arXiv subjects

S. A. Narawade

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

17 recordsLinked to original sources

arXiv2026

Dynamical analysis of the covariant $f(Q)$ gravity models

In this study, we explore the cosmological evolution of the Universe in the framework of covariant $f(Q)$ gravity, with a coupling function that evolves dynamically in proportion to the Hubble parameter. Two specific forms of the function are examined: a power-law model and a logarithmic model. By rewriting the cosmological field equations as an autonomous dynamical system, we determine and classify the corresponding critical points and analyze their stability. Our results show that both models are able to reproduce the sequence of cosmic evolution, including radiation, matter, and dark energy-dominated eras, along with the transitions between them. The physical properties at each critical point are described using key cosmological quantities such as the total EoS parameter, density parameters, and the deceleration parameter. The stability of the non-hyperbolic critical point is analyzed through center manifold theory. In addition, we present phase space trajectories along with the stability behavior of each critical point. The evolution plots for the density parameters of radiation, matter, and dark energy, along with the EoS parameter for the total, are illustrated for further analysis. Overall, the analysis suggests that the $f(Q)$ models considered here, within the context of covariant formulation, provide a consistent description of cosmic evolution and offer a promising approach to explaining the late-time acceleration of the Universe.

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arXiv2025

Cosmological Models with Symmetric Teleparallel Gravity and its Extension

This thesis investigates late-time cosmic acceleration using modified gravity theories with a focus on $f(Q)$ gravity, as an alternative to the $Λ$CDM model. The standard cosmological model attributes the acceleration to a cosmological constant, but it faces issues like the unexplained nature of dark matter and dark energy and discrepancies with certain observations. Modified gravity including $f(Q)$ gravity, offers a potential solution by incorporating dynamic dark energy or changes to gravitational interactions, avoiding the need for a constant cosmological term. Also, thesis evaluates the viability of $f(Q)$ gravity by analyzing observational data from Type Ia Supernovae, Hubble parameter measurements and other cosmological datasets. Using statistical tools like Markov Chain Monte Carlo (MCMC) analysis, this work constrains the parameters of $f(Q)$ gravity and compares it to the $Λ$CDM model.....

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arXiv2025

Exploring the universal $\bar{\mathcal{I}}-\mathcal{C}$ relations for relativistic stars in $f(Q)$ gravity

We investigate the properties of neutron stars within the framework of $f(Q)$ gravity by incorporating rotational effects through a slowly rotating metric. We derive the modified TOV equations and calculate the angular velocity profiles and moments of inertia (MOI) for linear, quadratic, exponential, and logarithmic $f(Q)$ models. Our results show that deviations in the MOI are more pronounced than those in the stellar mass profiles, suggesting that rotational observables are highly sensitive to geometric corrections. We also calculate a quasi-universal relation between the dimensionless MOI and compactness ($\bar{I}$-$C$). The linear and quadratic models are generally consistent with observational data from PSR J0737-3039A, although the deviations are small and difficult to distinguish from General Relativity due to inherent EoS variability. On other hand, the logarithmic and exponential models show larger deviations (over 20 %), exceeding the EoS-induced uncertainty reported by Suleiman & Read (2024), highlighting the relation's sensitivity to the $f(Q)$ gravity model. These results indicate that $f(Q)$ gravity could potentially be tested in the strong-field regime and point to a direction for future studies, such as investigating EoS-insensitive quasi-universal relations, like the $\bar{I}(Λ)$ relations, within the $f(Q)$ framework. Such relations may provide a clearer pathway for exploring possible signatures in strong-field gravity when combined with more precise future observations.

