arXiv subjects

Parthasarathi Majumdar

Publications and source records attributed to Parthasarathi Majumdar.

At least 55 records · Page 3Linked to original sources

arXiv2010

Electroweak Theory without a Higgs potential: Radiative Effects

We examine the one loop effective potential in a recently proposed (by Chernodub et. al.) alternative approach to mass generation in the Higgs-gauge sector of the electroweak theory, which does not make use of a classical Higgs potential. We show that the interpretation given by these authors, of the Higgs boson as the conformal degree of freedom in a background conformal gravity theory, is invalidated because genuine one loop radiative effects cancel the local functional measure of the Higgs field taken to be the basis of this interpretation. Functional evaluation of the one loop effective Higgs potential leads to a minimum away from the origin, thereby providing yet another radiative mass generation scheme for weak gauge bosons. In arriving at the one loop effective potential we make use of the gauge free formulation of electrodynamics introduced by us earlier, which obviates any need for gauge fixing.

hep-ph↗

arXiv2010

Circular Orbits in Extremal Reissner Nordstrom Spacetimes

Circular null geodesic orbits, in extremal Reissner-Nordstrom spacetimes, are examined with regard to their stability, and compared with similar orbits in the near-extremal situation. Extremization of the effective potential for null circular orbits shows the existence of a stable circular geodesic in the extremal spacetime, precisely {\it on} the event horizon, which coincides with its null geodesic generator. Such an orbit also emerges as a global minimum of the effective potential for circular {\it timelike} orbits. This type of geodesic is of course absent in the corresponding near-extremal spacetime, as we show here, testifying to differences between the extremal limit of a generic RN spacetime and the exactly extremal geometry.

gr-qc↗

arXiv2009

Entropy of Isolated Horizons revisited

The decade-old formulation of the isolated horizon classically and within loop quantum gravity, and the extraction of the microcanonical entropy of such a horizon from this formulation, is reviewed, in view of recent renewed interest. There are two main approaches to this problem: one employs an SU(2) Chern-Simons theory describing the isolated horizon degrees of freedom, while the other uses a reduced U(1) Chern-Simons theory obtained from the SU(2) theory, with appropriate constraints imposed on the spectrum of boundary states `living' on the horizon. It is shown that both these ways lead to the same infinite series asymptotic in horizon area for the microcanonical entropy of an isolated horizon. The leading area term is followed by an unambiguous correction term logarithmic in area with a coefficient $-\frac32$, with subleading corrections dropping off as inverse powers of the area.

gr-qc↗

arXiv2009

Holography, Gauge-Gravity Connection and Black Hole Entropy

The issues of holography and possible links with gauge theories in spacetime physics is discussed, in an approach quite distinct from the more restricted AdS-CFT correspondence. A particular notion of holography in the context of black hole thermodynamics is derived (rather than conjectured) from rather elementary considerations, which also leads to a criterion of thermal stability of radiant black holes, without resorting to specific classical metrics. For black holes that obey this criterion, the canonical entropy is expressed in terms of the microcanonical entropy of an Isolated Horizon which is essentially a local generalization of the very global event horizon and is a null inner boundary of spacetime, with marginal outer trapping. It is argued why degrees of freedom on this horizon must be described by a topological gauge theory. Quantizing this boundary theory leads to the microcanonical entropy of the horizon expressed in terms of an infinite series asymptotic in the cross-sectional area, with the leading `area-law' term followed by finite, unambiguously calculable corrections arising from quantum spacetime fluctuations.

gr-qc↗

arXiv2008

Holography, CFT and Black Hole Entropy

Aspects of holography or dimensional reduction in gravitational physics are discussed with reference to black hole thermodynamics. Degrees of freedom living on Isolated Horizons (as a model for macroscopic, generic, eternal black hole horizons) are argued to be topological in nature and counted, using their relation to two dimensional conformal field theories. This leads to the microcanonical entropy of these black holes having the Bekenstein-Hawking form together with finite, unambigious {\it quantum spacetime} corrections. Another aspect of holography ensues for radiant black holes treated as a standard canonical ensemble with Isolated Horizons as the mean (equilibrium) configuration. This is shown to yield a universal criterion for thermal stability of generic radiant black holes, as a lower bound on the mass of the equilibrium isolated horizon in terms of its microcanonical entropy. Saturation of the bound occurs at a phase boundary separating thermally stable and unstable phases with symptoms of a first order phase transition.

