College of Science
https://hdl.handle.net/10092/23
2021-07-31T05:55:18ZAveraging the inhomogeneous universe
https://hdl.handle.net/10092/102252
Averaging the inhomogeneous universe
Yao, Hui
We re-formulate and examine T. Buchert’s recent averaging scheme for scalars in cosmological applications of general relativity. The equation thus obtained can be used to describe the averaged quantities of an arbitrary inhomogeneous co-moving region and show the importance of back-reaction.
We also study the use of information theory in this averaging framework. Original extensions are mainly made along two lines: the information of inhomogeneity for different scales are compared; the possibility of use of Shannon’s entropy in inhomogeneous cosmology are investigated. We also discuss the non-locality of gravitational energy in inhomogeneous cosmology.
Examples of cosmological solutions of Buchert’s averaging scheme are studied.
2006-01-01T00:00:00ZApparent and average acceleration of the universe
https://hdl.handle.net/10092/102251
Apparent and average acceleration of the universe
Frost, William
We study two forms of deceleration parameter, one derived from supernova observations and the other from the Buchert averaging scheme. This work followed the analysis of Bolejko and Andersson in their paper “Apparent and Average Acceleration of the Universe”. We have recalculated the volume deceleration parameter, qvol, and the distance deceleration parameter, qdis, within Lemaître-Tolman models. Within the models studied, those which are realistic and fit supernova data are found to have qvol > 0, while those which Bolejko and Andersson found with qvol < 0 were deemed unrealistic. Mistakes in their paper were found and corrected. The deceleration parameter was found not to be directly related to the volume deceleration parameter.
2010-01-01T00:00:00ZPost-Newtonian cosmology
https://hdl.handle.net/10092/102250
Post-Newtonian cosmology
Williams, Michael
In cosmology, it is common to model the universe as being close to an exact solution of Ein- stein’s equations, the homogeneous and isotropic Friedmann–Lemaˆıtre–Robertson–Walker spacetime. The inhomogeneities in the real universe that arise due to structure formation are modelled using perturbation theory in two different ways: cosmological perturbation theory and post-Newtonian theory. We review these two approaches and their assumptions and restrictions. Perturbation theory introduces a gauge freedom, but a recent work by Clifton, Gallagher, Goldberg, and Malik found that some of the well-studied gauge choices in cosmological perturbation theory are not viable in post-Newtonian theory. We discuss the gauge problem, review the paper of Clifton et al., and extend its analysis to a new set of gauges, the Machian gauges of Bičák, Katz, and Lynden-Bell.
2020-01-01T00:00:00ZAsymptotic structure and symmetries of FLRW universes
https://hdl.handle.net/10092/102249
Asymptotic structure and symmetries of FLRW universes
Wilson, Joseph
The asymptotic structure and symmetries of asymptotically flat spacetime are closely related to the gravitational memory effect, whereby detectors are permanently displaced relative to one another due to the passing of gravitational radiation. We examine the asymptotic properties and structure of a class of non-asymptotically flat spacetimes, including the dust-filled spatially hyperbolic and decelerating spatially flat Friedmann–Lemaître–Robertson–Walker universes. In order to study asymptotic structure, we inspect the Bondi–Sachs criterion of asymptotic flatness with respect to these universes, and compare the rate of falloff in the deviation of outgoing radial light rays. We take the first steps toward studying gravitational memory in these universes by deriving their associated groups of asymptotic Killing vectors, and comparing them to the well-known case of asymptotically flat spacetime.
2019-01-01T00:00:00Z