Sharif Digital Repository

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... 

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... 

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 standard model of Cosmology (ΛCDM, based on the cosmological constant and cold dark matter) is the simplest model that many of its predictions match with observations. Large scale observations are one of the most important observational data in Cosmology. This type of observation reveals that the cosmos have a web-like structure at large scales called "Cosmic web". This web consists of dense regions full of galaxies (Knot), long filamentary structures (Filament), flat sheets (Sheet), and low-density vast regions (Void). The cosmic web is the most prominent representative of the distribution of galaxies and dark matter in large scales and studying it can provide cosmologists with valuable... 

The Standard Model ΛCDM is the simplest model that fits well with cosmic microwave background radiation observations and large-scale structures. In this model, dark matter halos form from the growth of density fluctuations and their collapse. The galaxies we see today reside in these halos and the relationship between the spatial distribution of the galaxies and the matter known as bias is important for relating observational data to theoretical models. Many methods have also been proposed to fully describe the distribution of galaxies and their clustering, but all of these methods are based on two-point functions. In this thesis, we study the spatial distribution and clustering of dark... 

The existence of primordial black holes has been proposed for about half a century, but so far we have not been able to have convincing observations to prove the existence of such black holes. With the discovery of gravitational waves, hopes for finding primordial black holes have increased again; So that in recent years, different ways have been proposed to reveal them. This matter becomes even more important when we know that these black holes can explain all or at least part of the missing matter of the universe known as dark matter. One of the proposed ways is to investigate the 21 cm line of hydrogen in the intergalactic medium. Over time, the temperature of the intergalactic medium is... 

Fuzzy dark matter ( FDM ) or wave dark matter is an alternative theory designed to solve small-scale problems faced by the standard cold dark matter, such as the missing satellites problem, core cusp problem, and too big to fail problem. FDM is made up of ultra-light ax- ions having mass 10−22eV that typically have de Broglie wavelength of several kpc, so in small-scales behave like a wave. For this thesis, I aim to explore how the sub-halo structure and its features change with FDM particles masses which may be able to solve small-scale problems. We derived the halo mass function for FDM by solving the excursion set problem with a mass-dependent barrier func- tion; then, we compared 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... 

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... 

The standard cosmological model has been successful in describing many observations from widely different epochs of the Universe, from primordial nucleosynthesis to the accelerat- ing expansion of the present day. With the improvement of the number and the accuracy of observations, some inconsistencies among key cosmological parameters of the model have emerged. Many alternative models are proposed in order to alleviate these tensions. Pecu- liar velocities, the deviation from the regular Hubble flow of the galaxies, are an excellent probe of large-scale structure universe. In a ΛCDM universe, peculiar velocities can be mea- sured using linear perturbation theory, which in known as bulkflow.... 

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 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... 

The purpose of this research is to provide a model to explain the dependence of the galaxy distribution within voids on redshift and the density of the environment. The study of the formation of large-scale structures in the universe, as one of the most important observables in cosmology, is of great importance. Investigating the formation of these structures is possible by going beyond the linear regime and using theories such as the Press-Schechter and the excursion set. Also, by developing this type of approach, theories have been made possible to study the statistics of halos in environments such as voids, which can be useful for a better understanding of the formation and evolution of... 

When we talk about the large-scale structure of the cosmos and the cosmic web, one of the important issues is to examine the location of the structures formed. We use statistical characteristics to check the cosmic web. Nearest neighbor distribution, two-point correlation, count in cell, etc. are some of the parameters that help us in this way. Our main problem in this project is to examine these statistical parameters in large scale structure for two different time periods. First, in the time period near the present age(z ≈ 0), we want to check these parameters for the dark matter halos that have formed the structure, and secondly, for the initial conditions and transition to high... 

One of the biggest challenges facing cosmologists is finding the nature of dark matter. The cosmological observations, like the Rotation Curve, Gravitational Weak Lensing and Cosmic Microwave Background radiation indicate that baryonic matter alone cannot explain the density of matter in the universe. In the context of standard cosmological models, it means that the universe is made of an unknown component(Dark Matter), which aprroximately has 27% of total energy-momentum of the universe. The Modify Gravity and General Relativity are alternative theories that introduced to describe this observation. In This thesis, we introduce the issue of the measurements of accelerated observers in... 

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... 

The standard model of Cosmology known as CDM is established as a cornerstone of modern cosmology. The Cosmic Microwave Background radiation (CMB) and Large Scale Structure (LSS) Observations indicate that almost all observations can be described by a 6-parameter standard model with an initial condition of almost Gaussian, isotropic, scale invariant and adiabatic which can be sourced by inflationary models. Recent observations of Planck collaboration shows almost no deviation from standard picture of single field slow roll inflation. Despite the success of the standard model, the physics of early universe, the nature of dark matter and dark energy is still unknown. The large scale structure... 

Today with respect to the particle physics experiments and astrophysical observations we know that Neutrinos are massive. Free streaming of light Neutrinos supresses the density field of matter and opposes gravitational growth, so that clusters formed at fixed mass are fewer and hence more biased than for a pure CDM density field. One way to study the effects of Neutrinos on the formation of large-scale structures is to examine the matter power-spectrum and corellation function in the presence of massive Neutrinos.In the presence of massive Neutrinos, the matter power-spectrum is suppressed in small scales and also correlation function and clustering bias are changed in comparison to the CDM... 

In the standard cold dark matter (CDM) family of cosmological models,the cosmological structures form hierarchically with the assumption of cold dark matter existence. The structure formation in the current CDM paradigm is capable of estimating the distribution function of the structures. It gives us a correct view from our observations of galaxies and structures. One of the successful methods to explain the statistics of cosmlogoical structures and their formation time is Excursion Set Theory. The excursion set theory considers the density field for each point in the universe smoothed with a spatial window function using the physics of random processes. The statistics of the structures can... 

In this thesis, we have investigated the light scalar field models for describing the dark sector(Dark Matter and Dark Energy) of the universe in cosmology. As a result of tiny mass, the de Broglie wavelength becomes large, and consequently, the quantum mechanical behavior is essential in large scales. Our main attempt is to investigate a model of scalar fields consisting of particles with a mass range of ($10^{-21}-10^{-23}$)eV, often called Fuzzy Dark Matter (FDM). In the FDM hypothesis, the dark matter halo consists of a solitonic core that formed Bose-Einstein-Condensation BEC at the halo's inner region and non-condensed dark matter particles at the outer region. We took into account...