Sharif Digital Repository

The finite-size effect in gravitational microlensing provides a possibility to measure the variation of surface brightness of stars from centre to limb which is called limb darkening profile. Measuring limb darkening profile provides information about star atmosphere. It is also useful in other fields of astrophysics. In this thesis we develop a numerical method to recover limb darkening of source stars from single lens microlensing light curves with finite source effect in which the lens transits the source disk. We use the Finite Element Method (FEM) as an inversion tool for discretization and inversion of the finite source effect integral equation. This method makes no explicit assumption... 

In this thesis, we investigate the imprint of gravitational microlensing effects on the light curves of Gamma-ray burst (GRBs) to estimate compact object mass in the form of black holes in the Universe. We assume a point mass model of the gravitational lens and a geometry in which the source−lens are at the same direction that manifests in the GRB light curve as superimposed peaks by the time delay between the two images and searches the Fermi/GBM catalog. According to the sensitivity of our detection method, we consider microlensing events with a time delay on the order of 10 s due to point-mass lenses in the range of 〖10〗^5- 〖10〗^7 M_(⊙ ). We found one microlensed GRB candidate out of... 

In this work we investigate the propagattion of light in inhomogenous cosmic spacetime within general relativity framework. We use Lemaitr-Tolaman-Bondi metric for modeling inhomogenous spacetime. For investigation of gravitational lensing we need to solve geodesic equaions in this model. Beacuse of nonlinearity and complexity of these equations we should apply numerical methods. At first we used Runge-Kutta method with adaptive step size, but beacuse of stiffness of these equaions, the results of this method did not match to approximation methods for low deflection angles. So we try semi-implicit Rosenbrock method. Then for inspection of thin lens approximation for the first time; we... 

The standard model of Cosmology (SM), based on Einstein’s theory of general relativity (GR), is the best model in describing the cosmic-scale observations. In galactic scales, however, galactic dynamics show a discrepancy between the observed luminous mass and the predicted value from the theory. Two approaches are suggested to eliminate this inconsistency: assuming the existence of dark matter within the context of GR, or modifying the theory of gravity in galactic scales, like MOND or MOG. In this thesis, we study the spherical collapse and the formation of shell galaxies in MOG, in comparison with SM. We introduce the action and the field equations of MOG and the equation of motion of a... 

Primordial black holes have attracted attention because of their possible role in explaining phenomena such as dark matter and gravitational waves. These observations require the primordial black holes to be in different mass ranges. We investigated the inflaton potentials in which the stochastic sequence of sharp step-like features on the slow-roll potential can produce primordial black holes in different mass ranges. To this end, we first studied the cumulative effects of consecutive steps and showed that there is a resonance mechanism that leads to an exponential enhancement of the power spectrum of certain wavelengths.
 

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... 

The present work has proposed an integrated methodology for combined gas & electricity distribution network expansion planning, which considers network expansion constraints (substation installation, feeder/pipeline installation, etc), load growth, and conversion technologies. The planning aims to determine the optimal reinforcement of existing energy network to meet the load growth in the planning horizon. To take the advantages of new technologies, CHP, fuel cell, & microturbine have been included in the problem.This paper employs gravitational search algorithm (GSA) to solve multi-objective problem, including minimization of cost, voltage drop, & pressure drop. GSA that introduced in... 

Gravitational Microlensing has been used as a powerful tool for astrophysical studies and exoplanet detections. In the gravitational microlensing, we have two images with negative and positive parities. The negative-parity image is a fainter image and is produced at a closer angular separation with respect to the lens star. In the case of a red-giant lens star and large impact parameter of lensing, this image can be eclipsed by the lens star. The result would be dimming the flux receiving from the combination of the source and the lens stars and the light curve resembles to an eclipsing binary system. In this work, we introduce this phenomenon and propose an observational procedure for... 

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... 

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... 

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... 

In this manuscript we used the method of coset construction to describe theories of spontaneous symmetry breaking. The mathematical details are included in chapter 2. The utility of the method is that without any knowledge of the UV part of a theory we can construct IR part by just knowing the symmetry breaking pattern. We used the method for some symmetry breaking patterns to construct general relativity, massive gravity and also some symmetry breaking patterns corresponding to conformal gravity  

When in a binary system one of the stars passes behind another star eclipsing binary will occur and light’s intensity will decrease. Now with assumption that one of its component be Black Hole When another star passes behind of it the star’s light is amplified by Microlensing.With detection of this amplification we can find black hole’s mass. Of course in this phenomenon the degeneracy will be broken completely and we can obtain all of the quantity. From another specification of this event we can call the shortness and repetitive of Einestein’time .The Einestein’s time is approximately a few hours and depending on system’s period this event will be repeated in specific time.Finally Black... 

In this thesis, we investigate two subjects in gravitational microlensing. In first part we study wave optics effect in light curve of a binary system. Gravitational lensing occurs when light deflects in a gravitational field. This deflection produces multi-images from real one. In microlensing regime, images can not be detected separately and gravitational lensing appears as magnifying source’s flux . Microlensing has been studied mostly in geometric optics regime. At larger wavelengths, mostly at radio wavelengths, we expect to see the effects of wave optics in microlensing. Stars at radio wave are faint and to detect them we need a powerful radio telescope. In the near future, SKA (Square... 

In this thesis, we have investigated the light scalar field models for describing the dark sector(Dark Matter and Dark Energy) of the universe in cosmology. As a result of tiny mass, the de Broglie wavelength becomes large, and consequently, the quantum mechanical behavior is essential in large scales. Our main attempt is to investigate a model of scalar fields consisting of particles with a mass range of ($10^{-21}-10^{-23}$)eV, often called Fuzzy Dark Matter (FDM). In the FDM hypothesis, the dark matter halo consists of a solitonic core that formed Bose-Einstein-Condensation BEC at the halo's inner region and non-condensed dark matter particles at the outer region. We took into account... 

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... 

At the beginning of the 20th century, Albert Einstein introduced the equations of general relativity, opening a new avenue for investigating various phenomena. One of the predictions of these equations, gravitational lensing, was observed by Eddington during his lifetime, confirming the theory's ability to describe nature. Gravitational lensing can be categorized into three types: strong lensing, weak lensing, and microlensing. The current focus of research is on observing the separation phenomenon in gravitational microlensing. The observation of gravitational microlensing provides valuable insights into the composition of celestial bodies in the universe. Calculating the mass of different... 

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... 

Intensity or phase Modulation of a wavefront is possible by means of optical devices. Placing a grating or lens on the way of a wavefront, will alter the phase or intensity of it and this will make a diffraction pattern. Spatial Light Modulator (SLM) is an optical device which is the most important part of a video projector. One can apply grating, lens and other phases on the wavefront using this device.In this project we used SLM to modify phase of laser beam and then we made many focal points such that they could displace independently. The result of this project can be used in designing and creating a holographic optical tweezers.Moreover, A. Einstein proved that the light could be... 

In my Thesis we are going to explain Large Scale Weak Lensing observations by weak field approximation of non-local modified gravity. We try to explain the weak lensing of galaxy clusters after collision without any need to dark matter, which could be an important test of the modified gravity model.One of the important questions of modern cosmology is the nature of dark matter,the concept which was created to explain what ordinary baryonic matter can’t explain.However, one alternative way is to modify Einstein’s General Relativity. One of the main pillars of General Relativity is the locality assumption. Non-local gravity puts aside this assumption. The Newtonian limit of this theory is...