On the Effects of Cosmic Structures in the Late Universe
Year: 2012
Type: dissertation
Abstract: During the last decade, the cosmological observations have indicated that the homogeneous and isotropic Friedmann models with linear perturbations fail to describe our universe at late times unless a dominant energy component with negative pressure called dark energy is introduced. In this thesis, we study the implications of the nonlinear nature of general relativity on the cosmological model building beyond the standard Friedmann models. Despite the well established observational status of cosmic structures, their effects have gained more attention only along with the dark energy debate. In particular, the fact that the start of the supposed dark energy domination coincides with the time the nonlinear inhomogeneities started to form on larger scales, motivates the study of the dynamics of the cosmic structures. In cosmology, the implication of the nonlinearity of gravity is that averages of inhomogeneous quantities do not evolve in time like the corresponding homogeneous quantities - a phenomenon referred to as the backreaction. Due to the new precision observations during the recent years, the evaluation of the backreaction in our universe is a topical, but complex task. In this thesis, rather than trying to fully quantify the backreaction, the emphasis is on the model building. We explicitly demonstrate the importance of the exact matching conditions in the solutions representing cosmic structures in the context of backreaction evaluation. Indeed, the cosmic web of structures is made of very differently behaving regions and the shear on the interface between the different regions seems to play an important role. The backreaction term emerging from averaging the Einstein equation is not the only effect that cosmic structures can have on the observations. Indeed, we also demonstrate that even though the backreaction would remain small, large effects can arise from the choice of the smoothing scale and, perhaps surprisingly, from perturbative models as well. As we find, at least the supernova data can be explained within a linearly perturbed Friedmann model - without dark energy. The key point is to take into account the effects of structures on the observable distance measures, ignored in the standard cosmological perturbation theory. Further inspection shows that the model is actually equivalent to a nonperturbative inhomogeneous solution, confirming that the supernova data does not necessarily imply additional nonperturbative corrections. Considering physical quantities such as the expansion rate of space and the matter density, there are large local variations in the cosmic web. The main question to answer is whether (and to what extent) the effects of the local variations average out or accumulate in the observables. It appears likely that when combining all the cosmological data, more sophisticated models than the perturbed Friedmann or the simplest spherically symmetric exact inhomogeneous solutions are required to fully quantify the effects of the structures on the cosmological observations.
Source: Työväentutkimus Vuosikirja
Author: Maria Mattsson
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W1151753491