Showing papers in "International Journal of Modern Physics D in 2000"

The Case for a Positive Cosmological Λ-Term

Abstract: Recent observations of Type 1a supernovae indicating an accelerating universe have once more drawn attention to the possible existence, at the present epoch, of a small positive Λ-term (cosmological constant). In this paper we review both observational and theoretical aspects of a small cosmological Λ-term. We discuss the current observational situation focusing on cosmological tests of Λ including the age of the universe, high redshift supernovae, gravitational lensing, galaxy clustering and the cosmic microwave background. We also review the theoretical debate surrounding Λ: the generation of Λ in models with spontaneous symmetry breaking and through quantum vacuum polarization effects — mechanisms which are known to give rise to a large value of Λ hence leading to the "cosmological constant problem." More recent attempts to generate a small cosmological constant at the present epoch using either field theoretic techniques, or by modelling a dynamical Λ-term by scalar fields are also extensively discussed. Anthropic arguments favouring a small Λ-term are briefly reviewed. A comprehensive bibliography of recent work on Λ is provided.

Finite field theory on noncommutative geometries

Abstract: The propagator is calculated on a noncommutative version of the flat plane and the Lobachevsky plane with and without an extra (Euclidean) time parameter. In agreement with the general idea of noncommutative geometry it is found that the limit when the two "points" coincide is finite and diverges only when the geometry becomes commutative. The flat four-dimensional case is also considered. This is at the moment less interesting since there has been no curved case developed with which it can be compared.

On Gravitational Fluctuations and the Semiclassical Limit in Minisuperspace Models

Abstract: An attempt is made to go beyond the semiclassical approximation for gravity in the Born–Oppenheimer decomposition of the wave-function in minisuperspace. New terms are included which correspond to quantum gravitational fluctuations on the background metric. They induce a back-reaction on the semiclassical background and can lead to the avoidance of the singularities the classical theory predicts in cosmology and in the gravitational collapse of compact objects.

Boson Stars with Generic Self-Interactions

Abstract: We study boson star configurations with generic, but not nontopological, self-interaction terms. When compared with the usual λ|ψ|4 potential, similar results for masses and number of particles appear. However, changes are noticed in the order of a few percent of the star mass. We explore, using catastrophe theory, the stability properties of the configurations and look for possible observational outputs that may differentiate one model from the other: gravitational redshifts, rotation curves of accreted particles, and lensing phenomena are computed for our new cases.

GEODESICS IN THE γ SPACETIME

Abstract: Geodesics are studied in the spacetime described by the γ metric. Their behaviour is compared with the spherically symmetric situation, bringing out the sensitivity of the trajectories to deviations from spherical symmetry.

Gravitational lensing statistical properties in general FRW cosmologies with dark energy component(s): Analytic results

Abstract: Various astronomical observations have been consistently making a strong case for the existence of a component of dark energy with negative pressure in the universe. It is now necessary to take the dark energy component(s) into account in gravitational lensing statistics and other cosmological tests. By using the comoving distance we derive analytic but simple expressions for the optical depth of multiple image, the expected value of image separation and the probability distribution of image separation caused by an assemble of singular isothermal spheres in general FRW cosmological models with dark energy component(s). We also present the kinematical and dynamical properties of these kinds of cosmological models and calculate the age of the universe and the distance measures, which are often used in classical cosmological tests. In some cases we are able to give formulae that are simpler than those found elsewhere in the literature, which could make the cosmological tests for dark energy component(s) more convenient.

Large quantum gravity effects: cylindrical waves in four dimensions

Abstract: Linearly polarized cylindrical waves in four-dimensional vacuum gravity are mathematically equivalent to rotationally symmetric gravity coupled to a Maxwell (or Klein–Gordon) field in three dimensions. The quantization of this latter system was performed by Ashtekar and Pierri in a recent work. Employing that quantization, we obtain here a complete quantum theory which describes the four-dimensional geometry of the Einstein–Rosen waves. In particular, we construct regularized operators to represent the metric. It is shown that the results achieved by Ashtekar about the existence of important quantum gravity effects in the Einstein–Maxwell system at large distances from the symmetry axis continue to be valid from a four-dimensional point of view. The only significant difference is that, in order to admit an approximate classical description in the asymptotic region, states that are coherent in the Maxwell field need not contain a large number of photons anymore. We also analyze the metric fluctuations on the symmetry axis and argue that they are generally relevant for all of the coherent states.

