arXiv subjects

Ghanashyam Date

Publications and source records attributed to Ghanashyam Date.

At least 19 recordsLinked to original sources

arXiv2026

Field Theoretic Aspects of Gravity From Direct Action to Quantum Fields

One of the technical anchors of JVN's research was the "action-at-a-distance" view of gravitational interaction, driven by "Mach's Principle" addressing the origin of inertia. This view exorcises the concept of a field as a mediator of force between separated "particles". However, within a quantum framework, the concept of field as a mediator transcends to field as an autonomous physical entity. Beginning with a comparison between direct action view and field theoretic view in classical framework and moving over to quantum framework, I sketch the strides made in quantum field theory including the new features brought in by gravity. This is intended as a conceptual trace rather than a review.

gr-qc↗

arXiv2026

Consistent Truncation of Linearized gravitational waves in de Sitter space-time

An important step in using observations of gravitational waves from bounded sources is to relate the observed waveforms far away from a source to the local dynamics and environment of the source. To isolate and identify different features of the source, multipole expansion of both the radiation field and the source distribution are crucial. At a practical level, these expansions are truncated at some finite multipole order and a consistency issue arises. This was flagged in \cite{CHK} as the emergence of disallowed $\log(r)$ terms in certain asymptotic forms and a particular quadrupolar truncation was proposed which was consistent with the source conservation. In \cite{NKD80}, it was noted (incorrectly) that the particular solution does not satisfy the generalized harmonic gauge condition and this may be the source of appearance of the log(r) terms. In this work the consistency issue is analyzed by explicitly integrating the gauge conditions and the source conservation equations over the conformal time. This gives a general procedure for a consistent truncation. The procedure is verified in generalized harmonic gauge in conformal chart and in the Bondi gauge in Bondi chart.

gr-qc↗

arXiv2024

Linearized gravitational waves in de Sitter space-time

This is a brief note on a recently flagged issue \cite{CHK} regarding the precise characterization of quadrupolar truncation of the linearized gravitational waves in de Sitter space-time. An apparent inconsistency was noted while transforming the linearized solution truncated to quadrupolar contribution to the Bondi-Sachs form. A consistent truncation was identified as one where the transformation of the truncated solution does not generate logarithmic terms inadmissible in the Bondi-Sachs asymptotic form. I point out the role of the gauge conditions and the solution of the homogeneous equation in obviating the apparent inconsistency issue.

gr-qc↗

arXiv2022

Lectures on Introduction to Quantum Field Theory

These are lecture notes of the QFT-I course I gave in an online mode at Chennai Mathematical Institute. The course focussed on the free relativistic quantum fields, their interactions in the perturbative scattering framework, standard computations of QED processes, radiative corrections at 1-loop with renormalization and an introduction to the toolbox of path integrals.

hep-th↗

arXiv2017

Cosmological Horizon and the Quadrupole Formula in de Sitter Background

An important class of observables for gravitational waves consists of the fluxes of energy, momentum and angular momentum carried away by them and are well understood for weak gravitational waves in Minkowski background. In de Sitter background, the future null infinity, $\mathcal{J}^+$, is space-like which makes the meaning of these observables subtle. A spatially compact source in de Sitter background also provides a distinguished null hypersurface, its {\em cosmological horizon}, $\mathcal{H}^+$. For sources supporting the short wavelength approximation, we adopt the Isaacson prescription to define an effective gravitational stress tensor. We show that the fluxes computed using this effective stress tensor can be evaluated at $\mathcal{H}^+$, match with those computed at $\mathcal{J}^+$ and also match with those given by Ashtekar et al at $\mathcal{J}^+$ {\em at a coarse grained level}.

gr-qc↗

arXiv2016

Gravitational Waves from Compact Sources in de Sitter Background

The concordance model of cosmology favours a universe with a tiny positive cosmological constant. A tiniest positive constant curvature, profoundly alters the asymptotic structure, forcing a re-look at a theory of gravitational radiation. Even for compact astrophysical sources, the intuition from Minkowski background is challenged at every step. Nevertheless, at least for candidate sources such as compact binaries, it is possible to quantify influence of the cosmological constant, as small corrections to the leading order Minkowski background results. Employing suitably chosen Fermi normal coordinates in the static patch of the de Sitter background, we compute the field due to a compact source to first order in $Λ$. For contrast, we also present the field in the Poincare patch where the leading correction is in $\sqrtΛ$. We introduce a gauge invariant quantity, {\em deviation scalar}, containing polarization information and compute it in both charts for a comparison.

