Simulating Large Scale Structure For Models Of Cosmic Acceleration

Simulating Large Scale Structure for Models of Cosmic Acceleration explores alternative cosmological models and how we can learn from these as well as differentiate them from the standard cosmic model. The book also looks at the ways in which techniques can be used to accurately develop and test the model to produce new observations. This self-contained book provides a fundamental guide to researchers looking to enter the field of cosmological simulations. Postgraduate students will also find it of use as the need for numerical simulations and astronomical surveys increases. The book contains a significant amount of Professor Li's own research as well as assistance from other experts and collaborators in the field and will certainly encourage others to explore the ever-expanding world of cosmic acceleration.

Line intensity mapping (LIM) is an observational technique that probes the large-scale structure of the Universe by collecting light from a wide field of the sky. This book demonstrates a novel analysis method for LIM using machine learning (ML) technologies. The author develops a conditional generative adversarial network that separates designated emission signals from sources at different epochs. It thus provides, for the first time, an efficient way to extract signals from LIM data with foreground noise. The method is complementary to conventional statistical methods such as cross-correlation analysis. When applied to three-dimensional LIM data with wavelength information, high reproducibility is achieved under realistic conditions. The book further investigates how the trained machine extracts the signals, and discusses the limitation of the ML methods. Lastly an application of the LIM data to a study of cosmic reionization is presented. This book benefits students and researchers who are interested in using machine learning to multi-dimensional data not only in astronomy but also in general applications.

This book contains a series of lectures given at the NATO Advanced Study Institute (ASI) "Structure Formation in the Universe", held at the Isaac Newton Institute in Cambridge in August, 1999. The ASI was held at a critical juncture in the development of physical cosmology, when a flood of new data concerning the large scale structure of the Universe was just be coming available. There was an air of excitement and anticipation: would the standard theories fit the data, or would new ideas and models be re quired? Cosmology has long been a field of common interest between East and West, with many seminal contributions made by scientists working in the former Soviet Union and Eastern bloc. A major aim of the ASI was to bring together scientists from across the world to discuss exciting recent developments and strengthen links. However, a few months before the meeting it appeared that it might have to be cancelled. The war in the former Yugoslavia escalated and NATO began a protracted bombing cam paign against targets in Kosovo and Serbia. Many scientists felt uneasy about participating in a NATO-funded meeting in this situation. After a great deal of discussion, it was agreed that the developing East West conflict only heightened the need for further communication and that the school should go ahead as planned, but with a special session devoted to discussion of the legitimacy of NATO's actions.

In this work we use the N-body resimulation technique to address aspects of structure formation. In the first chapter we study the influence of the local environment of DM haloes on their properties. In the second chapter we address the so-called "substructure problem" which is one of the major challenges of the CDM model of cosmology. We perform ultra-high resolution simulations of the assembly of a Milky Way type dark matter halo within its full cosmological context and propose a new analytical fitting formula (SWTS) which provides a better fit to the simulated Milky Way halo than the NFW or Moore profiles do. In the third chapter we use our ultra-high resolution simulations to study the possible -ray signal from dark matter annihilation. If such a signal was detected, the nature of the dark matter, the answer to one of the most important questions of modern cosmology, would be known. (urn: nbn: de: bvb:19-16446)"

In the last century, new observational techniques and discoveries such as the Cosmic Microwave Background Radiation have brought a new dimension of knowledge about the Universe. Therefore new theories and models have been proposed to explain the observed Universe. Computer simulations are a very important tool because they lay a bridge between theory, often over-simpli ed, and observations, which reveal the complexity of our Universe. In this thesis, it is given a review of observations including the most important discoveries and results that help to describe the Universe and have been used to develop the models considered nowadays. The cosmological theory behind the large-scale structure formation is explained, from the basis of the Friedman model to the formation of structures through the linear, quasi-linear and non-linear regime, including the Zeldovich approximation and the spherical collapse model. Furthermore, the di erent types of codes used for cosmological simulations are introduced, focusing on the N-body codes and presenting the code used in this thesis, developed by Klypin & Holtzman (1997). The tools used to analyse the results: density plots, power spectrum and mass variance are described as well. Three main sets of simulations have been performed: a basic simulation (RUN0) with standard cosmological parameters, simulations of CDM and simulations of Hot+Cold Dark Matter (HCDM). All the simulations use 323 particles, while di erent cosmological parameters have been changed e.g. 8, m, and n. Thus, it is observed that higher values of m and low values of lead to more clustering and hence more developed structures. Moreover, the e ect of 8 appears to be critical, since it determines the amplitude of the density uctuations at the initial redshift of the simulation. When studying the presence of hot dark matter, the main di erence comes from the cut-o in the power spectrum due to the hot dark matter free-streaming, resulting in less developed structures. Similarly to the previous case, the e ects of the cosmological parameters are explained for this model. Finally, some additional simulations regarding dark halos populations and density pro- les are included in the Appendix.

