Research Publications
Permanent URI for this communityhttp://localhost:4000/handle/11007/1
Browse
13 results
Search Results
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 Ideal gas in a strong gravitational field: Area dependence of entropy(American Physical Society, 2011-03-24) Kolekar, Sanved; Padmanabhan, T.Item Is gravity an intrinsically quantum phenomenon ? Dynamics of Gravity from the Entropy of Spacetime and the Principle of Equivalence(World Scientific Publishing Company, 2002-05-21) Padmanabhan, T.The two surprising features of gravity are (a) the principle of equivalence and (b) the connection between gravity and thermodynamics. Using principle of equivalence and special relativity in the local inertial frame, one could obtain the insight that gravity must possess a geometrical description. We show that, using the same principle of equivalence, special relativity and quantum theory in the local Rindler frame one can obtain the Einstein{Hilbert action functional for gravity and thus the dynamics of the space{ time. This approach, which essentially involves postulating that the horizon area must be proportional to the entropy, uses the local Rindler frame as a natural extension of the local inertial frame and leads to the interpretation that the gravitational action represents the free energy of the space{time geometry. As an aside, one also obtains a natural explanation as to: (i) why the covariant action for gravity contains second derivatives of the metric tensor and (ii) why the gravitational coupling constant is positive. The analysis suggests that gravity is intrinsically holographic and even intrinsically quantum mechanical.Item Gravity: the inside story(Springer, 2008-07-23) Padmanabhan, T.It is well known that one could determine the kinematics of gravity by using the Principle of Equivalence and local inertial frames. I describe how the dynamics of gravity can be similarly understood by suitable thought experiments in a local Rindler frame. This approach puts in proper context several unexplained features of gravity and describes the dynamics of spacetime in a broader setting than in Einstein’s theory.Item Gravity from Spacetime Thermodynamics(Kluwer Academic Publishers, 2003-03-12) Padmanabhan, T.The Einstein-Hilbert action (and thus the dynamics of gravity) can be obtained by: (i) combining the principle of equivalence, special relativity and quantum theory in the Rindler frame and (ii) postulating that the horizon area must be proportional to the entropy. This approach uses the local Rindler frame as a natural extension of the local inertial frame, and leads to the interpretation that the gravitational action represents the free energy of the spacetime geometry. As an aside, one obtains an insight into the peculiar structure of Einstein-Hilbert action and a natural explanation to the questions: (i) Why does the covariant action for gravity contain second derivatives of the metric tensor? (ii)Why is the gravitational coupling constant positive? Some geometrical features of gravitational action are clarified.Item Gravity as elasticity of spacetime: A paradigm to understand horizon thermodynamics and cosmological constant(World Scientific Publishing Company, 2004-05-20) Padmanabhan, T.It is very likely that the quantum description of spacetime is quite di erent from what we perceive at large scales, l (G~=c3)1=2. The long wavelength description of spacetime, based on Einstein's equations, is similar to the description of a continuum solid made of a large number of microscopic degrees of freedom. This paradigm provides a novel interpretation of coordinate transformations as deformations of \spacetime solid" and allows one to obtain Einstein's equations as a consistency condition in the long wave- length limit. The entropy contributed by the microscopic degrees of freedom reduces to a pure surface contribution when Einstein's equations are satis ed. The horizons arises as \defects" in the \spacetime solid" (in the sense of well-de ned singular points) and contributes an entropy which is one quarter of the horizon area. Finally, the response of the microstructure to vacuum energy leads to a near cancellation of the cosmological constant, leaving behind a tiny uctuation which matches with the observed value.Item Gravity: A new holographic perspective(World Scientific Publishing Company, 2005-12-15) Padmanabhan, T.A general paradigm for describing classical (and semiclassical) gravity is presented. This approach brings to the centre-stage a holographic relationship between the bulk and surface terms in a general class of action functionals and provides a deeper insight into several aspects of classical gravity which have no explanation in the conventional approach. After highlighting a series of unresolved issues in the conventional approach to gravity, I show that (i) principle of equivalence, (ii) general covariance and (iii)a reasonable condition on the variation of the action functional, suggest a generic Lagrangian for semiclassical gravity of the form L=QabcdRabcd with ∇b Qabcd=0. The expansion of Qabcd in terms of the derivatives of the metric tensor determines the structure of the theory uniquely. The zeroth order term gives the Einstein-Hilbert action and the first order correction is given by the Gauss-Bonnet action. Any such Lagrangian can be decomposed into a surface and bulk terms which are related holographically. The equations of motion can be obtained purely from a surface term in the gravity sector. Hence the field equations are invariant under the transformation Tab → Tab + λ gab and gravity does not respond to the changes in the bulk vacuum energy density. The cosmological constant arises as an integration constant in this approach. The implications are discussed.Item Gravity and the thermodynamics of horizons(Elsevier Science Publishers, 2004-12-08) Padmanabhan, T.Spacetimes with horizons showa resemblance to thermodynamic systems and it is possible to associate the notions of temperature andentrop y with them. Several aspects of this connection are reviewedin a manner appropriate for broadread ership. The approach uses two essential principles: (a) the physical theories must be formulatedfor each observer entirely in terms of variables any given observer can access and(b) consistent formulation of quantum field theory requires analytic continuation to the complex plane. These two principles, when usedtogether in spacetimes with horizons, are powerful enough to provide several results in a unified manner. Since spacetimes with horizons have a generic behaviour under analytic continuation, standardresults of quantum fieldtheory in curvedspacetimes with horizons can be obtainedd irectly (Sections 3–7). The requirements (a) and(b) also put strong constraints on the action principle describing the gravity and, in fact, one can obtain the Einstein–Hilbert action from the thermodynamic considerations (Section 8). The review emphasises the thermodynamic aspects of horizons, which couldbe obtainedfrom general principles andis expectedto remain valid, independent of the microscopicdescription (‘statistical mechanics’) of horizons.Item Vacuum fluctuations of energy density can lead to the observed cosmological constant(Institute of Physics Publishing, 2005-12-10) Padmanabhan, T.