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arXiv2025

Late time behavior in $f(R,\mathcal{L}_{m})$ gravity through Gaussian reconstruction and dynamical stability

In this paper, we explore modified gravity in the framework of $f(R, \mathcal{L}_m)$ theories by reconstructing the function $f(\mathcal{L}_m)$, where $\mathcal{L}_m = ρ$ is the matter Lagrangian, under the assumption of a pressureless, matter-dominated Universe. Using a non-parametric Gaussian process reconstruction technique applied to Hubble data, we obtain two viable models of $f(\mathcal{L}_m)$ : (i) a power-law model $f_1(\mathcal{L}_m) = α\mathcal{L}_m^{b_1}$ with $b_1 \in [0.018, 0.025]$ and (ii) an exponential model $f_2(\mathcal{L}_m) = α\mathcal{L}_{m0} \left(1 - e^{-b_2 \sqrt{\mathcal{L}_m/\mathcal{L}_{m0}}} \right)$ with $b_2 \in [2.3, 3.0]$. We then fix the parameter values within these reconstructed ranges and analyze the corresponding dynamical systems within the matter-dominated epoch by constructing autonomous equations. Phase-space analysis reveals the presence of stable critical points in both models, suggesting viable cosmic evolution within their domains of validity. Both the models exhibit stable attractor solution at late time, reinforcing their viability in explaining the late time cosmic acceleration without explicitly invoking a cosmological constant. Our results indicate that $f(R, \mathcal{L}_m)$ gravity with data-driven matter-sector modifications can offer a compelling alternative description of cosmic dynamics during the matter-dominated era.

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arXiv2025

Stable $f(Q)$ gravity model through non-trivial connection

This study effectively reconstructs a cosmological model utilizing covariant $f(Q)$ gravity within Connection-III and FLRW spacetime. The dynamic behavior of the reconstructed model is thoroughly analyzed using the Hubble parameter $H(z)$ and various observational datasets. Our robust findings demonstrate that the model displays quintessence behavior at the present epoch and converges to the $Λ$CDM model at late time. It is confirmed through comprehensive evaluations against energy conditions that the Null Energy Condition remains positive throughout cosmic evolution, and the Dominant Energy Condition is consistently satisfied. The Strong Energy Condition is initially fulfilled in the early Universe but violated in the late epoch. Moreover, scalar perturbations extensively assess stability, affirming the strength of the model with respect to the Hubble parameter. This research offers compelling insights into cosmic acceleration, suggesting that $f(Q)$ gravity can effectively displace the $Λ$CDM model and provides a convincing alternative explanation for the current accelerating expansion of the Universe without relying on the cosmological constant.

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arXiv2024

Neutron Star in Covariant $f(Q)$ gravity

Assuming static and spherically symmetric stars with perfect fluid matter, we used realistic equations of state to study neutron stars in covariant $f(Q)$ gravity. The structure profiles and properties of neutron stars such as mass, radius and compactness are obtained through numerical methods using quadratic, exponential, and logarithmic $f(Q)$ models. The results indicate that nonmetricity affects the interior profile deviations of the star, which in turn influence the properties of stars, as illustrated in the mass-radius relation diagram. This effect allows the star to accommodate either more or less matter compared to GR, resulting in a different total mass. For the quadratic model, we cannot generate larger masses, whereas the other two models can give consistent results for both smaller and larger masses of the observed stars. By tuning model parameters, we obtain $\mathcal{M}-\mathcal{R}$ diagrams that are compatible with observational constraints from NICER and LIGO.

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arXiv2024

Modelling the Accelerating Universe with $f(Q)$ Gravity: Observational Consistency

In this paper, we present a cosmological model within the framework of symmetric teleparallel gravity, focusing on $f(Q)$ gravity, where $Q$ represents the non-metricity scalar. Utilizing cosmological datasets, we derive an accelerating cosmological model by constraining its free parameters. To achieve this, we determine the parametric form of the Hubble parameter using a well-motivated $f(Q)$ function. Remarkably, all obtained values fall within the range suggested by cosmological observations. By employing the best-fit parameters, we calculate the present geometrical parameters and demonstrate the accelerating behaviour of the Universe. Furthermore, we thoroughly examine the evolutionary behaviours of the Universe, noting that our model converges to the $Λ$CDM model at late times. Finally, we investigate the energy conditions and find a violation of the strong energy condition, which could provide a valuable understanding of the nature of dark energy.

