Sharif Digital Repository

Shell galaxies are understood to form through the collision of a dwarf galaxy with an elliptical galaxy. Shell structures and kinematics have been noted to be independent tools to measure the gravitational potential of the shell galaxies. We compare theoretically the formation of shells in Type I shell galaxies in different gravity theories in this work because this is so far missing in the literature. We include Newtonian plus dark halo gravity, and two non-Newtonian gravity models, MOG and MOND, in identical initial systems. We investigate the effect of dynamical friction, which by slowing down the dwarf galaxy in the dark halo models limits the range of shell radii to low values. Under... 

We study the minimally and non-minimally coupled scalar field models as possible alternatives for dark energy (DE), the mysterious energy component that is driving the accelerated expansion of the universe. After discussing the dynamics at both the background and perturbation level, we confront the two models with the latest cosmological data. After obtaining updated constraints on their parameters, we perform model selection using the basic information criteria. We found that the ACDM model is strongly favoured when the local determination of the Hubble constant is not considered and that this statement is weakened once local H0 is included in the analysis. We calculate the parameter... 

In this work, we investigate the MOdified Gravity (MOG) theory for dynamics of the Universe and compare the results with the Lambda cold dark matter (ΛCDM) cosmology. We study the background cosmological properties of the MOG model and structure formation at the linear perturbation level. We compare the two models with the currently available cosmological data by using statistical Bayesian analyses. After obtaining updated constraints on the free parameters, we use some methods of model selection to assist in choosing the more consistent model such as the reduced chi-squared (χ 2red) and a number of the basic information criteria such as the Akaike Information Criterion (AIC), the Bayes... 

As an alternative to dark matter models, Modified Gravity (MOG) theory is a covariant modification of Einstein gravity. The theory introduces two additional scalar fields and one vector field. The aim is to explain the dynamics of astronomical systems based only on their baryonic matter. The effect of the vector field in the theory resembles a Lorentz force where each particle has a charge proportional to its inertial mass. The weak field approximation of MOG is derived by perturbing the metric and the fields around Minkowski space-time. We obtain an effective gravitational potential which yields the Newtonian attractive force plus a repulsive Yukawa force. This potential, in addition to the... 

The excursion set approach is a framework for estimating how the number density of nonlinear structures in the cosmic web depends on the expansion history of the universe and the nature of gravity. A key part of the approach is the estimation of the first-crossing distribution of a suitably chosen barrier by random walks having correlated steps: The shape of the barrier is determined by the physics of non-linear collapse, and the correlations between steps by the nature of the initial density fluctuation field. We describe analytic and numerical methods for calculating such first up-crossing distributions. While the exact solution can be written formally as an infinite series, we show how to... 

We study the possibility of magnetic mass detection using the gravitational microlensing technique. Recently, the theoretical effect of magnetic mass in NUT space on the microlensing light curve has been studied. It has been shown that in the low photometric signal-to-noise ratio and sampling rate of MACHO experiment light curves, no signature of the NUT factor has been found. In order to increase the sensitivity of magnetic mass detection, we propose a systematic search for microlensing events, using the currently running alert systems and complementary telescopes for monitoring Large Magellanic Clouds stars. This observation strategy provides the lowest observable limit of the NUT factor,... 

We study the theoretical signature of magnetic masses on the light curve of the gravitational microlensing effect in NUT space. The light curves for microlensing events in NUT space are presented and contrasted with those caused by lensing produced by normal matter. In the next step, associating the magnetic mass to the massive astrophysical compact halo objects (MACHOs), we try to see the effect on the light curves of microlensing candidates observed by the MACHO group. The presence or absence of this feature in the observed microlensing events can shed light on the question of the existence of magnetic masses in the Universe  

Modeling the structure formation in the universe, we extend the spherical-collapse model in the context of modified Newtonian dynamics (MOND) starting with the linear Newtonian structure formation followed by the MONDian evolution. In MOND, the formation of structures speeds up without a need for dark matter. Starting with the top-hat overdense distribution of the matter, the structures virialize with a power-law profile of the distribution of matter. We show that the virialization process takes place gradually from the center of the structure to the outer layers. In this scenario, the smaller structures enter the MONDian regime earlier and evolve faster, hence they are older than larger...