arXiv subjects

Abhik Kumar Sanyal

Publications and source records attributed to Abhik Kumar Sanyal.

At least 19 recordsLinked to original sources

arXiv2025

Vulnerability of f(Q) gravity theory and a possible resolution

Both the generalized teleparallel theories of gravity suffer from some serious problems. The strong coupling issue appearing as a consequence of extra degrees of freedom in the `generalized metric teleparallel gravity' theory, prompted to consider `generalized symmetric teleparallel gravity' theory (GSTG). Unfortunately, recent perturbative analysis in the background of maximally symmetric space-time revealed that GSTG also suffers from the strong coupling issue and the ghost degrees of freedom. It has also been cognised that GSTG does not admit diffeomorphic invariance in general. Lately, it has been shown that except for the first, the other two connections associated with spatially flat Robertson-Walker metric do not even admit GSTG, while the first connection leads to an eerie Hamiltonian upon ensuing Dirac-Bergmann constraint analysis. Here we show that the only existing non-flat connection is also not viable in the same sense. Thus GSTG happens to be jeopardized. These problems do not showup in $f(R,Q)$ theory of gravity. Modified Dirac-Bergmann constraint analysis is deployed to formulate the phase-space structure. Quantization, probabilistic interpretation and semi-classical approximation connote that such a theory is well behaved in the context of early inflation, which has also been studied.

gr-qc↗

arXiv2025

Phase space structure of symmetric teleparallel theory of gravity

The `Generalized Symmetric Teleparallel Gravity' (GSTG) does not admit diffeomorphic invariance, since the auxiliary field as well as the shift vector act as non-propagating dynamical variables carrying 1/2 degrees of freedom each. We show that in a minisuperspace model, which is devoid of the shift vector, the problem is alleviated for locally Lorentz invariant GSTG theory, and diffeomorphic invariance is established at least for one connection. However, the eerie structure of the Hamiltonian constructed even in the background of spatially flat isotropic and homogeneous Robertson-Walker space-time, can not be maneuvered. In contrast, the other two spatially flat connections containing an arbitrary time dependent function, doesn't admit non-linear extension to `Symmetric Teleparallel Equivalent to General Relativity (STEGR). We therefore construct the phase-space structure with three different spatially flat connections for the `Lorentz invariant' linear-scalar-vector-tensor GSTG action. Diffeomorphic invariance is established and the associated Hamiltonians are found to be well behaved for all the three cases.

gr-qc↗

arXiv2024

Probing symmetric teleparallel gravity in the early universe

General theory of relativity can be equivalently formulated on a flat space-time associating a torsion-free affine connection of non-vanishing non-metricity scalar $Q$. In this paper, we present an extension of this, viz., the $f(Q)$ theory of gravity, and explore the early evolution of the universe in the background of anisotropic Bianchi-I model. The $f(Q)$ theory in the current setting through its geometric modification is quite successful in explaining the late time accelerated expansion. Here we note that it accommodates latest released constraints on the inflationary parameters by Planck's collaboration group with excellent precession, but fails to produce a viable decelerated expansion in the radiation dominated era.

gr-qc↗

arXiv2024

Reconstructing modified and alternative theories of gravity

A viable radiation dominated era in the early universe is best described by the standard (FLRW) model of cosmology. In this short review, we demonstrate reconstruction of the forms of F(R) in the modified theory of gravity and the metric compatible F(T) together with the symmetric F(Q) in alternative teleparallel theories of gravity, from different perspectives, primarily rendering emphasis on a viable FLRW radiation era. Inflation has also been studied for a particular choice of the scalar potential. The inflationary parameters are found to agree appreciably with the recently released observational data.

