The Physics of the Early Universe

I briefly review our current understanding of dark matter and dark energy.The first part of this paper focusses on issues pertaining to dark matterincluding observational evidence for its existence, current constraints and the`abundance of substructure' and `cuspy core' issues which arise in CDM. I alsobriefly describe MOND. The second part of this review focusses on dark energy.In this part I discuss the significance of the cosmological constant problemwhich leads to a predicted value of the cosmological constant which is almost$10^{123}$ times larger than the observed value $\la/8\pi G \simeq10^{-47}$GeV$^4$. Setting $\la$ to this small value ensures that theacceleration of the universe is a fairly recent phenomenon giving rise to the`cosmic coincidence' conundrum according to which we live during a specialepoch when the density in matter and $\la$ are almost equal. Anthropicarguments are briefly discussed but more emphasis is placed upon dynamical darkenergy models in which the equation of state is time dependent. These includeQuintessence, Braneworld models, Chaplygin gas and Phantom energy. Modelindependent methods to determine the cosmic equation of state and theStatefinder diagnostic are also discussed. The Statefinder has the attractiveproperty $\atridot/a H^3 = 1 $ for LCDM, which is helpful for differentiatingbetween LCDM and rival dark energy models. The review ends with a briefdiscussion of the fate of the universe in dark energy models.Comment: 40 pages, 11 figures, Lectures presented at the Second Aegean Summer School on the Early Universe, Syros, Greece, September 2003, New References added Final version to appear in the Proceeding