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Soumitra Sengupta

Publications and source records attributed to Soumitra Sengupta.

16 recordsLinked to original sources
arXiv2026

Phase space analysis of $f(R)$ cosmologies: revisiting attractor-to-GR

Phase space analysis of $f(R)$-Gravity, a simple generalization of Einstein's General theory of Relativity has been of interest especially in the context of Cosmology. Beyond its success in providing some viable descriptions of both early and late time cosmology, $f(R)$ gravity is considered to be an ideal set up for investigating some generic features of modified gravitational dynamics because of its mathematical simplicity, freedom from higher-derivative ghost instabilities and also having a natural dual description as scalar-tensor gravity in the Einstein frame representation. In this work, we have considered a class of $f(R)$-Gravity theories with $f(R)=R+\alpha R^n$ for both positive and negative integral values of $n$ and studied some general aspects of their cosmological phase space dynamics in the Einstein frame. We have used standard techniques from dynamical systems to obtain finite fixed points and assess their stability non-perturbatively by the Lyapunov method, whereas fixed points at infinity have been addressed using Poincar\'e projection. We have introduced techniques from Hybrid dynamical systems to the models with odd $n$, which do not admit standard methods of analysis in the Einstein frame representation due to peculiarities of the conformal transformation involved. Based on both analytical and numerical results we show the claimed convergence of all such $f(R)$ theories to GR (plus a dark energy like component for inverse power models) in a universe with a future-singularity free expansion history, assuming an isotropic and homogeneous background - via the dynamical freeze-out of the extra scalar degree of freedom to stable fixed points. At the end we discuss some observational constraints on the parameters of such theories and discuss their consistency with respect to our work.

gr-qc↗
arXiv2025

Bouncing Cosmologies in modified gravity with space time torsion

We explore the possibility of realizing a non-singular bounce in the early universe within the framework of modified gravity with spacetime torsion. In Einstein Cartan theory, torsion is embedded in the spacetime by adding an antisymmetric part in affine connection . We consider generalized version of the framework as $f(\bar{R})$, $\bar{R}$ being the scalar of the modified curvature tensor. $f(\bar{R})$ gravity is recast in Einstein frame as non-minimally coupled scalar tensor theory where the scalar field gets coupled with a rank 2 antisymmetric torsion field through derivative couplings. We investigate whether the introduction of three additional torsion-dependent terms in Einstein frame help to realize a bounce. We first explore this cosmological system in the background of a homogeneous and isotropic FRW spacetime but inclusion of the torsion terms are insufficient to produce a bounce in this symmetric setting. Motivated by this limitation, we relax the symmetry and generalize the background to include inhomogeneity and anisotropy. In this setup, the dynamics is modified in such a way that a bouncing solution is possible without invoking phantom fields or energy condition violations. We have found the exact solutions of all the fields and reconstructed the modified gravity form. We have addressed the behaviour of the fields under perturbation and investigated the stability of the solutions. Constraints on the model parameters have also been derived based on cosmological observations.

gr-qc↗
arXiv2024

Absence of antisymmetric tensor fields : Clue from f(R) model of gravity

One of the surprising aspects of the present Universe, is the absence of any noticeable observable effects of higher-rank antisymmetric tensor fields in any natural phenomena. Here, we address the possible explanation of the absence of the higher rank antisymmetric tensor fields within the framework of a general class of $f(R)$ gravity represented by $f (R) = R +\alpha_n R^n$. We explore the setup in Einstein frame, where the higher curvature is manifested in terms of a scalar field with a potential through a conformal transformation. The evolution of different cosmological parameters is studied in the background of FRW universe. We show that while different cosmological parameters mimic their standard behaviour at different epochs for different forms of higher curvature gravity (i.e. different values of n ), the positive values of the scalar field in the models provide an additional suppression for the massless modes of higher rank antisymmetric field, The Starobinsky model identified with $n=2$, provides heavier suppression compared to the others $n\neq2$. The result does not change even with the inclusion of the Cosmological Constant. Thus, our result reveals that a general class of modified gravity models can successfully explain the suppression of the massless modes of higher rank antisymmetric tensor fields leading to their invisibility in the present universe.

astro-ph.CO↗
arXiv2022

New $W$-Boson mass in the light of doubly warped braneworld model

The recent observation by CDF collaboration has disclosed a modification in the mass of the $W$ boson. In this regard we show that this modification in the mass of the $W$ boson can be well explained in the background of a 6-dimensional warped geometry model, where the double warping is associated with the two extra spatial dimensions. We consider that all the Standard Model fields are residing in the bulk, where the bulk Higgs field gives rise to the spontaneous symmetry breaking in the 6-dimensional spacetime. Allowing a little hierarchy between the two moduli we exactly obtain the observed mass for the $W$ boson, which is identified as the lowest lying Kaluza-Klein mass mode of the bulk $W$ boson on the $(3+1)$ dimensional visible brane. The essential feature of the 5-dimensional Randall-Sundrum scenario such as the resolution of the gauge hierarchy problem without introducing any intermediate scale between the Planck and the TeV scale, remains intact.

hep-ph↗
arXiv2016

Graviton Kaluza Klein modes in non-flat branes with stabilised modulus

We consider a generalised two brane Randall Sundrum model where the branes are endowed with non-zero cosmological constant. In this scenario, we re-examine the modulus stabilisation mechanism and the nature of Kaluza Klein (KK) graviton modes. Our result reveals that while the KK mode graviton masses may change significantly with brane cosmological constant, the Goldberger-Wise stabilisation mechanism, which assumes negligible backreaction on the background metric, continues to hold even when the branes have large cosmological constant. The possibility of having a global minimum for the modulus is also discussed.