gr-qc↗

arXiv2023

Inflation and cosmological evolution with $F(R,G)$ gravity theory

In the last decade Planck PR4 data together with ground-based experimental data such as, BK18, BAO and CMB lensing, tightened constraint of the tensor to scalar ratio, starting form $r < 0.14$ to $r < 0.032$, while the spectral index lies within the range $0.9631 < n_s < 0.9705$. Viability of modified gravity theories, proposed as alternatives to the dark-energy issue, should therefore be tested in the light of such new result. Here, we explore $F(R,G)$ gravity theory in regard of the early universe and have shown that, it is not compatible with newly released constraints on $r$ and $n_s$ simultaneously. Further, it also fails to produce a feasible radiation dominated era. It therefore questions the justification of using the model for resolving the cosmic puzzle.

gr-qc↗

arXiv2023

A viable form of the metric Teleparallel F(T) theory of gravity

Unlike F(R) gravity, pure metric F(T) gravity in the vacuum dominated era, ends up with an imaginary action and is therefore not feasible. This eerie situation may only be circumvented by associating a scalar field, which can also drive inflation in the very early universe. We show that, despite diverse claims, F(T) theory admits Noether symmetry only in the pressure-less dust era in the form F(T) proportional to the nth power of T, n being odd integers. A suitable form of F(T), admitting a viable Friedmann-like radiation dominated era, together with early deceleration and late-time accelerated expansion in the pressure-less dust era, has been proposed.

gr-qc↗

arXiv2023

Perusing Buchbinder--Lyakhovich canonical formalism for Higher-Order Theories of Gravity

Ostrogradsky's, Dirac's and Horowitz's techniques of higher order theories of gravity produce identical phase-space structures. The problem is manifested in the case of Gauss-Bonnet-dilatonic coupled action in the presence of higher-order term, in which case, classical correspondence can't be established. Here, we explore yet another technique developed by Buchbinder and his collaborators (BL) long back and show that it also suffers from the same disease. However, expressing the action in terms of the three-space curvature, and removing "the total derivative terms", if Horowitz's formalism or even Dirac's constraint analysis is pursued, all pathologies disappear. Here we show that the same is true for BL formalism, which appears to be the simplest of all the techniques, to handle.

gr-qc↗

arXiv2022

The issue of Branched Hamiltonian in F(T) Teleparallel Gravity

As in the case of Lanczos-Lovelock gravity, the main advantage of F(T) gravity is said to be that it leads to second order field equations, while F(R) gravity theory leads to fourth order equations. We show that it is rather a disadvantage, since it leads to the unresolved issue of `Branched Hamiltonian'. The problem is bypassed in F(R,T) gravity theory.

gr-qc↗

arXiv2022

New conserved tensors and Brans-Dicke type field equation, using integrability condition

We explore some new off-shell and on-shell conserved quantities for a scalar field in Minkowski space, using integrability condition. The off-shell conserved tensors are related to the kinematics of the field, while a linear combination of the off-shell and the on-shell conserved tensors ends up with the energy-momentum tensor for the scalar field. In the curved background, using Ricci and Bianchi identities, Brans-Dicke type field equations emerge, without requiring the principle of equivalence. Further, starting from the curvature scalar and using these identities, the field equations for modified gravity (Einstein-Hilbert action in the presence of higher-order terms) follows.

gr-qc↗

arXiv2022

Inflation -- a Comparative Study Amongst Different Modified Gravity Theories

In the recent years, a host of modified gravity models have been proposed as alternatives to the dark energy. A quantum theory of gravity also requires to modify `General Theory of Relativity'. In the present article, we consider five different modified theories of gravity, and compare inflationary parameters with recent data sets released by two Planck collaboration teams. Our analysis reveals that the scalar-tensor theory of gravity is the best alternative.

gr-qc↗

arXiv2021

Inflation with F(T) Teleparallel Gravity

We study early universe with a particular form of F(T) Telleparallel gravity theory, in which inflation is driven by a scalar field. To ensure slow rollover, two different potentials are chosen in a manner, such that they remain almost flat for large initial value of the scalar field. Inflationary parameters show wonderful fit with the presently available Planck's data set. The energy scale of inflation is sub-Planckian and graceful exit from inflation is also administered. The chosen form of F(T) administers late-time cosmic acceleration too. In the process, unification of the early inflation with late-time acceleration is ensured. Unfortunately, a decelerated radiation dominated era is only possible with a different form of (quartic) potential, which being devoid of a flat section does not admit slow rollover.

