Dark matter and dark energy

Abstract

Abstract. I briefly review our current understanding of dark matter and dark en- ergy. The first part of this paper focusses on issues pertaining to dark matter includ- ing observational evidence for its existence, current constraints and the ‘abundance of substructure’ and ‘cuspy core’ issues which arise in CDM. I also briefly describe MOND. The second part of this review focusses on dark energy. In this part I dis- cuss the significance of the cosmological constant problem which leads to a predicted value of the cosmological constant which is almost 10123 times larger than the ob- served value Λ/8πG ≃ 10−47 GeV4 . Setting Λ to this small value ensures that the acceleration of the universe is a fairly recent phenomenon giving rise to the ‘cosmic coincidence’ conundrum according to which we live during a special epoch when the density in matter and Λ are almost equal. Anthropic arguments are briefly dis- cussed but more emphasis is placed upon dynamical dark energy models in which the equation of state is time dependent. These include Quintessence, Braneworld models, Chaplygin gas and Phantom energy. Model independent methods to deter- mine the cosmic equation of state and the Statefinder diagnostic are also discussed. The Statefinder has the attractive property ... a /aH3 = 1 for LCDM, which is helpful for differentiating between LCDM and rival dark energy models. The review ends with a brief discussion of the fate of the universe in dark energy models.