Sharif Digital Repository

One of the biggest challenges facing cosmologists is finding the nature of dark matter. The cosmological observations, like the Rotation Curve, Gravitational Weak Lensing and Cosmic Microwave Background radiation indicate that baryonic matter alone cannot explain the density of matter in the universe. In the context of standard cosmological models, it means that the universe is made of an unknown component(Dark Matter), which aprroximately has 27% of total energy-momentum of the universe. The Modify Gravity and General Relativity are alternative theories that introduced to describe this observation. In This thesis, we introduce the issue of the measurements of accelerated observers in... 

the gravitational wave (GW) which is a prediction of General Relativity has entered a new era after being detected by Ligo observatory in 2015. new discoveries are still being made. the discovery of GW is a new opportunity for studying gravity, astrophysical sources of GWs like binary black-holes or binry neutron stars as well as testing cosmology models.on the other hand, it should be mentioned that the astronomy of gravitational waves is in its dawn stage. in future having numerous precise detectors and by accumulation of data, speaking about exact phenomena and statistical analysis will become meaningful. as in the case of cosmic microwave background physics, in which researchers started... 

A new heuristic controller, called nonlinear model predictive control based on gravitational search algorithm, is proposed for nonlinear systems. Nonlinear model predictive control involves the solution at each sampling instant of a finite horizon optimal control problem subject to nonlinear system dynamics and constraints. The new controller formulates nonlinear model predictive control problem as a single stochastic dynamic optimization problem and uses a system of virtual masses to find the best control signal at each sampling instant. For this purpose, a cost function is defined to evaluate each point of the search space. This function minimizes simultaneously the tracking error, control... 

Gravitational microlensing is one of the experimental tests to confirm the theory of general relativity. In this phenomenon, the light of the source star is deflected by the gravitational field of the foreground star which plays the role of the lens. As a result of this deviation, images of source star are formed on the sky plane. The observer records the light of the source star as a variable with time. The gravitational microlensing illustrates the possibility to discover extrasolar planets, study the atmosphere of stars, and probe the nature of Dark matter. In this thesis, at first, we introduce the basic concepts of gravitational microlensing and its history. Then by simulation of... 

The standard model of Cosmology known as CDM is established as a cornerstone of modern cosmology. The Cosmic Microwave Background radiation (CMB) and Large Scale Structure (LSS) Observations indicate that almost all observations can be described by a 6-parameter standard model with an initial condition of almost Gaussian, isotropic, scale invariant and adiabatic which can be sourced by inflationary models. Recent observations of Planck collaboration shows almost no deviation from standard picture of single field slow roll inflation. Despite the success of the standard model, the physics of early universe, the nature of dark matter and dark energy is still unknown. The large scale structure... 

Discovery of the effect of a massive object on the light path, reaching its exact formalism through general relativity and utilizing this phenomenon as a natural lens, opened a new window in astronomical research. Gravitational lensing occurs in three different ways; strong lensing, weak lensing, and microlensing which we focus on the third one. Gravitational microlensing is useful in studying dark matter candidates, discovering exoplanets, finding the surface properties of stars, and calculating the mass distribution of galactic objects. From a typical microlensing event, Einstein crossing time is obtained which is the time scale of source star passing by the lens. This is the only... 

The discovery of gravitational waves by LIGO has undoubtedly opened a new window on our observations of the universe. In this way, valuable observational information can be obtained from near astrophysical sources to distant cosmic structures. In addition to the source of gravitational wave propagation, with the help of phenomena such as the magnification of light intensity by intermediate objects in the path of the gravitational wave to the observer, useful information can be obtained from the distribution of cosmic matter. The study of this effect is possible with the help of frequency changes per unit time. Therefore, what can be measured are the magnification effects of the... 

The purpose of this study is to investigate the effect of radiation pressure on accretion disks. The radiation density causes the separation of electrons from ions which leads to the creation of an internal electric dipole. In this study Radiation density is calculated using the Eddington limit. Afterwards radiation pressure is calculated with the use of collisions between photons and electrons in the plasma. Collisions are assumed to happen only between photons and electrons, due to the larger cross section of electrons compared to protons. The mentioned assumption causes the creation of an electric dipole and with it an internal electric field. The separation of the charges is assumed to... 

In present thesis, the effect of cosmological constant, Λ, in bending of light by spherical symmetric mass has been studied. At first we obtain the Schwarzschild-de Sitter solution as spherical symmetric solution of General relativity field equations with cosmological constant, without we suppose the solution is static. Then we obtain gravitational lens equation by considering the space geometry and we benefited from several approach to specify the contribution of cosmological constant to the deflection angle. Furthermore we put new upper bounds on the value of the lambda independently of cosmological constraints. Finally for study the contribution of lambda in gravitational lensing... 

Einstein's theory of General Relativity is the cornerstone in many modern physics areas in which the strong gravitational field is engaged, i.e. modern cosmology, neutron star and black hole physics, generation of gravitational waves, and several other cosmic phenomena. Despite its simple appearance, the theory of general relativity is practically rigorous. Einstein's field equations are consist of 10 coupled, non-linear, partial differential equations in 4 space-time dimensions. Except for some highly symmetric space-times, general relativity does not provide exact analytical solutions to real problems with a low level of symmetry, since the field equations would be strongly complex for... 

