arXiv subjects

Bibhas Ranjan Majhi

Publications and source records attributed to Bibhas Ranjan Majhi.

At least 19 recordsLinked to original sources

arXiv2026

Entropic route to Brown-York tensor: A unified framework for null and timelike hypersurfaces

Building on Padmanabhan's entropy functional, originally introduced to derive Einstein's equations and highlight the emergent nature of gravity, we demonstrate its robustness in a broader context. Using the same entropy density, we show that the Brown-York tensor is associated with a tangential projection of the entropy-functional polymomentum, thereby providing a common construction applicable to both timelike and null hypersurfaces. This perspective places the two (null and timelike) cases on the same variational footing and clarifies the structural differences of the null Brown-York tensor, including its generally non-symmetric character, without resorting to separate or ad hoc prescriptions in the formulation. We further extend the analysis to scalar-tensor theories, showing that the entropy-based formulation reproduces the expected equations of motion along with the corresponding BY tensor. Our results provide a coherent variational interpretation of quasi-local gravitational quantities and reveal a common underlying structure linking bulk dynamics and boundary momentum.

gr-qc↗

arXiv2026

Quantum speed limit time of a uniformly accelerated Unruh-DeWitt detector

Within the open quantum system formalism, using the Markovian process, we investigate the {\it quantum speed limit time} (QSLT) of a uniformly accelerated Unruh-DeWitt detector, interacting with the massless real scaler field. Three specific scenarios are being studied -- the field is in Minkowski spacetime (i) without any reflecting boundary but at a finite temperature, (ii) presence of single boundary with the detector is accelerating parallel to the plane of it, and (iii) presence of two boundaries with the detector is moving between them along the parallel direction. Interestingly, the behavior of QSLT appears to be very distinctive around a particular value of $a$. For the first scenario QSLT shows very noticeable change around a specific $a$ and the required value of $a$ corresponding to this change decreases with the increase of field temperature. In the other two scenarios, QSLT starts to fluctuate after a critical value of $a$. If we further increase $a$, more vigorous fluctuation emerges. The value of $a$ required for the starting of the oscillation is much less for the double boundary case compared to the single boundary. Taking into account specific values of the distance between the boundaries and the detector's frequency, we estimate that the critical value of $a$ can be as low as $\sim 10^5 \text{m}/\text{s}^2$ for the double boundary situation. We argue that such a distinctive feature in QSLT can be a fruitful tool to diagnose the Unruh effect.

gr-qc↗

arXiv2026

Detector's response to coherent Rindler and Minkowski photons

We observe that the transition probability in a static two-level quantum detector interacting with a coherent Rindler field mode differs from that of the Rindler detector interacting with a coherent Minkowski field mode. The situation does not change in the quantum detector's response in the semiclassical limit of the field state. This we investigate in $(1+1)$ and $(3+1)$-spacetime dimensions. Interestingly, in $(1+1)$-dimensions, the transition probabilities of the ``classical'' detector in the semiclassical limit of the field state for these two scenarios appear to be identical when the field mode and detector frequencies are taken to be the same. However, in $(3+1)$ dimensions, the detector transition probabilities calculated under the large-acceleration condition do not exhibit such a signature. The implications of these observations are also discussed.

quant-ph↗

arXiv2026

A time-like window into tensionless worldsheets

Rindler worldsheets are well known to transition into a tensionless regime in the infinite acceleration limit, acquiring an emergent Carrollian structure. In this work, we uncover an entirely new manifestation of such tensionless dynamics by constructing a time-evolving Milne worldsheet within the future (expanding) and past (contracting) regions of the background target spacetime. We show that as the Milne worldsheet approaches its null horizons, it reveals a deep and previously unexplored interplay between tensionless string dynamics and an emergent Carrollian structure, governed by an ultra-high frequency limit in time evolution. Our results demonstrate that non-inertial string dynamics can emerge in two structurally independent yet complementary settings--governed either by worldsheet acceleration (Rindler) or by the frequency of its intrinsic time evolution (Milne). Remarkably, we uncover a non-trivial duality-like correspondence between Rindler and Milne worldsheets near their respective null horizons, both exhibiting identical tensionless physics despite their distinct causal and dynamical origins. This discovery positions the Milne construction as a genuine time-like counterpart to the ultra-relativistic (Carrollian) or infinite-boost limit, offering new foundational insights into the deep structure of non-inertial string theory.