hep-ph↗
arXiv2015

Higher rank antisymmetric tensor fields in Klebanov-Strassler geometry

In string theory, higher rank antisymmetric tensor fields appear as massless excitations of closed strings. Till date there is no experimental support in favour of their existence. In a stringy framework, starting from a warped throat-like Klebanov-Strassler geometry, we show that all the massless higher rank antisymmetric tensor fields are heavily suppressed due to the background fluxes leading to their invisibility in our Universe.

hep-th↗
arXiv2014

Radion cosmology and stabilization

We solve the Einstein's equation in five-dimensional spacetime for Randall Sundrum Brane world model with time dependent radion field to study variation of brane scale factor with time. We have shown that as radion field decreases with time compactifying the extra dimension, the scale factor increases exponentially with time leading to inflationary scenario. We have also proposed a time dependent generalization of Goldberger-Wise moduli stabilization mechanism to explain the time evolution of the radion field to reach a stable value after which the scale factor on the brane exits from inflationary expansion.

gr-qc↗
arXiv2014

Solar system constraints on alternative gravity theories

The perihelion precession of planetary orbits and the bending angle of null geodesics are estimated for different gravity theories in string-inspired models. It is shown that, for dilaton coupled gravity, the leading order measure in the angle of bending of light comes purely from vacuum expectation value of the dilaton field which may be interpreted as an indicator of a dominant stringy effect over the curvature effect. We arrive at similar results for spherically symmetric solution in quadratic gravity. We also present the perihelion shift and bending of light in the Einstein-Maxwell-Gauss-Bonnet theory with special reference to the Casimir effect and Damour-Polyakov mechanism. Numerical bounds to different coupling parameters in these models are estimated.

gr-qc↗
arXiv2011

Tunneling across dilaton coupled black holes in anti de Sitter spacetime

Considering generalised action for dilaton coupled Maxwell-Einstein theory in four dimensions, Gao and Zhang obtained black holes solutions for asymptotically anti de Sitter (Ads) and de Sitter (ds) spacetimes. We study the Hawking radiation in Parikh-Wilczek's tunneling formalism as well as using Bogoliubov transformations. We compare the expression of the Hawking temperature obtained from these two different approaches. Stability and the extremality conditions for such black holes are discussed. The exact dependences of the Hawking temperature and flux on the dilaton coupling parameter are determined. It is shown that the Hawking flux increases with the dilaton coupling parameter. Finally we show that the expression for the Hawking flux obtained using Bogoliubov transformation matches exactly with flux calculated via chiral gauge and gravitational anomalies. This establishes a correspondence among all these different approaches of estimating Hawking radiation from these class of black holes.

gr-qc↗
arXiv2010

Strong gravitational lensing across dilaton anti-de Sitter black hole

In this work we investigate gravitational lensing effect in strong field region around a dilaton black holes in an anti de Sitter (ADS) space. We also analyse the dependence of the radius of the photon sphere and deflection angle on dilaton coupling and cosmological constant in this black hole space time. Finally the values of minimum impact parameter, the separation between the first and the other images as well as the ratio between the flux of the first image and the flux coming from all the other images are determined to characterize some possible distinct signatures of such black holes.

gr-qc↗
arXiv2000

Does a Kalb-Ramond field make spacetime optically active

A spacetime with torsion produced by a Kalb-Ramond field coupled gravitationally to the Maxwell field, in accordance with a recent proposal by two of us (PM and SS), is argued to lead to an optical activity in synchrotron radiation from cosmologically distant radio sources. We suggest that this could qualitatively explain observational data from a large number of radio sources displaying such polarization asymmetry (after eliminating effects of Faraday rotation due to magnetized galactic plasma). Possible implications for heterotic string theory are also outlined.

gr-qc↗
arXiv1999

Duality Invariance of Cosmological Solutions with Torsion

We show that for a string moving in a background consisting of maximally symmetric gravity, dilaton field and second rank antisymmetric tensor field, the $O(d) \otimes O(d)$ transformation on the vacuum solutions gives inequivalent solutions that are not maximally symmetric. We then show that the usual physical meaning of maximal symmetry can be made to remain unaltered even if torsion is present and illustrate this through two toy models by determining the torsion fields, the metric and Killing vectors. Finally we show that under the $O(d) \otimes O(d)$ transformation this generalised maximal symmetry can be preserved under certain conditions. This is interesting in the context of string related cosmological backgrounds.

hep-th↗
arXiv1998

A geometrical interpretation of parity violation in gravity with torsion

In a space-time with torsion, the action for the gravitational field can be extended with a parity-violating piece. We show how to obtain such a piece from geometry itself, by suitably modifying the affine connection so as to include a pseudo-tensorial part. A consistent method is thus suggested for incorporating parity-violation in the Lagrangians of all matter fields with spin in a space-time background with torsion.

hep-th↗
arXiv1997

Construction of Maximally Symmetric Solutions for the Metric

We construct new maximally symmetric solutions for the metric. We then show that for a string moving in a background consisting of maximally symmetric gravity, dilaton field and second rank antisymmetric tensor field, the O(d) $\otimes$ O(d) transformation on the vacuum solutions, in general, gives inequivalent solutions that are not maximally symmetric.

hep-th↗
arXiv1997

The Meaning of Maximal Symmetry in Presence of Torsion

We show that the usual physical meaning of maximal symmetry can be made to remain unaltered even if torsion is present. All that is required is that the torsion fields satisfy some mutually consistent constraints. We also give an explicit realisation of such a scenario by determining the torsion fields, the metric and the associated Killing vectors.

hep-th↗