Sharif Digital Repository

The standard model of cosmology with Cold Dark Matter (CDM) and cosmological constant ΛCDM, Despite its achievement and success, confronts some important open questions and challenges such as Dark Matter (DM) nature, small scale structure challenge, $H_0$ tension and, etc. In this dissertation, we study the formation and growth of Large Scale Structure (LSS) as one of the most important cosmological observables. Then we propose some solutions to these questions and challenges. We use the Excursion Set Theory (EST) to study the LSS and calculate the LSS observable such as first up-crossing of trajectories from density contrast barrier, the number density of DM halos, conditional number... 

The importance of using a fast and automatic method to classify variable stars for large amounts of data is undeniable. There have been many attempts for classifying variable stars by traditional algorithms, which require long pre-processing time. In recent years, neural networks as classifiers have come to notice. This thesis proposes the Hierarchical Classification technique, which contains several models with the same network structure. Our pre-processing method produces input data by using light curves and the period. We use OGLE-IV variable stars database to train and test the performance of Convolutional Neural Networks based on the Hierarchical Classification technique. We see that... 

The luminosity gap between the two brightest galaxies of the galaxy groups and clusters is a statistics criteria for studying the time of the formation of these gravitational systems. So far, the luminosity gap of galaxy systems is studied in local universe (z~0). This parameter is a good criteria to understand the structure formation models. In this project we investigate the evolution of luminosity gap statistic in high redshift universe (z~0.4). The evolution of luminosity with redshift is a proper tool for investigating the evolution of galaxies in different environments during the time  

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... 

The number of discovered extrasolar planetary systems has increased immensely during the past 15 years. The various outcomes of planetary formation are a result of the complicated interaction between gravitational, hydrodynamical and accretion processes a considerable amount of which is not yet understood. Thus far the effect of a planet embedded in a protoplanetary disk has been studied both analytically and using numerical simulations. These suggest that under suitable circumstances tidal effects result in the formation of a gap in the disk about the orbit of the planet. This result has been particularly used to detect massive extrasolar planets (massive enough to form gaps).In this study... 

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... 

Fossil galaxy groups, famed to have undisturbed inter-galactic gas due to lack of recent group scale mergers, are energetically and morphologically ideal environments to study the IGM heating. In this article, we study the role of active galactic nuclei in heating the IGM in a sample of five fossil galaxy groups by employing their GMRT observations at 610 MHz and 1.4 GHz. We find that only two of the dominant galaxies in fossil groups are associated with the radio lobes. We evaluate the PdV work of the radio lobes and their corresponding heating power and compare to the X-ray emission loss within cooling radius. Our results show that the power due to mechanical heating is not sufficiently... 

Dark matter is one of the biggest unsolved problems in physics and cosmology. Almost 85 percent of the gravitational matter is not luminous and it’s properties is unknown. In recent decades variety of investigations regarding dark matter has been done which we will mention in the beginning part of this thesis. We continue testing MOdified gravity (MOG), Non-Local-Gravity (NLG) and MOdified Newtonian Dynamics (MOND) in a different scale by using the rotation curves of dwarf galaxies. In addition we compare MOG, MOND and NLG together to see which one better fit observed data. In another investigation that is reported in this thesis, we fit the observed acceleration data of galaxy cluster Abell... 

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 every theory with local symmetries, observales are mainly the gauge invariant quantities and fields are defined up to a local transformation.By choosing boundary conditions on field configurations and on the generating functions of gauge transformations, we can associate conserved charges to a particular subset of the generating functions. Thus field combinations which are related by these particular gauge transformations are physically distinct. These set of conserved charges usually form infinite-dimensional algebras such as Virasoro or BMS algebra. In addition, sets of all combinations of the associated fields form a phase space with a symplectic structure induced form the action of...