hep-th↗

arXiv2026

Mach's principle in atomic transitions

We investigate the atomic transition probabilities in atom-mirror set-ups that are in circular motion. In one scenario, the atom is in circular motion inside a static cylindrical mirror. In the other scenario, the cylindrical mirror rotates around its central axis while the atom remains static. We report structural similarity in the atomic transition probabilities between these two cases -- these probabilities are equivalent upon interchanging the field frequencies between the two scenarios. We interpret such an observation as a semi-classical phenomenon analogous to the classical Mach's principle.

quant-ph↗

arXiv2026

New-born strings are tensionless

We report a physical origin of tensionless strings, obtained by formulating string dynamics in finite-lifetime settings for the first time. Departing from the conventional paradigm of eternal strings, we construct an inertial worldsheet confined to a finite region of two-dimensional Minkowski spacetime, known as a causal diamond. This reveals a striking result: tensionless strings arise only at the moment of their birth. The ultra-shrinking limit of the diamond worldsheet realizes this birth configuration, thereby uncovering a new tensionless string phase characterized by a global, ultra-local Carrollian structure.

hep-th↗

arXiv2026

State-Selective Signatures of Quantum and Classical Gravitational Environments

A unified framework is developed for determining whether a gravitational-wave (GW) background behaves as a classical field or as a genuinely quantum environment. Unified here means that both descriptions originate from the same tidal coupling derived from geodesic deviation, which yields an identical quadratic interaction Hamiltonian for the detector; the only distinction lies in whether the GW degrees of freedom are modeled as classical phase-randomized coherent states or as quantized graviton modes. Within this common framework, the reduced dynamics of a quantum harmonic oscillator exhibit a sharp structural contrast: a quantized graviton bath preserves coherence within the lowest phonon-number manifold, forming a protected sector at leading order, whereas a classical stochastic GW field inevitably induces decoherence even inside this subspace. This difference provides an operational criterion for diagnosing the classical or quantum nature of gravitational waves using mesoscopic optomechanical systems. Our results establish decoherence structure - not merely its magnitude - as a sensitive probe of gravitational quantumness and delineate the experimental regimes under which such tests may become feasible.

gr-qc↗

arXiv2026

Entropy of matter on the Carroll geometry

Two prescriptions for the construction of Carroll geometries, the expansion of geometric variables near horizon and the expansion of metric with zero limit of the expansion parameter $c$ (speed of light in vacuum), are known to complement each other. The entropy of an ideal gas, confined in a box and kept very close to the horizon, depends on the transverse area of the container. We show this by using the Carroll geometry constructed through the expansion of the metric and then taking the zero limit of the expansion parameter $c$. Therefore, the present analysis assures the complementing nature of two ways of finding the Carroll geometry from the thermodynamical point of view.

gr-qc↗

arXiv2026

Transonic accretion flow in the mini discs of a binary black hole system

We study the general relativistic transonic accretion flow around the primary black hole, which forms the circumprimary disc (CPD), within a binary black hole (BBH) system. The BBH spacetime is characterized by the mass ratio ($q$) and the separation distance ($z_2$) between the two black holes. We numerically solve the radial momentum and energy equations to obtain the accretion solutions. It is observed that the CPD can exhibit shock solutions, which exist for a wide range parameter space spanned by flow specific angular momentum ($λ$) and energy ($E$). We find that the shock parameter space is modified by $q$ and $z_2$. Investigations show that $q$ and $z_2$ also affect various shock properties, such as density compression and temperature compression across the shock fronts. Moreover, we calculate the spectral energy distributions (SEDs) of the CPD and examine how the SEDs are modified by $q$ and $z_2$ for both shock-free and shock-induced accretion solutions. SED is found to be nearly independent of the binary parameters. We essentially show that although $q$ and $z_2$ alter the effective horizon area of the primary black hole located at the center of the CPD, they have a minimal impact on the dynamical and spectral properties of the accretion flow around the primary black hole.

astro-ph.HE↗

arXiv2025

Timelike transitions in an atom by a mirror in light cone and Kruskal-Szekeres regions: a status of quantum equivalence

