arXiv subjects

Harkirat Singh Sahota

Publications and source records attributed to Harkirat Singh Sahota.

12 recordsLinked to original sources

arXiv2026

Cavity-controlled Inhibition of Decoherence in Accelerated Quantum Detectors

Vacuum fluctuations of quantum fields provide an unavoidable environment for any quantum system coupled to it. We study the interplay between boundary conditions and acceleration in determining decoherence of a two-level Unruh-DeWitt detector coupled to a scalar field in a cylindrical cavity. We show that the decoherence rate closely follows the emission profile, and exhibits {\it Purcell-like} enhancement for both inertial and uniformly accelerated detectors. The acceleration induces an effective smearing of the resonant density of states, diluting the resonance enhancement for large accelerations while replacing the inertial off-resonant decay with an oscillatory behavior for small accelerations. For moderate accelerations, this interplay between cavity-induced and acceleration-assisted effects results in an extended region of cavity parameters where decoherence is strongly suppressed, particularly in regimes where the inertial detector otherwise experience strong decoherence. Thus, contrary to naive expectations, the Unruh thermality in a suitably engineered cavity can enhance rather than degrade quantum coherence, providing a very uncharacteristic feature of quantum fields in non-inertial frames.

gr-qc↗

arXiv2026

Unitary quantum matter-bounce in a universe with a positive cosmological constant

We analyze the Wheeler-DeWitt quantization of a spatially flat Friedmann-Lemaître-Robertson-Walker universe containing pressureless dust and a positive cosmological constant ($Λ> 0$). Following relational time framework, we establish a direct mathematical correspondence between the cosmological Hamiltonian and the radial Schrödinger equation for the scattering states of the non-relativistic hydrogen atom. This exact solvability allows us to rigorously construct the physical Hilbert space and ensure the self-adjointness of the Hamiltonian. As a concrete result, we show that the wave packets unitarily evolve depicting a non-singular quantum bounce, systematically replacing the classical Big Bang singularity. Finally, we discuss the physical relevance of this exact solution within the matter-bounce scenario. We demonstrate that this framework provides a robust quantum origin for a bounce during a dust-dominated contracting phase -- a necessary prerequisite for generating a scale-invariant spectrum of primordial perturbations -- derived from the unitary dynamics of the quantized background.

gr-qc↗

arXiv2026

Cavity-Induced Suppression of Entanglement and Enhancement of Quantum Discord

We study correlations between two Unruh-DeWitt detectors coupled to a scalar field in a cylindrical cavity. Boundary conditions strongly modify the detector-correlation dynamics relative to free space. The entanglement negativity is suppressed in the cavity and vanishes for smaller separation as compared to the free space. Increasing the cavity radius does not recover the free-space behavior of the negativity. In contrast, mutual information and quantum discord remain nonzero over much larger separations. While the mutual information decays monotonically with separation, the quantum discord is enhanced near the cavity boundary. Our results demonstrate that geometric confinement can selectively suppress distillable entanglement while preserving and even enhancing more general non-classical correlations, providing a controlled setting to probe the hierarchy of correlations in quantum field theory.

quant-ph↗

arXiv2026

A dynamical systems perspective on the thermodynamics of late-time cosmology

A thermodynamic description of cosmological spacetimes may provide insights into the fundamentals of the cosmic evolution that remain otherwise obscure, similar to `black hole thermodynamics'. We investigate the thermodynamic properties of late-time cosmological evolution using the dynamical systems approach, focusing on $Λ${}CDM model and scalar field models with exponential potentials. Thermodynamic quantities obtained through the Hayward-Kodama formalism are mapped onto the phase-space of these models. Specifically, we express the thermodynamic quantities as functions of the phase-space variables, allowing us to study the thermodynamic behavior across the phase space, particularly at the critical points. We focus on thermodynamic stability and phase transitions, analyzed in an initial condition-independent manner. In these models, the universe inevitably undergoes a thermodynamic phase transition, marked by diverging specific heats, irrespective of its initial configuration. We further demonstrate that the thermodynamic stability can occur only during an accelerating phase of the universe. For $Λ$CDM and quintessence models, the necessary stability conditions are never satisfied anywhere in the phase space, rendering both models thermodynamically unstable within the Hayward-Kodama framework and the canonical ensemble based stability criteria. Interestingly, the phantom models, although dynamically unstable, allow for the universe to attain thermodynamic stability in its asymptotic future. This can indicate the limitations of applying canonical ensemble based thermodynamic stability criteria to cosmological horizons. Through these archetypal descriptions of late-time cosmology, we show that the dynamical system approach is a robust framework to probe the thermodynamic aspects of cosmological evolution.

gr-qc↗

arXiv2026

Quantum Cosmology as a Hydrogen atom: Discrete $Λ$ and cyclic Universes from Wheeler-DeWitt quantization

