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Saurya Das

Publications and source records attributed to Saurya Das.

At least 19 recordsLinked to original sources
arXiv2026

Spinning test particles in the spacetime of a global monopole

We investigate the motion of spinning test particles in the spacetime of a global monopole in the framework of the Mathisson-Papapetrou-Dixon equations. By making use of the symmetries of the spacetime, we obtain a general exact solution to the equations of motion. We show that the particle's trajectories, momenta, and spin can be expressed in terms of three specific functions of the polar and azimuthal angles. We also show that the system is completely integrable. We obtain the general non-geodesic trajectory of the particle, and also examine the particular cases of radial and planar motion. We compare the non-geodesic trajectories of a spinning particle with a non-spinning particle.

gr-qc↗
arXiv2026

Quantum resolution of the Schwarzschild singularity

We revisit the Schwarzschild singularity in a semiclassical setting where the background geometry is classical and quantum effects enter through Bohmian (quantal) trajectories associated with a Klein Gordon wave packet. Using the Madelung-Bohm decomposition of the Klein Gordon wavefunction, we show that the quantum-modified motion is equivalent to geodesic motion in an effective metric conformally related to Schwarzschild, with a conformal factor fixed by the wavefunction amplitude. Solving the wavefunction equation near $r\to 0$ determines this factor and yields finite curvature invariants, in suitable coordinates the interior extends smoothly and the effective spacetime is geodesically complete. This suggests that quantum dynamics on a fixed classical background can regularize the Schwarzschild singularity without a full theory of quantum gravity.

gr-qc↗
arXiv2026

Closing the Cosmographic Hierarchy: Dynamical Attractors from Inflation to Reheating

We develop a potential-independent cosmographic framework, in which cosmographic parameters are promoted to dynamical variables within a closed autonomous system. Although the cosmographic hierarchy is formally infinite, we achieve closure by mapping potential slow-roll parameters onto the kinematic phase space within General Relativity with a minimally coupled scalar field. Within this framework, we perform a stability analysis and show that inflationary (quasi-de Sitter) solutions arise as natural attractors, while stiff-fluid configurations act as repellers without invoking the slow-roll approximation. To describe the transition to standard Big Bang evolution, we extend the system to include a radiation component and a phenomenological decay term. This leads to a generalized, potential-independent description of reheating characterized by an effective equation of state $w_{\rm eff}$. We demonstrate that the radiation-dominated phase is the late-time attractor of the extended system. These results provide a unified kinematical description of the expansion history from inflation through reheating, bridging cosmography and scalar field dynamics.

gr-qc↗
arXiv2025

A novel violation of the equivalence principle

It is generally assumed that any discrepancy between an object's inertial and gravitational masses, leading to a violation of the equivalence principle, arises from the nature of its internal constituents and their interactions. We show here that the difference can instead be a function of the distance of the object from a gravitating body, and suggest ways of testing this, illustrating side-by-side a covariant framework for the same.

gr-qc↗
arXiv2025

Arrival Time -- Classical Parameter or Quantum Operator?

The question of how to interpret and compute arrival-time distributions in quantum mechanics remains unsettled, reflecting the longstanding tension between treating time as a quantum observable or as a classical parameter. Most previous studies have focused on the single-particle case in the far-field regime, where both approaches yield very similar arrival-time distributions and a semi-classical analysis typically suffices. Recent advances in atom-optics technologies now make it possible to experimentally investigate arrival-time distributions for entangled multi-particle systems in the near-field regime, where a deeper analysis beyond semi-classical approximations is required. Even in the far-field regime, due to quantum non-locality, the semi-classical approximation cannot generally hold in multi-particle systems. Therefore, in this work, two fundamental approaches to the arrival-time problem -- namely, the time-parameter and time-operator approaches -- are extended to multi-particle systems. Using these extensions, we propose a feasible two-particle arrival-time experiment and numerically evaluate the corresponding joint distributions. Our results reveal regimes in which the two approaches yield inequivalent predictions, highlighting conditions under which experiments could shed new light on distinguishing between competing accounts of time in quantum mechanics. Our findings also provide important insights for the development of quantum technologies that use entanglement in the time domain, including non-local temporal interferometry, temporal ghost imaging, and temporal state tomography in multi-particle systems.

