2004 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/625
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Item Quintessential inflation on the brane and the Relic Gravity wave background(2011-07-06) Sami, M.; Sahni, VarunPune 411 007, India. Quintessential inflation describes a scenario in which both inflation and dark energy (quintessence) are described by the same scalar field. In conventional braneworld models of quintessential inflation gravitational particle production is used to reheat the universe. This reheating mechanism is very inefficient and results in an excessive production of gravity waves which violate nucleosynthesis constraints and invalidate the model. We describe a new method of realizing quintessential inflation on the brane in which inflation is followed by ‘instant preheating’ (Felder, Kofman & Linde 1999). The larger reheating temperature in this model results in a smaller amplitude of relic gravity waves which is consistent with nucleosynthesis bounds. The relic gravity wave background has a ‘blue’ spectrum at high frequencies and is a generic byproduct of successful quintessential inflation on the brane.Item Dark matter and dark energy(2011-07-06) Sahni, VarunAbstract. 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.Item Case for dynamical dark energy revisited(2011-07-06) Alam, Ujjaini; Sahni, Varun; Starobinsky, A. A.We investigate the behaviour of dark energy using the recently released supernova data of Riess et al., 2004 and a model independent parameterization for dark energy (DE). We find that, if no priors are imposed on Ω0m and h, DE which evolves with time provides a better fit to the SNe data than ΛCDM. This is also true if we include results from the WMAP CMB data. From a joint analysis of SNe+CMB, the best-fit DE model has w0 < ∼ − 1 at the present epoch and the transition from deceleration to acceleration occurs at zT = 0.39±0.03. However, DE evolution becomes weaker if the ΛCDM based CMB results Ω0m = 0.27 ± 0.04, h = 0.71 ± 0.06 are incorporated in the analysis. In this case, zT = 0.57±0.07. Our results also show that the extent of DE evolution is sensitive to the manner in which the supernova data is sampled.