49. Leszek M. Sokołowski
Stability of a metric f(R) gravity theory implies the Newtonian limit
Acta Phys. Polon., vol. B39, pp. 2879-2901 (2008).
[abstract] [preprint] [journal]

Abstract:
We show that the existence of the Newtonian limit cannot work as a selection rule for choosing the correct gravity theory fromm the set of all L=f(R) ones. To this end we prove that stability of the ground state solution in arbitrary purely metric f(R) gravity implies the existence of the Newtonian limit of the theory. And the stability is assumed to be the fundamental viability criterion of any gravity theory. The Newtonian limit is either strict in the mathematical sense if the ground state is flat spacetime or approximate and valid on length scales smaller than the cosmological one if the ground state is de Sitter or AdS space. Hence regarding the Newtonian limit a metric f(R) gravity does not differ from GR with arbitrary Lambda. This is exceptional to Lagrangians solely depending on R and/or Ricci tensor. An independent selection rule is necessary.

50. Michael Heller, Zdzisław Odrzygóźdź, Leszek Pysiak and Wiesław Sasin
Gravitational Aharonov-Bohm Effect
International Journal of Theoretical Physics, vol. 47, pp. 2566-2575 (2008).
[abstract] [journal] [download]

Abstract:
We study a purely gravitational Aharonov-Bohm effect. The space-time curvature is concentrated in the quasiregular singularity of a cosmic string, outside of which space-time is (locally) flat. The symmetries of this field configuration are described by the groupoid symmetries rather than by the usual group symmetries. The groupoid in question is formed by homotopy classes of piecewise smooth paths in the cosmic string region. A gravitational counterpart of the Aharonov-Bohm effect occurs if the symmetry of the system, with respect to the groupoid action, is broken down.

51. Piotr T. Chruściel, Sebastian J. Szybka
On the Ernst electro-vacuum equations and ergosurfaces
Acta Phys. Pol., B , vol. 39, pp. 59-75 (2008).
[abstract] [preprint] [journal]

Abstract:
The question of smoothness at the ergosurface of the space-time metric constructed out of solutions (E,phi) of the Ernst electro-vacuum equations is considered. We prove smoothness of those ergosurfaces at which Re(E) provides the dominant contribution to f=-(Re(E)+|phi|^2) at the zero-level-set of f. Some partial results are obtained in the remaining cases: in particular we give examples of leading-order solutions with singular isolated ``ergocircles".

52. Sebastian J. Szybka, Tadeusz Chmaj
Fractal Threshold Behavior in Vacuum Gravitational Collapse
Phys. Rev. Lett., vol. 100, p. 101102 (2008).
[abstract] [preprint] [journal] [download]

Abstract:
We present the numerical evidence for fractal threshold behavior in the five dimensional vacuum Einstein equations satisfying the cohomogeneity-two triaxial Bianchi type-IX ansatz. In other words, we show that a flip of the wings of a butterfly may influence the process of the black hole formation.

53. Andrzej Woszczyna
Dispersion of density waves in the Universe with positive cosmological constant
Conference report (2008).
[abstract] [Mathematica 5.2]

Abstract:
Marie Curie Host Fellowships for the Transfer of Knowledge (TOK) Project MTKD-CT-2005-029466: PARTICLE PHYSICS AND COSMOLOGY: THE INTERFACE, Fourth Workshop 13.02 - 16.02.2008, Warszawa

54. Sebastian J. Szybka
Chaos, Gravity and Wave Maps with Target SU(2)
Proceedings of the MG11 Meeting on General Relativity (2008).
[abstract] [journal]

Abstract:
We present the numerical evidence for chaotic solutions and fractal threshold behavior in the Einstein equations coupled to a wave map (with target SU(2)). This phenomenon is explained in terms of heteroclinic intersections.