arXiv subjects

Malay K. Nandy

Publications and source records attributed to Malay K. Nandy.

At least 19 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.

gr-qc↗

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

Dynamical Baryogenesis in Rainbow Cosmology

We investigate baryogenesis in the framework of rainbow cosmology employing a complex scalar field with a softly broken global $U(1)$-symmetry. The modified dispersion relation of rainbow cosmology leads to energy-dependent modification to the FLRW metric components, that modifies the Friedmann equation and the scalar field dynamics. We thereby obtain analytical solutions for the scalar field evolution in the radiation-dominated epoch, and show that baryon asymmetry is generated dynamically even from an initially baryon-symmetric state. We find that the baryon-to-photon ratio asymptotically approaches a constant value in the long-time limit, which is proportional to the symmetry-breaking coupling strength $λ$, and scales with the scalar field mass as $M^{-5/2}$. Our results demonstrate that requiring consistency with the observed baryon asymmetry constrains the $λ$ and $M$ values within reasonable ranges, thus providing a viable dynamical mechanism for baryogenesis during the radiation-dominated era without invoking supersymmetry.

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arXiv2026

Parametric Resonance in $ϕ^4$ Preheating: An Exact Numerical Study

Preheating after inflation proceeds through parametric resonance, leading to efficient particle production in scalar field models. In this work, we investigate the structure of parametric resonance in the $ϕ^4$ chaotic inflationary model during the preheating phase by performing a fully numerical analysis of the coupled dynamical equations governing the inflaton field and the mode function of the produced particles, thereby avoiding the approximations commonly employed in earlier studies. Our results reveal resonance patterns that differ significantly from those obtained with approximate analytical treatments. In the weak coupling regime, short-wavelength modes rapidly settle into oscillations with nearly constant amplitude, while the corresponding occupation numbers approach saturation. However, the long-wavelength modes exhibit gradual amplitude growth, with occupation numbers transitioning into a non-linear oscillatory regime. As the coupling strength increases, the dynamics becomes increasingly nonlinear, leading to the emergence of stochastic behavior. In the strong coupling regime, short-wavelength modes display a step-like (staircase) evolution in the occupation number, indicative of intermittent bursts of particle production. However, the long-wavelength modes exhibit a more gradual, monotonic growth with small superimposed fluctuations. These findings highlight the rich, coupling-dependent, structure of parametric resonance in the quartic inflationary model and underscore the importance of exact numerical treatment in accurately capturing preheating dynamics.

astro-ph.CO↗

arXiv2026

Singularity Resolution in Quantum Cosmology via Page-Wootters Formalism

We investigate the problem of classical big bang singularity in a plane-symmetric Bianchi type-I universe within the Wheeler-DeWitt (WDW) framework of quantum gravity. To address the problem of time, we employ the Page-Wootters formalism, which provides a relational notion of dynamics by conditioning the global state on a clock subsystem. Using Misner variables, the WDW equation assumes a Klein-Gordon (KG) type form. Its general solution is constructed as a Gaussian superposition of momentum eigenstates, resulting in an entangled global state between the clock and the remaining subsystem. Within this relational framework, we construct conditional states and obtain the corresponding probability density consistent with the KG-type inner product. The resulting conditional probability density vanishes in the limit of zero volume for all clock values, indicating quantum resolution of the classical singularity. We further show that positivity of the probability density imposes constraints on the admissible clock values, which depend on the parameters of the Gaussian wavepacket. These results highlight the essential role of quantum correlations in the emergence of relational dynamics, and demonstrate that the Page-Wootters formalism provides a consistent and nonsingular probabilistic description of quantum cosmology.

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arXiv2026

Non-Supersymmetric Baryogenesis from $U(1)$-Breaking Scalar Dynamics

We present a non-supersymmetric mechanism for baryogenesis driven by the nonlinear dynamics of a complex scalar field with generalized self-interaction potentials that explicitly break the global $U(1)$ symmetry. Specifically, three representative forms of the interaction potential are considered, which give rise to intricate nonlinear source terms in the evolution of the field components. In all cases, we show that these nonlinear source terms dynamically generate a nonzero Noether charge density from symmetric initial conditions, providing a purely dynamical origin of charge asymmetry. At late times, the charge density scales as $\sim t^{-3/2}$, leading to a constant baryon-to-photon ratio through dynamical freeze-in. While the qualitative behavior is robust across models, the quantitative features depend sensitively on the interaction structure. We find that one class of potentials yields a viable parameter space over a wide range of scalar masses, whereas another requires unrealistically suppressed mass scales. A third scenario stands out in that the final asymmetry is independent of the scalar mass and depends only on the coupling parameter, enhancing predictivity and allowing compatibility across a broad range of energy scales. Assuming efficient transfer of the generated asymmetry to the Standard Model sector and negligible washout effects, the mechanism can account for the observed baryon asymmetry.

