Sharif Digital Repository

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... 

Cosmology is a knowledge that deals with the matter of creation of the world, its current structure and its end. This scientific field that was a subject discussed by the scientific, philosophical and religious groups has nowadays taken the attention of many scientists and researchers. The philosophers of science, also, are not exemptions from theses groups and cosmology, as a topic common between philosophy and science, is one of the points that are under discussion in the field of philosophy of science. On the other hand, the religions, also, have noticed this issue. Koran that is the most complete divine book sent from God, has taken account of the creation and the Creator as the axis of... 

The cosmological observations, like the luminosity distance of type Ia Supernovas, Cosmic Microwave Background radiation (CMB), Baryon Acoustic Oscillations (BAO) and Large Scale Structure (LSS) observations indicate that the Universe is nearly flat and also it is in a phase of acceleration. In the context of standard cosmological models, it means that the Universe is made of an unknown component (Dark Energy), which approximately has 70% of total energy-momentum of the Universe. The Cosmological Constant(CC) in the paradigm of ΛCDM model is the easiest model to describe the acceleration of the Universe. On the other hand the fine tuning (CC problem) and coincidence problems open new... 

For more than a century, physicists have recognized the contrast between the asymmetry of thermodynamic phenomena and T-symmetry of the fundamental laws, and they have referred to it as the arrow of time. In this thesis we tried to give an overview of the past and present status of this problem, using various theories. It is shown that the problem becomes a question about the boundary conditions of thermodynamic systems, and then a question about cosmology, and finally it becomes the problem of initial conditions of universe. An analysis of initial condition theories is presented for knowing how each of them tries to solve the problem of arrow of time. Different views about the relationship... 

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... 

Multiverse scenario has been recently addressed by physicists and they have found it useful in many areas of physics. In this dissertation, I have treated some issues about multiverse such as the kinds of multiverses, the arguments for or against multiverse, areas in which multiverse is introduced, etc. This dissertation has six chapters and an appendix. In the chapter one, some classifications of multiverse is introduced, and also some definitions of the concept of a world is presented. Then, my classification and definition of a multiverse and a world is introduced. In the chapter two, the concept of possible worlds in philosophy and its relationship with multiverse hypothesis is examined.... 

The idea of "vacuum energy" with definition of the "Aether" has many historical backgrounds from Aristotelian philosophy to 19th century and the advent of quantum vacuum. Some assert vacuum which is defined as "pure void" and others have got contrary viewpoints. It can be said that the history of vacuum is as old as history of physics. With the advent of quantum mechanics, vacuum has taken a specific definition. Vacuum from vision of quantum mechanics describes that vacuum is the minimum possible energy state, because if the energy of "quantum vacuum state" became zero, Heisenberg uncertainty principle would be violated. The important consequence of quantum mechanics will be in apparent... 

This thesis is divided into two major parts. In the first half of the thesis we introduce and study primordial non-Gaussianity in the context of Inflationary Cosmology. Most recent cosmological observations indicate that Universe has experienced an inflationary phase during its very early moments of existence. Not only Inflation is a possible solution of FRW cosmology problems, but also it acts as the generator of all the structure in the Universe. According to standard Inflationary cosmology, the primordial fluctuations which are considered to be the seeds of the entire structure in the Universe are scale-invariant, adiabatic and Gaussian.However there is the possibility to have an... 

One of the most important studies in cosmology is the study of Large Scale Structure of cosmos, where one of the useful and valuable tools is Excursion Set Theory(EST). EST or Extended Press-Schechter is a theory based on stochastic processes which gives us the distribution function(mass function) of dark matter halos. In order to having smoothed density feld Excursion Set Theory uses a specifc window function(flter) called sharp k-space, which is a top-hat in Fourier space. If one use this specifc window function the process of density feld with respect to variance will be a Markov process, with any other window function the process won’t be a Markov process. Excursion Set Theory using the... 

Whilst often phrased in terms of accelerating expansion of the Universe and discovering the nature of the dark energy, maybe the first thing that we can think about, is measuring distances or the geometric observations. The Baryon Acoustic Oscillations (BAO) are the remaining relic of the baryon and photon decoupling epoch. These oscillations offer a powerful method of measuring cosmological distances as a standard ruler. In this thesis we first review the theoretical, observational and statistical aspects of the BAO as a statistical standard ruler and examine the impact of BAO that will have on our understanding of dark energy. Then we calculate this scale by LRGs data and constraints on... 