Detection of gravitational waves from inspiraling, compact binaries using a network of interferometric detectors

Abstract: We formulate the data analysis problem for the detection of the Newtonian waveform from an inspiraling, compact binary by a network of arbitrarily oriented and arbitrarily located laser interferometric gravitational-wave detectors. We obtain for the first time the relation between the optimal statistic and the magnitude of the network correlation vector, which is constructed from the matched network-filter.

Perturbative analysis of generalized einstein theories

Abstract: The hypothesis that the energy–momentum tensor of ordinary matter is not conserved separately, leads to a nonadiabatic expansion and, in many cases, to an Universe older than usual. This may provide a solution for the entropy and age problems of the Standard Cosmological Model. We consider two different theories of this type, and we perform a perturbative analysis, yielding analytical expressions for the evolution of gravitational waves, rotational modes and density perturbations. One of these theories exhibits satisfactory properties at this level, while the other one should be discarded.

A Comparative Study of Bondi-Type and Radiative Outflows Around Compact Objects

Abstract: Astrophysical jets are observed to come out with relativistic speed and are extremely collimated. Here we study the effect of momentum deposition by hot photons from the torus like, puffed up inner region of the accretion disc on radial outflows. We calculate the radiative momentum deposition force on the outflows and find that there is a focusing effect. The size of the radiating torus completely determines the location where the radiative force is maximum. We solve the steady state equations of motion of the radial outflow, in Schwarzschild geometry to show that the acceleration achieved is appreciable, when it is compared with Bondi-type outflow. We also show that, the photon momentum deposition force results in bringing the sonic point closer to the black hole thereby resulting in a more energetic outflows as compared to purely thermally driven outflow. Particularly interesting is that in many cases, where bound matters are freed and driven off as outflows after sufficient momentum is deposited.

A generalized equivalence principle

Abstract: Gauge field theories may quite generally be defined as describing the coupling of a matter-field to an interaction-field, and they are suitably represented in the mathematical framework of fiber bundles. Their underlying principle is the so-called gauge principle, which is based on the idea of deriving the coupling structure of the fields by satisfying a postulate of local gauge covariance. The gauge principle is generally considered to be sufficient to define the full structure of gauge-field theories. This paper contains a critique of this usual point of view: firstly, by emphazising an intrinsic gauge theoretic conventionalism which crucially restricts the conceptual role of the gauge principle and, secondly, by introducing a new generalized equivalence principle — the identity of inertial and field charge (as generalizations of inertial and gravitational mass) — in order to give a conceptual justification for combining the equations of motion of the matter-fields and the field equations of the interac...

Investigation of radiative outflows around compact objects

Abstract: Winds and outflows form in active galaxies and in binary systems which are known to harbour compact objects such as black holes. Matter starting subsonically from a disc must be accelerated very close to the black hole in order to reach a velocity comparable to the velocity of light, which is actually observed. In the absence of magnetic fields, winds forming in inner regions of accretion discs could primarily be accelerated by radiations emitted from this region where centrifugal force is important. We study critical point behaviour of outflows in presence of this radiative acceleration. We show that the momentum deposition term changes the character of the solution drastically depending on the magnitude and the location of the deposition. We discuss the implications of these solutions in detail. Particularly important is the fact that matter were found to be pushed to infinity, even when they were originally bound energetically. We perform numerical simulations by smoothed particle hydrodynamics (SPH), and show that these new solutions are stable.

Initial Operation of the International Gravitational Event Collaboration

Abstract: The International Gravitational Event Collaboration, IGEC, is a coordinated effort by research groups operating gravitational wave detectors working towards the detection of millisecond bursts of gravitational waves. Here we report on the current IGEC resonant bar observatory, its data analysis procedures, the main properties of the first exchanged data set. Even though the available data set is not complete, in the years 1997 and 1998 up to four detectors were operating simultaneously. Preliminary results are mentioned.

