Research Papers (TP)
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Item Drift, Drag and Brownian motion in the Davies-Unruh bath(2013-08-06) Kolekar, Sanved; Padmanabhan, T.An interesting feature of the Davies-Unruh effect is that a uniformly accelerated observer sees an isotropic thermal spectrum of particles even though there is a preferred direction in this context, determined by the direction of the acceleration g. We investigate the thermal fluctuations in the Unruh bath by studying the Brownian motion of particles in the bath, especially as regards to isotropy. We find that the thermal fluctuations are anisotropic and induce different frictional drag forces on the Brownian particle depending on whether it has a drift velocity along the direction of acceleration g or in a direction transverse to it. Using the fluctuation-dissipation theorem, we argue that this anisotropy arises due to quantum correlations in the fluctuations at large correlation time scales.Item Noether current from the surface term of gravitational action, Virasoro algebra and horizon entropy(2013-08-06) Majhi, Bibhas Ranjan; Padmanabhan, T.We describe a simple way of obtaining horizon entropy using the approach based on the Virasoro algebra and central charge. We show that the Virasoro algebra defined by the Noether currents corresponding to the surface term of gravitational action, for the diffeomorphisms which leave the horizon structure unaltered, has a central extension that directly leads to the horizon entropy. In this approach there are no ambiguities in the calculation of the central charge.We explain why this approach is physically well motivated and could provide greater insight into the nature of horizon entropy.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 Equipartition energy, Noether energy and boundary term in gravitational action(Springer, 2012-05-25) Padmanabhan, T.In the study of horizon thermodynamics and emergent gravity two natu-ral expressions for energy, E=2TS(equipartition energy) and E=TS(Noether energy) arise which differ by a factor 2. I clarify the role of these two expressions in different contexts and show howE=TSis also closely related to the Noether charge arising from theboundary termof the Einstein–Hilbert action.Item Secret Life of the Spacetime(World scientific, 2012-03-28) Padmanabhan, T.Just as the thermal properties of normal matterdemandsthe existence of microscopic degrees of freedom, the thermal properties of null surfaces — perceived as local Rindler horizons by accelerated observers — demands the existence of microscopic degrees of freedom to spacetime. The distortion of the null surfaces, just like the deformation of an elastic solid, costs entropy. I show how, just like in the case of an elastic solid, one can describe the dynamics of thespacetime solid by introducing an entropy density to the distortion of null surfaces in the spacetime.Item Entropy increase during physical processes for black holes in Lanczos-Lovelock gravity(American physical society, 2012-07-03) Padmanabhan, T.; Kolekar, Sanved; Sarkar, SudiptaWe 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.Item Noether Current, Horizon Virasoro Algebra, and Entropy(American physical society, 2012-04-08) Bibhas, Ranjan Majhi; Padmanabhan, T.We provide a simple and straightforward procedure for defining a Virasoro algebra based on the diffeomorphisms near a null surface in a space-time and obtain the entropy density of the null surface from its central charge.We use the off-shell Noether current corresponding to the diffeomorphism invariance of a gravitational Lagrangian Lðgab; RabcdÞ and define the Virasoro algebra from its variation. This allows us to identify the central charge and the zero-mode eigenvalue with which we obtain the entropy density of the Killing horizon. Our approach works for all Lanczos-Lovelock models and reproduces the correct Wald entropy. The entire analysis is done off-shell without using the field equations and allows us to define an entropy density for any null surface which acts as a local Rindler horizon for a particular class of observers.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 Automated classification of sloan digital sky survey (SDSS) stellar spectra using artificial neural networks(Astrophys Space Sci, 2008-04-21) Bazarghan, Mahdi; Gupta, RanjanAutomated techniques have been developed to automate the process of classification of objects or their analysis. The large datasets provided by upcoming spectroscopic surveys with dedicated telescopes urges scientists to use these automated techniques for analysis of such large datasets which are now available to the community. Sloan Digital Sky Survey (SDSS) is one of such surveys releasing massive datasets. We use Probabilistic Neural Network (PNN) for automatic classification of about 5000 SDSS spectra into 158 spectral type of a