Sharif Digital Repository

Bonding gravity with Quantum mechanics has always been a challenging issue. So that, accomplishing a theory of quantum gravity is one of the most important research topics in High-Energy Physics.Information paradox in black holes is among such issues which connect quantum mechanics with gravity. Black holes as a natural solution of Einstein equations have a distinction, with other solutions, since there exists an event horizon that imparts space-time into two causally disconnected regions.According to Hawking’s calculations, black holes would evaporate over time by the thermal radiation. This resulting thermal radiation has less information than the initial collapsed matter in a pure state.... 

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 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... 

A variety of early universe models predict the formation of primordial black holes. Primordial black holes can be a candidate for a fraction or total dark matter in the universe. Moreover, mergers of these black holes can produce gravitational waves. After re-entering the horizon, having a sufficiently large amplitude, the initial fluctuations of cosmic matter can collapse to primordial black holes. This thesis studies an inflationary potential that can amplify the power spectrum of curvature perturbations up to several orders of magnitude via imposing some features on the potential. This power amplification on small scales can lead to the formation of primordial black holes. For simplicity,... 

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... 

The use of MANETs in many areas is rapidly spreading. Due to dynamic nature,and limited energy of this networks, they are vulnerable to some attacks including blackhole attack. Blackhole attack can disrupt the commiunication between hosts and cause denial of service. One of countermeasures to this attack which has been recently proposed is the ActiveTrust. ActiveTrust is a routing protocol which relys on malicious node detection using trust calculations; this scheme has been proposed for WSNs. In this work we adopt the ActiveTrust’s idea and try to apply it into MANETs. Then we simulate the resulting proposal in NS-3 and compare the results  

Isolated black holes in equilibrium are intrinsically simple objects: They are much simpler than ordinary matter around us because they can be described by only a few parameters such as their mass, angular momentum, and charge. This simplicity, however, is apparent only because one never has an isolated black hole. Black holes in centers of galaxies or intermediate-mass black holes always have complex distributions of matter around them, such as galactic nuclei, accretion disks, strong magnetic fields, other stars, planets, etc., and are therefore actively interacting with their surroundings. However, even having been removed from all macroscopic objects and fields in space, a black hole... 

Nitrogen-Vacancy (NV) consists of a substitutional nitrogen located at a lattice site adjacent to a carbon vacancy. It has attracted much attention due to its application in solid state quantum information processing. Its long coherence time and ability to be used in ambient temperature made it a major candidate for quantum bit (Qbit). Due to its importance, a lot of theoretical studies for the NV’s properties based on the DFT and other single particle approaches have been done, while many-body approaches are not expanded so. For the first time, a generalized Hubbard Hamiltonian was used as a many-body approach to study NV defects. The obtained wavefunctions and states result to good... 

In the era of precision cosmology it is desirable to know if the collapse of an overdensity region within a FRW cosmological model leads to a black hole similar to that of Schwarzschild. Even the simple question of an event horizon within a dynamical universe is not a trivial one. This subject of studying collapsing over-densities within an expanding universe is called ”cosmological black holes”. Our aim is to construct such a black hole in the simplest possible scenario and study its similarities and differences to the familiar Schwarzschild one in vacuum. Therefore questions of rotating or charged black holes are not dealt with in our work. We insist, however, that the solution is dynamic... 

In this thesis by studying the extreme physical conditions near black hole horizons and singularity, we attack intriguing problems in the exciting era of quantum black holes and gravitational wave astronomy. We considered the backreaction of the energy due to quantum fluctuations of the background fields for a Schwarzschild black hole, in semi-classical gravity. We have shown that it results in modification of the horizon and also formation of an inner horizon. The process of the collapse of a thin shell stops before formation of the singularity. After the collapse stops the reverse process takes place. We also demonstrate the existence of a cold remnant for the black hole. We generalized... 