Generalized scalar tensor theory in four and higher dimension

Abstract: In this paper, we have considered generalized scalar–tensor theory for four-dimensional Bianchi-I model and also for a five-dimensional cosmological model. We have studied both exponential and power law solutions, considering a bulk viscous fluid. To solve the complicated coupled field equations, we have made assumptions among the physical parameters and solutions have been discussed.

Nucleosynthesis in a universe with a linearly evolving scale factor

Abstract: Primordial nucleosynthesis is regarded as a success story of the standard big bang (SBB) cosmology. In this article nucleosynthesis is explored in models in which the cosmological scale factor R(t) increases linearly with time. This relationship of R(t) and t continues from the period when nucleosynthesis begins until the present time. It turns out that weak interactions remain in thermal equilibrium upto temperatures which are two orders of magnitude lower than the corresponding (weak interaction decoupling) temperatures in SBB. Inverse beta decay of the proton can ensure adequate production of helium while producing primordial metallicity much higher than that produced in SBB. Attractive features of such models are the absence of the horizon, flatness and age problems as well consistency with classical cosmological tests.

Gravitational collapse of newtonian stars

Abstract: We investigate spherically symmetric solutions for nonrelativistic cosmological fluid equations and thermodynamic equation of state for Newtonian stars of ideal gas. Using simple ansatze it is shown that the assumption of a polytrope, , at the center of the star only suffices to obtain analytic solutions. We find collapse behavior for γ≤4/3 and oscillatory behavior for γ>4/3 along with the discussion on their mechanisms. For the oscillatory behavior we obtained the frequency of small oscillation ω which is times that obtained by Zel'dovich and Novikov.

A power filter for the detection of burst sources of gravitational radiation in interferometric detectors

Abstract: We present a filter for detecting gravitational wave signals from burst sources. This filter requires only minimal advance knowledge of the expected signal: i.e. the signal's frequency band and time duration. It consists of a threshold on the total power in the data stream in the specified signal band during the specified time. This filter is optimal (in the Neyman–Pearson sense) for signal searches where only this minimal information is available.

Rome all-sky search for periodic sources

Abstract: Optimal methods cannot be applied in the search for periodic gravitational sources, because of the high computing power needed. Some ideas on the hierarchical method based on the Hough transform, proposed by the Rome Virgo group, are presented.

Signature for the shape of the universe

Abstract: If the universe has a nontrivial shape (topology) the sky may show multiple correlated images of cosmic objects. These correlations can be couched in terms of distance correlations. We propose a statistical quantity which can be used to reveal the topological signature of any Robertson–Walker (RW) spacetime with nontrivial topology. We also show through computer-aided simulations how one can extract the topological signatures of flat, elliptic, and hyperbolic RW universes with nontrivial topology.

Development of the electroweak phase transition and baryogenesis

Abstract: We investigate the evolution of the electroweak phase transition, using a one-Higgs effective potential that can be regarded as an approximation for the Minimal Supersymmetric Standard Model. The phase transition occurs in a small interval around a temperature Tt below the critical one. We calculate this temperature as a function of the parameters of the potential and of a damping coefficient related to the viscosity of the plasma. The parameters that are relevant for baryogenesis, such as the velocity and thickness of the walls of bubbles and the value of the Higgs field inside them, change significantly in the range of temperatures where the first-order phase transition can occur. However, we find that in the likely interval for Tt there is no significant variation of these parameters. Furthermore, the temperature Tt is in general not far below the temperature at which bubbles begin to nucleate.

Matching characteristic codes: exploiting two directions

Abstract: Combining incoming and outgoing characteristic formulations can provide numerical relativists with a natural implementation of Einstein's equations that better exploits the causal properties of the spacetime and gives access to both null infinity and the interior region simultaneously (assuming the foliation is free of caustics and crossovers). We discuss how this combination can be performed and illustrate its behavior in the Einstein–Klein–Gordon field in 1D.

Global Monopoles in Lyra Geometry

Abstract: We present the solutions for the metric outside a monopole resulting from the breaking of a global 0(3) symmetry within the framework of Lyra geometry. We have shown that monopole exerts gravitational force on matter around it.