gr-qc↗

arXiv2007

Generalized Hawking-Page Phase Transition

The issue of radiant spherical black holes being in stable thermal equilibrium with their radiation bath is reconsidered. Using a simple equilibrium statistical mechanical analysis incorporating Gaussian thermal fluctuations in a canonical ensemble of isolated horizons, the heat capacity is shown to diverge at a critical value of the classical mass of the isolated horizon, given (in Planckian units) by the {\it microcanonical} entropy calculated using Loop Quantum Gravity. The analysis reproduces the Hawking-Page phase transition discerned for anti-de Sitter black holes and generalizes it in the sense that nowhere is any classical metric made use of.

gr-qc↗

arXiv2006

Thermal stability of radiant black holes

Beginning with a brief sketch of the derivation of Hawking's theorem of horizon area increase, based on the Raychaudhuri equation, we go on to discuss the issue as to whether generic black holes, undergoing Hawking radiation, can ever remain in stable thermal equilibrium with that radiation. We derive a universal criterion for such a stability, which relates the black hole mass and microcanonical entropy, both of which are well-defined within the context of the Isolated Horizon, and in principle calculable within Loop Quantum Gravity. The criterion is argued to hold even when thermal fluctuations of electric charge are considered, within a {\it grand} canonical ensemble.

gr-qc↗

arXiv2005

Kalb-Ramond field interactions in a braneworld scenario

Electromagnetic and (linearized) gravitational interactions of the Kalb-Ramond (KR) field, derived from an underlying ten dimensional heterotic string in the zero slope limit, are studied in a five dimensional background Randall-Sundrum I spacetime with standard model fields confined to the visible brane having negative tension. The warp factor responsible for generating the gauge hierarchy in the Higgs sector is seen to appear inverted in the KR field couplings, when reduced to four dimensions. This leads to dramatically enhanced rotation, {\it far beyond observational bounds}, of the polarization plane of electromagnetic and gravitational waves, when scattered by a homogeneous KR background. Possible reasons for the conflict between theory and observation are discussed.

hep-th↗

arXiv2005

Universal criterion for black hole stability

It is shown that a non-rotating macroscopic black hole with very large horizon area can remain in stable thermal equilibrium with Hawking radiation provided {\it its mass, as a function of horizon area, exceeds its microcanonical entropy, i.e., its entropy when isolated, without thermal radiation or accretion, and having a constant horizon area} (in appropriate units). The analysis does not use properties of specific classical spacetimes, but depends only on the plausible assumption that the mass is a function of the horizon area for large areas.

gr-qc↗

arXiv2004

Mass and charge fluctuations and black hole entropy

The effects of thermal fluctuations of the mass (horizon area) and electric charge, on the entropy of non-rotating charged {\it macroscopic} black holes, are analyzed using a grand canonical ensemble. Restricting to Gaussian fluctuations around equilibrium, and assuming a power law type of relation between the black hole mass, charge and horizon area, characterized by two real positive indices, the grand canonical entropy is shown to acquire a logarithmic correction with a positive coefficient proportional to the sum of the indices. However, the root mean squared fluctuations of mass and charge relative to the mean values of these quantities turn out to be independent of the details of the assumed mass-area relation. We also comment on possible cancellation between log (area) corrections arising due to {\it fixed area} quantum spacetime fluctuations and that due to thermal fluctuations of the area and other quantities.

gr-qc↗

arXiv2004

Parity-violating Kalb-Ramond-Maxwell interactions and CMB anisotropy in a braneworld

Following up on a recent paper by two of us (DM and SS), demonstrating the large enhancement in observable optical activity in radiation from high redshift sources arising from the string-based coupling of bulk Kalb-Ramond field to the Maxwell Chern Simons three-form on the brane in a Randall-Sundrum braneworld, we exhibit here a similar enhancement in parity-violating temperature-polarization correlations, yet unseen, in the CMB anisotropy due to a generalized parity-violating Kalb-Ramond axion-photon interaction proposed earlier by one of us (PM). The non-observation of such correlations in CMB anisotropies would necessitate unnatural fine tuning of the Kalb-Ramond axion parameters. As a stringy realization of Randall-Sundrum braneworld scenario is yet to be understood properly, our work indicates the need of a careful investigation to establish the connection between string-based phenomenological models and the Randall-Sundrum braneworld scenario

