Dark energy problem and possible solutions

Abstract

The end of the twentieth century is marked by a revolutionary discovery in cosmology: The expansion of the universe is accelerating, as revealed by measurements of type Ia supernovae at z 1. This most likely means that the energy content of the universe goes beyond that of matter; baryonic or dark. The simplest explanation of this “dark energy” is the cosmological constant , which was originally proposed by Albert Einstein at the beginning of the century to support his static universe model. However is problematic because of its unusually low energy scale and its recent occurrence as the dominant energy component. To overcome these fine-tuning problems alternative theories are proposed, ranging from scalar field theories to braneworld scenarios. In this thesis, after an introduction to the basic concepts in general relativity and cosmology, we review the recent findings about the universe. Then we go over the alternative theories of dark energy and show how they can be responsible for the latetime cosmic acceleration. We briefly discuss reconstruction of cosmological parameters from data and methods to distinguish between proposed models. The future of the universe is also examined, a classification is given for possible future singularities.