arXiv subjects

A. S. Agrawal

Publications and source records attributed to A. S. Agrawal.

At least 19 recordsLinked to original sources

arXiv2026

Bouncing Cosmology and Cosmological Dynamics in $f(Q,T)$ Gravity

We propose a reconstructed cosmological model in the framework of $f(Q,T)$ gravity, that provides a unified description of the early- and late-time evolution of the Universe. The model exhibits a non-singular asymmetric bounce, smoothly connecting an initial contracting phase to the subsequent expanding Universe and naturally evolving into a late-time dark energy-dominated epoch. Our study focuses on the progression of the Hubble parameter, energy density, pressure, and the parameter. This analysis aims to define the various stages of cosmic evolution and explore the characteristics of dark energy. The analysis of energy conditions reveals that the essential conditions for achieving a non-singular bounce are violated. Overall, the $f(Q, T)$ gravity model, once reconstructed, effectively captures the cosmic dynamics surrounding the bounce. It offers a cohesive theoretical framework that reliably explains the Universe's evolution during both its early and late stages.

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arXiv2025

Cosmological Dynamics in $f(R,L_m,T)$ Modified Gravity

In this paper, we investigate the accelerating phase of the Universe within the context of $f(R,L_m,T)$ gravity theory, where $R$, $L_m$, and $T$ represent the Ricci scalar, matter Lagrangian, and the trace of the energy-momentum tensor, respectively. We focus on a particular form of modified gravity defined by $f(R,L_m,T) = R - μL_m T - γ$, with $μ$ and $γ$ being positive constants. The matter sector is characterized by the Lagrangian density $L_m = -ρ$, where $ρ$ denotes the energy density of the cosmological fluid. We conduct an in-depth examination of the model using phase space analysis, thoroughly evaluating the evolution of cosmological solutions with dynamical system techniques. The results is illustrated through graphs in the phase space, the characteristics of critical points and the stable attractors within the proposed modified gravity $f(R,L_m,T)$ cosmological framework. We investigate the transition from the initial decelerating phase of the universe to its current accelerating phase. The behaviour of the EoS, deceleration parameter with the appropriate initial conditions have been investigated.

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arXiv2025

Exploring Wormhole Structures within the Framework of $f(R,L_{m})$ Gravity

In this work, we investigate wormhole geometries within the framework of $f(R,\mathcal{L}_{m})$ gravity by considering a specific form of the model. From the corresponding field equations, the shape function is derived, and the traversability conditions are examined for suitable choices of the model parameters. The obtained shape function is shown to satisfy all the necessary requirements for a traversable wormhole, including the energy conditions. The geometric properties of the wormhole are analyzed in detail, and particular attention is given to the requirement that the proper radial distance $l(r)$ remains finite throughout the space-time, thereby ensuring the consistency of the geometry.

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arXiv2025

Bouncing Cosmology in Interacting Scalar-Torsion Gravity

In this study we demonstrate the interacting teleparallel gravity models, to describe the matter bounce scenario. We discussed two interacting models and find both are suitable choice to describe the bouncing phenomena. The co-moving Hubble radius, is demonstrated to check the establishment of the matter bounce scenario. All the energy conditions and the behaviour of EoS parameter is analysed. The violation of NEC at bounce epoch is one of the crucial result to establish bouncing behaviour is found to be obeyed. The other energy conditions behaviour is in agreement with the EoS parameter which lies in the phantom region at the bounce epoch in both the models.

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arXiv2024

A model-independent compact dynamical system formulation for exploring bounce and cyclic cosmological evolutions in $f(R)$ gravity

Using the dynamical systems approach together with the cosmographic parameters, we present a model-independent dynamical system formulation for cosmology in f(R) gravity. The formulation is model-independent in the sense that one needs to specify not a particular functional form of f(R) a-priori, but rather a particular cosmological evolution, which fixes the cosmography. In a sense, our approach is the way around the reconstruction method. This is shown using both non-compact and compact dynamical variables. The focus in this paper is on the compact analysis since we demonstrate the applicability of this formulation using examples of bouncing and cyclic cosmology. In particular, our analysis reveals, in a model-independent manner, the problem of achieving such cosmologies when the universe is globally spatially flat and devoid of matter.

