International Journal of Modern Physics D

In this review we summarize the current understanding of the gravitational-wave driven instability associated with the so-called r-modes in rotating neutron stars. We discuss the nature of the r-modes, the detailed mechanics of the instability and its potential astrophysical significance. In particular we discuss results regarding the spin-evolution of nascent neutron stars, the detectability of r-mode gravitational waves and mechanisms limiting the spin-rate of accreting neutron stars in binary systems.

Possibility of misidentification of emission lines of QSOs and thereby missing detection of blueshifts have been suggested and demonstrated in literature. It is shown here that absorption lines observed in the spectra of QSOs may also have been misidentified. Plenty search lines are available at higher wavelengths (red/IR regions) for identification with observed absorption lines, although lines from lower wavelengths (UV/blue regions) are always considered for identification. Re-examination of the published spectra of ten QSOs shows that the observed absorption lines can be interpreted as blueshifted, with same or better accuracy and sometimes more convincingly than redshifts. Objects on the sight line are identified with same emission blueshift as the blueshift of the absorption system of the QSO, the emission spectrum of the QSO itself being of course interpreted as blueshifted as well. It is proposed that the absorbers are probably linked with the QSO either physically or tidally/gravitationally as a result of some ejection mechanism. The observed blueshifts of spectral lines in emission and absorption can be explained in a model comprising of multiple expanding universes.

In order to detect the gravitomagnetic clock effect by means of two counter-orbiting satellites placed on identical equatorial and circular orbits around the Earth with radius 7000 km their radial and azimuthal positions must be known with an accuracy of δr=10^-1 mm and δϕ=10^-2 mas (milliarcseconds) per revolution. In this work we investigate if the radial and azimuthal perturbations induced by the dynamical and static parts of the Earth's gravitational field meet these requirements. While the radial direction is affected only by harmonic perturbations with periods up to some tens of days, the azimuthal location is perturbed by a secular drift and very long period effects. It results that the present level of accuracy in the knowledge both of the Earth solid and ocean tides, and of the static part of the geopotential does not allow an easy detection of the gravitomagnetic clock effect at least by using short arcs only.

The sensitivity achievable by a pair of VIRGO detectors to stochastic and isotropic gravitational wave backgrounds produced in pre-big-bang models is discussed in view of the development of a second VIRGO interferometer. We describe a semi-analytical technique allowing to compute the signal-to-noise ratio for (monotonic or non-monotonic) logarithmic energy spectra of relic gravitons of arbitrary slope. We apply our results to the case of two correlated and coaligned VIRGO detectors and we compute their achievable sensitivities. We perform our calculations both for the usual case of minimal string cosmological scenario and in the case of a non-minimal scenario (originally suggested by Gasperini) where a long dilaton dominated phase is present prior to the onset of the ordinary radiation dominated phase. In this framework, we investigate possible improvements of the achievable sensitivities by selective reduction of the thermal contributions (pendulum and pendulum's internal modes) to the noise power spectra of the detectors. Since a reduction of the shot noise does not increase significantly the expected sensitivity of a VIRGO pair (in spite of the relative spatial location of the two detectors) our findings support the experimental efforts directed towards a substantial reduction of thermal noise.

We discuss some new problems of the modern cosmology which arose after the BOOMERANG and MAXIMA-1 successful missions. Statistics of high peaks of the CMB anisotropy is analyzed and we discuss possible inner structure of such peaks in the observational data of future MAP and PLANCK missions. We have investigated geometrical and statistical properties of the CMB polarization around such high isolated peaks of anisotropy in the presence of a polarized pixel noise and point sources. The structure of polarization fields in the vicinity of singular points with zero polarization is very sensitive to the level of pixel noises and point sources in the CMB maps.

The gravitational fields of vacuumless global and gauge strings have been studied in Brans–Dicke theory under the weak field assumption of the field equations. It has been shown that both global and gauge string can have repulsive as well as attractive gravitational effect in Brans–Dicke theory which is not so in General Relativity.

The Einstein–Maxwell dilaton field equations are constructed for four-dimensional Bianchi III and five-dimensional FRW models. The general solutions have been obtained and their asymptotic behavior have been studied.