astro-ph.CO↗

arXiv2021

Conflict between some higher-order curvature invariant terms

A viable quantum theory does not allow curvature invariant terms of different higher orders to be accommodated in the gravitational action. We show that there is indeed a conflict between the curvature squared and Gauss-Bonnet squared terms from the point of view of hermiticity. This means one should choose either, in addition to the Einstein-Hilbert term, but never the two together. We explore early cosmic evolution with Gauss-Bonnet squared term.

gr-qc↗

arXiv2021

String cosmology in Bianchi I space-time

Some cosmological solutions of massive strings are obtained in Bianchi I space-time following the techniques used by Letelier and Stachel. A class of solutions corresponds to string cosmology associated with/without a magnetic field and the other class consists of pure massive strings, obeying the Takabayashi equation of state.

gr-qc↗

arXiv2021

Viscoelastic flow past an infinite plate with suction and constant heat flux

While studying the viscoelastic flow past an infinite plate with suction and constant heat flux between fluid and plate, Raptis and Tziyanidis gave the solution of a pair of equations for velocity and temperature as functions of distance. They then gave some approximate solutions. This letter shows that the approximations are not justified and presents an exact analytical study.

physics.flu-dyn↗

arXiv2021

Bianchi II, VIII, and IX viscous fluid cosmology

In this paper we study the exact solutions for a viscous fluid distribution in Bianchi II, VIII, and IX models. The metric is simplified by assuming a relationship between the coefficients and the metric tensor. Solutions are obtained in two special cases: in one, an additional assumption is made where the matter density and the expansion scalar have a definite relation and in the other a barotropic equation of state between the matter density and the thermodynamic pressure is assumed. While the Bianchi II solutions are already found in the literature the other two classes of solutions are apparently new.

gr-qc↗

arXiv2021

Canonical equivalence, quantization and anisotropic inflation in higher order theory of gravity

We construct phase-space structure of a typical higher-order theory of gravity, in the background of anisotropic Bianchi-1 mini-superspace, following `Modified Horowitz Formalism' as well as applying `Dirac Algorithm' (after taking care of the divergent terms), and establish equivalence. Canonical quantization, and semiclassical approximation are performed to expatiate the fact that such a quantum theory transits successfully to a classical de-Sitter universe. Inflation has thereafter been studied. The numerical values of the inflationary parameters show excellent agreement with the latest released Planck's data.

gr-qc↗

arXiv2021

Bianchi VI0 viscous fluid cosmology with magnetic field

A spatially homogeneous Bianchi type VI0 model containing a viscous fluid in the presence of an axial magnetic field has been studied. A barotropic equation of state together with a pair of linear relations among the square root of matter density, shear scalar, and expansion scalar have been assumed. Solutions are obtained in the presence of a magnetic field, only in two special cases, which are comparatively simpler. The complete solutions for this model in the absence of a magnetic field are also obtained. The presence of a magnetic field in the former case however, does not in effect cause any major modification in the fundamental nature of the initial singularity of the expanding model.

gr-qc↗

arXiv2021

Homogeneous Anisotropic Cosmological Models with Viscous Fluid and Magnetic Field

The paper presents some exact solutions of Bianchi types I, III and Kantowski-Sachs cosmological models consisting of a dissipative fluid along with an axial magnetic field. A barytropic equation of state between the thermodynamic pressure and the matter density, together with a pair of linear relations between the matter density, the shear scalar, and the expansion scalar have been assumed for simplicity. The solutions are basically of two different types, one for the Bianchi-I and the other for Bianchi-III and Kantowski-Sachs type. The presence of the magnetic field, however, does not change the fundamental nature of the initial singularity.

gr-qc↗