Iranian Journal of Physics Research
Year:2005 | Volume:5 | Issue:3|Papers Count:14

We elaborate on some recent results on a solution of the Hilbert-space problem in minisuperspace quantum cosmology and discuss the consequences of making the (geometry of the) Hilbert space of ordinary no relativistic quantum systems time-dependent. The latter reveals a remarkable similarity between Quantum Mechanics and General Relativity.

A null thin shell immersed in a generic spherically symmetric space-time is studied within the distributional formalism. It has been shown that the distributional formalism leads to the same result as the conventional Barrabes-Israel formalism.

Einstein and transport synchronizations of infinitesimally spaced and distant clocks are considered in a general Riemannian space time. It is shown that infinitesimally spaced clocks can always be synchronized. In general one can not find observers for whom distant clock are Einstein synchronized; but transport synchronized observers do always exit. Whenever both procedures are possible, they are equivalent.

In this paper, the rotating black brane solutions with zero curvature horizon of classical gravity with first order string corrections are introduced. Although these solutions are not asymptotically anti de Sitter, one can use the counterterm method in order to compute the conserved quantities of these solutions. Here, by reviewing the counterterm method for asymptotically anti de Sitter spacetimes, the conserved quantities of these rotating solutions are computed. Also a Smarr-type formula for the mass as a function of the entropyand the angular momenta is obtained, and it is shown that the conserved and thermodynamic quantities satisfy the first law of thermodynamics. Finally, a stability analysis in the canonical ensemble is performed, and it is shown that the system is thermally stable. This is in commensurable with the fact that there is no Hawking-Page phase transition for black object with zero curvature horizon.

Gravitational multipole moments of the Sun are still poorly known. Theoretically, the difficulty is mainly due to the differential rotation for which the velocity rate varies both on the surface and with the depth. From an observational point of view, the multipole moments cannot be directly measured. However, recent progresses have been made proving the existence of a strong radial differential rotation in a thin layer near the solar surface (the leptocline). Applying the theory of rotating stars, we will first compute values of J2 and J4 taking into account the radial gradient of rotation, then we will compare these values with the existing ones, giving a more complete review. We will explain some astrophysical outcomes, mainly on the relativistic Post Newtonian parameters. Finally we will conclude by indicating how space experiments (balloon SDS flights, Golf NG, Beppi-Colombo, Gaia...) will be essential to unambiguously determine these parameters.

Allowing the energy of a gravitational field to serve partially as its own source allows gravitating bodies to exhibit stronger fields, as if they were more massive. Depending on degree of compaction of the body, the field could be one to five times larger than the Newtonian field. This is a comfortable range of increase in field strength and may prove to be of convenience in the study of velocity curves of spirals, of velocity dispersions in clusters of galaxies and in interpreting the Tully-Fisher or Faber-Jackson relations in galaxies or systems of galaxies. The revised gravitation admits of superposition principle but only approximately in systems whose components are widely separated. The revised dynamics admits of the equivalence principle in that, the effective force acting on a test particle is derived from a potential, and could be elimhnated in a freely falling frame of reference.

Which place is an appropriate site for the construction of the Iranian National Observatory (INO)? In this paper part of the site selection process is reported. The emphasis is on the measuring of the seeing parameter for the pre-selected regions. These regions are examined by meteorological and geophysical studies and finally Kashan, Kerman, Qom and Ferdows sites were selected among 31 regions. To decide on the best site the seeing parameter had to be measured for the regions. To measure this parameter, one needs an astronomical seeing monitor. Designing and making a kind of seeing monitor and preparing the corresponding software is demonstrated and analysing of a sample of seeing data is discussed.

The stepwise Sine-Gordon equation in which the value of the system parameter is different for negative and positive value of the scalar field is investigated. By applying appropriate junction conditions, the relation between solution velocities before and after collision is derived. It turns out that a heavy soliton anti-soliton pair can transform into a light pair moving at right velocities. We call this phenomenon "soliton gun". Some of the result which were obtained before numerically, are derived analytically in this paper.

Replacing the wormhole geometry with an equivalent medium using the perturbation theory of scattering and the Born approximation, we have calculated the differential scattering cross section of electromagnetic waves by a traversable wormhole. It is shown that scattering at long wavelenghts can essentially distinguish wormhole from ordinary scattering object. Some of the zeros of the scattering cross section are determined which can be used for estimating the radius of the throat of wormholes. The known result that in this kind of scattering the linear polarization remains unchanged is verified here.

Although, the hot solar corona was discovered more than sixty years ago, however, the exact dissipation mechanism that heats the corona is still unknown. Resonant absorption and damping of Alfven waves appear to be one of the major candidates in this respect. The corona is highly structured and inhomogeneous medium, containing a large number of discrete magnetic loops. In this paper a cylindrical magnetic flux tube with a specified density profile is considered, and the ideal equation of motion in coronal condition is obtained. The problem is reduced to solving a wave equation for the component of magnetic perturbation along the tube axis. The mathematical formalism is identical with that of the propagation of electromagnetic waves in optical fibers with a varying index of refraction through the cross section of the fiber. The ideal equations of magnetic and velocity fields in the global modes are singular and are solved numerically, using a modified shooting scheme. Resistive and viscous dissipation rates as an exponential time decay of disturbances are obtained. The results show that the amount of energy produced in the resonant layer inside the tube seems to be responsible for heating the corona.

In this paper we consider the action of higher derivative gravity up to the second order terms in the scalars made from the Ricci scalar, Ricci and Riemann tensors. We use the Bach- Lanczos identity of the Weyl tensor in four dimensions and show that the solutions of 4-dimensional Einstein equations with cosmological constant term in vacuum, which are known as Einstein metrics, satisfy the field equations of higher derivative gravity. We also find that the field equations of higher derivative gravity are not satisfied by the solutions of Einstein equations in higher dimensions or in the presence of matter.