published_or_final_version / abstract / Physics / Master / Master of Philosophy

I have presented an evolving wormhole solution in braneworld model. Moreover I have shown that there are observational signatures for using braneworld to solve dark matter problem. Braneworld assumed that four dimensional spacetime embedded into five dimensional bulk. Gravity is a five dimensional interaction and the usual four dimensional description is reproduced by geometric projection. It predicts correction terms to the Einstein equation, for instance, the higher dimensional Weyl curvature projected on the brane. This projected Weyl curvature could make braneworld wormhole satisfies the Null Energy Condition (NEC), which is impossible in general relativity. We considered inflating braneworld wormhole that enlarge with the Universe, the wormhole is supported by the Weyl curvature so that it satisfies NEC. We demonstrated how this wormhole embedded into a five dimensional bulk and studied how it evolve with our Universe. The result is that inflating wormhole satisfying general initial condition will collapse into black hole when the scalar field oscillates. In addition, the projected Weyl curvature introduces a new source of gravity. This Weyl fluid of geometrical origin (reducing in the spherically symmetric, static configuration to a dark radiation and dark pressure) modifies space-time geometry around galaxies and has been used to explain the flatness of galactic rotation curves. Independent observations for discerning between the Weyl fluid and other dark matter models are necessary. Gravitational lensing could provide such a test. Therefore we study null geodesics and weak gravitational lensing in the dark radiation dominated region of galaxies in a class of spherically symmetric braneworld metrics. We find that the lensing profile in the braneworld scenario is distinguishable from dark matter lensing, despite both the braneworld scenario and dark matter models can fit the rotation curve data. In particular, in the asymptotic regions light deflection is 18% enhanced as compared to dark matter halo predictions. For a linear equation of state of the Weyl fluid we further find a critical radius, below which braneworld effects reduce, while above it they amplify light deflection. This is in contrast to any dark matter model, which always increases the deflection angle. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

The Noether charge algebra of brane actions is typically modified by a topological anomalous term. The underlying cohomology of this anomalous term is investigated, and it is shown that the anomalous term possesses a gauge freedom. The result is that the anomalous term generates a parameterized family of topological charge algebras. When fermionic charges are taken to be nonvanishing, the known algebras underlying extended superspace formulations of the action appear in these families. This phenomena is investigated for minimal p-branes, Dp-branes and (p, q)-strings. The algebras resulting from the D-brane actions are shown to allow the construction of extended superspace actions without worldvolume gauge fields. It is shown that the actions are !-symmetric, and that the symmetry is generated by a right action. The global and local symmetry transformations of the Born-Infeld gauge field are thus shown to be described geometrically by left/right actions of the underlying extended supertranslation group. An equivalence class construction is proposed for the description of compact fermionic dimensions. In this construction, open strings in extended superspace translate to closed strings in compact superspace, and fermionic topological charges may be realized by closed strings. The differential underlying the descent construction for Noether charge algebras is shown to be naturally described as a dual of the de Rham differential. The ghost fields used in the construction are shown to be described geometrically as a vielbein with respect to this differential.

published_or_final_version / Physics / Master / Master of Philosophy

No description available.

Thesis (M. Phil.)--University of Hong Kong, 2010. / Includes bibliographical references (leaves 80-82). Also available in print.

Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, Discipline of Physics, 2005. / "March 2005" Includes bibliographical references. Also available in a print form.

Thesis (M.Sc.). / Written for the Dept. of Physics. Title from title page of PDF (viewed 2008/05/13). Includes bibliographical references.

Thesis (Ph.D.). / Written for the Dept. of Physics. Title from title page of PDF (viewed 2009/06/09). Includes bibliographical references.