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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

Bohmian singularity resolution and quantum relaxation in Bianchi type-I quantum cosmology

We investigate cosmological singularity resolution and relaxation dynamics within the Bohmian mechanics via the plane-symmetric Bianchi type-I minisuperspace model in the Wheeler-DeWitt framework of quantum cosmology by constructing wave functions as Gaussian and Lorentzian wavepackets. Our analyses of the corresponding Bohmian trajectories reveal that Gaussian superposition predominantly yields classical singular solutions, with only a low fraction of small-amplitude cyclic trajectories. On the other hand, the Lorentzian wavepacket, characterized by the power-law momentum tail, generates stronger quantum potential barrier and a substantially rich velocity field, producing a significant fraction of non-singular bounce trajectories over extended volume ranges. We further examine quantum relaxation by evolving non-equilibrium distributions under the corresponding guidance dynamics. The Gaussian superposition exhibits laminar flow leading to boundary accumulation and incomplete relaxation, with non-monotonic decay of the $H$-function followed by saturation. In contrast, the Lorentzian wavepacket induces more complex trajectories, yielding monotonic decay of the $H$-function and better, though still incomplete, approach to Born-rule equilibrium. These results demonstrate that the inherent structure of the wave packet governs both singularity resolution and quantum relaxation through the nature of the Bohmian velocity field.

gr-qc↗

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

Dynamical Evolution and Graceful Exit in Quartic Warm Inflation

In this study, we investigate the full nonlinear dynamics of warm inflation driven by the quartic inflaton potential, avoiding any simplifying approximations. The thermal backreaction is incorporated through a dissipation coefficient that depends linearly on the temperature, and the model parameters are chosen to remain consistent with Planck observational constraints. By numerically integrating the complete set of three coupled, nonlinear differential equations that describe the evolution of the inflaton field, radiation energy density, and background expansion, we obtain an exact description of the system's dynamics. Our results reveal that, while transition to radiation domination is suppressed in the weak regime, the strong dissipative regime leads to a smooth and natural transition to a hot, radiation-dominated Universe, thereby confirming graceful-exit within warm inflation in the quartic scenario. The reheating temperature is extracted directly from the nonlinear evolution of warm inflation, yielding a temperature of approximately 10^13 GeV at the end of inflation, which cools to about 10^12 GeV near radiation-inflaton equality, whence the Universe transitions into a radiation-dominated era.

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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.

gr-qc↗

arXiv2025

Supermassive Neutron Stars in Starobinsky Gravity with Causal Hybrid Stellar Matter

We investigate the stellar structure of neutron stars in the framework of Starobinsky gravity, characterized by a quadratic correction to the Einstein-Hilbert action, $f(R) = R + αR^2$. In order to {\em preserve causality throughout\/} the star, we adopt a two-phase hybrid construction for the stellar matter, in which the core region consists of deconfined quark matter described by the MIT bag model, while the outer layers are composed of hadronic matter represented by unified equations of state such as SLy4, BSk20, and BSk21. Within this framework, we derive the modified field equations with static spherical symmetry, and numerically integrate the corresponding Tolman-Oppenheimer-Volkoff (TOV) equations with the chosen hybrid equations of state. Our analyses show that, unlike in general relativity, the Ricci scalar remains nonzero outside the stellar surface, and gradually falls to zero beyond 50 km, while the stellar surface remains within 10--12 km for the hybrid equations of state considered. This extended Ricci scalar profile arises from the extra degree of freedom (scalaron) inherent in Starobinsky gravity, which also contributes to the gravitational mass outside the star, causing the ADM mass measured at infinity to exceed the stellar mass at the surface. Nevertheless, the key physical relationships, such as the mass-central density and mass-radius curves, remain consistent with what is expected physically. Notably, we find that the maximum stable mass of neutron stars increases with the Starobinsky parameter $α$, with the combined MIT-BSk21 model supporting an ADM mass of up to $2.07 \, M_\odot$ for $α= 10\,r_g^2$. This theoreical limit for a nonrotating neutron star suggests that a rotating configuration could reach mass thresholds in the range of $2.48$ to $2.59 \, M_\odot$, considering that rapid rotation can enhance the maximum mass by approximately 20--25\%.

astro-ph.HE↗

arXiv2025

Trapped string states in AdS$_5$ black hole geometry: A path toward Hawking radiation