The distance measurement is the corner stone of the modern cosmology. They are measured via ”Doppler Effect”, accordingly the distances in cosmology are obtained in red-shift space. Despite the cosmological background expansion, the peculiar velocities due to the local density contrast distort these measurements. One of the main challenges in upcoming cosmological studies is to distinguish the peculiar velocity effect from the background expansion.Supernova Type Ia as standard candles can be think of a crucial observation to address the above problem. In this thesis, we study the effect of cosmological perturbations on distance-redshift relation of the SNIa. The corrective effects are from weak... 

We know the matter can curve the space-time according to the general theory of relativity. On the other hand Quantum Field Theory has been formulated in a flat space-time and we cannot use most of its techniques within a curved space-time. Formulating the Quantum Field Theory in curved space-time is important for various subjects in physics like early universe or black hole physics. We will review the formulating QFT in cosmological background and will look for observable consequences on Cosmological Microwave Background in this thesis  

In the second half of the 20th century, the standard cosmology was encountered some problems like flatness problem and horizon problem. The inflation theory proposed by Alan Guth in the 1980s, not only solved these problems but also predicted some primordial fulctuations for the universe that as a source for the fluctuations of cosmic microwave background, could explain our observations.The fluctuations predicted by simple inflationary models, are Gaussain. Although we have not found a valid observational result suggesting Non-Gaussianity in the primordial fluctuations so far, but these simple single scalar field inflationary models, are very specific. So it is very probable that in the... 

The standard model of cosmology with nearly Gaussian, isotropic, scale invariant and adiabatic initial conditions describes the cosmological observations. However, the study of any deviation from the mentioned conditions will open up a new window to the physics of early universe. In this work we study the effect of the oscillatory and step in potential inflationary models in late time large scale structure observations. Almost we study the matter power spectrum, number density of the structures, the halo bias an the bulk flow in mentioned models. We assert that inflationary models with step functions which deviates from the standard model in small scales l ≤ 1Mpc can be confirmed by Large... 

The observations of the cosmic background radiation and large-scale structures show that the standard cosmological model is in good agreement with observational data. The standard model is based on the cosmological constant, dark matter, and Gaussian, nearly scale-invariant, adiabatic, and isotropic initial conditions. Although this model has been phenomenologically successful, but the problems of dark energy, dark matter, missing baryons and early universe remain unresolved. One of the most important theoretical questions in cosmology is whether the cosmological constant is the reason for accelerated cosmic expansion. In other words, is Einsteinian gravity the correct theory on the cosmic... 

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... 

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... 

Observations of cosmic microwave background (CMB) and large scale structure show that standard model of cosmology ΛCDM is consistant with observational data. The standard model is defined by cold dark matter, cosmological constant and also Gaussian inital conditions which are almost scale-invariant, adiabatic and isotropic.while this model is consistant with observational data, there exist a couple of problems namely the problems of dark energy, dark matter, missing baryon and early universe. One of the most recent methods to analyse cosmological models which is tightly ascociated with structure formation is the use of galaxy clusters distribution function and the effect of such structures... 

Cosmic Microwave Background (CMB) and large scale structure observations are in good agreement with standard model of cosmology know as Lambda CDM. Standard model is based on cold dark matter and Cosmological constant. It also assumes that the cosmic initial conditions are Gaussian, nearly scale invariant, adiabatic and isotropic. Although this model is in good agreement with observational data, there are some important unsolved problems such as dark energy, dark matter and Missing Baryon Problem (MBP). Also there are some observational anomalies such as dipole and quadrapole alignment and hemispherical asymmetry in CMB fluctuations. Distribution of baryons in the universe and particularly... 

The statistics of Dark matter halos and their relation to luminous matter is one of the most important observational and theoretical tools in putting constraint on Cosmological models.There are two approaches for studying the statistics of dark matter halos in the context of non-linear structure formation: Excursion set theory (density contrast- variance space), and Peak theory (density contrast- position space). The goal of this thesis is studying the combination of these two approaches, which is known as Excursion Set of Peaks. We also address the ”cloud in cloud” problem, proposing an approximation for up-crossing problem. Then,we will address the bias parameter for matter-halo in the...