Since the event horizon has some drawbacks because of its global definition, a quasilocal horizon has been introduced. It is a hypersurface foliated by marginally trapped surfaces on which expansion of the outer null normal congruence vanishes. Different types of quasilocal horizons will be listed, i.e. apparent horizon, trapping horizon and isolated and dynamical horizons. Analysis of quasilocal horizons in two dynamical spacetimes used as inhomogeneous cosmological models will also be presented. We discovered future and past horizon in spherically symmetric Lemaître spacetime. Both horizons are null and have locally the same geometry as the horizons in the LTB spacetime. Then we studied Szekeres-Szafron spacetime with no symmetries, particularly its subfamily with non-zero beta,z, and we derived the equation of the horizon. However, because of the lack of symmetries the space-time is not adapted to double-null foliation, therefore our attempts to estimate the equation´s solution were unsuccessful.
Roughly speaking, cosmologists study large, linear scales and small, non-linear scales in two different ways: relativistic perturbation theory around a homogeneous and isotropic background describes scales where the growth of structures is at an early stage. At smaller scales, well inside the Hubble horizon, the relativistic effects are supposed to be completely negligible and General Relativity is replaced by Newtonian gravity. A post-Newtonian type approximation is a crucial improvement of this simple paradigm as it bridges the gap between relativistic perturbation theory and Newtonian structure formation. I focus on the relativistic corrections corrections for both the Eulerian and the Lagrangian approaches to gravitational dynamics in standard perturbation theory and in the post-Newtonian approximation. I finally present the post-Newtonian extension of the Zel'dovich solution for the plane parallel dynamics and its application in the context of the cosmological back-reaction proposal.
In the task of describing the history of our Universe - considered as a solution of the field equations of General Relativity - we analyse the issue of including the spacetime geometry as a thermodynamic actor along with the matter degrees of freedom. We show how it is possible to associate thermodynamic features to the free gravitational field in a purely geometrical way, following a proposal conceptualized by Penrose and recently formalized by different authors. We apply the resulting framework in the context of structure formation under the effect of gravity: the chain of processes leading from small matter inhomogeneities to black holes is shown to satisfy a "second law" of non-decreasing entropy. An interesting link to the thermodynamics of horizons is also presented. Eventually we contemplate the possibility of using the second law as a guiding principle for selecting thermodynamically favourable gravitational configurations: in this regard, an application to the Cosmic Censorship of naked singularities is briefly discussed.
We describe the properties of disk sources of the gravitational field matching the first three members of the Tomimatsu-Sato family of stationary axisymmetric, asymptotically flat vacuum spacetimes (among them is the well-known Kerr solution). We cut out the central region containing singularities and by a proper identification we get a disk-like distribution of stress-energy tensor. If the cut-out region is large enough, realistic models of disk material can be found, e.g. in the form of counter-rotating streams of particles. We discuss how energy conditions become violated and possibility to find realistic models of disk material is lost once the cut-out region becomes small enough and how it relates to the cosmic censorship conjecture. We show that this prevents the construction of 'realistic' disks surrounded by regions of closed time-like curves which appear in the the Tomimatsu-Sato spacetimes.
We study geometrical properties of null congruences generated by an aligned null direction of the Weyl tensor (WAND) in spacetimes of the Weyl and Ricci type N in an arbitrary dimension. We prove that non-aligned Ricci type N spacetimes of the genuine Weyl type III or N do not exist and for aligned Weyl type III and N, Ricci type N spacetimes, the multiple WAND is geodetic. For aligned Weyl and Ricci type N spacetimes, the canonical form of the optical matrix in the twisting and non-twisting case is derived and the dependence of the Weyl tensor and the Ricci tensor on an affine parameter of the geodetic null congruences generated by the WAND is obtained. Finally, examples of higher-dimensional Weyl and Ricci type N spacetimes with various possible geometries of their multiple WANDs are constructed as direct products of four-dimensional pure radiational metrics with the Euclidean space.
Extremal black holes tend to expel magnetic field lines from the horizon. Since the temperature of extremal black hole vanishes, this effect is analogous to the well-known Meissner effect for superconductors. The presence of the black hole Meissner effect has been demonstrated in many particular situations, especially (but not only) in the cases when the electromagnetic field is treated as a test field. In this talk, we present an exact result that EM field is expelled from the horizon of arbitrary stationary, axially symmetric black hole, provided that the field shares the symmetries of the spacetime.