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arXiv2024

Constraining Parameters for the Accelerating Universe in $f(R,\mathcal{L}_{m})$ Gravity

In the paper, we present an accelerating cosmological model in $f(R,\mathcal{L}_{m})$ gravity with the parameter constrained through the cosmological data sets. At the beginning, we have employed a functional form of $f(R,\mathcal{L}_{m}) =\frac{R}{2}+αR^2+\mathcal{L}_{m}^β$, where $α$ and $β$ are model parameters. This model is well motivated from the Starobinsky model in $f(R)$ gravity and the power law form of $f(\mathcal{L}_{m})$. The Hubble parameter has been derived with some algebraic manipulation and constrained by Hubble data and Pantheon$^{+}$ data. With the constraint parameters, present value of deceleration parameter has been obtained to as $q_{0}\approx-0.63$ with the transition at $z_{t}\approx0.7$. It shows the early deceleration and late time acceleration behaviour. The present value of other geometric parameters such as the jerk and snap parameter are obtained to be $j_{0}\approx0.78$ and $s_{0}\approx 0.1$ respectively. The state finder diagnostic test gives the quintessence behaviour at present and converging to $Λ$CDM at late times. Moreover the $Om(z)$ diagnostics gives negative slope which shows that the model favours the state finder diagnostic result. Also the current age of Universe has been obtained as, $t_{0} = 13.64~~Gyrs$. The equation of state parameter also shows the quintessence behaviour. Based on the present analysis, it indicates that the $f(R,\mathcal{L}_{m})$ gravitational theory may be another alternative to study the dark energy models.

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arXiv2024

Baryon asymmetry constraints on Extended Symmetric Teleparallel Gravity

In this paper, we have explored the observed matter-antimatter asymmetry in the Universe to constrain the model parameters in extended symmetric teleparallel gravity (STG) or $f(Q,T)$ gravity, where $Q$ be the nonmetricity and $T$ be the trace of energy momentum tensor. We have considered two functional forms of $f(Q,T)$ to find the baryon asymmetry to entropy ratio calculated at a decoupling temperature. Two different data sets namely Hubble data set and the Hubble+BAO+Pantheon data sets are used to constrain the scale factor and the constrained model is used to obtain the baryon asymmetry to entropy ratio. It is observed that, model constrained from the Hubble data set favour a narrow range of the $f(Q,T)$ gravity parameters to reproduce the observed baryon asymmetry.

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arXiv2023

Constrained cosmological model in $f(Q,T)$ gravity with non-linear non-metricity

The $f(Q,T)$ cosmological model has emerged as a promising framework for understanding various aspects of cosmic evolution. In this study, we focused on obtaining the constraints of the free parameters in the non-linear form of non-metricity in $f(Q,T)$ gravity using the $Hubble$, $Pantheon$, and $BAO$ datasets. To determine the best-fit values for the model parameters and the equation of state (EoS) parameter, we employed an MCMC analysis. By examining the error bar plots, we observed that both the model curve and the $Λ$CDM curve successfully passed through the range obtained from the datasets. Additionally, we studied the state finder diagnostics and energy conditions to gain insights into the properties of the model. Furthermore, we conducted an analysis using the $Om(z)$ diagnostic, which provides a null test for the validity of the $Λ$CDM model.

