International Journal of Modern Physics D

Some historical remarks concerning the strange stars are briefly discussed. The recent developments in physics and dynamical behavior of strange stars are reviewed. Especially, various observational effects in distinguishing strange stars from neutron stars and related interesting astrophysical phenomena are also discussed.

The Einstein's field equations of FRW universes filled with a dissipative fluid described by full theory of causal transport equations are analyzed. New exact solutions are found using a nonlocal transformation on the nonlinear differential equation for the Hubble factor. The stability of the de Sitter and asymptotically Friedmannian solutions are analyzed using Lyapunov function method.

The meaning of 'tunneling' in a timeless theory such as quantum cosmology is discussed. A recent suggestion of 'tunneling' of the macroscopic universe at the classical turning point is analyzed in an anisotropic and inhomogeneous toy model. This 'inhomogeneous tunneling' is a local process which cannot be interpreted as a tunneling of the universe.

We apply the causal interpretation of quantum mechanics to homogeneous quantum cosmology and show that the quantum theory is independent of any time-gauge choice and there is no issue of time. We exemplify this result by studying a particular minisuperspace model where the quantum potential driven by a prescribed quantum state prevents the formation of the classical singularity, independently on the choice of the lapse function. Hence, within the framework of the causal interpretation of quantum cosmology, the fast-slow-time gauge conjecture is incorrect for homogeneous minisuperspace models.

The Einstein equation in D dimensions, if restricted to the class of spacetimes possessing n = D - 2 commuting hypersurface-orthogonal Killing vectors, can be equivalently written as metric-dilaton gravity in two dimensions with n scalar fields.

For n = 2, this result reduces to the known reduction of certain four-dimensional metrics which include gravitational waves. Here, we give such a representation which leads to a new proof of the Birkhoff theorem for plane-symmetric spacetimes, and which leads to an explanation, in which sense two (spin zero-) scalar fields in two dimensions may incorporate the (spin two-) gravitational waves in four dimensions. (This result should not be mixed up with well–known analogous statements where, however, the four-dimensional spacetime is supposed to be spherically symmetric, and then, of course, the equivalent two-dimensional picture cannot mimic any gravitational waves.)

Finally, remarks on hidden symmetries in two dimensions are made.

The evolution of a soliton star filled with fermions is studied in the framework of general relativity. Such a system could be described by the surface tension σ₀, the bag constant B, and the fermion number density ρ. One of the parameters mentioned above could prevail in the system and thus affect the spacetime inside the bubble. Whether it is described by Friedman or de Sitter metric depends on the prevailing parameter. In broad outline, the whole spacetime is divided by the surface of the soliton into the false vacuum region inside the soliton and the true vacuum region outside, the latter being described by the Schwarzschild line element. The aim of this paper is to study the equations of motion of the domain wall in two cases. In both of them, the Schwarzschild metric is outside the soliton. The de Sitter metric describes the interior in the first case, and in the second case, it is replaced by the Friedman metric. Analyzing obtained equations, one can draw conclusions concerning the further evolution of the soliton star.

A method for obtaining a class of complex solutions of the Ernst equation is described which is based on a set of linear equations. This method is applied to generate families of unpolarized vacuum and electrovac G₂ cosmologies and nondiagonal solutions describing colliding plane gravitational and gravito-electromagnetic waves.

The metrical Hamiltonian is investigated for a nonconformal scalar field with arbitrary coupling in a nonstationary spatially homogeneous isotropic gravitational field. It is shown that this Hamiltonian may be diagonalized by the canonical Bogoliubov transformations of creation-annihilation operators only for the values of coupling coefficient 0 ≤ ξ ≤ 1/6. The quasienergy of quasiparticles diagonalizing the Hamiltonian is shown to differ for the nonconformal case from the effective oscillator frequency of the wave equation. Different definitions of the initial vacuum state for the nonconformal scalar field are discussed.

The time-dependent, spherically symmetric, Wyman sector of the Unified Field Theories is shown to be equivalent to a self-gravitating scalar field with a positive-definite, repulsive interaction potential. A homothetic symmetry is imposed on the fundamental tensor, and the resulting autonomous system is numerically integrated. Near the critical point (between the collapsing and noncollapsing spacetimes) the system displays an approximately periodic alternation between collapsing and dispersive epochs.

Superluminal signal transfer is studied in the context of a preferred cosmic frame of reference provided by the galactic background. The receding galaxies constitute a frame of absolute rest, in which the energy of tachyons (faster-than-light particles) is unambi-guously defined as a positive quantity. The causality violation which arises in relativistic tachyonic theories is avoided. We define interactions of particles and tachyons in terms of elastic head-on collisions and energy-momentum conservation. To compare the theory developed with existing relativistic theories, tachyons are studied at first in a Minkowski universe, and the causality of a superluminal communication process is analyzed. Then we discuss the dynamics of tachyons in a Robertson–Walker universe with linear expansion factor and negatively curved three-space. We point out the consequences that the space expansion has on tachyons, like a finite life-time in the frame of absolute rest, and multiple images in the rest frames of moving observers.

This work introduces a new space of 'vertex-smooth' states for use in the loop approach to quantum gravity. Such states provide a natural domain for Euclidean Hamiltonian constraint operators of the type introduced by Thiemann (and using certain ideas of Rovelli and Smolin). In particular, such operators map into itself, and so are actual operators in this space. Their commutator can be computed on and compared with the classical hypersurface deformation algebra. Although the classical Poisson bracket of Hamiltonian constraints yields an inverse metric times an infinitesimal diffeomorphism generator, and despite the fact that the diffeomorphism generator has a well-defined nontrivial action on , the commutator of quantum constraints vanishes identically for a large class of proposals.