Research Papers (TP)
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Item Cosmological constant—the weight of the vacuum(Elsevier Science Publishers, 2003-03-01) Padmanabhan, T.Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(Gℏ/c3)≈10−123. This review discusses several aspects of the cosmological constant both from the cosmological (Sections 1–6) and field theoretical (Sections 7–11) perspectives. After a brief introduction to the key issues related to cosmological constant and a historical overview, a summary of the kinematics and dynamics of the standard Friedmann model of the universe is provided. The observational evidence for cosmological constant, especially from the supernova results, and the constraints from the age of the universe, structure formation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar field, …) from different perspectives. The latter part of the review (Sections 7–11) concentrates on more conceptual and fundamental aspects of the cosmological constant like some alternative interpretations of the cosmological constant, relaxation mechanisms to reduce the cosmological constant to the currently observed value, the geometrical structure of the de Sitter spacetime, thermodynamics of the de Sitter universe and the role of string theory in the cosmological constant problem.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 Why gravity has no choice: Bulk spacetime dynamics Is dictated by information entanglement across horizons¹(Springer, 2003-05-21) Padmanabhan, T.Item Thermodynamic structure of lanc zos-lovelock field equations from near-horizon symmetries(American Physical Society, 2009-05-15) Kothawala, Dawood; Padmanabhan, T.It is well known that, for a wide class of spacetimes with horizons, Einstein equations near the horizon can be written as a thermodynamic identity. It is also known that the Einstein tensor acquires a highly symmetric form near static, as well as stationary, horizons. We show that, for generic static spacetimes, this highly symmetric form of the Einstein tensor leads quite naturally and generically to the interpretation of the near-horizon field equations as a thermodynamic identity. We further extend this result to generic static spacetimes in Lanczos-Lovelock gravity, and show that the near-horizon field equations again represent a thermodynamic identity in all these models. These results confirm the conjecture that this thermodynamic perspective of gravity extends far beyond Einstein’s theory.Item Thermodynamics and/of horizons: A comparison of schwarschild, rindler and deSitter spacetimes(World Scientific Publishing Company, 2002-04-28) Padmanabhan, T.