Research Papers (TP)
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Item Cosmology with tachyon field as dark energy(American Physical Society, 2003-03-14) Bagla, J. S.; Jassal, H. K.; Padmanabhan, T.We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with nonrelativistic matter and radiation, concentrating on the inverse square potential and the exponential potential for the tachyonic scalar field. A distinguishing feature of these models ~compared to other cosmological models! is that the matter density parameter and the density parameter for tachyons remain comparable even in the matter dominated phase. For the exponential potential, the solutions have an accelerating phase, followed by a phase with a(t)}t2/3 as t!`. This eliminates the future event horizon present in cold dark matter models with a cosmological constant (LCDM) and is an attractive feature from the string theory perspective. A comparison with supernova type Ia data shows that for both the potentials there exists a range of models in which the universe undergoes an accelerated expansion at low redshifts which are also consistent with the requirements of structure formation. They do require fine-tuning of parameters but not any more than in the case of L CDM models or quintessence models.Item Cosmological constant—the weight of the vacuum(Elsevier Science Publishers, 2003-03-01) Padmanabhan, T.Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(Gℏ/c3)≈10−123. This review discusses several aspects of the cosmological constant both from the cosmological (Sections 1–6) and field theoretical (Sections 7–11) perspectives. After a brief introduction to the key issues related to cosmological constant and a historical overview, a summary of the kinematics and dynamics of the standard Friedmann model of the universe is provided. The observational evidence for cosmological constant, especially from the supernova results, and the constraints from the age of the universe, structure formation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar field, …) from different perspectives. The latter part of the review (Sections 7–11) concentrates on more conceptual and fundamental aspects of the cosmological constant like some alternative interpretations of the cosmological constant, relaxation mechanisms to reduce the cosmological constant to the currently observed value, the geometrical structure of the de Sitter spacetime, thermodynamics of the de Sitter universe and the role of string theory in the cosmological constant problem.Item Hubble Expansion for Pedestrians(Indian Academy of Sciences, 2009-03-12) Padmanabhan, T.Many features of the expanding universe, which should be legitimately discussed using general relativity, can be sneaked in by using Newtonian physics in an expanding coordinate system. In this special issue on Hubble, I describe several of these features along with some cautionary comments.Item Why do we observe a small but nonzero cosmological constant?(IOP Publishing, 2002-08-16) Padmanabhan, T.Item Viable cosmology with a scalar field coupled to the trace of the stress tensor(American Physical Society, 2003-05-12) Sami, M.; Padmanabhan, T.Item Vanishing of the cosmological constant in nonfactorizable geometry(American Physical Society, 2001-04-26) Padmanabhan, T.; Shankaranarayanan, S.We generalize the results of Randall and Sundrum to a wider class of four-dimensional space-times includ-ing the four-dimensional Schwarzschild background and de Sitter universe. We solve the equation for graviton propagation in a general four dimensional background and find an explicit solution for a zero mass bound state of the graviton. We find that this zero mass bound state is normalizable only if the cosmological constant is strictly zero, thereby providing a dynamical reason for the vanishing of cosmological constant within the context of this model. We also show that the results of Randall and Sundrum can be generalized without any modification to the Schwarzschild background.Item Theoretician’s analysis of the supernova data and the limitations in determining the nature of dark energy(Wiley-Blackwell, 2003-06-02) Padmanabhan, T.; Choudhury, T. RoyItem Effects of anticorrelation on gravitational clustering(Wiley-Blackwell, 2001-01-28) Kanekar, Nissim; Padmanabhan, T.We use non-linear scaling relations (NSRs) to investigate the effects arising from the existence of negative correlations on the evolution of gravitational clustering in an expanding universe. It turns out that such anti-correlated regions have important dynamical effects on ıt all scales. In particular, the mere existence of negative values for the linear two-point correlation function ξbL over some range of scales starting from l = Lo, implies that the non-linear correlation function is bounded from above at ıt all scales x < Lo. This also results in the relation ξb ∝ x-3, at these scales, at late times, independent of the original form of the correlation function. Current observations do not rule out the existence of negative ξb for 200 h-1 Mpc ła ξb ła 1000 h-1 Mpc; the present work may thus have relevance for the real Universe. The only assumption made in the analysis is the ıt existence of the NSR; the results are independent of the form of the NSR as well as of the stable clustering hypothesis.Item Casimir effect confronts cosmological constant(Elsevier Science Publishers, 2006-08-23) Mahajan, Gaurang; Sarkar, Sudipta; Padmanabhan, T.It has been speculated that the zero-point energy of the vacuum, regularized due to the existence of a suitable ultraviolet cut-off scale, could be the source of the on-vanishing cosmological constant that is driving the present acceleration of the universe. We show that the presence of such a cut-off can significantly alter the results for the Casimir force between parallel conducting plates and even lead to repulsive Casimir force when the plate separation is smaller than the cut-off scale length. Using the current experimental data we rule out the possibility that the observed cosmological constant arises from the zero-point energy which is made finite by a suitable cut-off. Any such cut-off which is consistent with the observed Casimir effect will lead to an energy density which is at least about 10¹² times larger than the observed one, if gravity couples to these modes. The implications are discussed.