Exact Classical Correspondence in Quantum Cosmology

We find a Friedmann model with appropriate matter/energy density such that the solution of the Wheeler-DeWitt equation exactly corresponds to the classical evolution. The well-known problems in quantum cosmology disappear in the resulting coasting evolution. The exact quantum-classical correspondence is demonstrated with the help of the de Broglie-Bohm and modified de Broglie-Bohm approaches to quantum mechanics. It is reassuring that such a solution leads to a robust model for the universe, which agrees well with cosmological expansion indicated by SNe Ia data.Comment: Accepted for publication in Gravitation and Cosmolog

Was There a Decelerating Past for the Universe?

In this talk, I argue that the method of analysing the m-z data of Type Ia Supernovae (SNe Ia) by assuming exotic energy densities with strange equations of state is misleading and the reasonable remaining option is to make a model-independent analysis of SNe data, without reference to the energy densities. Using this cosmographic method, we address an important question relevant to cosmology: Was there a decelerating past for the universe? To answer this, the Bayes's probability theory is employed, which is the most appropriate tool for quantifying our knowledge when it changes through the acquisition of new data. The cosmographic approach helps to sort out models which were always accelerating from those which decelerated for at least some time in the period of interest. Bayesian model comparison technique is used to discriminate these rival hypotheses with the aid of recent releases of supernova data. It is argued that the lessons learned using Bayesian theory are extremely valuable to avoid frequent U-turns in cosmology.Comment: Based on a talk given in the 1st Crisis in Cosmology Conference (CCC-1), June 23-25, 2005 at Moncao, Portugal. To appear in the proceeding