Research Publications

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Start date: 2010Subject: search.filters.subject.Gravity

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Now showing 1 - 7 of 7
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    The bending of light by gravity
    (Physics News, 2015-04-10) Narlikar, J. V.
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    Structural Aspects Of Gravitational Dynamics And The Emergent Perspective Of Gravity
    (2013-08-06) Padmanabhan, T.
    I describe several conceptual aspects of a particular paradigm which treats the field equations of gravity as emergent. These aspects are related to the features of classical gravitational theories which defy explanation within the conventional perspective. The alternative interpretation throws light on these features and could provide better insights into possible description of quantum structure of spacetime. This review complements arXiv:1207.0505, which describes space itself as emergent in the cosmological context.
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    Emergent perspective of Gravity and Dark Energy
    (2012-07-02) Padmanabhan, T.
    There is sufficient amount of internal evidence in the nature of gravitational theories to indicate that gravity is an emergent phenomenon like, e.g, elasticity. Such an emergent nature is most apparent in the structure of gravitational dynamics. It is, however, possible to go beyond the field equations and study the space itself as emer-gent in a well-defined manner in (and possibly only in) the context of cosmology. In the first part of this review, I describe various pieces of evidence which show that gravitational field equations are emergent. In the second part, I describe a novel way of studying cosmology in which I interpret the expansion of the universe as equivalent to the emergence of space itself. In such an approach, the dynamics evolves towards a state of holographic equipartition, characterized by an equality in the number of bulk and surface degrees of freedom in a region bounded by the Hubble radius. This prin-ciple correctly reproduces the standard evolution of a Friedmann universe. Further, (a) it demands the existence of an early inflationary phase as well as late time accelera-tion for its successful implementation and (b) allows us to link the value of late time cosmological constant to the e-folding factor during inflation.
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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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    Some aspects of field equations in generalised theories of gravity
    (American Astronomical Society, 2011-12-19) Padmanabhan, T.
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    Surface density of spacetime degrees of freedom from equipartition law in theories of gravity
    (American Physical Society, 2010-06-22) Padmanabhan, T.
    I show that the principle of equipartition, applied to area elements of a surface @V which are in equilibrium at the local Davies-Unruh temperature, allows one to determine the surface number density of the microscopic spacetime degrees of freedom in any diffeomorphism invariant theory of gravity. The entropy associated with these degrees of freedom matches with theWald entropy for the theory. This result also allows one to attribute an entropy density to the spacetime in a natural manner. The field equations of the theory can then be obtained by extremizing this entropy. Moreover, when the microscopic degrees of freedom are in local thermal equilibrium, the spacetime entropy of a bulk region resides on its boundary.
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    Finite entanglement entropy from the zero-point area of spacetime
    (American Physical Society, 2010-12-13) Padmanabhan, T.
    The calculation of entanglement entropy S of quantum fields in spacetimes with horizon shows that, quite generically, S is (a) proportional to the area A of the horizon and (b) divergent. I argue that this divergence, which arises even in the case of Rindler horizon in flat spacetime, is yet another indication of a deep connection between horizon thermodynamics and gravitational dynamics. In an emergent perspec- tive of gravity, which accommodates this connection, the fluctuations around the equipartition value in the area elements will lead to a minimal quantum of area Oð1ÞL2 P, which will act as a regulator for this divergence. In a particular prescription for incorporating the L2 P as zero-point-area of spacetime, this does happen and the divergence in entanglement entropy is regularized, leading to S / A=L2 P in Einstein gravity. In more general models of gravity, the surface density of microscopic degrees of freedom is different which leads to a modified regularization procedure and the possibility that the entanglement entropy—when appropriately regularized—matches the Wald entropy.