With the first detection of Gravitational Waves (GWs) in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO), there opened a new window to perceive the universe.Gravitational Waves Astronomy is a new area in cosmology, which its aim is to study the sources of Gravitational Waves, their distribution in the universe and its connection to formation and evolution of structures. In this thesis, we investigate the statistics of background radiation of GWs as a phenomenon in the context of stochastic processes, which radiated form indistinct sources. For this purpose, we will first evaluate the angular distribution function of background radiation of GWs and their relation to the... 

In this thesis, inspired by foundational discussions about the concepts of energy and entropy in the gravitational theory, we study the definitions of these concepts and the roles they play in that theory. The definitions considered in this thesis are derived from canonical formalism. To this end, first of all, the variational principle and the canonical structure in the gravitational theory, especially in situations where the spacetime is bounded, is carefully studied and the boundary and corner actions as well as the canonical structure on the boundaries in three cases of non-null, null and general boundaries are derived. Based on this study, two canonical approaches, namely the... 

The physical description of particles states in the quantum theory are characterized by means of theirs mass and spin. The inertial and gravitational properties of mass are already well known in mechanics and gravitation theories as well as their quantum counterparts. In spite of mass, the inertial and gravitational properties of intrinsic spin are not studied that much.In this thesis we review the inertial and gravitational properties of intrinsic spin. Initially we explore the general procedure to formulate Dirac equation in arbitrary curved spacetime. Then we write Dirac equation in Kerr spacetime to draw physical conclusions about the interaction of Dirac particles with the Kerr... 

The information paradox is one of the most important questions around black hole physics. The problem is raised since the operators which are responsible for the evolutions in quantum mechanics are unitary while the black hole evaporation seems to be a nonunitary evolution (Since a black hole which has been formed from the collapse of matter in a pure state evaporates to a mixed thermal state eventually). This loss of information during the evaporation process is known as the information paradox.Recently many attempts have been made to investigate this problem holographically, actually to see if the information is really lost and if not how one can recover the information.Our main goal is to... 

In this thesis we first discuss the final fate of a spherical symmetric distribution of matter which has undergone a gravitational collapse, by reviewing some classical collapse models. Our emphasis will be on the visibility of singularity after it forms. Then we will mention some general aspects of quantum field theories in curved space-time and after that we investigate the quantum background effects on the evolution of a spherical symmetric and homogeneous distribution of dust. In fact by calculating the quantum corrections—due to massless scalar fields which are arbitrarily coupled to the background geometry— on the geometry of collapsing sphere, one can obtain the quantum corrected... 

In this thesis first we will review gravitational lensing and microlensing, then we will introduce all direct and indirect methods for detecting exo-planets. gravitational microlensing is one of these methods in detecting exo-planets. In the second part first we will explain a powerful numerical method for calculating magnification, position and the shape of images by combining of tree code and inverse rays hooting algorithm. Then we simulated the caustic crossing of Hot Jupiters using that algorithm. Finally with performing Monte-Carlo simulation we will obtain the detection efficiency for detecting this class of exo-planets during caustic crossing and we will estimate the number of these... 

After Supernovae and gamma-ray bursts, Quasars are the brightest in the Universe. They are powered by supermassive black holes which lie at the center of the galaxies. Perturbations in space-time metrics by massive galaxies near the line of sight of the quasar can lead to strong lensing and create multiple pictures. Gravitationally lensed quasar systems yield abundant data sets for astrophysical and cosmological experimentation. In this thesis survey results of the multiply lensed SDSS J1206+4332 is studied. Candidate phenomenons are microlensing and gravitational grating. Gravitational grating studies the interaction of electromagnetic wave and gavitational wave in both geometric and wave... 

In this thesis, we first review MOdified Gravity (MOG) theory as an alternative to dark matter models and then investigate some of the phenomena related to galaxy dynamics and gravitational lensing within the framework of this theory. In the last step, we present a model for the interaction of the fifth force field and the electromagnetic field. This model is based on the definition of a current density of matter using the energy momentum tensor in which the Lorentz symmetry is broken. This is due to the existence of a constant coefficient in the interaction, which can be considered as a fixed background field or observer 4-velocity that couples to energy momentum tensor. After introducing... 

This thesis introduces a fast and accurate two-phase reservoir flow simulation procedure. The speed up is achieved by an upscaling technique where coarsening is applied to the detailed geological reservoir descriptions. This process results to two major error sources affecting the simulation accuracy significantly. The homogenization error is appeared when coarse scale data, such as equivalent permeability, is calculated from the available fine grid model information. It is shown that the error can be reduced adjusting coarse grid (non-uniform Cartesian) distribution. For the first time in this study, the grid distribution optimization is done based on “vorticity” map. In particular, the... 

In this study numerical methods are used for modeling flow and simulation of aerosol transport and deposition in alveolar zone of human lung. The flow modeling is done by FLUENT code, using its moving boundaries feature. For particle tracking we developed a code that uses results of velocity field obtained by FLUENT code. Two-dimensional the axisymmetric model was investigated in this study, and was exhibited that rhythmically expanding walls in alveolar duct model has significant influence on flow patterns and particles transport mechanism. In this study we also developed a three-dimensional model of a single alveolated duct with moving boundaries allowing for the cyclic expansion and...