The accelerating expansion of the universe is considered to be a well-established fact. However, a physical explanation of its origin is still missing. While the cosmological constant, [Lambda], is the favorite candidate, a multitude of other theories have been proposed. Rather than testing every theory against data, one can adapt phenomenological approaches aimed at testing [Lambda]. Adopting a model-independent approach to studying dark energy, we have investigated the utility of wavelets for constraining the redshift evolution of the dark energy equation of state from a combination of the type Ia supernovae, cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) data. We have shown that sharp deviations from w[subscript {Lambda}]=-1 can be detected efficiently. Applying this method to the "Constitution" SNe data, combined with the CMB data from WMAP and BAO data from SDSS, provided only weak hints of dark energy dynamics. Future weak lensing surveys will have the ability to measure the growth of large scale structure with accuracy sufficient for discriminating between different theories of dark energy and modified gravity. The growth of structure can be tested, in a model-independent way, by parametrizing the evolution equations of cosmological perturbations. At the linear level, this can be achieved by introducing two scale- and time-dependent functions (MG functions). We have consistently implemented the parametrized equations in the commonly used public codes, CAMB and CosmoMC, while preserving the covariant conservation of the energy-momentum. As a demonstration, we have obtained joint constraints on the neutrino mass and parameters of a scalar-tensor gravity model from the CMB, SNe and the correlation of CMB with large scale structure. We have performed a Principal Component Analysis (PCA) to find the eigenmodes and eigenvalues of the forecasted covariance matrix of the MG functions for surveys like DES and LSST. By examining the eigenmodes, we can learn about the scales and redshifts where the surveys are most sensitive to modification of the growth. We have considered the impact of some of the systematic effects expected in weak lensing surveys. We have demonstrated the utility of the PCA as an efficient way of storing information about the linear growth of perturbations.

In this thesis, we harness the power of modern scientific computing to explore the formation and evolution of cosmological structure in a wide variety of astrophysical scenarios. We explore the nonlinear dynamics associated with the interplay between cold dark matter (CDM), baryons, ionizing radiation, and cosmic neutrinos, within regimes where analytic calculations necessarily fail. We begin by providing an overview of structure formation and its connections to the fields of study considered here: the epoch of reionization, galactic substructure evolution, and cosmic neutrinos. We then present a rigorous numerical convergence study of cosmological hydrodynamics simulations post-possessed with radiative transfer to study the impact of small-scale absorption systems within the intergalactic medium (IGM) during the onset of reionization. We present converged statistics of the IGM on smaller scales and earlier times than previously considered. Moreover, we provide strict resolution limits for hydrodynamic simulations to properly resolve the unheated IGM. Next we study the infall and dynamical evolution of CDM halos in a galactic host. We find the behaviour of low-mass subhalos is qualitatively different than previously described for high-mass subhalos. In particular, the evolution of low-mass subhalos, with masses less than 0.1 per cent that of the host, is mainly driven by their concentration. This presents an opportunity to use concentration as a predictive indicator of substructure evolution. We finish this thesis with an investigation of a recently proposed method for constraining individual neutrino mass from cosmological observations. Such a detection depends on the ability to reconstruct the CDM-neutrino relative velocity, which we show can be accomplished using linear transformations of an observed galaxy field. Based on this, we perform the world's largest cosmological N-body simulation and present preliminary results for the observational prospects of cosmic neutrinos.