International Journal of Modern Physics D

We study the cosmological evolution of string/M moduli fields T. We use T-duality to fix the potential and show that the superpotential W is a function of the duality invariant function j(T) only. If W is given as a finite polynomial of j then the moduli fields do not give an accelerating universe, i.e. they cannot be used as quintessence. Furthermore, at T ≫ 1 the potential is given by a double exponential potential leading to a fast decaying behavior at large times. For moduli potentials with a finite v.e.v. of T the energy density redshift is model dependent but if T has a finite mass, m < ∞, then the moduli energy density redshifts faster or equal to matter. Only if the moduli mass is infinite can the moduli energy density dominate the universe independently of the initial conditions.

A generalized scalar–tensor (GST) theory is investigated whose cosmological (or quintessence) term depends on both a scalar field and its time derivative. A correspondence with solutions of five-dimensional Space–Time–Matter (STM) theory is noted. Analytic solutions are found for the scale factor, scalar field and cosmological term. Models with free parameters of order unity are consistent with recent observational data and could be relevant to both the dark matter and cosmological "constant" problems.

The multidimensional gravity on the principal bundle with the SU(2) gauge group is considered. The numerical investigation of the spherically symmetric metrics with the center of symmetry is made. The solution of the gravitational equations depends on the boundary conditions of the "SU(2) gauge potential" (off-diagonal metric components) at the symmetry center and on the type of symmetry (symmetrical or antisymmetrical) of these potentials. In the chosen range of the boundary conditions it is shown that there are two types of solutions: wormhole-like and flux tube. The physical application of such kind of solutions as quantum handles in a spacetime foam is discussed.

We intend to clarify the interplay between boundary terms and conformal transformations in scalar-tensor theories of gravity. We first consider the action for pure gravity in five dimensions and show that, on compactifing a la Kaluza–Klein to four dimensions, one obtains the correct boundary terms in the Jordan (or String) Frame form of the Brans–Dicke action. Further, we analyze how the boundary terms change under the conformal transformations which lead to the Pauli (or Einstein) frame and to the nonminimally coupled massless scalar field. In particular, we study the behaviour of the total energy in asymptotically flat spacetimes as it results from surface terms in the Hamiltonian formalism.

The behavior of a arbitrary coupled quantum scalar field is studied in the background of the Gödel spacetime. Closed forms are derived for the effective action and the vacuum expectation value of quadratic field fluctuations by using ζ-function regularization. Based on these results, we argue that causality violation presented in this spacetime cannot be removed by quantum effects.

A rotating universe represented by the Gödel metric in spacetimes with Cartan torsion is investigated. The spin–torsion coupling is shown to contribute to the structure formation in an apppreciable way only at the early stages of the universe. To demonstrate these conjectures, we have made use of a spinning fluid model in Einstein–Cartan gravity. We also show that autoparallel equation in Riemann–Cartan spacetime leads to the evolution equation of the cosmological perturbation where the spin-rotation coupling contributes to the growth of inhomogeneities through a constant term. From this idea, one is able to estimate that torsion in the Planck era is around 10¹⁵s^-1 where COBE constraint on temperature fluctuations is used in this computation. A spin density of the universe of 10⁵³g · cm^-1s^-1 is also obtained in our framework.

We formulate conditions on the geometry of a nonexpanding horizon Δ which are sufficient for the spacetime metric to coincide on Δ with the Kerr metric. We introduce an invariant which can be used as a measure of how different the geometry of a given nonexpanding horizon is from the geometry of the Kerr horizon. Directly, our results concern the spacetime metric at Δ at the zeroth and the first orders. Combined with the results of Ashtekar, Beetle and Lewandowski, our conditions can be used to compare the spacetime geometry at the nonexpanding horizon with that of Kerr to every order. The results should be useful to numerical relativity in analyzing the sense in which the final black hole horizon produced by a collapse or a merger approaches the Kerr horizon.

We examine several higher spin modes of the Poincaré gauge theory (PGT) of gravity using the Hamiltonian analysis. The appearance of certain undesirable effects due to nonlinear constraints in the Hamiltonian analysis are used as a test. We find that the phenomena of field activation and constraint bifurcation both exist in the pure spin 1 and the pure spin 2 modes. The coupled spin-0^- and spin-2^- modes also fail our test due to the appearance of constraint bifurcation. The "promising" case in the linearized theory of the PGT given by Kuhfuss and Nitsch¹ likewise does not pass. From this analysis of these specific PGT modes we conclude that an examination of such nonlinear constraint effects shows great promise as a strong test for this and other alternate theories of gravity.

The well known general relativistic Lense–Thirring drag of the orbit of a test particle in the stationary field of a central slowly rotating body is generated, in the weak-field and slow-motion approximation of General Relativity, by a gravitomagnetic Lorentz-like acceleration in the equations of motion of the test particle. In it the gravitomagnetic field is due to the central body's angular momentum supposed to be constant. In the context of the gravitational analogue of the Larmor theorem, such acceleration looks like a Coriolis inertial term in an accelerated frame. In this paper the effect of the variation in time of the central body's angular momentum on the orbit of a test mass is considered. It can be shown that it is analogue to the inertial acceleration due to the time derivative of the angular velocity vector of an accelerated frame. The possibility of detecting such effect in the gravitational field of the Earth with LAGEOS-like satellites is investigated. It turns out that the orbital effects are far too small to be measured.

We study whether the black hole entropy can be understood in terms of thermodynamical properties of a fluid membrane in the Membrane Paradigm. In particular, by using local observable quantities of the membrane and thermodynamical relations, we calculate the entropy of the membrane in a phenomenological manner. It is shown that one can assign the entropy of black hole to that of the membrane such that the entropy, together with other physical quantities of the membrane, composes thermodynamics consistently. This result concerning entropy gives a new support for the Membrane Paradigm, and consequently, supports a kind of holographic viewpoint about black hole for external observers, i.e., black hole entropy for external observers can be regarded consistently as a measure of some degrees of freedom associated with the horizon boundary. In addition, our result also suggests that the generalized second law of thermodynamics hold in quite general process.