We investigate the timelike transitions in a two-level atom in the presence of an infinite reflecting mirror in the future-past light cone regions of a Minkowski spacetime, as well as in the region interior of a $(1+1)$ dimensional Schwarzschild black hole. In particular, when considering the light cone regions, two specific scenarios are dealt with -- $(i)$ a mirror is static in Minkowski spacetime while the atom is attached to a frame confined inside the future light cone region, $(ii)$ an atom is static in Minkowski spacetime, and the mirror is confined inside the future light cone region. For both situations, the atom is interacting with field modes defined in the mirror's frame. Analogous configurations are considered in the black hole spacetime: in one case, the mirror carries field modes represented by the Kruskal time, while the atom follows the Schwarzschild time defined inside the black hole; in the other case, the situations are reversed. The analyses, depending on the frame of the atom, are respectively done within the light cone, Minkowski, Schwarzschild, and Kruskal time-interaction pictures. In all of these scenarios, we observe that the excitation probabilities contain a thermal factor and depend periodically on the separation between the atom and the mirror. At the level of transition probabilities, the aforesaid two scenarios in $(1+1)$ dimensional Minkowski-light cone regions appear to be the same for the equal field and atomic frequencies. However, the same is not true when we consider the $(3+1)$ dimensional Minkowski-light cone or the Schwarzschild interior regions. We also estimate the de-excitation probabilities and encounter similar situations. However, we observe that the excitation to de-excitation ratios (EDRs) corresponding to analogous scenarios are equal for equal atomic and field frequencies.

gr-qc↗

arXiv2025

A note on gravity and fluid dynamic correspondence on a null hypersurface

In the extensive literature on fluid-gravity correspondence formulated on null hypersurfaces, the Carrollian and membrane paradigm approaches have predominantly employed a timelike foliation. By contrast, within the null foliation formalism, only the momentum conservation law, expressed through the Damour-Navier-Stokes (DNS) equation, has been established. In this work, we revisit the null foliation formalism for a generic null hypersurface and extend it to include the energy conservation law, continuity equation, and related relations, all derived from the covariant conservation of an appropriately defined energy-momentum tensor. This development complements the existing literature on the fluid description of gravitational dynamics in the null foliation framework.

gr-qc↗

arXiv2025

A novel quantum memory effect and thermal modulation in graviton-mediated entanglement

A central challenge in probing the quantum nature of gravity is to distinguish effects that are genuinely quantum from those that can be explained classically. In this work, we study how quantized gravitational waves interact with thermal quantum systems, modeled as harmonic oscillators. We show that, unlike classical waves, quantized gravitons generate entanglement and leave behind a persistent ``graviton-induced quantum memory'' even after the wave has passed. This effect is further shaped by the presence of thermal noise, which does not simply wash out quantum correlations but can in fact amplify them in distinctive ways. Our analysis reveals clear signatures - such as nonlinear thermal corrections and a prethermal time-crystal-like phase-that cannot arise from any classical treatment. These results identify experimentally relevant markers of gravitons and provide a framework for exploring how finite-temperature environments may help uncover the quantum nature of gravity.

gr-qc↗

arXiv2025

Thermal scalar field stress tensor on a two dimensional black hole and its near horizon properties

We calculate the thermal renormalized energy-momentum tensor components of a massless scalar field, leading to trace anomaly, on a $(1+1)$ dimensional static black hole spacetime. Using these, the energy density and flux, seen by both static and freely-falling observers, are evaluated. Interestingly for both these observers the aforementioned quantities in the thermal version of Unruh and Boulware states are finite at the horizon when the scalar field is in thermal equilibrium with the horizon temperature (given by the Hawking expression). Whereas in Hartle-Hawking thermal state both the observers see finite energy-density and flux at the horizon, irrespective of the value of field temperature. Particularly in the case of Schwarzschild spacetime a freely falling observer, starts with initial zero velocity, finds its initial critical position $r^c_i = (3/2)r_H$, where $r_H$ is the horizon radius for which energy-density vanishes.

gr-qc↗

arXiv2025

A note on entropy of matter in presence of gravity: status of extensivity of entropy

Entropy of matter in a very strong gravity depends on cross-sectional area of the container of the system -- is being further bolstered by calculating entropy of a monoatomic gas kept under uniform strong gravity at Newtonian scale. This bypasses the earlier analysis where existence of horizon is crucial. Also the extensivity of this entropy has been discussed in the light of the same of a two-space dimensional ultra-relativistic non-interacting gas without gravity. The whole analysis, as far as entropy is concerned, indicates that under strong gravity the microscopic degrees of freedom of the system are effectively contributed by the cross-sectional area of the system.

gr-qc↗

arXiv2025

Gravitationally induced entanglement at finite temperature: A memory-driven time-crystalline phase?

