IUCAA Preprints
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Item Can brane cosmology with a vanishing /\ explain the observations?(2011-07-05) Vishwakarma, R. G.; Singh, ParampreetA plethora of models of the universe have been proposed in recent years claiming that the present universe is accelerating, being driven by some hypothetical source with negative pressure collectively known as dark energy which though do not appear to resemble any known form of matter tested in the laboratory. These models are motivated by the high redshift supernovae Ia observations. Though low density models, without dark energy, also appear to fit the SN Ia data reasonably well, however, they are ruled out by the CMB observations. In this paper, we present a warped brane model with an additional surface term of brane curvature scalar in the action. This results in shifting the dynamical curvature of the model from its geometrical counterpart, which creates profound consequences. Even for Λ = 0, the low energy decelerating model successfully explains the observed locations of the peaks in the angular power spectrum of CMB. This model also fits the high redshift supernovae Ia observations, taken together with the recently observed SN 1997ff at z ≈ 1.7, very well. Additionally, it also fits the data on the angular size and redshift of the compact radio sources very well.Item Avoidance of future singularities in loop quantum cosmology(2006-05-21) Sami, M.; Singh, Parampreet; Tsujikawa, ShinjiWe consider the fate of future singularities in the effective dynamics of loop quantum cosmology. Non-perturbative quantum geometric effects which lead to ρ2 modification of the Friedmann equation at high energies result in generic resolution of singularities whenever energy density ρ diverges at future singularities of Friedmann dynamics. Such quantum effects lead to the avoidance of a Big Rip, which is followed by a recollapsing universe stable against perturbations. Resolution of sudden singularity, the case when pressure diverges but energy density approaches a finite value depends on the ratio of the latter to a critical energy density of the order of Planck. If the value of this ratio is greater than unity, the universe escapes the sudden future singularity and becomes oscillatory.