Das, Saurya
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Browsing Das, Saurya by Subject "Black holes"
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- ItemBlack hole emission rates and the AdS/CFT correspondence(SISSA, 1999) Das, Saurya; Dasgupta, ArundhatiWe study the emission rates of scalar, spinor and vector particles from a 5 dimensional black hole for arbitrary partial waves. The solution is lifted to 6 dimensions, and the near horizon BTZ S3 geometry of the black hole solution is probed to determine the greybody factors. We show that the exact decay rates can be reproduced from a (1 + 1)-dimensional conformal eld theory which lies on the boundary of the near horizon geometry. The AdS/CFT correspondence is used to determine the dimension of the CFT operators corresponding to the bulk elds. These operators couple to plane waves incident on the CFT from in nity to produce emission in the bulk.
- ItemBlack hole thermodynamics: entropy, information and beyond(2015-12-16) Das, SauryaWe review some recent advances in black hole thermodynamics, including statistical mechanical origins of black hole entropy and its leading order corrections, from the viewpoints of various quantum gravity theories. We then examine the information loss problem and some possible approaches to its resolution. Finally, we study some proposed experiments which may be able to provide experimental signatures of black holes.
- ItemCharged black holes in generalized dilaton-axion gravity(2015-12-16) Sur, Sourav; Das, Saurya; SenGupta, SoumitraWe study generic Einstein-Maxwell-Kalb-Ramond-dilaton actions, and derive conditions under which they give rise to static, spherically symmetric black hole solutions. We obtain new asymptotically flat and non-flat black hole solutions which are in general electrically and magnetically charged. They have positive definite and finite quasi-local masses. Existing non-rotating black hole solutions (including those appearing in low energy string theory) are recovered in special limits.
- ItemConserved quantities in Kerr-anti-de Sitter spacetimes in various dimensions(SISSA, 2000) Das, Saurya; Mann, Robert B.We compute the conserved charges for Kerr anti-de Sitter spacetimes in various dimensions using the conformal and the counterterm prescriptions. We show that the conserved charge corresponding to the global timelike killing vector computed by the two methods di er by a constant dependent on the rotation parameter and cosmological constant in odd spacetime dimensions, whereas the charge corresponding to the rotational killing vector is the same in either approach. We comment on possible implications of our results to the AdS/CFT correspondence.
- ItemEikonal particle scattering and dilaton gravity(American Physical Society, 1997) Das, Saurya; Majumdar, ParthasarathiApproximating light charged pointlike particles in terms of ~nonextremal! dilatonic black holes is shown to lead to certain pathologies in Planckian scattering in the eikonal approximation, which are traced to the presence of a ~naked! curvature singularity in the metric of these black holes. The existence of such pathologies is confirmed by analyzing the problem in an ‘‘external metric’’ formulation where an ultrarelativistic point particle scatters off a dilatonic black hole geometry at large impact parameters. The maladies disappear almost trivially upon imposing the extremal limit. Attempts to derive an effective three-dimensional ‘‘boundary’’ field theory in the eikonal limit are stymied by four-dimensional ~bulk! terms proportional to the light-cone derivatives of the dilaton field, leading to nontrivial mixing of electromagnetic and gravitational effects, in contrast with the case of general relativity. An eikonal scattering amplitude, showing decoupling of these effects, is shown to be derivable by resummation of graviton, dilaton, and photon exchange ladder diagrams in a linearized version of the theory for an asymptotic value of the dilaton field which makes the string coupling constant nonperturbative.
- ItemEntanglement entropy in all dimensions(2015-12-15) Braunstein, Samuel L.; Das, Saurya; Shankaranarayanan, S.It has long been conjectured that the entropy of quantum fields across boundaries scales as the boundary area. This conjecture has not been easy to test in spacetime dimensions greater than four because of divergences in the von Neumann entropy. Here we show that the R´enyi entropy provides a convergent alternative, yielding a quantitative measure of entanglement between quantum field theoretic degrees of freedom inside and outside hypersurfaces. For the first time, we show that the entanglement entropy in higher dimensions is proportional to the higher dimensional area. We also show that the R´enyi entropy diverges at specific values of the R´enyi parameter q in each dimension, but this divergence can be tamed by introducing a mass to the quantum field.