gr-qc↗

arXiv2013

Polymer quantization and Symmetries

Polymer quantization was discovered during the construction of Loop Quantum Cosmology. For the simplest quantum theory of one degree of freedom, the implications for dynamics were studied for the harmonic oscillator as well as some other potentials. For more degrees of freedom, the possibility of continuous, kinematic symmetries arises. While these are realised on the Hilbert space of polymer quantum mechanics, their infinitesimal versions are not supported. For an invariant Hamiltonian, these symmetry realizations imply infinite degeneracy suggesting that the symmetry should be spontaneously or explicitly broken. The estimation of symmetry violations in some cases have been analysed before. Here we explore the alternative of shifting the arena to the distributional states. We discuss both the polymer quantum mechanics case as well as polymer quantized scalar field.

gr-qc↗

arXiv2012

Matter in Loop Quantum Gravity

Loop quantum gravity, a non-perturbative and manifestly background free, quantum theory of gravity implies that at the kinematical level the spatial geometry is discrete in a specific sense. The spirit of background independence also requires a non-standard quantum representation of matter. While loop quantization of standard model fields has been proposed, detail study of its implications is not yet available. This review aims to survey the various efforts and results.

gr-qc↗

arXiv2011

Revisiting canonical gravity with fermions

Fermions constitute an important component of matter and their quantization in presence of dynamical gravity is essential for any theory of quantum gravity. We revisit the classical formulation adapted for a background free quantization. The analysis is carried out with the Hilbert-Palatini form for gravity together with the Nieh-Yan topological term which keeps the nature of Barbero-Immirzi parameter independent of inclusion of arbitrary matter with arbitrary couplings. With dynamical gravity, a priori, there are two distinct notions of `parity' - orientation reversing diffeomorphisms and improper Lorentz rotations. The invariance properties of the action and the canonical framework are different with respect to these and gravitational origin of parity violation seems ambiguous.

gr-qc↗

arXiv2010

Lectures on Constrained Systems

These lecture notes were prepared as a basic introduction to the theory of constrained systems which is how the fundamental forces of nature appear in their Hamiltonian formulation. Only a working knowledge of Lagrangian and Hamiltonian formulation of mechanics is assumed. These notes are based on the set of eight lectures given at the {\em Refresher Course for College Teachers} held at IMSc during May-June, 2005. These are submitted to the arxiv for an easy access to a wider body of students.

gr-qc↗

arXiv2010

Lectures on LQG/LQC

A School on Loop Quantum Gravity was held at the IMSc during Sept 8 -- 18, 2009. In the first week a basic introduction to LQG was provided while in the second week the focus was on the two main application, to cosmology (LQC) and to the black hole entropy. These notes are an expanded written account of the lectures that I gave. These are primarily meant for beginning researchers.

gr-qc↗

arXiv2009

Effective Actions from Loop Quantum Cosmology: Correspondence with Higher Curvature Gravity

Quantum corrections of certain types and relevant in certain regimes can be summarised in terms of an effective action calculable, in principle, from the underlying theory. The demands of symmetries, local form of terms and dimensional considerations limit the form of the effective action to a great extent leaving only the numerical coefficients to distinguish different underlying theories. The effective action can be restricted to particular symmetry sectors to obtain the corresponding, {\em reduced effective action}. Alternatively, one can also quantize a classically (symmetry) reduced theory and obtain the corresponding effective action. These two effective actions can be compared. As an example, we compare the effective action(s) known in isotropic loop quantum cosmology with the Lovelock actions, as well as with more general actions, specialized to homogeneous isotropic space-times and find that the $\barμ$-scheme is singled out.