hep-th↗

arXiv2004

Universal canonical entropy for gravitating systems

The thermodynamics of general relativistic systems with boundary, obeying a Hamiltonian constraint in the bulk, is argued to be determined solely by the boundary quantum dynamics, and hence by the area spectrum. Assuming, for large area of the boundary, (a) an area spectrum as determined by Non-perturbative Canonical Quantum General Relativity (NCQGR), (b) an energy spectrum that bears a power law relation to the area spectrum, (c) an area law for the leading order microcanonicai entropy, leading thermal fluctuation corrections to the canonical entropy are shown to be logarithmic in area with a universal coefficient. Since the microcanonical entropy also has univeral logarithmic corrections to the area law (from quantum spacetime fluctuations, as found earlier) the canonical entropy then has a universal form including logarithmic corrections to the area law. This form is shown to be independent of the index appearing in assumption (b). The index, however, is crucial in ascertaining the domain of validity of our approach based on thermal equilibrium.

gr-qc↗

arXiv2003

Universal canonical black hole entropy

Non-rotating black holes in three and four dimensions are shown to possess a canonical entropy obeying the Bekenstein-Hawking area law together with a leading correction (for large horizon areas) given by the logarithm of the area with a {\it universal} finite negative coefficient, provided one assumes that the quantum black hole mass spectrum has a power law relation with the quantum area spectrum found in Non-perturbative Canonical Quantum General Relativity. The thermal instability associated with asymptotically flat black holes appears in the appropriate domain for the index characterising this power law relation, where the canonical entropy (free energy) is seen to turn complex.

gr-qc↗

arXiv2003

Black hole entropy: quantum vs thermal fluctuations

The relation between logarithmic corrections to the area law for black hole entropy, due to thermal fluctuations around an equilibrium canonical ensemble, and those originating from quantum spacetime fluctuations within a microcanonical framework, is explored for three and four dimensional asymptotically anti-de Sitter black holes. For the BTZ black hole, the two logarithmic corrections are seen to precisely cancel each other, while for four dimensional adS-Schwarzschild black holes a partial cancellation is obtained. We discuss the possibility of extending the analysis to asymptotically flat black holes.

gr-qc↗

arXiv2003

Reflection coefficient for superresonant scattering

We investigate superresonant scattering of acoustic disturbances from a rotating acoustic black hole in the low frequency range. We derive an expression for the reflection coefficient, exhibiting its frequency dependence in this regime.

gr-qc↗

arXiv2002

Phenomenological equation of state and late-time cosmic acceleration

Assuming a flat Friedmann-Robertson-Walker cosmology with a single perfect fluid, we propose a pressure-density ratio that evolves as a specific universal function of the scale parameter. We show that such a ratio can indeed be consistent with several observational constraints including those pertaining to late-time accelerated expansion. Generic dynamical scalar field models of Dark energy (with quadratic kinetic terms in their Lagrange density) are shown to be in accord with the proposed equation-of-state ratio, provided the current matter density parameter $Ω_{m0} < 0.23$ - a value {\it not} in agreement with recent measurements.

gr-qc↗

arXiv2002

`Superresonance' from a rotating acoustic black hole

Using the analogy between a shrinking fluid vortex (`draining bathtub'), modelled as a (2+1) dimensional fluid flow with a sink at the origin, and a rotating (2+1) dimensional black hole with an ergosphere, it is shown that a scalar sound wave is reflected from such a vortex with an {\it amplification} for a specific range of frequencies of the incident wave, depending on the angular velocity of rotation of the vortex. We discuss the possibility of observation of this phenomenon, especially for inviscid fluids like liquid HeII, where vortices with quantized angular momentum may occur.

gr-qc↗

arXiv2002

New parity-violating photon-axion interaction

A variant of an earlier proposal by the author and SenGupta, to describe four dimensional Maxwell electrodynamics in Einstein-Cartan spacetimes through a Kalb-Ramond field as an intermediary, is shown to lead to a new Maxwell-Kalb-Ramond coupling that violates spatial parity, even when the KR gauge field has its standard parity assignment. One consequence of this coupling seems to be a modulation, independent of wavelength but dependent on the KR field strength, of the intensity of synchrotron radiation observed from distant galactic sources.

hep-th↗