Özet:
The aim of this dissertation is to explore the vast phenomenological area between the cosmological observations and the theoretical approaches. We are speci cally 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 brie y 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 nd that the acceleration history of the universe cannot be reliably determined in this approach due to the irregularity and parametrisationdependence 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 a ect 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 uids. Lastly, inspired by the web-like large scale structure we introduce an inhomogeneous equation of state from uid dynamics and examine its cosmological implications.
