• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 792
  • 280
  • 194
  • 46
  • 29
  • 22
  • 18
  • 15
  • 8
  • 7
  • 6
  • 6
  • 6
  • 6
  • 6
  • Tagged with
  • 1668
  • 599
  • 465
  • 363
  • 275
  • 257
  • 257
  • 248
  • 242
  • 223
  • 218
  • 207
  • 205
  • 179
  • 172
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Average cosmic evolution in a lumpy universe

Duley, James January 2011 (has links)
The procedure of averaging and coarse-graining of the gravitational field equations with sources are investigated in both Newtonian gravity and in general relativity. In particular the schemes of Buchert and Korzyńnski are examined and compared in both situations. In Newtonian gravity it is shown how to calculate the tidal tensor given boundary conditions for it and how to average it given those boundary conditions. It is also shown that one can always choose boundary conditions to make the average tidal tensor vanish or take any value. The problems of coarse-graining tensors in general relativity are critically examined, and a set of relevant conditions for such a procedure are enumerated. Korzyńnski's covariant coarse-graining procedure is reviewed and applied to a particular case. For the case of the Lemaître-Tolman-Bondi model it is shown that the backreaction was always zero for a centred spherical coarse-graining domain. Wiltshire's timescape model, which applies a particular observational interpretation to Buchert's averaging scheme, is reviewed. The dust timescape model of Wiltshire is extended by the addition of a homogeneous radiation source. This model is solved numerically and it is shown not to vary significantly from the dust model since the redshift z ≈ 30, which is when the backreaction and radiation density are equal. The model is integrated back in time from the surface of last scattering with results indicating a breakdown in aspects of the model at early times.
2

New cosmological model

Shen, Po-Yu January 1970 (has links)
The present situation in cosmology is discussed. We try to explain the observed distribution of quasars in terms of an inhomogeneous model universe that consists of inner and outer Friedmann zones separated by a transition zone. The number - red-shift relation is derived, and numerical calculations are performed on the assumption that the transition zone is negligible. When the results are compared with observation, it is found that the dearth of quasars with red-shifts greater than 2 is easily explained, but that one cannot account for their anisotropic distribution. Modifications of the model are suggested. / Science, Faculty of / Physics and Astronomy, Department of / Graduate
3

Heating the Early Universe : Numerical Methods and Their Analysis

Lee, Kai Yan January 2016 (has links)
During the epoch when the first collapsed structures formed (6&lt;z&lt;50) our Universe went through an extended period of changes. Some of the radiation from the first stars and accreting black holes in those structures escaped and changed the state of the Intergalactic Medium (IGM). The era of this global phase change in which the state of the IGM was transformed from cold and neutral to warm and ionized, is called the Epoch of Reionization.In this thesis we focus on numerical methods to calculate the effects of this escaping radiation. We start by considering the performance of the cosmological radiative transfer code C2-Ray. We find that although this code efficiently and accurately solves for the changes in the ionized fractions, it can yield inaccurate results for the temperature changes. We introduce two new elements to improve the code. The first element, an adaptive time step algorithm, quickly determines an optimal time step by only considering the computational cells relevant for this determination. The second element, asynchronous evolution, allows different cells to evolve with different time steps. An important constituent of methods to calculate the effects of ionizing radiation is the transport of photons through the computational domain or ``ray-tracing''. We devise a novel ray tracing method called PYRAMID which uses a new geometry - the pyramidal geometry. This geometry shares properties with both the standard Cartesian and spherical geometries. This makes it on the one hand easy to use in conjunction with a Cartesian grid and on the other hand ideally suited to trace radiation from a radially emitting source. A time-dependent photoionization calculation not only requires tracing the path of photons but also solving the coupled set of photoionization and thermal equations. Several different solvers for these equations are in use in cosmological radiative transfer codes. We conduct a detailed and quantitative comparison of four different standard solvers in which we evaluate how their accuracy depends on the choice of the time step. This comparison shows that their performance can be characterized by two simple parameters and that the C2-Ray generally performs best. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Submitted.</p>
4