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arXiv2024

Non-interacting String and Holographic Dark Energy Cosmological Models in f(R) Theory of Gravitation

In this paper, a new class of string and holographic dark energy (HDE) cosmological model in the context of the $f(R)$ theory of gravity using the Kasner metric is considered. The exact solution of the field equations is obtained using the relation between the average scale factor and the scalar function $f(R)$. It is observed that the universe is accelerating and expanding. The string phase of the universe is present at an early stage of the evolution of the universe. The universe is dominated by quintessence type HDE at present. The effect of the curvature function $f(R)$ is also observed on dynamical parameters.

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arXiv2024

Matter Geometry Coupling and Casimir Wormhole Geometry

In this study, we investigate traversable wormhole solutions within the setup of $f(R,\mathcal{L}_{m})$ gravity, a modified theory of gravity where the gravitational action relies upon the matter Lagrangian $\mathcal{L}_{m}$ and the Ricci scalar $R$. In General Relativity (GR), stability issues in traversable wormholes necessitate the existence of exotic matter that violates the null energy condition (NEC). In contrast, we explore wormhole solutions that align with the criteria for Casimir wormholes, which do not necessarily require NEC violation. Our analysis demonstrates that in the context of $f(R,\mathcal{L}_{m})$ gravity, exotic matter can sustain these wormholes. We further examine the traversability conditions of the wormhole, considering both scenarios with and without the Generalized Uncertainty Principle (GUP) correction. Additionally, the stability of the wormhole is assessed based on equilibrium conditions. Our findings suggest that $f(R,\mathcal{L}_{m})$ gravity offers a viable framework for the existence of stable, traversable wormholes sustained by exotic matter, potentially expanding the landscape of viable wormhole solutions beyond the confines of GR.

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arXiv2024

Black Holes and Wormholes Beyond Classical General Relativity

In the paper, only Static Spherically Symmetric space-times in four dimensions are considered within modified gravity models. The non-singular static metrics, including black holes not admitting a de Sitter core in the center and traversable wormholes, are reconsidered within a class of higher-order $F(R)$, satisfying the constraints $F(0)=\frac{dF}{dR}(0)=0$. Furthermore, by making use of the so-called effective field theory formulation of gravity, the quantum corrections to Einstein-Hilbert's action due to higher-derivative terms related to curvature invariants are investigated. In particular, in the case of Einstein-Hilbert action plus cubic curvature Goroff-Sagnotti contribution, the second-order correction in the Goroff-Sagnotti coupling constant is computed. In general, it is shown that the effective metrics, namely Schwarzschild expression plus small quantum corrections, are related to black holes and not to traversable wormholes. In this framework, within the approximation considered, the resolution of singularity for $r=0$ is not accomplished. The related properties of these solutions are investigated.

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arXiv2024

Wormholes in the f(R,L,T) theory of gravity

Morris and Thorne developed wormhole solutions in the late 1980s when they discovered a recipe that wormholes must follow for travelers to cross them safely. They describe exotic matter as satisfying $-p_{r} > ρ$, where $p_{r}$ is the radial pressure and $ρ$ is the energy density of the wormhole. This is a notable characteristic of the General Relativity Theory. The current article discusses traversable wormhole solutions in $f(R, L, T)=R+αL+βT$, with $α$ and $β$ are model parameters. The wormhole solutions presented here satisfy the metric constraints of traversability while remarkably avoiding the exotic matter condition, indicating that $f(R, L, T)$ gravity wormholes can be filled with ordinary matter. The derived solutions for the shape function of the wormhole meet the required metric conditions. They exhibit behavior that is comparable to that of wormholes reported in earlier references, which is also the case for our solutions for the energy density of such objects.

