Sharif Digital Repository

Gravitational Microlensing is one of the most powerful methods of detecting very low mass objects like Exoplanets and Brown dwarfs. The most important parameter that we can extract from a microlensing event is Einstein radius crossing time t_E. In this thesis, with performing Monte-Carlo simulation, we will obtain t_E distribution for brown dwarf population. Then we will show that this population can be a good candidate for very short microlensing events with t_E<2 days. The data set used in this analysis was taken in 2006 and 2007 seasons by the MOA-II survey, with the 1.8-m MOA-II telescope located at the Mt. John University Observatory, New Zealand  

Star Formation History (SFH) is one of the important quantities to constrain galaxy formation and evolution scenarios and star formation models. In this thesis we aim to derive SFH for solar neighbourhood stars. We can use absolute magnitude-color diagram of a stellar system to derive the SFH. The method is to consider different functions for SFH and compare the resulting colormagnitude diagram with observational data. We used HIPPARCOS satellite data in this thesis. Hipparcos has measured astrometric parameters for about 120000 stars very accurately, which enables us to compare the color-magnitude of those stars with theoretical color-magnitudes. In this thesis, using Monte Carlo method, we... 

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... 

One of the important concepts in cosmology is that all of the observations is done on the past light-cone. In general, cosmological linear perturbation theory is done in a particular gauge and often on spacelike hyperplanes. Then in order to link quantities calculated in perturbation theory with observables, we have to take into account effects coming from projecting these quantities onto past light-cone. We aim in this thesis to study these effects and to calculate corrections over observable quantities such as, two point correlation function and power spectrum of matter perturbations in cosmos. For a particular observational example, in the end we study clustering of structures in... 

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... 

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... 

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... 

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... 

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... 

We investigate the possibility of exoplanet detection orbiting source stars in microlensing events through the Spitzer and WFIRST follow-up observations.We perform a Monto Carlo simulation on the detection rate of exoplanets via microlensing, assuming that each source star has at least one exoplanet. The exoplanet can reflect part of the light from the parent star or emit internal thermal radiation. In this new detection channel, we use microlensing as an amplifier to magnify the reflection light from the planet.In the literature, this mode of detecting exoplanets has been investigated much less than the usual mode in which the exoplanets are considered as one companion in binary lens... 

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 increasing acceleration of the universe is one of the most challenging problems that cosmology has ever faced. Several models are presented to explain this increasing acceleration and one of the most famous models is “Dark Energy”. According to this model, all of the universe is filled by a fluid with a negative pressure and the equation of state of . Despite all of the achievements in explaining the observations, this model has very important problems in its basis. As a result, recently some other models have been considered to explain this increasing acceleration, including f(R) generalized gravity. In this final thesis, f(R) generalized gravity within Palatini formalism by a Yukawa... 

In this thesis, we have reviewed some of the theories which describe distribution of astriods in the solar system. The secular mechanism has been considered to the lowest order of inclination and eccentricity. In addition, the Kozai mechanism, which influences small bodies on highly inclined orbits, has been investigated. With some simulations, we have shown that distribution of astroids is mainly a dynamical phenomenon. Our simulations in the case of Hila group astroids, gave similar results like the astronomical data. Finally we investigated distribution of Neptune's Trojans, and realized that theories which are useful in analyzing distribution of astroids, can be of great help in... 

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...