Research Papers (TP)
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Item 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.Item 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.Item Two Aspects of Black hole entropy in Lanczos-Lovelock models of gravity(American physical society, 2012-03-06) Padmanabhan, T.; Kothawala, Dawood; Kolekar, SanvedWe 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.Item Some aspects of field equations in generalised theories of gravity(American Astronomical Society, 2011-12-19) Padmanabhan, T.Item Conformal invariance, gravity and massive gauge theories(IOP Publishing, 1985-05-07) Padmanabhan, T.A gauge theory is constructed maintaining the invariance of the action of massless scalar fields under local conformal transformations. The theory leads to gravity in a natural fashion and also allows for the gauge field to be massive.Item Thermodynamic route to field equations in lanczos-lovelock gravity(American Physical Society, 2006-11-10) Padmanabhan, T.Item 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.Item Semiclassical approximations for gravity and the issue of backreaction(IOP Publishing, 1988-08-28) Padmanabhan, T.Semiclassical approximations, which are useful in the study of a quantum system interacting with a classical system, are studied and compared. A toy quantum mechanical model with two degrees of freedom (which mimics the features of gravity interacting with quantum fields) is used for illustration. In particular, we consider the Born-Oppenheimer approximation (BOA) (corresponding to G-O at fixed h), the effective action approach (h - 0 at fixed G) and their combinations. We show that in the strict BOA limit there is no backreaction on gravity. Gravity is described by classical equations and the fields are quantised in that background. In the effective action approach one can obtain a semiclassical description for gravity, if certain stringent requirements are satisfied. In most situations of interest these conditions will not be met and the O(h) contribution from gravitons will be comparable to that from quantum fields. We study the system using both the Schrodinger equation and path integrals and indicate the correspondence.Item Duality and Zero-Point Length of Spacetime(American Physical Society, 1997-03-10) Padmanabhan, T.The action for a relativistic free particle of mass m receives a contribution 2mds from a path of infinitesimal length ds. Using this action in a path integral, one can obtain the Feynman propagator for a spinless particle of mass m. Assuming that the path integral amplitude is invariant under the “duality” transformation ds ! L2 P yds, one can calculate the modified Feynman propagator. I show that this propagator is the same as the one obtained by assuming that quantum effects of gravity lead to modification of the spacetime interval sx 2 yd2 to sx 2 yd2 1 L2 P . The implications of this result are discussed.Item C-field cosmology: A possible solution to singularity, horizon and flatness problems(American Physical Society, 1985-10-15) Narlikar, J. V.; Padmanabhan, T.A solution of Einstein's equations which admits radiation and a negative-energy massless scalar creation field as a source is presented. It is shown that the cosmological model based on this solution satisfies all the observational tests and thus is a viable alternative to the standard big-bang model. The presentation model is from singularity and particle horizon and provides a natural explanation for the flatness problem. We argue that these features make the creation-field cosmological model theoretically superior to the big-bang model.