International Journal of Modern Physics D

The mathematical simplicity of black holes, combined with their links to some of the most energetic events in the universe, means that black holes are key objects for fundamental physics and astrophysics. Until recently, it was generally believed that black holes in nature appear in two broad mass ranges: stellar-mass (M~3–20 M_⊙), which are produced by the core collapse of massive stars, and supermassive (M~10⁶–10¹⁰ M_⊙), which are found in the centers of galaxies and are produced by a still uncertain combination of processes. In the last few years, however, evidence has accumulated for an intermediate-mass class of black holes, with M~10²–10⁴ M_⊙. If such objects exist they have important implications for the dynamics of stellar clusters, the formation of supermassive black holes, and the production and detection of gravitational waves. We review the evidence for intermediate-mass black holes and discuss future observational and theoretical work that will help clarify numerous outstanding questions about these objects.

In this paper we investigate Charged Domain Walls (CDW's), topological defects that acquire surface charge density Q induced by fermion states localized on the walls. The presence of an electric and magnetic field on the walls is also discussed. We find a relation in which the value of the surface charge density Q is connected with the existence of such topological defects.

A possible solution to the problem of providing a spacetime description of the transmission of signals for quantum entangled states is obtained by using a bimetric spacetime structure, in which quantum entanglement measurements alter the structure of the classical relativity spacetime. A bimetric gravity theory locally has two lightcones, one which describes classical special relativity and a larger lightcone which allows light signals to communicate quantum information between entangled states, after a measurement device detects one of the entangled quantum states. This theory would remove the tension that exists between macroscopic classical, local gravity and macroscopic nonlocal quantum mechanics.

Several boundary conditions for the universe have been hitherto suggested, basing on different philosophical approaches. In particular, one may choose between the notions that the universe was created either from nothing or by itself. The quantum state of a universe created from nothing has been already formulated under distinct standpoints by Hartle, Hawking, Vilenkin and others. In this paper we have concentrated on deriving a quantum theory for a self-created universe. Thus, we have first considered the spacetime structure of a six-dimensional de Sitter space with a multiply connected region and, by using a cutting and pasting procedure, we have then been able to show that one can introduce a four-brane in such a spacetime whose evolution can also be considered within the context of ekpyrotic and cyclic universes, and quantized in terms of the sum-over-histories formulation, according to the rules of the generalized quantum theory developed by Hartle.

A linear cosmological perturbation theory of an almost homogeneous and isotropic perfect fluid Universe with dynamically evolving Newton constant G and cosmological constant Λ is presented. A gauge invariant formalism is developed by means of the covariant approach, and the acoustic propagation equations governing the evolution of the comoving fractional spatial gradients of the matter density, G, and Λ are thus obtained. Explicit solutions are discussed in cosmologies where both G and Λ vary according to renormalization group equations in the vicinity of a fixed point.

We study quintessence models with a constant (effective) equation of state. It is investigated whether such quintessence models are consistent with a negative spatial curvature of the Universe with respect to the anisotropy of the cosmic microwave background radiation measured by the WMAP mission. If the reionization is negligibly small, it is found that such models with negative curvature are admissible due to a geometrical degeneracy. However, a very high optical depth τ to the surface of last scattering, as indicated by the polarization measurements of WMAP, would rule out such models.

The bounded orbital motion of a massive spinless test particle in the background of a Kerr Brans–Dicke geometry is analysed in terms of worldlines that are auto-parallels of different metric compatible spacetime connections. In one case the connection is that of Levi–Civita with zero-torsion. In the second case the connection has torsion determined by the gradient of the Brans–Dicke background scalar field. The calculations permit one in principle to discriminate between these possibilities.

An exact analytical solution describing the interior of a charged strange quark star is found under the assumption of spherical symmetry and the existence of a one-parameter group of conformal motions. The solution describes a unique static charged configuration of quark matter with radius R=9.46 km and total mass M=2.86M_⊙.

If the causality condition [the speed of sound always remains less than that of light in vacuum, i.e. v≤c=1] is imposed on the spheres of homogeneous energy density, the "ratio of the specific heats", γ≤2.59457, constraints the compaction parameter, u[≡(M/a), mass to size ratio in geometrized units] of the dynamically stable configurations ≤0.34056 [corresponding to a surface redshift (z_a)≤0.771]. Apparently, the maximum value of u obtained in this manner belongs to an absolute upper bound, and gives: (i) The maximum value for static neutron star masses as 5.4 M_⊙, if we substitute the density at the surface of the configuration equal to the average nuclear density, E=2×10¹⁴g cm^-3 [e.g. Nature, 259, 377 (1976)]. (ii) However, if the density of the static configuration is constrained to the value 1.072×10¹⁴g cm^-3, by imposing the empirical result that the minimum rotation period of the fastest rotating pulsar known to date, PSR 1937+21, is 1.558 ms, the maximum mass value for static neutron stars exceed upto 7.4 M_⊙. These masses have important implications for the massive compact objects, such as Cyg X-1, Cyg XR-1, LMC-X3, etc., which may not, necessarily, represent black holes. (iii) The minimum rotation periods for a static 1.442 M_⊙ neutron star to be 0.3041 ms. (iv) A suitable stable model of ultra-compact objects [u>(1/3)] which has important astrophysical significance.

We present a new reformulation of the canonical quantum geometrodynamics, which allows one to overcome the fundamental problem of the frozen formalism and, therefore, to construct an appropriate Hilbert space associate to the solution of the restated dynamics. More precisely, to remove the ambiguity contained in the Wheeler–DeWitt approach, with respect to the possibility of a (3+1)-splitting when space–time is in a quantum regime, we fix the reference frame (i.e. the lapse function and the shift vector) by introducing the so-called kinematical action. As a consequence the new super-Hamiltonian constraint becomes a parabolic one and we arrive to a Schrödinger-like approach for the quantum dynamics. In the semiclassical limit our theory provides General Relativity in the presence of an additional energy–momentum density contribution coming from non-zero eigenvalues of the Hamiltonian constraints. The interpretation of these new contributions comes out in natural way that soon as it is recognized that the kinematical action can be recasted in such a way that it describes a pressureless, but, in general, non-geodesic perfect fluid.

The principal features in the detection of non-stationary undulatory metric disturbances based on interference effects are considered. The advantage for these the Aharonov–Bohm effect in superconductors in comparison with conventional optical interference is shown. Schemes of interferometric detectors with SQUID are offered.