Constructing hyperbolic systems in the ashtekar formulation of general relativity

Abstract: Hyperbolic formulations of the equations of motion are essential technique for proving the well-posedness of the Cauchy problem of a system, and are also helpful for implementing stable long time evolution in numerical applications. We, here, present three kinds of hyperbolic systems in the Ashtekar formulation of general relativity for Lorentzian vacuum spacetime. We exhibit several (I) weakly hyperbolic, (II) diagonalizable hyperbolic, and (III) symmetric hyperbolic systems, with each their eigenvalues. We demonstrate that Ashtekar's original equations form a weakly hyperbolic system. We discuss how gauge conditions and reality conditions are constrained during each step toward constructing a symmetric hyperbolic system.

Stable inflationary dissipative cosmologies

Abstract: The stability of the de Sitter era of cosmic expansion in spatially curved homogeneous isotropic universes is studied The source of the gravitational field is an imperfect fluid such that the parameters that characterize it may change with time In this way we extend our previous analysis for spatially-flat spaces as well as the work of Barrow

About the detection of gravitational wave bursts

Abstract: Several filtering methods for the detection of gravitational wave bursts in interferometric detectors are presented. These are simple and fast methods which can act as online triggers. All methods are compared to matched filtering with the help of a figure of merit based on the detection of supernovae signals simulated by Zwerger and Muller.

Monitoring and adaptive removal of the power supply harmonics applied to the virgo read-out noise

Abstract: The VIRGO signal read-out system is based on a sixteen channels architecture. The system is now installed at the site and the noise of eight of the channels has been measured. The noise at 50 Hz and all its harmonics is coherent with a simultaneous measurement of the power supply. The power supply measurement has been used to remove the 50 Hz line and all its harmonics from the signal read-out noise.

Oscillating universe as eigensolutions of cosmological schrödinger equation

Abstract: We propose a cosmological model which could explain, in a very natural way, the apparently periodic structures of the universe, as revealed in a series of recent observations. Our point of view is to reduce the cosmological Friedman–Einstein dynamical system to a sort of Schrodinger equation whose bound eigensolutions are oscillating functions. Taking into account the cosmological expansion, the large scale periodic structure could be easily recovered considering the amplitudes and the correlation lengths of the galaxy clusters.

Orbit design and analysis for the astrod mission concept

Abstract: The ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) mission concept is to conduct high-precision measurement of relativistic effects, better determination of the orbits of major asteroids and other solar system parameters, improvement in the measurement of , measurement of solar angular momentum via Lense–Thirring effect, and the detection of low-frequency gravitational waves and solar oscillations in a single mission. It will be realized by placing a fleet of drag-free spacecraft in solar orbits together with an Earth reference system. Two spacecraft launched into separate solar orbits, as a simple implementation, can reach the opposite side of Sun after traveling about 2.5 years. In this paper, we describe the orbit design process for this simple implementation including the two-body model, initial velocity determination, and optimization consideration. Through fine tuning of the initial velocity, we can have the two spacecraft nearly return to the 2.5 years positions at 7.5 years mission time, which means that the ASTROD mission can have a second good chance to observe Shapiro time delay precisely and to measure the solar Lense–Thirring effect if the mission lasts over 7.5 years. We also calculate the light traveling time and the Shapiro time delay.

Distinguishing marks of simply-connected universes

Abstract: A statistical quantity suitable for distinguishing simply-connected Robertson–Walker (RW) universes is introduced, and its explicit expressions for the three possible classes of simply-connected RW universes with an uniform distribution of matter are determined. Graphs of the distinguishing mark for each class of RW universes are presented and analyzed. There sprout from our results an improvement on the procedure to extract the topological signature of multiply-connected RW universes, and a refined understanding of that topological signature of these universes studied in previous works.

Gravitational waves emitted by extrasolar planetary systems

Abstract: In this paper we consider the Extrasolar Planetary Systems recently discovered in our Galaxy as potential sources of gravitational waves. We estimate the frequency and characteristic amplitude of the radiation they emit due to the orbital motion, using the quadrupole formalism. In addition, we check whether the conditions needed for the resonant excitation of the f- and g-modes of the central star can be fulfilled. By a Roche-lobe analysis, we show that there could exist systems in which the low-order g-modes could be excited, although this does not happen in the systems discovered up to now.