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arXiv2024

Bouncing Scenario and Cosmic Dynamics in Modified Theories of Gravity

The main objective of this study is to investigate the phenomenon of the bouncing scenario of the universe. The most widely recognized cosmological framework is the standard cosmological model, sometimes referred to as the Big Bang model. This is mainly because of its inherent properties and its consistent alignment with recent observational studies. However, the standard cosmological model faces some challenges concerning the physical conditions at the initial epochs. Some of these issues include the initial singularity problem, flatness problem, horizon problem, etc. Some of these challenges could potentially be addressed by incorporating the inflationary scenario into the cosmological framework of the universe. However, the inflationary mechanism is not able to tackle the occurrence of the initial singularity. The bouncing cosmology offers a probable solution to this initial singularity issue. In addition, it is capable of addressing some other issues that may arise during the early stages. Hence, in the modified gravity theory, bounce cosmology has been discussed.

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arXiv2023

Global phase space analysis for a class of single scalar field bouncing solutions in general relativity

We carry out a compact phase space analysis of a non-canonical scalar field theory whose Lagrangian is of the form $F(X)-V(ϕ)$ within general relativity. In particular, we focus on a kinetic term of the form $F(X)=βX^m$ with power law potential $V_0 ϕ^n$ and exponential potential $V_0 e^{-λϕ/M_{Pl}}$ of the scalar field. The main aim of this work is to investigate the genericity of nonsingular bounce in these models and to investigate the cosmic future of the bouncing cosmologies when they are generic. A global dynamical system formulation that is particularly suitable for investigating nonsingular bouncing cosmologies is used to carry out the analysis. We show that when $F(X)=βX^m$ ($β<0$), nonsingular bounce is generic for a power law potential $V(ϕ) = V_0 ϕ^n$ only within the parameter range $\left\lbrace \frac{1}{2}<m<1,\,n<\frac{2m}{m-1}\right\rbrace$ and for an exponential potential $V(ϕ) = V_0 e^{-λϕ/M_{Pl}}$ only within the parameter range $\left\lbrace\frac{1}{2}<m\leq1\right\rbrace$. Except in these cases, nonsingular bounce in these models is not generic due to the non-existence of global past or future attractors. Our analysis serves to show the importance of a global phase space analysis to address important questions about nonsingular bouncing solutions, an idea that may and must be adopted for such solutions even in other theories.

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arXiv2023

Observationally constrained accelerating cosmological model with higher power of non-metricity and squared trace

In this paper, a cosmological model of the Universe is presented in $f(Q,T)$ gravity and the parameters are constrained by cosmological data sets. Initially, a generalised form of $f(Q,T)$ model is used as $f(Q,T)=-λ_{1} Q^{m}-λ_{2} T^2$, where $λ_{1}$, $λ_{2}$ and $m$ are model parameters. With some algebraic manipulation, the Hubble parameter is obtained in terms of redshift. Then, using MCMC analysis, the model parameters are constrained using the most current Hubble and Pantheon$^{+}$ data. The model parameters are also verified through the BAO data set. The model shows an early deceleration transitioning to an accelerating phase of the Universe. The $Om(z)$ diagnostics indicate a positive slope, favouring the model to be in a phantom field dominated phase.

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arXiv2023

Bouncing cosmological models in a functional form of F(R) gravity

We have investigated some bouncing cosmological models in an isotropic and homogeneous space time with the F(R) theory of gravity. Two functional forms of F(R) have been investigated with a bouncing scale factor. The dynamical parameters are derived and analysed along with the cosmographic parameters. The analysis in both the models show the occurrence of bouncing scenario. The violation of strong energy conditions in both models is also shown. In the stability point of view we have analysed the behaviour of F_R = dF/dR with respect to cosmic time and both the models exhibit stable behaviour.

