singularity have a much milder behavior of the curvature than in Einstein's theory. In synchronized time, the metric is given by ds^2 = dt^2 - t^{6/5(dx^2 + dy^2+ dz^2). On the technical side we show that two different conformal transformations make L_cs asymptotically equivalent to the Gurovich-ansatz L= | R |^{4/3} on the one hand, and to Einstein's theory with a minimally coupled scalar field with self-interaction on the other.

gr-qc↗