2.30 pm Monday 26 November 2001
Recent observations suggest the universe is spatially flat, while the matter density is only about 30% of the critical value. Supernovae experiments further suggest that the expansion of the universe is presently accelerating, implying that the missing 70% of the energy density possesses a significantly negative pressure. Of the two possible candidates, cosmological constant and quintessence, the latter is more theoretically attractive due to the ability of quintessence, under general conditions, to track attractor solutions. Finally, the robustness of the quintessence scenario in the context of more general cosmological models that derive from high-energy physics is considered. Overall, the conditions for a successful model become more complex in these settings. However, most of the cases where tracking is disrupted are those in which the cosmological model is itself non-viable due to other constraints. In this sense tracking remains robust in models that are cosmologically viable.