quant-ph↗
arXiv2025

Interaction of exact gravitational waves with matter

We consider interactions of exact (i.e., solutions of full nonlinear field equations) gravitational waves with matter by using the Einstein-Boltzmann equation. For a gravitational wave interacting with a system of massless particles, we compute the perturbed energy-momentum tensor and obtain explicit form of a set of Einstein-Boltzmann equations. We find solution to this system of equations to obtain the gravitational wave profile. The interaction superposes a static term on the gravitational wave profile which depends on the difference between square of the temperatures of the system in the absence and in the presence of the wave. We compute this perturbed term when the states of the system obey Bose-Einstein, Fermi-Dirac, and Maxwell-Boltzmann distributions, respectively. The relative strength of this term is roughly half for the Fermi-Dirac, and one-third for the Maxwell-Boltzmann distributions compared with that of the Bose-Einstein distribution. We consider both Minkowski and flat Friedmann-Robertson-Walker backgrounds.

gr-qc↗
arXiv2025

Power-Law Bounces in $f(R)$ Gravity: Analysis of the Ekpyrosis and Accelerating Regimes

We investigate the dynamics of the Friedmann-Lema\^itre-Robertson-Walker spacetime within the framework of $f(R)$ gravity using a compact, model-independent dynamical systems approach. By assuming a power-law scale factor, we explore ekpyrotic and accelerating solutions to address the big bang singularity. Our analysis demonstrates that a cosmological bounce, characterized by a transition from contraction to expansion, possibly avoids the singularity without directly using the Raychaudhuri equation, unlike previous approaches using specific $f(R) \simeq R^n$ forms. We identify a key fixed point in the phase space corresponding to the bounce, supported by perturbation analysis and qualitative description of trajectories in the phase space. The results suggest that $f(R)$ gravity provides a robust framework for non-singular cosmologies.

gr-qc↗
arXiv2025

Testing the Quantum Equivalence Principle with Gravitational Waves

We study modifications of gravitational wave observables, such as the wave amplitude and frequency, which follow from the quantum equivalence principle, and are expressed in terms of the inertial, gravitational and rest masses of the LIGO/Virgo mirrors. We provide bounds on the violations of the quantum equivalence principle by comparing the results with the most resolved gravitational wave events observed by the LIGO/Virgo collaboration. The formalism is equally applicable to other future ground and space-based gravitational wave detectors.

gr-qc↗
arXiv2025

Test of Quantum Gravity in Optical Magnetometers

In this work, quantum gravity effects, which can potentially be measured in magnetometers through the Larmor frequency of atoms in an external magnetic field, are estimated. It is shown that the thermal motion of atoms can, in principle, produce measurable quantum gravity effects, given the precision of modern magnetometers. If the particle velocities are caused by some other mechanism, such as convection, it is shown that the quantum gravity effects may be observed with the magnetometer's proposed detection threshold. An actual state-of-the-art optical magnetometer experiment is being designed to search for these effects and is described in a companion paper by Maldaner et al. (2023) [1].

gr-qc↗
arXiv2024

Varying Newton's constant: a cure for gravitational maladies?

We show that a slowly varying Newton's constant, consistent with existing bounds, can potentially explain a host of observations pertaining to gravitational effects or phenomena across distances spanning from planetary to the cosmological, relying neither on the existence of Dark Matter or (and) Dark Energy, nor on any expected high proportions of either of them in the Universe. It may also have implications at very short distances or quantum gravity scales.

gr-qc↗
arXiv2024

Superradiance of anyons

In this paper, we investigate superradiance of anyons from a (2+1)-dimensional Ba\~nados, Teitelboim and Zanelli (BTZ) black hole. Our analysis demonstrates that the superradiance condition for anyons mirrors that of a neutral scalar field within a BTZ black hole. Furthermore, we explore the feasibility of observing this phenomenon in analogue black holes and formulate the corresponding superradiance condition for acoustic black holes.

gr-qc↗
arXiv2024

Cosmological Singularity and Power-Law Solutions in Modified Gravity

A bouncing Universe avoids the big-bang singularity. Using the time-like and null Raychaudhhuri equations, we explore whether the bounce near the big-bang, within a broad spectrum of modified theories of gravity, allows for cosmologically relevant power-law solutions under reasonable physical conditions. Our study shows that certain modified theories of gravity, such as Stelle gravity, do not demonstrate singularity resolution under any reasonable conditions, while others including $f(R)$ gravity and Brans-Dicke theory can demonstrate singularity resolution under suitable conditions. For these theories, we show that the accelerating solution is slightly favoured over ekypyrosis.