gr-qc↗

arXiv2008

Topological Interpretation of Barbero-Immirzi Parameter

We set up a canonical Hamiltonian formulation for a theory of gravity based on a Lagrangian density made up of the Hilbert-Palatini term and, instead of the Holst term, the Nieh-Yan topological density. The resulting set of constraints in the time gauge are shown to lead to a theory in terms of a real SU(2) connection which is exactly the same as that of Barbero and Immirzi with the coefficient of the Nieh-Yan term identified as the inverse of Barbero-Immirzi parameter. This provides a topological interpretation for this parameter. Matter coupling can then be introduced in the usual manner, {\em without} changing the universal topological Nieh-Yan term.

gr-qc↗

arXiv2008

Loop Quantization of Polarized Gowdy Model on $T^3$: Kinematical States and Constraint Operators

In this second paper on loop quantization of Gowdy model, we introduce the kinematical Hilbert space on which appropriate holonomies and fluxes are well represented. The quantization of the volume operator and the Gauss constraint is straightforward. Imposition of the Gauss constraint can be done on the kinematical Hilbert space to select subspace of gauge invariant states. We carry out the quantization of the Hamiltonian constraint making specific choices. Alternative choices are briefly discussed. It appears that to get spatial correlations reflected in the Hamiltonian constraint, one may have to adopt the so called `effective operator viewpoint'.

gr-qc↗

arXiv2008

Loop Quantization of Polarized Gowdy Model on $T^3$: Classical Theory

The vacuum Gowdy models provide much studied, non-trivial midi-superspace examples. Various technical issues within Loop Quantum Gravity can be studied in these models as well as one can hope to understand singularities and their resolution in the loop quantization. The first step in this program is to reformulate the model in real connection variables in a manner that is amenable to loop quantization. We begin with the unpolarized model and carry out a consistent reduction to the polarized case. Carrying out complete gauge fixing, the known solutions are recovered.

gr-qc↗

arXiv2007

Singularity Resolution in Isotropic Loop Quantum Cosmology: Recent Developments

Since the past Iagrg meeting in December 2004, new developments in loop quantum cosmology have taken place, especially with regards to the resolution of the Big Bang singularity in the isotropic models. The singularity resolution issue has been discussed in terms of physical quantities (expectation values of Dirac observables) and there is also an ``improved'' quantization of the Hamiltonian constraint. These developments are briefly discussed. This is an expanded version of the review talk given at the 24$^{\mathrm{th}}$ IAGRG meeting in February 2007.

gr-qc↗

arXiv2006

On obtaining classical mechanics from quantum mechanics

Constructing a classical mechanical system associated with a given quantum mechanical one, entails construction of a classical phase space and a corresponding Hamiltonian function from the available quantum structures and a notion of coarser observations. The Hilbert space of any quantum mechanical system naturally has the structure of an infinite dimensional symplectic manifold (`quantum phase space'). There is also a systematic, quotienting procedure which imparts a bundle structure to the quantum phase space and extracts a classical phase space as the base space. This works straight forwardly when the Hilbert space carries weakly continuous representation of the Heisenberg group and recovers the linear classical phase space $\mathbb{R}^{\mathrm{2N}}$. We report on how the procedure also allows extraction of non-linear classical phase spaces and illustrate it for Hilbert spaces being finite dimensional (spin-j systems), infinite dimensional but separable (particle on a circle) and infinite dimensional but non-separable (Polymer quantization). To construct a corresponding classical dynamics, one needs to choose a suitable section and identify an effective Hamiltonian. The effective dynamics mirrors the quantum dynamics provided the section satisfies conditions of semiclassicality and tangentiality.

gr-qc↗

arXiv2005

Pre-classical solutions of the vacuum Bianchi I loop quantum cosmology

Loop quantization of diagonalized Bianchi class A models, leads to a partial difference equation as the Hamiltonian constraint at the quantum level. A criterion for testing a viable semiclassical limit has been formulated in terms of existence of the so-called pre-classical solutions. We demonstrate the existence of pre-classical solutions of the quantum equation for the vacuum Bianchi I model. All these solutions avoid the classical singularity at vanishing volume.

gr-qc↗