Probing primordial non-Gaussianity using large scale structure

Fantaye, Yabebal January 2009 (has links)
Includes abstract. / Includes bbliographical references (leaves 65-69). / Recent evidence from the WMAP satellite has lead to a tentative detection of non-Gaussianity. Using the bispectrum statistic applied to the MegaZ catalogue of over 600,000 luminous red galaxies we find new bounds on the large-scale nonGaussianity. We constrain the fNL parameter using a particular type of triangular configuration as well as the combination of all the possible triangles in harmonic space. The constraint on fNL from the combination of all possible triangular configurations is ffV'ial = 57 ± 52 with 68% confidence limit, which is consistent with vanishing primordial non-Gaussianity, although some triangular configurations on their own suggest a non-zero bispectrum which, if confirmed, would have a profound effect on modern cosmology.
5

Rendering dark energy void

February, Sean Phillip January 2009 (has links)
Includes abstract. / Includes bibliographical references (leaves 114-120). / The current model of cosmology, the Friedman-Lemaitre-Robertson-Valker model, assumes that the universe is approximately homogeneous and isotropic on very large scales. Further assuming flatness and dark energy in the form of Einstein's cosmological constant A then implies that the latter contributes roughly 73% of the total energy of the universe, cold dark matter (CD'I) 23SiC, and baryons, the matter we are made, only 4%.
6

Probing the new cosmology

Zentner, Andrew Ronald 06 August 2003 (has links)
No description available.
7

Inhomogeneous conformal cosmological models /

Campbell, Robert Alan. January 1985 (has links) (PDF)
Thesis (Ph. D.)--University of Adelaide, 1986. / Includes bibliographical references (leaves 152-156).
8

Flawed nature cosmology

Pederson, Steven Patrick January 2008 (has links)
[Abstract]: This thesis presents a new cosmological model based on a blank-slate reconsideration of the issue of the first cause in cosmology. It is proposed that a pre-Big Bang evolution of nature occurred that removes the need for postulating the existence of matter, energy and time. This new approach leads to an underlyingconceptualisation of nature consistent with quantum mechanics.The problems of first cause and initial conditions in cosmology are reconsidered. It is proposed that the initial conditions were flawed and evolved toproduce the Big Bang as a natural response to these flaws. This contrasts with the traditional approach of postulating a homogeneous initial state requiring perturbation by an additional first cause.In flawed nature cosmology the origin of time occurs as a natural response to the flawed set of initial conditions, and removes the need to postulate time. The development of causality remains an ongoing process rather than being fully determined by the first cause. Ongoing development of causality provides a conceptual understanding of the probabilistic nature of quantum mechanics and itsrelationship with classical physics.Flawed nature cosmology is used to examine pre-Big Bang evolution, in order to justify rather than postulate a set of conditions leading to the Big Bang. Thisexamination of pre-Big Bang evolution also introduces a structured method to start addressing the question of the origin of matter and the forces of nature.Flawed nature cosmology reconsiders the issues that introduced the manyuniverses concept into physics such as spontaneous first causes, the many-worlds interpretation of quantum mechanics, brane cosmology’s use of the extra dimensions in string theory, and parallel universes to solve the fine tuning problem. The manyuniversesconcept has found favour, as much of the puzzling behaviour of the universe can be avoided by simply stating that if there are many universes, one couldmatch our experience. In contrast, flawed nature cosmology demonstrates that the universe we experience is the unique product of its evolutionary history.
9

Does the universe require a cause?

Sickler, Bradley L. January 2002 (has links) (PDF)
Thesis (M.A.)--Trinity International University, 2002. / Abstract. Includes bibliographical references (leaves 101-105).
10

La cosmologie des pa Koua et L'astronomie moderne. prévision d'une nouvelle planète ...

Liu, Tzǔ-hua. January 1940 (has links)
Thèse--Université de Paris. / "Bibliographie": p. [142]-151.

Page generated in 0.1004 seconds