Publications (23)92.53 Total impact
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ABSTRACT: The quantum genesis of Hawking radiation is a longstanding puzzle in black hole physics. Semiclassically one can argue that the spectrum of radiation emitted by a black hole look very much sparse unlike what is expected from a thermal object. It was demonstrated through a simple quantum model that a quantum black hole will retain a discrete profile, at least in the weak energy regime. However, it was suggested that this discreteness might be an artifact of the simplicity of eigenspectrum of the model considered. Different quantum theories can, in principle, give rise to different complicated spectra and make the radiation from black hole dense enough in transition lines, to make them look continuous in profile. We show that such a hope from a geometryquantized black hole is not realized as long as large enough black holes are dubbed with a classical mass area relation in different gravity theories which ranges from Lanczos–Lovelock to f(R) gravity. We show that the smallest frequency of emission from black hole in any quantum description, is bounded from below, to be of the order of its inverse mass. That leaves the emission with only two possibilities. It can either be nonthermal, or it can be thermal only with the temperature being much larger than 1/M.  [Show abstract] [Hide abstract]
ABSTRACT: The quantum genesis of Hawking radiation is a longstanding puzzle in black hole physics. Semiclassically one can argue that the spectrum of radiation emitted by a black hole look very much sparse unlike what is expected from a thermal object. It was demonstrated through a simple quantum model that a quantum black hole will retain a discrete profile, at least in the weak energy regime. However, it was suggested that this discreteness might be an artifact of the simplicity of eigenspectrum of the model considered. Different quantum theories can, in principle, give rise to different complicated spectra and make the radiation from black hole dense enough in transition lines, to make them look continuous in profile. We show that such a hope from a geometryquantized black hole is not realized as long as large enough black holes are dubbed with holographic relation which tells that the entropy of the black hole can be obtained from the area of the horizon and they have a classical mass area relation. We show that the smallest frequency of emission from black hole in any quantum description, is bounded from below, to be of the order of its inverse mass. That leaves the emission with only two possibilities. It can either be nonthermal, or it can be thermal only with the temperature being much larger than 1/M. In order to populate the emission profile with the holographic relation preserved, one must change the area mass relation of the holes or vice versa.  [Show abstract] [Hide abstract]
ABSTRACT: The crux of the black hole information paradox is related to the fact that the complete information about the initial state of a quantum field in a collapsing spacetime is not available to future asymptotic observers, belying the expectations from a unitary quantum theory. We study the imprints of the initial quantum state, contained in the distortions of the black hole radiation from the thermal spectrum, which can be detected by the asymptotic observers. We identify the class of instates which can be fully reconstructed from the information contained in the distortions at the semiclassical level. Even for the general instate, we can uncover a specific amount of information about the initial state. For a large class of initial states, some specific observables defined in the initial Hilbert space are completely determined from the resulting final spectrum. These results suggest that a \textit{classical} collapse scenario ignores this richness of information in the resulting spectrum and a consistent quantum treatment of the entire collapse process might allow us to retrieve all the information from the spectrum of the final radiation.  [Show abstract] [Hide abstract]
ABSTRACT: We study gravitational lensing by a recently proposed black hole solution in Loop Quantum Gravity. We highlight the fact that the quantum gravity corrections to the Schwarzschild metric in this model evade the `mass suppression' effects (that the usual quantum gravity corrections are susceptible to) by virtue of one of the parameters in the model being dimensionless, which is unlike any other quantum gravity motivated parameter. Gravitational lensing in the strong and weak deflection regimes is studied and a sample consistency relation is presented which could serve as a test of this model. We discuss that though the consistency relation for this model is qualitatively similar to what would have been in BransDicke, in general it can be a good discriminator between many alternative theories. Although the observational prospects do not seem to be very optimistic even for a galactic supermassive black hole case, time delay between relativistic images for billion solar mass black holes in other galaxies might be within reach of future relativistic lensing observations.  [Show abstract] [Hide abstract]
