IUCAA Preprints

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Subject: search.filters.subject.Cosmological constant

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    Machian model of dark energy
    (2002-03-02) Vishwakarma, R. G.
    Einstein believed that Mach’s principle should play a major role in finding a meaningful spacetime geometry, though it was discovered later that his field equations gave some solutions which were not Machian. It is shown, in this essay, that the kinematical Λ mod- els, which are invoked to solve the cosmological constant problem, are in fact consistent with Mach’s ideas. One particular model in this category is described which results from the microstructure of space- time and seems to explain the current observations successfully and also has some benefits over the conventional models. This forces one to think whether the Mach’s ideas and the cosmological constant are interrelated in some way.
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    Living with Lambda
    (2000-10-24) Sahni, Varun
    This talk presents a brief overview of recent results pertaining to the cosmological con- stant ‘ ’. I summarize the observational situation focussing on observations of high redshift Type Ia supernovae which suggest > 0. Observations of small angular anisotropies in the cosmic mi- crowave background complement Type Ia supernovae observations and both CMB and Sn can be combined to place strong constraints on the value of . The presence of a small -term increases the age of the universe and slows down the formation of large scale structure. I also review recent the- oretical attempts to generate a small -term at the current epoch and a model independent approach for determining the cosmic equation of state.
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    Cosmological constant - The weight of the vacuum
    (2011-07-06) Padmanabhan, T.
    Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(G~/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 for- mation, 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 space- time, thermodynamics of the de Sitter universe and the role of string theory in the cosmological constant problem.
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    Steep inflation followed by born-infeld reheating
    (2011-07-05) Sami, M.; Dadhich, Naresh
    We discuss a model in which high energy brane corrections allow a single scalar field to describe inflation at early epochs and quintessence at late times. The reheating mechanism in the model originates from Born-Infeld matter whose energy density mimics cosmological constant at very early times and manifests itself as radiation subsequently. For most of the inflationary evolution the Born- Infeld matter remains subdominant to the the scalar field. Shortly before the end of inflation driven by the scalar field, the energy density of Born-Infeld matter starts scaling as radiation and drops by several orders of magnitudes at the epoch inflation ends. The problem of over production of gravity wave background in scenarios based upon reheating through gravitational particle production is successfully resolved by suitably fixing the initial value of radiation energy density at the end of inflation. No additional fine tuning of the parameters is required for a viable evolution.
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    Higher dimensional dust collapse with a cosmological constant
    (2005-07-01) Ghosh, S. G.; Deshkar, D. W.
    The general solution of the Einstein equation for higher dimensional (HD) spherically symmetric collapse of inhomogeneous dust in presence of a cosmological term, i.e., exact interior solutions of the Einstein field equations is presented for the HD Tolman-Bondi metrics imbedded in a de Sitter background. The solution is then matched to exterior HD Scwarschild-de Sitter. A brief discussion on the causal structure singularities and horizons is provided. It turns out that the collapse proceed in the same way as in the Minkowski background, i.e., the strong curvature naked singularities form and that the higher dimensions seem to favor black holes rather than naked singularities.
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    Five dimensional dust collaspe with cosmological constant
    (2006-07-31) Ghosh, S. G.; Deshkar, D. W.; Saste, N. N.
    We study five dimensional spherical collapse of a inhomogeneous dust in presence of a positive cosmological constant. The general interior solutions, in the closed form, of the Einstein field equations, i.e., the 5D Tolman-Bondi-de Sitter, is obtained which in turn is matched to exterior 5D Scwarschild-de Sitter. It turns out that the collapse proceed in the same way as in the Minkowski background, i.e., the strong curvature naked singularities form and thus violate the cosmic censorship conjecture. A brief discussion on the causal structure singularities and horizons is also given.
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    Dark energy
    (2006-01-10) Sahni, Varun
    The cosmological constant problem as well as the case for dark energy are briefly reviewed and some theoretical models of dark energy are discussed in detail. These include: the cosmological constant, quintessence, the Chaplygin gas and Braneworld models. I also discuss model independent measures of dark energy and conclude by mentioning some properties of the Statefinder diagnostic which can successfully differentiate between different families of dark energy models.
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    Casimir effect confronts cosmological constant
    (2006-04-02) 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 non-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 osmological 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 about 1012 times larger than the observed one, if gravity couples to these modes. The implications are discussed.