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arXiv2023

Unimodular Gravity Traversable Wormholes

Wormholes are outstanding solutions of Einstein's General Relativity. They were worked out in the late 1980's by Morris and Thorne, who have figured out a recipe that wormholes must obey in order to be traversable, that is, safely crossed by travelers. A remarkable feature is that General Relativity Theory wormholes must be filled by {\it exotic matter}, which Morris and Thorne define as matter satisfying $-p_r>ρ$, in which $p_r$ is the radial pressure and $ρ$ is the energy density of the wormhole. In the present article, we introduce, for the first time in the literature, traversable wormhole solutions of Einstein's Unimodular Gravity Theory. Unimodular Gravity was proposed by Einstein himself as the theory for which the field equations are the traceless portion of General Relativity field equations. Later, Weinberg has shown that this approach elegantly yields the solution of the infamous cosmological constant problem. The wormhole solutions here presented satisfy the metric conditions of "traversability" and remarkably evade the exotic matter condition, so we can affirm that Unimodular Gravity wormholes can be filled by ordinary matter.

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arXiv2023

Matter Bounce Scenario in Extended Symmetric Teleparallel Gravity

In this paper, we have shown the matter bounce scenario of the Universe in an extended symmetric teleparallel gravity, the f(Q) gravity. Motivated from the bouncing scenario and loop quantum cosmology (LQC), the form of the function $f(Q)$ has been obtained at the backdrop of Friedmann-Lemaitre-Robertson Walker (FLRW) space time. Considering the background cosmology dominated by dust fluid, the e-folding parameter has been expressed, which contains the nonmetricity term. Since the slow roll criterion in the bouncing context is not valid, we used a conformal equivalence between f(Q) and scalar-tensor model to apply the bottom-up reconstruction technique in the bouncing model. The dynamics of the model has been studied through the phase space analysis, where both the stable and unstable nodes are obtained. Also, the stability analysis has been performed with the first order scalar perturbation of the Hubble parameter and matter energy density to verify the stability of the model.

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arXiv2022

f(R) wormholes embedded in a pseudo--Euclidean space $E^{5}$

This work is devoted to the study of analytic wormhole solutions within the framework of $f(R)$ gravity theory. To check the possibility of having wormhole structures satisfying energy conditions, by means of the class I approach the pair $\{Φ(r), b(r)\}$ describing the wormhole geometry has been obtained. Then, in conjunction with a remarkably $f(R)$ gravity model, the satisfaction of the null and weak energy conditions at the wormhole throat and its neighborhood is investigated. To do so, some constant parameters have been bounded restricting the space parameter. In this concern, the $f(R)$ gravity model and its derivatives are playing a major role, specially in considering the violation of the non--existence theorem. Furthermore, the shape function should be bounded from above by the Gronwall--Bellman shape function, where the red--shift function plays a relevant role. By analyzing the main properties at the spatial stations and tidal accelerations at the wormhole throat, possibilities and conditions for human travel are explored.

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arXiv2022

Role of Extended Gravity Theory in Matter Bounce Dynamics

In this work, we have studied some bouncing cosmologies in the frame work of $f(R,T)$ gravity. The bouncing scenario has been formulated to avoid the big bang singularity. The physical and geometrical parameters are investigated. The effect of the extended gravity theory on the dynamical parameters of the model is investigated. It is found that, the $f(R,T)$ gravity parameter affects the cosmic dynamics substantially. We have also, tested the model through the calculation of the cosmographic coefficients and the $Om(z)$ parameter. A scalar field reconstruction of the bouncing scenario is also carried out. The stability of the model are tested under linear, homogeneous and isotropic perturbations.

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arXiv2021

Traversable wormhole models in $f(R)$ gravity

In this work, we analyze the wormhole solutions in $f(R)$ gravity. Specifically we sought for wormhole geometry solutions for the following three shape functions: (i) $b(r)=r_{0}+ρ_{0}r_{0}^{3}\ln\left(\frac{r_{0}}{r}\right)$, (ii) $b(r)=r_{0}+γr_{0}\left(1-\dfrac{r_{0}}{r}\right)$, and (iii) $b(r)=α+βr$, under some legitimate physical conditions on the parameters as well as constants involved here with in the shape functions. It is observed from the graphical plots that the behaviour of the physical parameters are interesting and viable.

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