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arXiv2023

Accelerating cosmological models in $f(Q)$ gravity and the phase space analysis

The dynamical aspect of accelerating cosmological model has been studied in this paper in the context of modified symmetric teleparallel gravity, the $f(Q)$ gravity. Initially, we have derived the dynamical parameters for two well known forms of $f(Q)$ such as: (i) log-square-root form and (ii) exponential form. The equation of state (EoS) parameter for the dark energy in the $f(Q)$ gravity in both the models emerges into a dynamical quantity. At present model-I shows the quintessence behavior and behave like the $Λ$CDM at the late time whereas model-II shows phantom behaviour. Further, the dynamical system analysis has been performed to determine the cosmological behaviour of the models along with its stability behaviour. For both the models the critical points are obtained and analysed the stability at each critical points with phase portraits. The evolutionary behaviour of density parameters for the matter-dominated, radiation-dominated, and dark energy phases are also shown for both the models.

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arXiv2023

Observational Constraints on Hybrid Scale Factor in f(Q,T) Gravity with Anisotropic Space-Time

In this paper, we present an accelerating cosmological model by constraining the free parameters using the cosmological datasets in an extended symmetric teleparallel gravity for the flat and anisotropic space-time. We employ a time variable deceleration parameter that behaves early deceleration and late time acceleration in the form of Hybrid Scale Factor (HSF). We obtain the present values of deceleration parameter and analyse the late time behavior of the Universe based on the best-fit values of free parameters. We derive the dynamical parameters of the model and obtain the equation of state parameter at present in the quintessence region; however at late time it approaches to $Λ$CDM. The energy conditions are also analysed to validate the modified gravity and we find that strong energy condition is violating. We establish the importance of hybrid scale factor in the late time cosmic phenomena issue.

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arXiv2023

Phantom cosmological model with observational constraints in $f(Q)$ gravity

In this paper, the cosmological model of the Universe has been presented in $f(Q)$ gravity and the parameters are constrained from the cosmological data sets. At the beginning, we have employed a well motivated form of $f(Q) = α+ βQ^{n}$, where $α$, $β$ and $n$ are model parameters. We have obtained the Hubble parameter in redshift with some algebraic manipulation from the considered form of $f(Q)$. Then we parameterize with the recent $Hubble$ data and $Pantheon + SHOES$ data using $\textit{MCMC}$ analysis. We validate our obtained model parameter values with $\textit{BAO}$ data set. A parametrization of the cosmographic parameters shows the early deceleration and late time acceleration with the transition at $z_t\approx0.75$. The $Om(z)$ diagnostics gives positive slope which shows that the model in the phantom phase. Also the current age of Universe has been obtained as, $t_{0} = 13.85~~Gyrs$. Based on the present analysis, it indicates that the $f(Q)$ gravity may provide an alternative to dark energy for addressing the current cosmic acceleration.

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arXiv2023

Evolutionary behaviour of cosmological parameters with dynamical system analysis in f (Q, T) gravity

We have investigated the accelerating behaviour of the universe in $f(Q,T)$ gravity in an isotropic and homogeneous space-time. We have initially derive the dynamical parameters in the general form of $f(Q,T)=αQ^{m}+βT$ [Xu et al., Eur. Phys. J. C, \textbf{79}, 708 (2019)] and then split it into two cases (i) one with $m=1$ and the (ii) other with $β=0$. In the first case, it reduces to the linear form of the functional $f(Q,T)$ and second case leads to the higher power of the nonmetricity $Q$. In an assumed form of the hyperbolic scale factor, the models are constructed and its evolutionary behaviours are studied. The geometrical parameters as well the equation of state parameter are obtained and found to be in the preferred range of the cosmological observations. Marginal variation has been noticed in the behaviour of $ω$ and $ω_{eff}$ at present time. The violation of strong energy conditions in both the cases are shown. The dynamical system analysis for the models has been performed.

