arXiv subjects

Nihar Ranjan Ghosh

Publications and source records attributed to Nihar Ranjan Ghosh.

8 recordsLinked to original sources

arXiv2026

Dynamical Selection of Horizon-BMS Goldstone Modes in Evaporating Black Holes

We investigate the dynamics of horizon soft degrees of freedom associated with near-horizon BMS supertranslations in an evaporating Vaidya-Schwarzschild black hole spacetime. Considering the dynamic nature of the supertranslation parameter, we derive its effective action directly from the Einstein-Hilbert action. The resulting Goldstone sector is intrinsically coupled to the evolving black hole dynamics, with the mass function entering the evolution of the Goldstone modes while the Goldstone configuration contributes to the dynamical evolution of the black hole mass. Azimuthal periodicity and regularity at the poles select the physically admissible angular sector and yields a mass-dependent $selection\ rule$ on the azimuthal modes. Consequently, the spectrum of horizon-supported Goldstone modes evolves with the black hole mass; as the horizon shrinks during evaporation, the horizon is left with a progressively reduced set of lower-order modes. The evaporation thus induces a dynamical filtering of the horizon soft sector, arising intrinsically from the black hole mass dynamics. These results provide an effective gravitational framework linking near-horizon BMS symmetry, dynamical Goldstone modes, and black hole evaporation, suggesting a direct connection between macroscopic horizon evolution with the microscopic organization of the gravitational soft degrees of freedom residing on the horizon.

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arXiv2026

Near-Horizon BMS Symmetry and Implications on Black Hole Entropy

Thermodynamic significance of near-horizon symmetries remains an important open question in black hole physics, particularly in the context of black hole evaporation and information recovery. In this work, we investigate the role of horizon-adapted Bondi-van der Burg-Metzner-Sachs (BMS)-like supertranslations in the thermodynamic description of a dynamical Schwarzschild black hole. Working in a near-horizon Rindler coordinate system, we construct a class of diffeomorphisms that preserve the horizon structure and promote the associated supertranslation parameter to a Goldstone-like mode arising from the breaking of horizon symmetry. By expanding the Einstein-Hilbert action around the background geometry, we obtain the effective action for the Goldstone mode and identify the corresponding conserved horizon charge from the surface contribution of the action. The relevant horizon is defined at the future outer trapping horizon, while the surface gravity is computed using the Kodama-vector construction appropriate for dynamical spacetimes. We show that the horizon supertranslation mode contributes non-trivially to the surface gravity and modifies the thermodynamic description of the black hole beyond the stationary limit. Using the associated Noether charge, we derive the entropy of the horizon-BMS transformed geometry and find that the Bekenstein-Hawking area law is recovered at leading order, while subleading corrections depend explicitly on the supertranslation sector and the dynamical evolution of the black hole.

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arXiv2026

BMS transformed Quantum String Dynamics near a Black Hole

Asymptotic symmetries are expected to leave subtle but physically meaningful imprints on quantum probes of gravity, yet their manifestation in near-horizon dynamics remains incompletely understood. We examine this question for a closed bosonic string propagating in the near-horizon geometry of a five-dimensional Schwarzschild black hole subjected to a generalized Bondi-van der Burg-Metzner-Sachs (BMS) supertranslation. The extended nature of the string makes it especially sensitive to the resulting anisotropic geometric distortions, and this sensitivity appears most clearly in the angular sector of the worldsheet dynamics. Under the gauge and falloff conditions adopted here, the temporal and radial sectors remain unaffected by the supertranslation, while the angular deformation breaks the original SO(4) symmetry of the background. The radial equation is governed by modified Bessel modes, with a nonvanishing radial conserved current, indicating transport-like propagation. Radial squeezing driven by gravity and anisotropic angular spreading induced by supertranslations provide a dynamical realization of string spreading near the horizon. Thus this analysis demonstrates that probe string dynamics encodes nontrivial signatures of BMS-induced deformations, providing a dynamical probe of symmetry structures in higher-dimensional black hole spacetimes.

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arXiv2026

Constraints on BMS Transformations via Energy Conditions and implications on black hole geometry

We investigate whether the formally infinite-dimensional supertranslation sector of the Bondi-Metzner-Sachs (BMS) group remains fully physically admissible once classical energy conditions are enforced. Working in a perturbative framework $g_{ab}\rightarrow g_{ab}+h_{ab}$, we first develop a general toolkit by expanding the curvature tensors and the Ricci scalar in powers of the perturbation $h_{ab}$ and recast the strong, weak, null and dominant energy conditions (SEC, WEC, NEC and DEC, respectively) as explicit inequalities on $h_{ab}$ following from the Raychaudhuri equation. The formalism is general, but to obtain concrete constraints we specialize to the standard BMS form on a Schwarzschild background and parametrize $h_{ab}=\mathcal{L}_ηg_{ab} $ by a supertranslation function $f(θ,ϕ)$. We find that the SEC and WEC impose nontrivial angular restrictions on $f$ already at next-to-leading order (NLO) in the perturbation, whereas the NEC and DEC are preserved at linear order and acquire their first nontrivial contributions only at next-to-next-to-leading order (NNLO). Notably, the NNLO NEC reduces to a purely angular condition (independent of the radial coordinate), providing the strongest constraint on admissible supertranslations. Thus, imposing energy conditions substantially reduces the space of physically admissible supertranslations; the allowed sector, although remains infinite-dimensional in principle, is substantially constrained in practice.