ABSTRACT: Can certain degrees of freedom of a closed physical system, described by a timeindependent Hamiltonian, become more and more classical as they evolve from 1 some state? This question is important because our universe seems to have done just that! We construct an explicit, simple, example of such a system with just two degrees of freedom, of which one achieves `spontaneous classicalization'. It starts from a quantum state and under the usual Hamiltonian evolution, becomes more and more classical (in a welldefined manner in terms of the Wigner function) as time progresses. This is achieved without the usual procedures of integrating out a large number of environmental degrees of freedom or conventional decoherence. We consider different ranges of parameter space and identify the conditions under which spontaneous classicalization occurs in our model. The mutual interaction between the subsystems of a larger system can indeed drive some of the subsystems to a classical configuration, with a phase space trajectory of evolution. We also argue that the results of our toy model may well be general characteristics of certain class of interacting systems. Several implications are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: The appearance of the inertial vacuum state in Rindler frame has been extensively studied in the literature, both from the point of view of QFT developed using Rindler foliation and using the response of an UnruhDewitt Detector (UDD). In comparison, less attention has been devoted to the study of inertial nonvacuum states when viewed from the Rindler frame. We provide a comprehensive study of this issue in this paper. We first present a general formalism describing characterization of an arbitrary inertial state when described using (i) an arbitrary foliation and (ii) the response of UDD moving along an arbitrary trajectory. We use this formalism to explicitly compute the results for the Rindler frame and uniformly accelerated detectors. Any arbitrary inertial state will always have a thermal component in the Rindler frame with additional contributions arising from the nonvacuum nature of the inertial state. We classify the nature of the additional contributions in terms of functions characterizing the inertial state. We establish that for all physically well behaved normalizable inertial states, the correction terms decay with the energy of the Rindler mode so that the high frequency limit is dominated by the thermal noise. However, inertial states which are not strictly normalizable, lead to a constant contribution at all high frequencies in the Rindler frame. A similar behavior arises in the response of the UDD as well. In the case of the detector response, we provide a physical interpretation for the constant contribution at high frequencies in terms of total detection rate of comoving inertial detectors. We discuss two different approaches for defining a transition rate for the UDD, when the twopoint function lacks the time translation invariance and discuss several features of different definitions of transition rates. The implications are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: It has been suggested in the literature that spatial coherence of the wave function can be dynamically suppressed by fluctuations in the gravity of spacetime. These fluctuations represent the minimal uncertainty that is present when one probes spacetime geometry with a quantum probe. Two similar models have been proposed, one by Di\'osi [Dmodel] and one by Karolyhazy and collaborators [Kmodel], based on apparently unrelated minimal spacetime bounds. The two models arrive at somewhat different expressions for the dependence of the localization coherence length on the mass and size of the quantum object. In the present article we compare and contrast the two models. We show that under certain conditions the minimal spacetime bounds in the two models can be derived one from the other. We also derive the twopoint correlation for the fluctuation potential in the Kmodel, and show that it is nonwhite noise, unlike in the Dmodel, where the corresponding correlation is white noise in time. This seems to be the origin of the different results in the two models. We derive the nonMarkovian master equation for the Kmodel. We argue that the minimal spacetime bound cannot predict the noise correlation uniquely, and additional criteria are necessary to accurately determine the effects of gravitationally induced decoherence. 
