arXiv subjects

Dalia Saha

Publications and source records attributed to Dalia Saha.

12 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

Issue of Branched Hamiltonian, Inflation and indispensability of scalar field in generalized Teleparallel theories

Generalized Teleparallel gravity theories were proposed as alternatives to the dark energy and modified theories of gravity. However, minimal generalization of TEGR and STEGR lead to the issue of branched Hamiltonian. Here, in the FLRW background we fix the coupling parameters of the said model in view of energy condition and show that the theory is free from the issue of Branched Hamiltonian unlike second-order Lanczos-Lovelock gravity. We also study scalar field driven inflation, which shows excellent agreement with recent observations. Nonetheless, a viable radiation era requires the very presence of the scalar field, which is retained at the late universe as dark energy. Thus the fundamental claim to act as an alternative to the dark energy is dubious.

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

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

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↗

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

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

Early universe in view of a modified theory of gravity

We study the quantum evolution of the early universe, its semi-classical analogue together with inflationary regime, in view of a generalized modified theory of gravity. The action is built by supplementing the non-minimally coupled scalar-tensor theory of gravity with scalar curvature squared term and a Gauss-Bonnet-dilatonic coupled term. It is generalized, since all the parameters are treated as arbitrary functions of the scalar field. It is interesting to explore the fact that instead of considering additional flow parameters, an effective potential serves the purpose of finding inflationary parameters. The dilaton stabilization issue appears here as a problem with reheating. Addition of a cosmological constant term alleviates the problem, and inflation is effectively driven by the vacuum energy density. Thus Gauss-Bonnet term might play a significant role in describing late-time cosmic evolution.

hep-th↗

arXiv2020

Probing the early universe with a generalized action

Possibly, the most general action in the background of isotropic and homogeneous space-time has been considered to study the quantum evolution of the early universe, apart from a cosmological constant. The hermiticity of the effective Hamiltonian operator in the presence of curvature squared terms suggests unitary time evolution of the quantum states, assuring conservation of probability. The oscillatory behaviour of the semi-classical wavefunction around a de-Sitter solution signals that the theory is classically allowed, and the universe enters an inflationary regime just after Planck's era. In view of a hierarchy of Hubble flow parameters, and using a redefined effective potential, the complicated classical field equations in the presence of several coupling parameters, reduce to standard general-relativistic equations with a single scalar field. As a result, inflation has been studied without considering any additional flow parameters. Inflationary parameters lie very much within the presently available Planck's data, and the model admits graceful exit from inflation.

hep-th↗

arXiv2020

Inflation with scalar-tensor theory of gravity

The latest released data from Planck in 2018, put up tighter constraints on inflationary parameters. In the present article, the in-built symmetry of the non-minimally coupled scalar-tensor theory of gravity is used to fix the coupling parameter, the functional Brans-Dicke parameter, and the potential of the theory. It is found that all the three different power-law potentials and one exponential, pass these constraints comfortably, and also gracefully exit from inflation.

astro-ph.CO↗