We investigate here the behavior of a few spherically symmetric static acclaimed black hole solutions in respect of tidal forces in the geodesic frame. It turns out that the forces diverge on the horizon of cold black holes (CBH) while for ordinary ones, they do not. It is pointed out that Kruskal-like extensions do not render the CBH metrics nonsingular. We present a CBH that is available in the Brans–Dicke theory for which the tidal forces do not diverge on the horizon and in that sense it is a better one.

The fermionic free energy and entropy are calculated in Garfinkle–Horowitz–Strominger dilatonic black hole background spacetime, by using 't Hfoot's brick wall model. It turns out that the fermionic entropy in Garfinkle–Horowitz–Strominger dilatonic black hole back ground spacetime is 7/2 times the Bosonic entropy.

The superstring and superbrane theories include gravity as a necessary and fundamental part of a (future) unified field theory. Thus it is important to consider the alternative representations of general relativity as well as the alternative models of gravity.

We study the coframe teleparallel theory of gravity with a most general quadratic Lagrangian. The coframe field on a differentiable manifold is a basic dynamical variable. A metric tensor as well as a metric compatible connection is generated by a coframe in a unique manner.

The Lagrangian is a general linear combination of Weitzenböck's quadratic invariants with free dimensionless parameters ρ₁, ρ₂, ρ₃. Every independent term of the Lagrangian is a global SO(1,3)-invariant 4-form. For a special choice of parameters which confirms with the local SO(1,3) invariance this theory gives an alternative description of Einsteinian gravity — teleparallel equivalent of GR.

The field equations of the theory is studied by a "diagonal" coframe ansatz which is a subclass of a most general spherical-nsymmetric Einstein–Mayer ansatz. The restricted Lagrangian depends only on two free parameters ρ₁, ρ₃.

We obtain a formula for scalar curvature of a pseudo-Riemannian manifold with a metric constructed from the static "diagonal" solution of the field equation. It is proved that the sign of the scalar curvature depends only on a relation between the parameters ρ₁ and ρ₃. Thus by a specific choice of free parameters a manifold of positive or negative curvature can be obtained. The scalar curvature vanishes only for a subclass of models with ρ₁=0. This subclass includes the teleparallel equivalent of GR.

We obtain the explicit form of all spherically symmetric static solutions of the "diagonal" type to the field equations for an arbitrary choice of free parameters. We prove that the unique asymptotic-flat solution with Newtonian limit is the Schwarzschild solution that holds for a subclass of teleparallel models with ρ₁=0. Thus the Yang–Mills-type term of the general quadratic coframe Lagrangian should be rejected.

The metric for the two-dimensional dilaton gravity can be writen in an alternative form, similar to the two-dimensional Schwarzschild metric, and allow us the identification of some quantitiies with those equivalent in the Schwarzschild solution. This new form, however, presents a nonphysical singularity at the horizon in the same way that in the realistic four dimensional case. We show a procedure to eliminate this horizon singularity and, as an application, the resulting metric is used to obtain the associated Hawking temperature. We discuss also some differents between this metric and the Schwarzschild one.

We present an approximate solution of the Einstein equations for the metric of a global string based on Lyra's geometry. The geodesic of a test particle is studied due to the gravitational field of the string. Finally, the angle of deficit has been calculated for the string solution.

We study multidimensional cosmology to obtain the wavefunction of the universe using wormhole dominance proposal. Using a prescription for time we obtain the Schrödinger–Wheeler–DeWitt equation without any reference to WD equation and WKB ansatz for WD wavefunction. It is found that the Hartle–Hawking or wormhole-dominated boundary conditions serve as a seed for inflation as well as for Gaussian type ansatz to Schrödinger–Wheeler–DeWitt equation.

The Wheeler–DeWitt equation in quantum gravity is timeless in character. In order to discuss quantum to classical transition of the universe, one uses a time prescription in quantum gravity to obtain a time contained description starting from Wheeler–DeWitt equation and WKB ansatz for the WD wavefunction. The approach has some drawbacks. In this work, we obtain the time-contained Schrödinger–Wheeler–DeWitt equation without using the WD equation and the WKB ansatz for the wavefunction. We further show that a Gaussian ansatz for SWD wavefunction is consistent with the Hartle–Hawking or wormhole dominance proposal boundary condition. We thus find an answer to the small scale boundary conditions.