Research Papers (TP)

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Author: search.filters.author.Sarkar, Sudipta

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    Entropy increase during physical processes for black holes in Lanczos-Lovelock gravity
    (American physical society, 2012-07-03) Padmanabhan, T.; Kolekar, Sanved; Sarkar, Sudipta
    We study quasistationary physical process for black holes within the context of Lanczos-Lovelock gravity. We show that the Wald entropy of the stationary black holes in Lanczos-Lovelock gravity monotonically increases for quasistationary physical processes in which the horizon is perturbed by the accretion of positive energy matter and the black hole ultimately settles down to a stationary state. This result reinforces the physical interpretation of Wald entropy for Lanczos-Lovelockmodels and takes a step towards proving the analogue of the black hole area increase theorem in a wider class of gravitational theories.
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    Is gravitational entropy quantized?
    (American Physical Society, 2008-11-18) Kothawala, Dawood; Padmanabhan, T.; Sarkar, Sudipta
    In Einstein’s gravity, the entropy of horizons is proportional to their area. Several arguments given in the literature suggest that, in this context, both area and entropy should be quantized with an equallyspaced spectrum for large quantum numbers. But in more general theories (like, for example, in the black hole solutions of Gauss-Bonnet or Lanczos-Lovelock gravity) the horizon entropy is not proportional to area and the question arises as to which of the two (if at all) will have this property. We give a general argument that in all Lanczos-Lovelock theories of gravity, it is the entropy that has an equally-spaced spectrum. In the case of Gauss-Bonnet gravity, we use the asymptotic form of quasinormal mode frequencies to explicitly demonstrate this result. Hence, the concept of a quantum of area in Einstein- Hilbert gravity needs to be replaced by a concept of quantum of entropy in a more general context.
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    Thermodynamics of horizons from a dual quantum system
    (Entropy, 2007-08-20) Sarkar, Sudipta; Padmanabhan, T.
    It was shown recently that, in the case of Schwarschild black hole, one can obtain the correct thermodynamic relations by studying a model quantum system and using a partic-ular duality transformation. We study this approach further for the case a general spherically symmetric horizon. We show that the idea works for a general case only if we define the en-tropy S as a congruence (“observer”) dependent quantity and the energy E as the integral over the source of the gravitational acceleration for the congruence. In fact, in this case, one recov-ers the relation S = E = 2 T between entropy, energy and temperature previously proposed by one of us in gr-qc/0308070. This approach also enables us to calculate the quantum correc-tions of the Bekenstein-Hawking entropy formula for all spherically symmetric horizons.
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    Einstein’s equations as a thermodynamic identity: The cases of stationary axisymmetric horizons and evolving spherically symmetric horizons
    (Elsevier Science Publishers, 2007-07-19) Kothawala, Dawood; Sarkar, Sudipta; Padmanabhan, T.
    There is an intriguing analogy between the gravitational dynamics of the horizons and thermodynamics. In case of general relativity as well as for a wider class of Lanczos–Lovelock theories of gravity, it is possible to interpret the field equations near any spherically symmetric horizon as a thermodynamic identity T dS = dE + P dV. We study this approach further and generalize the results to two more generic cases: stationary axis-symmetric horizons and time dependent evolving horizons within the context of general relativity. In both the cases, the near horizon structure of Einstein equations can be expressed as a thermodynamic identity under the virtual displacement of the horizon. This result demonstrates the fact that the thermodynamic interpretation of gravitational dynamics is not restricted to spherically symmetric or static horizons but is quite generic in nature and indicates a deeper connection between gravity and thermodynamics.
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    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.