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... 

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... 

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... 

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 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... 

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... 

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 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... 

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.... 

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... 

considering the cosmic web's components as the environment of structure formation or considering them as structures, studying the cosmic web has advantages. But because of the non-linear nature of the cosmic web, extracting information from it has some challenges. The main challenge is that there are several ways of quantifying the cosmic web, in which there is no agreement between them.In this work, by introducing the excursion set of saddle points, we try to find a way for modelling cosmic filaments. By this model, which is an extension of the excursion of peaks, we calculate the critical density contrast of filaments with the help of the ellipsoidal collapse model and then we calculate... 

The inflation paradigm solves the so-called flatness problem of the standard cosmo- logical model and explains the homogeneity of the Universe by adding a period of exponential expansion at early times to the standard scenario. Moreover, it provides a mechanism for generating density perturbations, serving as the seed of large-scale structures. One of the successes of inflation is predicting primordial scale-invariant perturbations that agree with the observations of cosmic background radiation. Nev- ertheless, there will be a cold, matter-free universe after exponential expansion, so one needs a process that leads to a hot cosmos containing standard particles after the end of inflation, the... 

The theory of nonlocal gravity is a classical modification of Einstein’s general relativity, that explores nonlocal effects of gravity by putting aside the locality postulate, or the assumption that the non-inertial observer is equivalent to an inertial observer with the same speed at each instant of time. In this research we explore the local limit of this theory, or in other words, we consider nonlocality with only a short-term memory, with the goal of calculating the effects of nonlocality on cosmology in mind. We calculate these effects that are formulated inspired by the effects of materials in electrodynamics, on a homogenous universe in the local limit. We also reproduce the theory of... 

The Standard Model ΛCDM which includes cold dark matter and cosmological constant components provides a good fit to a large span of cosmological data and large scale structures. Recent results of local measurements of Hubble constant rely on observations of type 1A supernovae have shown 4σ tension comparing H0 to the value inferred from cosmic microwave background observations assuming ΛCDM. The Hubble parameter appears in the cosmic microwave background as one of the standard cosmological model parameters. If these observations are not the result of statistical or systematic error, They could represent new physics beyond the standard ΛCDM cosmological model. The purpose of this thesis is to... 

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... 

Not all effective theories in the low energy limit are compatible with quantum gravity. The goal of the swampland program is bringing the necessary criteria to test these effective theories and examine their compatibility with quantum gravity. This set of conjectures determines which theory is effective in the perspective of the self-consistent theory of quantum gravity which is actually string theory. A very important connection (through the swamp program) between high energy physics (UV) And the physics of the low energy limit (IR) can provide very important explanations for our questions. Among them are the cosmological constant problem in general relativity and the hierarchy problem in... 

n this thesis, the formation of cosmic structures in the presence of fuzzy dark matter (FDM) is investigated using the Gevolution code. In the FDM model, dark matter is composed of ultralight bosons. In fact, the mentioned bosons are the remnants of a scalar field present in the early universe. Simulations of structure formation in the presence of FDM that have been carried out so far (for example with Gadget and RAMSES) are based on Newtonian gravity, which have a number of limitations. The Gevolution code is an N-Body simulation that, unlike other existing simulations, is based on General Relativity, which can lead to interesting results. In fact, in this simulation, the weak field limit... 

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  

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...