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arXiv2025

Asymptotic Generation of Kerr Geometry from Schwarzschild via BMS Supertranslations

The Bondi-van der Burg-Metzner-Sachs (BMS) group, as the asymptotic symmetry group of asymptotically flat spacetimes, plays a central role in connecting infrared structures of gravity with soft theorems and gravitational memory. In this work, we investigate the extent to which BMS supertranslations can relate physically distinct black hole geometries. Focusing on the Schwarzschild and Kerr solutions, we show that the asymptotic structure of the Kerr spacetime can be generated from the Schwarzschild geometry via two successive supertranslations. These transformations yield a Kerr-like geometry at null infinity and reveal two distinct classes of supertranslation functions. The first, composed of $l=1$ spherical harmonics, corresponds to center-of-mass displacements and encodes the translational sector of the BMS group. The second, characterized by an infinite series of even-parity Legendre polynomials ($l \geq 2$), captures the intrinsic mass multipole structure of the Kerr spacetime. Our result illustrates how BMS supertranslations can act as symmetry transformations linking asymptotically flat black hole geometries, and that they encode physically meaningful soft hair consistent with the multipole structure of rotating black holes. This work supports a unified description of soft degrees of freedom in black hole spacetimes and underscores the role of infinite-dimensional asymptotic symmetries in gravitational physics.

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arXiv2025

Geodesically Complete Regularized Schwarzschild Black Holes

Classical general relativity predicts a singularity at the center of a black hole, where known laws of physics break down. This suggests the existence of deeper, yet unknown principles of Nature. Among various theoretical possibilities, one of the most promising proposals is a transition to a de Sitter phase at the BH core. This transition, originally proposed by Gliner and Sakharov, ensures the regularity of metric coefficients and avoids the singularity. In search for such a regular BH solution with finite curvature scalar, we propose a metric $g_{rr}$ that exhibits a dS core in the central region. An appealing feature of this metric is the existence of a $single$ event horizon resembling the Schwarzschild black hole. Furthermore, the entire spacetime geometry is determined by the black hole mass alone, in agreement with the Isarel-Carter $no-hair$ $theorem$ for a charge-less, non-rotating black hole. To determine the gravitational action consistent with such a solution, we consider a general Lagrangian density $f(R)$ in place of the Einstein-Hilbert action. By numerically solving the resulting field equation, we find that, in addition to the Einstein-Hilbert term, a Padé approximant in the Ricci scalar $R$ can produce such regular black hole solutions. To assess the physical viability of these black hole solutions, we verify that the proposed metric satisfies the principal energy conditions: DEC, WEC, and NEC, throughout spacetime. Furthermore, in agreement with Zaslavskii's regularity criterion, the metric satisfies the SEC in the range $r\geq r_h/2$, where $r_h$ is the event horizon. Furthermore, with the proposed regularized metric, the expansion scalar in the Raychaudhuri equation remains finite and its derivative vanishes at $r=0$, thereby preventing formation of caustic. This confirms that the spacetime is geodesically complete and free from true physical singularities.

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arXiv2024

Nonlinear Dynamics of the Inner Horizon in Reissner-Nordström Black Holes: Insights into Mass Inflation

The well-known instability of the inner horizon of a Reissner-Nordström black hole, first suggested by Simpson and Penrose, although studied extensively, has remained illusive so far as several studies led to varied conclusions about the dynamical nature of the inner horizon. In this work, we therefore focus upon the dynamic nature of the inner horizon in the course of mass inflation. We model this phenomenon with a massive chargeless scalar field minimally coupled with the Reissner-Nordström spacetime. Employing the Einstein-Maxwell field equation coupled with the Klein-Gordon equation, we obtain a nonlinear dynamical equation for the inner horizon coupled with the dynamics of the mass function and the scalar field. In the S-wave approximation, we develop a perturbative solution about the dynamic inner horizon and obtain an analytical solution as a polynomial of twelfth degree. Our detailed analysis shows that the inner horizon moves inward in the course of mass inflation. Higher the mass of the scalar field, faster are the shrinking rate of the inner horizon and the rate of mass inflation. Our solution for dynamic shrinking of the inner horizon suggests that a Reissner-Nordström spacetime tends towards a Schwarzschild-like geometry, in the infinite advanced time limit.

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arXiv2024

Mass Superinflation from Enhanced Cauchy Horizon Singularity in a Reissner-Nordström Black Hole

Ever since Penrose and Simpson contradicted Novikov's prediction that an infalling passenger would emerge into an asymptotically flat universe, there have been a continued interest in predicting the nature of singularity at the Cauchy horizon of a Reissner-Nordstrom blackhole. This prediction was first confirmed by Poisson and Israel using cross-stream of massless particles, suggesting the phenomenon of mass inflation. Ori however obtained a weaker singularity using a null shell of radiation. In this work, we consider a massive scalar field coupled to the Reissner-Nordstrom geometry and analyze the nature of singularity at the Cauchy horizon. To study the asymptotic behavior of the mass function and the scalar field near the Cauchy horizon, we perturbatively solve the coupled dynamical equations emplyoing the Adomian decomposition method. Our analysis shows that the mass function exhibits a very rapid and unbounded double-exponential growth, called herein mass superinflation, which is enormously stronger than previously obtained singularities. The scalar field is also found to undergo a very strong blueshift near the Cauchy horizon.

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