Building upon our recently established correspondence between quantum cosmology and the hydrogen atom [1], we investigate the specific sector of a negative cosmological constant ($Λ< 0$) in a flat FLRW universe with dust. While the positive $Λ$ sector [1] yields a continuous spectrum and a single bounce, we show here that the negative $Λ$ sector leads to a discrete spectrum of energy eigenvalues, effectively quantizing the cosmological constant. Within this dual description, the operator-ordering ambiguity parameter appears as the azimuthal quantum number of the hydrogen atom. A skewed Bohr correspondence emerges for the bound states, matching classical evolution at large volumes but deviating near the bounce. By constructing wave packets from these bound states, we demonstrate that the classical Big Bang and Big Crunch singularities are resolved, and the universe oscillates between quantum bounces and classical turnaround points. The expectation values of the observables indicate a cyclic universe -- with vanishing Hubble parameter at turnarounds -- undergoing quantum bounces. This exactly solvable model offers a tractable setting to explore quantum gravitational effects in cosmology. We analyze the properties of this cyclic universe, contrasting its bound-state dynamics with the scattering states of the de Sitter case.

gr-qc↗

arXiv2025

Exact path integrals on half-line in quantum cosmology with a fluid clock and aspects of operator ordering ambiguity

We perform $\textit{exact}$ half-line path integral quantization of flat, homogeneous cosmological models containing a perfect fluid acting as an internal clock, in a $D+1$ dimensional minisuperspace setup. We also discuss certain classes of operator ordering ambiguity inherent in such quantization procedures and argue that a particular ordering prescription in the quantum theory can preserve two symmetries, namely arbitrary lapse rescalings and general covariance, which are already present at the classical level. As a result of this imposition, a large class of quantum Hamiltonians differing by operator ordering produces the same inner products between quantum states. This imposition of the two symmetries of the classical minisuperspace models leads to a unique prescription for writing the quantum Hamiltonian for minisuperspace dimension $D>2$. Interestingly, in the case of $D=1$, the lapse rescaling symmetry is lost in the quantum theory, leading to an essentially ambiguous description of the canonical theory. We provide general proof of this in the context of both canonical quantization and path integrals. We supply concrete examples to validate our findings further.

gr-qc↗

arXiv2025

Non-local correlations of a test quantum field in gravitational collapse

Quantum correlations across the horizon could be pivotal in unveiling the puzzles surrounding quantum aspects of black holes and Hawking radiation. The peaks in the equal time correlation function are typically attributed to the entangled particle excitations. In this work, we have investigated the evolution of the correlations of a test quantum field on a dynamical background spacetime undergoing gravitational collapse. In the case of super-critical collapse, as the black hole and its horizon forms, correlated peaks are seen to appear across the horizon, representing an entangled Hawking pair. The outside peak moves away from the horizon as the system evolves, possibly representing outgoing Hawking flux. The implications of these non-local correlations are discussed in light of information paradox, quantum atmosphere and analogue black holes.

gr-qc↗

arXiv2025

Are accelerated detectors sensitive to Planck scale changes?

One of the foremost concerns in the analysis of quantum gravity is whether the locations of classical horizons are stable under a full quantum analysis. In principle, any classical description, when interpolated to the microscopic level, can become prone to fluctuations. The curious question in that case is if there indeed are such fluctuations at the Planck scale, do they have any significance for physics taking place at scales much away from the Planck scale? In this work, we address the question of small scales and address whether there are definitive signatures of Planck scale shifts in the horizon structure. In a recent work (arXiv:2107.03406), it was suggested that in a nested sequence of Rindler causal wedges, the vacua of preceding Rindler frames appear thermally populated to a shifted Rindler frame. The Bogoliubov analysis relies on the global notion of the quantum field theory and turns out to be insensitive to the local character of these horizon shifts. We investigate this system by means of the Unruh-DeWitt detector and see if this local probe of the quantum field theory is sensitive enough to the shift parameters to reveal any microscopic effects. For the case of infinite-time response, we recover the thermal spectrum, thus reaffirming that the infinite-time response probes the global properties of the field. On the other hand, the finite-time response turns out to be sensitive to the shift parameter in a peculiar way that any detector with energy gap $Ωc/a \sim 1$ and is operational for timescale $T a/c \sim 1$ has a measurably different response for a macroscopic and microscopic shift of the horizon, giving us a direct probe to the tiniest separation between the Rindler wedges. Thus, this study provides an operational method to identify Planck scale effects that can be generalized to various other interesting gravitational settings.

gr-qc↗

arXiv2024

Imprints of the operator ordering ambiguity on the dynamics of perfect fluid dominated quantum universe