- ItemGravitational anomalies, Hawking radiation, and spherically symmetric black holes(2016-01-06) Vagenas, Elias C.; Das, SauryaMotivated by the recent work of Robinson and Wilczek, we evaluate the gravitational anomaly of a chiral scalar field in a Vaidya spacetime of arbitrary mass function, and thus the outgoing flux from the time-dependent horizon in that spacetime. We show that this flux differs from that of a perfect blackbody at a fixed temperature. When this flux is taken into account, general covariance in that spacetime is restored. We also generalize their results to the most general static, and spherically symmetric spacetime.
- ItemGravitational anomalies: a recipe for Hawking radiation(2016-01-07) Das, Saurya; Robinson, Sean P.; Vagenas, Elias C.We explore the method of Robinson andWilczek for deriving the Hawking temperature of a black hole. In this method, the Hawking radiation restores general covariance in an effective theory of near-horizon physics which otherwise exhibits a gravitational anomaly at the quantum level. The method has been shown to work for broad classes of black holes in arbitrary spacetime dimensions. These include static black holes, accreting or evaporating black holes, charged black holes, rotating black holes, and even black rings. In the cases of charged and rotating black holes, the expected super-radiant current is also reproduced.
- ItemHigh energy effects on D-brane and black hole emission rates(American Physical Society, 1997) Das, Saurya; Dasgupta, Arundhati; Sarkar, TapobrataWe study the emission of scalar particles from a class of near-extremal five-dimensional black holes and the corresponding D-brane configuration at high energies. We show that the distribution functions and the black hole greybody factors are modified in the high energy tail of the Hawking spectrum in such a way that the emission rates exactly match. We extend the results to charged scalar emission and to four dimensions.
- ItemHigh frequency quasi-normal modes for black-holes with generic singularities(2015-12-23) Das, Saurya; Shankaranarayanan, S.We compute the high frequency quasi-normal modes (QNM) for scalar perturbations of spherically symmetric single horizon black-holes in (D+2)-space-time dimensions with generic curvature singularities and having metrics of the form ds2 = xp(dy2−dx2)+xqd 2 D near the singularity x = 0. The real part of the QN frequencies is shown to be proportional to log [1 + 2 cos ( [qD − 2] /2)] where the constant of proportionality is equal to the Hawking temperature for non-degenerate black-holes and inverse of horizon radius for degenerate black-holes. Apart from agreeing with the QN frequencies that have been computed earlier, our results imply that the horizon area spectrum for the general spherically symmetric black-holes is equispaced. Applying our results, we also find the QNM frequencies for extremal Reissner-Nordstr¨om and various stringy black-holes.
- ItemHigh frequency quasi-normal modes for black-holes with generic singularities: II. Asymptotically non-flat spacetimes(2016-01-05) Ghosh, Archisman; Shankaranarayanan, S.; Das, SauryaThe possibility that the asymptotic quasi-normal mode (QNM) frequencies can be used to obtain the Bekenstein-Hawking entropy for the Schwarzschild black hole — commonly referred to as Hod’s conjecture — has received considerable attention. To test this conjecture, using monodromy technique, attempts have been made to analytically compute the asymptotic frequencies for a large class of black hole spacetimes. In an earlier work, two of the current authors computed the high frequency QNMs for scalar perturbations of (D+2)-dimensional spherically symmetric, asymptotically flat, single horizon spacetimes with generic power-law singularities. In this work, we extend these results to asymptotically non-flat spacetimes. Unlike the earlier analyses, we treat asymptotically flat and de Sitter spacetimes in a unified manner, while the asymptotic anti-de Sitter spacetimes is considered separately. We obtain master equations for the asymptotic QNM frequency for all the three cases. We show that for all the three cases, the real part of the asymptotic QNM frequency – in general – is not proportional to ln(3) thus indicating that the Hod’s conjecture may be restrictive.
- ItemHow classical are TeV-scale black holes?(2015-12-23) Cavagalià, Marco; Das, SauryaWe show that the Hawking temperature and the entropy of black holes are subject to corrections from two sources: the generalized uncertainty principle and thermal fluctuations. Both effects increase the temperature and decrease the entropy, resulting in faster decay and “less classical” black holes. We discuss the implications of these results for TeV-scale black holes that are expected to be produced at future colliders.
- ItemMicrocanonical entropy of a black hole(2016-01-25) Bhaduri, Rajat K.; Tran, Muoi N.; Das, SauryaIt has been suggested recently that the microcanonical entropy of a system may be accurately reproduced by including a logarithmic correction to the canonical entropy. In this paper we test this claim both analytically and numerically by considering three simple thermodynamic models whose energy spectrum may be defined in terms of one quantum number only, as in a non-rotating black hole. The first two pertain to collections of noninteracting bosons, with logarithmic and power-law spectra. The last is an area ensemble for a black hole with equi-spaced area spectrum. In this case, the many-body degeneracy factor can be obtained analytically in a closed form. We also show that in this model, the leading term in the entropy is proportional to the horizon area A, and the next term is lnA with a negative coefficient.