We study the impact of thermal effects on gravity-induced entanglement (GIE) in a system of quantum harmonic oscillators interacting with classical linearly polarized gravitational waves (GWs). Specifically, we model the endpoints of interferometer arms in LIGO-like detectors as two-dimensional oscillators. Following the thermofield dynamics (TFD) approach, our analysis reveals that while thermal effects alone do not generate entanglement between independent oscillator modes, they serve as a catalyst, modifying the dynamical imprint of GWs. Notably, we identify a mixing of Bose-Einstein and Maxwell-Boltzmann distributions driven by thermal influences, which affects the statistical behavior of the quantum subsystem. Furthermore, gravitational interactions induce a quantum memory effect, leading to emergent periodic behavior in the reduced subsystem. This suggests a novel gravitationally induced breaking of time-translation symmetry, reminiscent of a prethermal time crystal (PTC). Our findings indicate that such effects could provide new theoretical insights into classical gravitational wave interactions.

gr-qc↗

arXiv2025

Effect of dark matter halo on transonic accretion flow around a galactic black hole

We investigate the transonic accretion flow in the spacetime of a supermassive black hole (BH) coupled to an anisotropic dark matter fluid, as proposed by Cardoso {\it et al}. We essentially compare the accretion properties of the Cardoso BH with those of an isolated Schwarzschild BH. The Cardoso BH is described by the halo mass ($M_{\rm H}$) and its characteristic length scale ($a_0$). Various classes of accretion solution topologies (e.g., A and W-types) are obtained by solving the dynamical equations of the flow in a fully general relativistic framework. We find that the global accretion solutions in the identified solution topologies are substantially influenced by the halo parameters ($M_{\rm H}, a_0$) when the halo mass is high or the dark matter distribution is concentrated near the black hole. In this high compactness regime, different observational signatures of the accretion disc, like the spectral energy distribution (SED) and bolometric disc luminosity, are found to exhibit considerable deviations from the known results in the Schwarzschild BH model. Furthermore, we obtain shock-induced accretion solutions, where different shock properties, such as the shock radius ($r_{\rm sh}$), flow mass density ($ρ$) compression, and electron temperature ($T_e$) compression across the shock front, are potentially altered from those in the Schwarzschild BH model when the halo compactness is high. Interestingly, the existing shock parameter space, defined by the flow specific angular momentum ($λ$) and energy ($E$), is largely reduced for higher halo compactness compared to that of the Schwarzschild BH. These unique features offer a possible valuable tool for characterizing the presence or absence of a dark matter halo around a galactic black hole.

astro-ph.HE↗

arXiv2025

Can spacetime fluctuations generate entanglement between co-moving accelerated detectors?

Recent studies [Class. Quant. Grav. 42, 03LT01 (2025); Phys. Rev. D 111, 045023 (2025)] indicate that in a nested sequence of Rindler wedges, vacuum of former Rindler frame appears to be thermally populated for an observer in shifted Rindler frame. Interestingly, this thermality is independent of shift parameter as long as it is non-zero and therefore arises even if the shift parameter is as small as Planck length. Building on this insight, we propose a set-up involving two atoms accelerating with identical acceleration. We find that if their Rindler frames (consequently their trajectories) get infinitesimally separated, the atoms become entangled. Remarkably again, this entanglement, like the perceived thermality, is independent of the shift parameter, provided it is non-vanishing. Further we observe the vanishing of mutual information and discord. It implies the absence of both classical and non-classical correlations which are not related to entanglement. We investigate the dependence of entanglement on acceleration of the detectors. The present study indicates that the entanglement between two detectors, moving on the same Rindler wedge, is possible. Moreover, small spacetime fluctuations can lead to entanglement between detectors, moving along same classical trajectory. Hence we feel that such theoretical prediction has potential to probe the Planck length nature of spacetime.

gr-qc↗

arXiv2025

Thermodynamic parameters of fluids on conformally connected spacetimes

Local thermal equilibrium generally implies the absence of heat flux within a fluid. We find the relations between a set of thermodynamic variables of a fluid on a general spacetime and those defined on a conformally connected spacetime, assuming both descriptions are at thermal equilibrium. The scaling relations appear to be consistent with Dicke's heuristic argument and the previous analysis done on the basis of various restrictions. Within the present framework, it is observed that the satisfaction of Klein's law on one of the spacetimes implies its validity on the other one. Moreover, our analysis bypasses some of the imposed restrictions and thereby reveals the generality of the earlier predictions. These relations are further shown to preserve the geometric structure of thermodynamics, known as geometrothermodynamics, such that the associated metrics on two conformally connected spacetimes are themselves conformally related. This provides an alternative consistency check as well as a distinct geometric interpretation of the relations.

gr-qc↗