Sharply peaked quantum states are conjectured to be conducive to the notion of a quantum-corrected spacetime. We investigate this conjecture for a flat-FLRW model with perfect fluid, where a generalized ordering scheme is considered for the gravitational Hamiltonian. We study the implications of different ordering choices on the dynamics of the quantum universe. We demonstrate that the imprints of the operator ordering ambiguity are minimal, and quantum fluctuations are small in the case of sharply peaked states, leading to a consistent notion of a quantum-corrected spacetime defined via the expectation value of the scale factor. Surprisingly, the ordering imprints survive far away from the singularity through the quantum fluctuations in the quantum-corrected spacetime for broadly peaked states.

gr-qc↗

arXiv2023

Analyzing quantum gravity spillover in the semiclassical regime

One of the standard approaches of incorporating the quantum gravity (QG) effects into the semiclassical analysis is to adopt the notion of a quantum-corrected spacetime arising from the QG model. This procedure assumes that the expectation value of the metric variable effectively captures the relevant QG subtleties in the semiclassical regime. We investigate the viability of this effective geometry approach for the case of dust dominated and a dark energy dominated universe. We write the phase space expressions for the geometric observables and construct corresponding Hermitian operators. A general class of operator ordering of these observables is considered, and their expectation values are calculated for a unitarily evolving wave packet. In the case of dust dominated universe, the expectation value of the Hubble parameter matches the "semiclassical" expression, the expression computed from the scale factor expectation value. In the case of Ricci scalar, the relative difference between the semiclassical expression and quantum expectation is maximum at singularity and decays for late time. For a cosmological constant driven universe, the difference between the semiclassical expressions and the expectation value is most pronounced far away from the bounce point, hinting at the persistent quantum effect at the late time. The parameter related to the shape of the distribution appears as a control parameter in these models. In the limit of a sharply peaked distribution, the expectation value of the observables matches with their semiclassical counterpart, and the usage of effective geometry approach is justified.

gr-qc↗

arXiv2023

Einstein and Jordan frame correspondence in quantum cosmology: Expansion-collapse duality

The conformal correspondence between FLRW universes in the Einstein and Jordan frames allows for an expansion-collapse duality -- an always expanding Einstein frame universe can have a dual Jordan frame description that is contracting forever. The scenario eventually runs into an apparent paradox. When a collapsing universe approaches singularity, the classical description of the spacetime becomes inadequate. The contracting Jordan frame universe is expected to develop quantum characteristics when its scale factor becomes sufficiently small. However, at the same time, the corresponding Einstein frame universe is expected to behave classically, due to the arbitrarily large size it has grown to. In this case, the conformal map appears to be providing a duality between a quantum effect-dominated universe and a universe behaving classically. We investigate the status of the conformal map at the quantum level in such a scenario, focusing on addressing this paradox. The Einstein and Jordan frame universes are quantized individually using the Wheeler-DeWitt prescription. We show that the classical conformal map holds true at the quantum level when compared through the expectation values of the scale factor operators in the two frames. The relative quantum fluctuation in the scale factor is found to be conformally invariant, and it increases in both the past and future directions according to the internal clock. Expectedly, the quantum fluctuations in the collapsing Jordan frame keep on increasing as it shrinks towards singularity. More surprisingly, the quantum fluctuations in the expanding Einstein frame keep on increasing as well, even as its classical scale factor becomes larger. Despite having drastically different cosmological evolutions, the rise in quantum characteristics in a collapsing frame implies the same in its expanding counterpart, thereby resolving the apparent paradox.

gr-qc↗

arXiv2021

Infrared signatures of quantum bounce in a minisuperspace analysis of Lema\^ıtre-Tolman-Bondi dust collapse

In the quantum treatment of collapsing matter, a viable mode analysis is supposed to give information regarding emission during the collapse. Nevertheless, partly owing to operator ordering ambiguities involved in a typical quantum gravity analysis, the availability of such well-defined modes is not guaranteed. We study the mode decomposition of the unitarily evolving wave packet constructed for the quantum model of spherically symmetric dust collapsing in a marginally bound Lema\^ıtre-Tolman-Bondi (LTB) model. We identify the observable depicting mode decomposition, and using the freedom of operator ordering ambiguity, we obtain the Hermitian extension of this operator alongside the Hermitian Hamiltonian. After identifying incoming and outgoing modes with this operator's eigenstates, we estimate their contributions to the radiation profile. The infrared sector of this process demonstrates some characteristic features which turn out to be highly sensitive to the near-bounce dynamics of the dust cloud. Near the epoch of classical singularity, there is a significant contribution from incoming/outgoing modes of small wavenumber in the expanding/collapsing phase of the dust cloud, which keeps on decreasing as one moves away from the singularity. The information of the bounce is carried over to the infrared modes through a flip from largely incoming to largely outgoing radiation as the evolution progresses from collapsing to expanding phase, much before the information of bounce comes about to any observer. In the infrared sector, the saturation value of the amplitude marks the bounce radius. Thus, we argue that the information of the short scale physics is essentially carried over to the longest wavelength in this quantum gravity model, which we argue is rather more prominent for low energy processes.

gr-qc↗