- ItemPlanckian scattering of D-branes(Elsevier BV North Holland, 1988) Das, Saurya; Dasgupta, Arundhati; Ramadevi, P.; Sarkar, TapobrataWe consider the gravitational scattering of point particles in four dimensions, at Planckian centre of mass energy and low momentum transfer, or the eikonal approximation. The scattering amplitude can be exactly computed by modelling point particles by very generic metrics. A class of such metrics are black hole solutions obtained from dimensional reduction of p-brane solutions with one or more Ramond-Ramond charges in string theory. At weak string coupling, such black holes are replaced by a collection of wrapped D-branes. Thus, we investigate eikonal scattering at weak coupling by modelling the point particles by wrapped D-branes and show that the amplitudes exactly match the corresponding amplitude found at strong coupling. We extend the calculation for scattering of charged particles.
- ItemQuantum mechanical spectra of charged black holes(Elsevier, 2003) Das, Saurya; Ramadevi, P.; Yajnik, U. A.; Sule, A.It has been argued by several authors, using different formalisms, that the quantum mechanical spectrum of black hole horizon area is discrete and uniformly spaced. Recently it was shown that two such approaches, namely the one involving quantization on a reduced phase space, and the algebraic approach of Bekenstein and Gour are equivalent for spherically symmetric, neutral black holes (hep-th/0202076). That is, the observables of one can be mapped to those of the other. Here we extend that analysis to include charged black holes. Once again, we find that the ground state of the black hole is a Planck size remnant.
- ItemQuantum mechanics of charged black holes(Elsevier, 2001) Barvinsky, Andrei; Das, Saurya; Kunstatter, GaborWe quantize the spherically symmetric sector of generic charged black holes. Thermal properties are incorporated by imposing periodicity in Euclidean time, with period equal to the inverse Hawking temperature of the black hole. This leads to an exact quantization of the area (A) and charge (Q) operators. For the Reissner–Nordström black hole, A = 4πG¯h(2n + p + 1) and Q= me, for integers n,p,m. Consistency requires the fine structure constant to be quantized: e2/¯h = p/m2. Remarkably, vacuum fluctuations exclude extremal black holes from the spectrum, while near extremal black holes are highly quantum objects. We also prove that horizon area is an adiabatic invariant.
- ItemShock wave mixing in Einstein and dilaton gravity(Elsevier BV North Holland, 1995) Das, Saurya; Majumdar, ParthasarathiWe consider possible mixing of electromagnetic and gravitational shock waves, in the Planckian energy scattering of point particles in Minkowski space, By boosting a Reissner-Nordstriim black hole solution to the velocity of light, it is shown that no mixing of shock waves takes place for arbitrary finite charge carried by the black hole. However, a similar boosting procedure for a charged black hole solution in dilaton gravity yields some mixing: the wave function of even a neutral test particle, acquires a small additional phase factor depending on the dilalonic black hole charge. Possible implications for poles in the amplitudes for the dilaton gravity case are discussed
- ItemSpectrum of rotating black holes and its implications for Hawking radiation(2015-12-16) Das, Saurya; Mukhopadhyay, Himan; Ramadevi, P.The reduced phase space formalism for quantising black holes has recently been extended to find the area and angular momentum spectra of four dimensional Kerr black holes. We extend this further to rotating black holes in all spacetime dimensions and show that although as in four dimensions the spectrum is discrete, it is not equispaced in general. As a result, Hawking radiation spectra from these black holes are continuous, as opposed to the discrete spectrum predicted for four dimensional black holes.
- ItemWhere are the degrees of freedom responsible for black hole entropy?(2015-12-15) Das, Saurya; Shankaranarayanan, S.; Sur, SouravConsidering the entanglement between quantum field degrees of freedom inside and outside the horizon as a plausible source of black hole entropy, we address the question: where are the degrees of freedom that give rise to this entropy located? When the field is in ground state, the black hole area law is obeyed and the degrees of freedom near the horizon contribute most to the entropy. However, for excited state, or a superposition of ground state and excited state, power-law corrections to the area law are obtained, and more significant contributions from the degrees of freedom far from the horizon are shown.