Sharif Digital Repository

Interactions between the members of an imaginary universe, where all the members are adaptive and have learning capability, is simulated numerically by using artificial neural networks. The universe is called λ-universe for brevity. It is shown here that in such a universe, rules governing the behavior of the members might be formed inside the universe by its own members and the randomness which is observed in the behavior of its members is a direct result of the learning capability of the members. Although this is not a simulation of the real universe, some fundamental concepts of astrophysics have been implemented in it  

In this paper, we study a particular deformation of the Jackiw-Teitelboim gravity recently considered by Maxfield, Turiaci, and independently by Witten. We will compute the partition function of this model as well as its higher order correlators to all orders in genus expansion in the low temperature limit for small perturbations. In this limit, the results match with those obtained from the Airy limit of a Hermitian random matrix ensemble. Using this result, we will also study the free energy of the model. One observes that although the annealed free energy has pathological behaviors, under certain assumptions, the quenched free energy evaluated by replica trick exhibits the desired... 

We present N-body simulations of unstable spiral modes in a dynamically cool collisionless disc. We show that spiral modes grow in a thin collisionless disk in accordance with the analytical perturbation theory. We use the particle-mesh code SUPERBOX with nested grids to follow the evolution of unstable spirals that emerge from an unstable equilibrium state. We use a large number of particles (up to N = 40 & 106) and high-resolution spatial grids in our simulations (1283 cells). These allow us to trace the dynamics of the unstable spiral modes until their wave amplitudes are saturated due to nonlinear effects. In general, the results of our simulations are in agreement with the analytical... 

We revisit the question of what mechanism is responsible for the spins of halos of dark matter. The answer to this question is of high importance for modeling galaxy intrinsic alignment, which can potentially contaminate current and future lensing data. In particular, we show that when the dark matter halos pass nearly each other in dense environments - namely halo assemblies - they swing and spin each other via exerting mutual tidal torques. We show that this has a significant contribution to the spin of dark matter halos comparable to that calculated by the so-called tidal torque theory. We use the results of the state-of-the-art simulation of Illutris to check the prediction of this... 

Various models for the Galactic distribution of massive compact halo objects (MACHOs) have been proposed for the interpretation of microlensing toward the Large Magellanic Cloud (LMC). A direct way to fit the best model is by measuring the lens parameters, which can be obtained by measuring the Einstein crossing time and the parallax effect on the microlensing light curve and by astrometry of centroids of images. In this work, the theoretical distribution of these parameters is obtained for the various power-law Galactic dark halo models and MACHO mass functions (MF). For self-lensing as one of the models for the interpretation of LMC events, the maximum shift of image centroids and the... 

Ultra-high-energy (E > 100 TeV) Extensive Air Showers (EASs) have been monitored for a period of five years (1997-2003), using a small array of scintillator detectors in Tehran, Iran. The data have been analyzed taking into account the dependence of source counts on zenith angle. During a calendar year different sources come into the field of view of the detector at varying zenith angles. Because of varying thickness of the overlaying atmosphere, the shower count rate is extremely dependent on zenith angle, which has been carefully analyzed over time (Bahmanabadi et al. 2002, Exp. Astron., 13, 39). High energy gamma-ray sources from the EGRET third catalogue where observed and the data were... 

In this study, control strategies based on time-varying LQR, Lyapunov-Floquet transformation (LFT), backstepping, feedback linearization, and constant gain feedback control are implemented for the linearized time periodic equations of spacecraft relative motion when the reference orbit is elliptic. Also, natural and nonnatural leader-follower two-spacecraft formations are studied. The stability of the closed-loop response, the control effort required, and the settling time are investigated and compared for all control strategies. Furthermore, using constant gain feedback, the estimated region of attraction of the closed-loop system is obtained analytically  

Aims. Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected at a longer time scale when the light of remote stars crosses an interstellar molecular cloud, but it has never been observed at optical wavelength. In a favorable case, the light of a background star can be subject to stochastic fluctuations on the order of a few percent at a characteristic time scale of a few minutes. Our ultimate aim is to discover or exclude these scintillation effects to estimate the contribution of molecular hydrogen to the Galactic baryonic hidden mass. This feasibility study is a pathfinder toward an observational strategy to search for scintillation, probing... 

This paper presents a novel and fast homography-based tracking method to segment and track the instructor placed in front of a class board using two synchronized views. In the proposed method we estimate the instructor as a planar object and track its planar parameters. At first, the homography of board and fundamental geometry of two views are extracted using six point correspondences. The homography transformation matrix of object is calculated by obtaining three point correspondences in parallax region introduced with homography of the board. The parameters to be tracked by Kalman filter are obtained frame by frame by object's homography decomposition. Our proposed method is robust...