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arXiv2023

Constrained $f(Q,T)$ gravity accelerating cosmological model and its dynamical system analysis

In this paper, we have presented an accelerating cosmological model of the Universe in an extended symmetric teleparallel gravity or $f(Q,T)$ gravity. The parametric form of the Hubble parameter is, $H\left(z\right) =H_{0}\left[ α+\left( 1-α\right) \left( 1+z\right) ^{n}\right] ^{\frac{3}{2n}}$, where $H_0$ and $n$ are constants and for $n=3$, the $Λ$CDM scenario can be obtained. We have considered the logarithmic form of $f(Q,T)$ as, $f(Q,T)=-Q+β\log\left(\frac{Q}{Q_{0}}\right)+γT$, where $β$ and $γ$ are the free model parameters. Using the Hubble, Baryon Acoustic Oscillations (BAO), and Type Ia Supernovae (SNe Ia) datasets, the present value of the Hubble parameter and other free parameters are constrained. Further other cosmographic and dynamical parameters are presented using the obtained constrained values of the Hubble and free parameters. The model shows the quintessence behavior of the Universe at the present time. The present value of the EoS parameter is obtained as, $ω_{0}=-0.56$ for the $Hubble+BAO+SNe$ datasets. The energy conditions are presented and the violation of the strong energy condition has been shown. We have performed the dynamical system analysis to validate the stability of the model. From the evolutionary plot obtained through the dynamical system variables, the present value of density parameters have been obtained as $Ω_m\approx0.3$ and $Ω_{de}\approx0.7$.

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arXiv2023

Weyl type $f(Q,T)$ gravity observational constrained cosmological model

In this paper, we have studied the dynamical aspects of the cosmological model of the Universe in the Weyl type $f(Q,T)$ gravity, which is an extension of symmetric teleparallel gravity. The non-metricity scalar $Q$ has been expressed in standard Weyl form and can be determined by a vector field $w_μ$ and the trace of energy momentum tensor denoted as $T$. The logarithmic form of the Hubble parametrization has been incorporated and the best fit values of the free parameters have been determined using $32~CC$ sample points, $1701~Pantheon^{+}$ and $6~BAO$ data points. The present value of the $H_{0}\approx 70.2\pm 4.6$, $H_{0}\approx 68.69_{-0.59}^{+0.67}$ and $H_{0}\approx 69.26_{-0.53}^{+0.57}$ respectively for $CC~Sample$, $CC + Pantheon^{+}$ and $CC + Pantheon^{+} + BAO$ datasets. With the constrained values of the free parameters, the cosmographic parameters are constrained and the present value of each parameter has been noted. The deceleration parameter for $CC~Sample$, $CC + Pantheon^{+}$ and $CC + Pantheon^{+} + BAO$ datasets provides $-0.5221$, $-0.5477$ and $-0.5691$ respectively at present time. We have considered exponential and non-linear form of the Weyl type function $f(Q,T)$ to assess the dynamical behaviour of the model. The accelerating cosmological models show the quintessence behaviour at present time as we get the present EoS parameter values obtained as $ω\approx -0.7068$ and $ω\approx -0.6828$ for $CC~Sample$, $ω\approx -0.6991$ and $-0.6949$ for $CC + Pantheon^{+}$ and $ω\approx -0.7001$ and $-0.7084$ for $CC + Pantheon^{+} + BAO$ respectively.

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arXiv2022

Dynamical system analysis for accelerating models in non-metricity $f(Q)$ gravity

Two accelerating cosmological models are presented in symmetric teleparallel $f(Q)$ gravity, $Q$ be the non-metricity. The models are constructed based on the assumptions of two different functional forms of $f(Q)$ and a dynamically changing nature of the deceleration parameter that shows transition at $t=2n\pm\sqrt{\frac{4n^2+1}{3}}$, $n$ being a positive constant. In both the models, the equation of state parameter for the dark energy in $f(Q)$ gravity becomes a dynamical quantity and crosses the phantom divide line. The violation of the strong energy condition and the null energy condition at late times are also established. In addition, the dynamical system analysis has been performed and three critical points in each model are identified. In each model, at least one stable node has been observed. To strengthen further, the stability analysis using homogeneous linear perturbations has been performed to ensure the stability of the models.

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