Research Papers (TP)

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Start date: 2010Subject: search.filters.subject.Black hole

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    Two Aspects of Black hole entropy in Lanczos-Lovelock models of gravity
    (American physical society, 2012-03-06) Padmanabhan, T.; Kothawala, Dawood; Kolekar, Sanved
    We consider two specific approaches to evaluate the black hole entropy which are known to produce correct results in the case of Einstein’s theory and generalize them to Lanczos- Lovelock models. In the first approach (which could be called extrinsic) we use a procedure motivated by earlier work by Pretorius, Vollick and Israel, and by Oppenheim, and evaluate the entropy of a configuration of densely packed gravitating shells on the verge of forming a black hole in Lanczos-Lovelock theories of gravity. We find that this matter entropy is not equal to (it is less than) Wald entropy, except in the case of Einstein theory, where they are equal. The matter entropy is proportional to the Wald entropy if we consider a specific m-th order Lanczos-Lovelock model, with the proportionality constant depending on the spacetime dimensions D and the order m of the Lanczos-Lovelock theory as (D−2m)/(D−2). Since the proportionality constant depends on m, the proportionality between matter entropy and Wald entropy breaks down when we consider a sum of Lanczos-Lovelock actions involving different m. In the second approach (which could be called intrinsic) we generalize a procedure, previ- ously introduced by Padmanabhan in the context of GR, to study off-shell entropy of a classof metrics with horizon using a path integral method. We consider the Euclidean action of Lanczos-Lovelock models for a class of metrics off-shell and interpret it as a partition function. We show that in the case of spherically symmetric metrics, one can interpret the Euclidean action as the free energy and read off both the entropy and energy of a black hole spacetime. Surprisingly enough, this leads to exactly the Wald entropy and the energy of the spacetime in Lanczos-Lovelock models obtained by other methods. We comment on possible implications of the result.
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    Hydrodynamics of the atoms of spacetime: Gravitational field equation is navier-stokes equation
    (World Scientific, 2011-05-30) Padmanabhan, T.
    There is consider able evidence to suggest that field equations of gravity have the same conceptual status as the equations of hydrodynamics or elasticity. We add further sup-port to this paradigm by showing that Einsteins field equations are identical in form to Navier –Stokes equations of hydrodynamics , when projected on to any null surface. In fact, these equations can be obtained directly by extremizing of entropy associated with the deformations of null surfaces thereby providing a completely thermodynamic route to gravitational field equations . Several curious features of this remarkable connection (including a phenomenon of dissipation without dissipation) are described and the implications for the emergent paradigm of gravity is highlighted.