The existance of Dark Matter (DM)has been proved by cosmological and astronomical observation. Yet, all the evidences are based on gravitational effects. It is plausible that DM has a gravitational nature. Any creature with a pure gravitational essence has a chance to bea DM candidate. Primordial Black holes are formed from the density perturbations that are induced by the quantum fluctuations of the vaccum during the inflationary expansion. It is probable that these creatures are the solution of DM problem. By holding the power spectrum that generates the abundance of PBH with the one resulting from observation based on the presumed inflationary model in common, considering the... 

There has been a growing interest in studying asymptotically AdS space times given the AdS/CFT conjecture. Among different models finding the one which decribes broader phenomena is important.The importance of a dynamical metric stems from the fact that one can see thermalization in its dual description. We introduce a dynamical metric and study apparent horizon formation and energy evolution as well. We plot its appparent horizion curve and show that it aproaches the static limit within our numerical accuracy. We then review quasi-normal modes of this space time  

One of the most important topics of interest to cosmologists and astrophysicists is the origin of supermassive black holes that form the center of most galaxies.In this research, we will introduce a mechanism for the growth of primordial black holes at the early universe, and we will argue that by this mechanism, with the assumptions that will be given in the relevant section, black holes of the order of 〖10〗^6 M_⊙ can be formed.Then assuming that there are supermassive black holes at the center of most galaxies, by calculating the number density of galaxies in comoving coordinate We will set the number density of galaxies in comoving coordinate and the number density of grown primordial... 

According to general theory of relativity, collapse of matter under gravitational force leads to the formation of black holes. Considering the quantum effect shows that black holes have thermal radiation which cause them to evaporate. After a complete evaporation, the state of radiation can not contain all information of initial matter. In other word, this process does not obey unitary evolution of quantum mechanics. This is called the black hole information paradox, which has been one of the most challenging problem of theoretical physics for several recent decades. Recently, a deep connections between asymptotic symmetries, soft graviton theorem and gravitational memory effect has been... 

We discuss and classify all four dimensional black hole solutions to Einstein-Maxwell-Lambda theory. These solutions contain a black hole region with an event horizon. The cosmological constant can be nonzero and the black hole may possess electric and magnetic charge. We classify solutions according to their asymptotic behavior. Three major parts are asymptotic-flat, asymptotic-de Sitter and asymptotic-Anti-de Sitter and in all of them we discuss Schwarzschild, Reissner-Nordstrom, Kerr and Kerr-Newman black holes separately. Black holes can also have NUT charge and acceleration parameter which make the solution singular in one direction. As a result, these solutions are asymptotically... 

In the most cosmological models which already have putted forth, metric has a spherical symmetry; however, according to the galaxy’s shape is not a real approximation. In order to build a more acqurate model, by applying a particular conformal transformation on the static vacuum solution, we introduce a new metric which is axially symmetric and also it approaches to FLRW metric at space infinity. The cosmological model according to this metric has especial features;The solution we have found represent a spacetime which is axially symmetric although it leads to a spherically symmetric Einstein tensor. Therefore, we have found a solution of Einstein equations representing a spherically... 

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.
 

We have tried this for the first time to reperesent a comprehensive description of the evolution of a perfect fluid in inhomogeneous relativistic models.We solve Einstein equations using numerical methods.In this procedure we can evaluate the effect of pressure in perfect fluid evolution.Also we look at the behavior of the various horizons, and in the end we will present a cosmological model to a black hole  

Nowadays, use of wireless networks and devices is inevitable due to their beneficial features, such as ease of use, portability, rapid and comfortable access to information. On the other hand, the security for wireless networks is a challenging matter by overwhelming increase over the number of mobile hardware and devices. Mobile Ad-hoc network (MANET) is a class of wireless networks, which forms dynamically by joining or leaving the nodes into/from the network without any fix infrastructure. This kind of network in comparison to other types of networks, is prone to various security threats or attacks due to its unique characteristics (dynamic topology, open medium, lack of central...