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ABSTRACT: Can certain degrees of freedom of a closed physical system, described by a timeindependent Hamiltonian, become more and more classical as they evolve from some state? This question is important because our universe seems to have done just that! We construct an explicit, simple, example of such a system with just two degrees of freedom, of which one achieves `spontaneous classicalization'. It starts from a quantum state and under the usual Hamiltonian evolution, becomes more and more classical (in a welldefined manner in terms of the Wigner function) as time progresses. This is achieved without the usual procedures of integrating out a large number of environmental degrees of freedom or conventional decoherence. We consider different ranges of parameter space and identify the conditions under which spontaneous classicalization occurs in our model. The mutual interaction between the subsystems of a larger system can indeed drive some of the subsystems to a classical configuration, with a phase space trajectory of evolution. We also argue that the results of our toy model may well be general characteristics of certain class of interacting systems. Several implications are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: Trace Dynamics is a classical theory of noncommuting matrices, which uses cyclic permutation inside a trace to define the derivative with respect to an operator. We have used the methods of Trace Dynamics to construct a noncommutative special relativity. We have defined a lineelement using the trace over spacetime coordinates, which are assumed to be operators. The lineelement is shown to be invariant under generalized Lorentz transformations, and is used to construct a noncommutative relativistic dynamics. We have been motivated for such an operator structure at a more fundamental level, and attempt to obtain an emergent picture of classical spacetime.  [Show abstract] [Hide abstract]
ABSTRACT: The inflationary paradigm provides a mechanism to generate the primordial perturbations needed to explain the observed large scale structures in the universe. Inflation traces back all the inhomogeneities to quantum fluctuations although the structures look classical today. Squeezing of primordial quantum fluctuations along with the mechanism of decoherence accounts for many aspects of this quantum to classical transition, although it remains a matter of debate as to whether this is sufficient to explain the issue of realization of a single outcome (i.e. the issue of macroobjectification) from a quantum ensemble given that the universe is a closed system. A similar question of emergence of classical behavior of macroscopic objects exists also for laboratory systems and apart from decoherence there have been attempts to resolve this issue through Continuous Spontaneous Localization (CSL), which is a stochastic nonlinear modification of the nonrelativistic Schr\"{o}dinger equation. Recently, Martin {\it et al.} have investigated whether a CSLlike mechanism with a constant strength parameter, when the MukhanovSasaki variable is taken as the "collapseoperator", can explain how the primordial quantum perturbations generated during inflation become classical. Within the scope of their assumptions they essentially come to a negative conclusion. In the present work, we generalize their analysis by allowing the CSL strength parameter to depend on physical scales so as to capture the CSL amplification mechanism. We show that such a generalization provides a mechanism for macroobjectification (i.e. classicalization) of the inflationary quantum perturbations, while also preserving scale invariance of the power spectrum and phase coherence of superhorizon perturbation modes in a particular class of these models.  [Show abstract] [Hide abstract]
ABSTRACT: Continuous Spontaneous Localization (CSL) model of Quantum Mechanics modifies Schr\"{o}dinger equation by adding nonlinear stochastic terms due to which the total energy of a system increases with a constant rate which is proportional to the collapse rate $\lambda$. Thus applying CSL model to cosmological scenarios can change the thermal behaviour of the particles during evolution and we will put constraints on $\lambda$ by considering several cosmological scenarios.  [Show abstract] [Hide abstract]
ABSTRACT: This brief article reviews stochastic processes as relevant to dynamical models of wavefunction collapse, and is supplemental material for the review article arXiv:1204.4325  [Show abstract] [Hide abstract]
ABSTRACT: We attempt to calculate the point separated Noise Kernel for self similar Tolman Bondi metric, using a method similar to that developed by Eftekharzadeh et. al for ultrastatic spacetimes referring to the work by Page. In case of formation of a naked singularity, the Noise Kernel thus obtained is found to be regular except on the Cauchy horizon, where it diverges. The behavior of the noise in case of the formation of a covered singularity is found to be regular. This result seemingly renders back reaction nonnegligible which questions the stability of the results obtained from the semiclassical treatment of the self similar Tolman Bondi metric.  [Show abstract] [Hide abstract]
ABSTRACT: Models of spontaneous wave function collapse modify the linear Schrödinger equation of standard quantum mechanics by adding stochastic nonlinear terms to it. The aim of such models is to describe the quantum (linear) nature of microsystems along with the classical nature (violation of superposition principle) of macroscopic ones. The addition of such nonlinear terms in the Schrödinger equation leads to nonconservation of energy of the system under consideration. Thus, a striking feature of collapse models is to heat nonrelativistic particles with a constant rate. If such a process is physical, then it has the ability to perturb the wellunderstood thermal history of the universe. In this article we will try to investigate the impacts of such heating terms, according to the continuous spontaneous localization model, on the standard evolution of nonrelativistic matter and on the formation of cosmic microwave background radiation. We will also put constraints on the continuous spontaneous localization collapse rate λ by considering that the standard evolution of nonrelativistic matter is not hampered and the observed precise blackbody spectrum of cosmic microwave background radiation would not get distorted (in the form of μtype and ytype distortions) so as to violate the observed bounds.  [Show abstract] [Hide abstract]
