The greatly anticipated era of precision cosmology has finally arrived! There is now near-certain evidence that we don't know what the universe is made of. Most of the matter content in and around galaxies is some kind of `dark matter' we can't see but can only gravitationally detect, while the expansion rate of the universe is counter-intuitively accelerating. The `dark energy' responsible for this accounts for some 70% of the energy content of the universe yet we don't know what or why this is. While there are many phenomenological models which can account for the existence of both of these components none, as yet, make much sense from a fundamental point of view. These models range from postulating new forms of matter, to generalising general relativity to more complex forms. In the coming years it is expected that we can differentiate quite precisely between these models, an exploration in which South Africa will play an important role.
Dark energy is proving to be one of the great scientific problems of our age. Even if the simplest model - a cosmological constant - is the appropriate description its origin is far from understood. Arising as an integration constant in Einstein's general relativity, its apparent magnitude is some 120 orders of magnitude smaller than one would expect. In any event it is increasingly likely that these problems - among others - will be properly understood only within the context of a theory of quantum gravity. In this context, cosmology and astrophysics is the area where we can hope to experimentally differentiate between differing theories.
theories of gravity
Some of our current research work is focused on the investigating cosmological models based on alternative theories of gravity. In particular we are investigating the non-linear gravity models in which higher order invariants are added to the Hilbert-Einstein Lagrangian. These theories have proved to be a good alternative to the standard Dark Energy models, but are very difficult to be analyzed because of the higher order terms and the extreme non-linearity of the evolution equations. Our approach to this problem exploits the qualitative theory of dynamical systems to find exact cosmological solutions and understand the global dynamics of these models. These exact solutions can be used as background solutions to develop the theory of cosmological perturbations and Cosmic Microwave Background anisotropies. This is particularly interesting because the study of these phenomena can give us new constraint on the validity range of General Relativity.
Recent developments in string/M-theory suggest that there may exist large extra dimensions accessible only by gravity at low energies. While models such as the braneworld are phenomenological in nature, they provide a theoretical framework for investigating some of the low energy observational characteristics we might expect from a full theory of quantum gravity.