gr-qc↗
arXiv2024

Quasinormal modes of anyons

We derive the quasinormal modes of anyons from (2+1)-dimensional Banados, Teitelboim, and Zanelli(BTZ) and analogue black holes, and discuss potential experiments to measure these quasinormal modes.

gr-qc↗
arXiv2024

Feasibility analysis of a proposed test of quantum gravity via novel optical magnetometry in xenon

We present an analysis of the sensitivity limits of a proposed experimental search for quantum gravity, using a novel approach based on optical magnetometry in the noble gas isotope $^{129}$Xe. The analysis relies on a general uncertainty principle model that is consistent with most formulations of quantum gravity theory, where the canonical uncertainty relations are modified by a leading-order correction term that is linear in momentum. In turn, this correction modifies the magnetic moment of the spin-polarized $^{129}$Xe atoms that are immersed in a magnetic field in the proposed experiment, which results in a velocity-dependent variation of their Larmour frequency, that is detected via two-photon laser spectroscopy. The thermal distribution of atomic velocities, in conjunction with the Doppler effect, is used to scan the interrogating laser over different atomic velocities, and search for a corresponding variation in their Larmor frequencies. We show that the existing bounds on the leading-order quantum gravity correction can be improved by $10^{7}$ with existing technology, where another factor of $10^{2}$ is possible with near-future technical capabilities.

cond-mat.quant-gas↗
arXiv2023

A Unified Cosmological Dark Sector from a Bose-Einstein Condensate

We examine the viability of cosmological solution(s) describing a unified picture of the dark side of the universe from a Bose-Einstein condensate (BEC) of light bosons. The energy density of the BEC, together with its quantum potential, can indeed account for such a unification, in the sense that the (dust-like) cold dark matter and the dark energy components emerge from the same source. In particular, the bulk of the dark energy can be attributed to the quantum potential, in the quantum corrected Raychaudhuri-Friedmann equation, when the `macroscopic' BEC wave-function is taken to be such that the corresponding probability density is construed as the energy density of the dusty fluid. However, there arises a purely quantum mechanical back-reaction effect, of even the visible baryons, on the effective dark energy and dark matter contents, which crucially determines the mass of the BEC. We determine the constraint on such a back-reaction, and hence on the BEC mass, from physical considerations, as well as estimate the same using recent observational data.

gr-qc↗
arXiv2023

A novel mechanism for probing the Planck scale with wavepackets following general distributions

It was also shown recently that GUP predicts potentially measurable corrections to the `doubling time' of freely moving Gaussian atomic and molecular wavepackets with a favorable combination of three parameters, {\it e.g.} mass, initial width and mean velocity of a travelling wavepacket. However, it is well known that such wavepackets can come with various shapes which correspond to variety of distributions. In this article, we generalize our earlier work for an {\it arbitrary distribution} and thereby accommodate any shape of the wavepacket. Mathematically, we build this formalism by exploiting a duality between quantum and statistical mechanics, by which (quantum mechanical) expectation values of the momentum operator can be expressed in terms of the derivatives of the characteristic functions of the dual statistical description. Equipped with this result, we go one step further and numerically study a few physical distributions. We find that large organic (TPPF152) wavepacket following the generalized normal distribution with parameter $κ=0.5$ offers one of the best-case scenarios, effectively scanning the whole GUP parameter space with current technologies. Although we do not say that the minimal length has to be near or at the Planck value, we mange improving our previous studies to scan the minimal length signatures down to hundred times the Planck value.

gr-qc↗
arXiv2023

General Formalism of the Quantum Equivalence Principle

A consistent theory of quantum gravity will require a fully quantum formulation of the classical equivalence principle. Such a formulation has been recently proposed in terms of the equality of the rest, inertial and gravitational mass operators, and for non-relativistic particles in a weak gravitational field. In this work, we propose a generalization to a fully relativistic formalism of the quantum equivalence principle, valid for all background space-times, as well as for massive bosons and fermions. The principle is trivially satisfied for massless particles. We show that if the equivalence principle is broken at the quantum level, it implies the modification of the standard Lorentz transformations in flat space-time and a corresponding modification of the metric in curved space-time by the different mass ratios. In other words, the observed geometry would effectively depend on the properties of the test particle. Testable predictions of potential violations of the quantum equivalence principle are proposed.

gr-qc↗