On the history of expansion and matter-energy content of the universe

The aim of this dissertation is to explore the vast phenomenological area between the cosmological observations and the theoretical approaches. We are specifically focusing on type Ia supernova data and what they tell us in the manner of expansion history of the universe. In this regard, we start by introducing the current status of cosmological observations including cosmic microwave background radiation, galaxy redshift surveys, supernova missions, etc. We briefly summarise the standard approaches of data analysis and cosmological parameter estimation. Then, we introduce our own approach, where we use only the cosmological principle and the assumption that gravitation is governed by a metric theory and try to reconstruct the expansion history from supernova data. We find that the acceleration history of the universe cannot be reliably determined in this approach due to the irregularity and parametrisation- dependence of the results. Therefore we add gamma ray bursts to our analysis and in doing so we become able to estimate the redshift of transition to cosmic acceleration and additionally –assuming Einstein gravity– put a lower limit to dark energy density. Next we introduce three ideas on the expansion characteristics of the universe and examine if they are compatible with observations. First we explore the idea of massive photons and how it would affect our observations. Secondly, we present an ansatz for the deceleration parameter for a model in which dark matter decays into dark energy, inspired by the theory of relativistic cosmic fluids. Lastly, inspired by the web-like large scale structure we introduce an inhomogeneous equation of state from fluid dynamics and examine its cosmological implications.