ABSTRACT: In this paper we discuss the entropy of quantum black holes in the LQG formalism when the number of punctures on the horizon is treated as a quantum hair, that is we compute the black hole entropy in the grand canonical (area) ensemble. The entropy is a function of both the average area and the average number of punctures and bears little resemblance to the BekensteinHawking entropy. In the thermodynamic limit, both the "temperature" and the chemical potential can be shown to be functions only of the average area per puncture. At a fixed temperature, the average number of punctures becomes proportional to the average area and we recover the BekensteinHawking areaentropy law to leading order provided that the BarberoImmirzi parameter, $\gamma$, is appropriately fixed. This also relates the chemical potential to $\gamma$. We obtain a subleading correction, which differs in signature from that obtained in the microcanonical and canonical ensembles in its sign but agrees with earlier results in the grand canonical ensemble.  [Show abstract] [Hide abstract]
ABSTRACT: We extend previous results on the reflection and transmission of selfgravitating dust shells across the apparent horizon during quantum dust collapse to nonmarginallybound dust collapse in arbitrary dimensions with a negative cosmological constant. We show that the Hawking temperature is independent of the energy function and that the wave functional describing the collapse is well behaved at the HawkingPage transition point. Thermal radiation from the apparent horizon appears as a generic result of nonmarginal collapse in AdS spacetime owing to the singular structure of the Hamiltonian constraint at the apparent horizon.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum mechanics is an extremely successful theory that agrees with every experiment. However, the principle of linear superposition, a central tenet of the theory, apparently contradicts a commonplace observation: macroscopic objects are never found in a linear superposition of position states. Moreover, the theory does not really explain as to why during a quantum measurement, deterministic evolution is replaced by probabilistic evolution, whose random outcomes obey the Born probability rule. In this article we review an experimentally falsifiable phenomenological proposal, known as Continuous Spontaneous Collapse: a stochastic nonlinear modification of the Schr\"{o}dinger equation, which resolves these problems, while giving the same experimental results as quantum theory in the microscopic regime. Two underlying theories for this phenomenology are reviewed: Trace Dynamics, and gravity induced collapse. As one approaches the macroscopic scale, the predictions of this proposal begin to differ appreciably from those of quantum theory, and are being confronted by ongoing laboratory experiments that include molecular interferometry and optomechanics. These experiments, which essentially test the validity of linear superposition for large systems, are reviewed here, and their technical challenges, current results, and future prospects summarized. We conclude that it is likely that over the next two decades or so, these experiments can verify or rule out the proposed stochastic modification of quantum theory.  [Show abstract] [Hide abstract]
ABSTRACT: There ought to exist a description of quantum field theory which does not depend on an external classical time. To achieve this goal, in a recent paper we have proposed a noncommutative special relativity in which spacetime and matter degrees of freedom are treated as classical matrices with arbitrary commutation relations, and a spacetime line element is defined using a trace. In the present paper, following the theory of Trace Dynamics, we construct a statistical thermodynamics for the noncommutative special relativity, and show that one arrives at a generalized quantum dynamics in which space and time are nonclassical and have an operator status. In a future work, we will show how standard quantum theory on a classical spacetime background is recovered from here.  [Show abstract] [Hide abstract]
ABSTRACT: We compute the canonical partition for quantum black holes in the approach of Loop Quantum Gravity (LQG). We argue that any quantum theory of gravity in which the horizon area is built of noninteracting constituents cannot yield qualitative corrections to the BekensteinHawking (BH) area law, but corrections to the area law can arise as a consequence additional constraints inducing interactions between the constituents. In LQG this is implemented by requiring spherical horizons. The canonical approach for LQG favours a logarithmic correction to the BH law with a coefficient of 1/2, independently of the area spectrum. Our initial calculation of the partition function uses certain approximations that, we show, do not qualitatively affect the expression for the black hole entropy. We later discuss the quantitative corrections to these results when the simplifying approximations are relaxed and the full LQG spectrum is dealt with. We show how these corrections can be recovered to all orders in perturbation theory. However, the convergence properties of the perturbative series remains unknown.
Publication Stats
167  Citations  
92.53  Total Impact Points  
Top Journals
Institutions

20142016

Savirtibai Phule Pune University
Poona, Maharashtra, India 
INFN  Istituto Nazionale di Fisica Nucleare
Frascati, Latium, Italy


20092015

Tata Institute of Fundamental Research
Mumbai, Maharashtra, India