We investigate the quantum dynamics of a closed bosonic string in the curved spacetime of an AdS$_5$-Schwarzschild black hole. Starting from the Polyakov action, we perform a canonical quantization of the string and formulate its quantum mechanical equation of motion in the Schrödinger (string coordinate) representation. This framework facilitates in obtaining quantum mechanical wave equation governing the radial and angular modes of the string. A central result of our analysis is the emergence of a trapping radius in the exterior region of the black hole. Near this radius, the radial motion of the string is governed by an effective potential that supports small, quantized oscillations, akin to a quantum harmonic oscillator. This behavior indicates a localization of the string at the trapping surface, where it becomes dynamically confined. The angular sector of the wave function is found to be governed by the confluent Heun equation, yielding confluent Heun functions as the angular part of the wave function. The emergence of a trapping surface is analogous to the stretched horizon proposed by Susskind in the context of black hole complementarity. The quantized harmonic oscillation of the string at the trapping radius complements with the Planck's black body whence the string can emit black-body radiation. Thus, the quantum dynamics of strings in black hole spacetimes offers a novel path to probing the quantum origin of Hawking radiation.

gr-qc↗

arXiv2025

The $α$-Attractor E-Model in Warm Inflation: Observational Viability from Planck 2018

We explore the inflationary evolution and observational viability of the $α$-attractor E-model in the framework of warm inflation, focusing on both weak and strong dissipative regimes, with a dissipation coefficient linear in temperature. In the strong regime, we account for the growth of inflaton fluctuations due to coupling with the radiation bath via two different forms for the dissipation enhancement function: one associated with plateau-like potentials, and another motivated by the warm little inflation scenario. Employing slow roll conditions, we analytically derive the expressions for the key inflationary observables, the spectral index $n_s$ and the tensor-to-scalar ratio $r$, in both dissipative regimes. The resulting theoretical trajectories on the $n_s$--$r$ plane are then juxtaposed with the contour plots obtained from Planck 2018 data in order to constrain the model parameter. Our analysis shows that the warm $α$-attractor E-model remains compatible with observations in both dissipative regimes, with dissipation playing a crucial role in shifting the predictions and enlarging the viable parameter space, highlighting observational robustness of the model when extended to warm inflation.

astro-ph.CO↗

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.

gr-qc↗

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.

gr-qc↗

arXiv2024

Charged black holes in Eddington-inspired Born-Infeld gravity: An in-depth analysis of the structure of spacetime geometry

In this paper, we focus upon the behaviour of spacetime of charged black holes described by Eddington-inspired Born-Infeld (EiBI) gravity. With a static and spherically symmetric metric, we solve the ensuing field equations obtained from the EiBI-Maxwell action in the Palatini formalism. Consequently we carry out, for the first time, an in-depth analysis of the structure of spacetime geometry in several regions of the charged EiBI black hole. In particular, we consider the analytical behaviours of the metric coefficients and the Kretschmann scalar by probing their asymptotic nature {\em analytically} in different regions of the black hole spacetime, such as, near the center, in the intermediate region, and near the horizon, for both positive and negative EiBI coupling. These analyses give a thorough understanding of the nature of spacetime of EiBI-Maxwell black holes. In order to aide our understanding further, we solve the EiBI-Maxwell field equation numerically with different values of the parameters involved. We find close agreement between the analytical behaviours and those obtained from numerical integration of the EiBI-Maxwell field equation.

gr-qc↗

arXiv2024

Stability of charged scalar hair on a Reissner-Nordström black hole

The Israel-Carter theorem (also known as the "no-hair theorem") puts a restriction on the existence of parameters other than mass, electric charge, and angular momentum of a black hole. In this context, Bekenstein proposed no-hair theorems in various black hole models with neutral and electrically charged scalar fields. In this paper, we take the Einstein-Maxwell-charged scalar model with an electrically charged scalar field gauge-coupled to the Maxwell field surrounding a charged black hole with a static spherically symmetric metric. In particular, we consider a quadratic scalar potential without any higher order terms and we do not impose any restriction on the magnitude of the scalar charge with respect to the black hole charge. With this setting, we ascertain the validity of all energy conditions coupled with the causality condition, suggesting the possibility of existence of charged hairy solutions. Consequently, we obtain, by exact numerical integration, detailed solutions of the field equations that incorporate backreaction on the spacetime due to the presence of the charged scalar field. The solutions exhibit damped oscillatory behaviours for the charged scalar hair. We also find that the electric potential is a monotonic function of the radial coordinate, as required by electrodynamics. In order to ascertain the existence of our charged hairy solutions, we carry out dynamic stability analyses against time-dependant perturbations about the static solutions. For a definite conclusion, we employ two different methodologies. The first methodology involves a Sturm-Liouville equation, whereas the second methodology employs a Schrödinger-like equation, for the dynamic perturbations. We find that our solutions are stable against time-dependant perturbations by both methodologies, confirming the existence of the charged hairy solutions.

gr-qc↗