arXiv subjects

G. K. Goswami

Publications and source records attributed to G. K. Goswami.

At least 19 recordsLinked to original sources

arXiv2026

Role of viscous fluid in FLRW model with observational constraints

In this study, we investigate the role of bulk viscosity in the evolution of a spatially flat Friedmann-Lemaitre-Robertson-Walker (FLRW) universe dominated by dust. We parameterize the bulk viscosity as p_tilde = -3[l + m(H'(t) + H^2) + nH^2]H, derive the modified Einstein field equations, and reformulate them in terms of redshift z. Using 46 Hubble parameter measurements and the Pantheon+ compilation of 1701 supernovae, we estimate the model parameters (H0, l, m, n) through chi2 minimization and refine them using Markov chain Monte Carlo (MCMC) simulations. The analysis gives a transition redshift z_t = 0.585 and a present deceleration parameter q0 = -0.705, consistent with Lambda CDM. The present age of the universe is estimated as t0 = 14.5734 Gyr. We obtain H0 = 68 km/s/Mpc from the Hubble datasets and approximately 73 km/s/Mpc from Pantheon+. These results illustrate the Hubble tension between late- and early-universe determinations of H0. We further investigate the evolution of the equation-of-state parameter omega(z), energy density rho(z), and pressure p(z). Statefinder diagnostics (r,s) demonstrate deviations from and convergence toward Lambda CDM at different epochs. The results highlight the potential role of bulk viscosity in describing the accelerated expansion of the universe and provide a useful framework for studying cosmological models.

physics.gen-ph↗

arXiv2026

Bianchi Type I Space -Time Geometry of the Universe with Time Dependent G and $Λ$ Within the Framework of General Relativity: Observational Aspects

Inspired by the latest progress in the hunt of acceptable cosmological model of the universe, present paper is devoted to explore a mathematical model of the universe having initial anisotropy and with accelerated evolution that attains isotropic character. We have considered a widely accepted anisotropic Bianchi type I space-time geometry of the universe to explore a physically viable model. To find an acceptable model, we adopted a hyperbolic form for the scale factor as $ a(t)= \left( sinh bt \right)^{\frac{1}{n}} $ and a relation $ Λ=Λ_0 \frac{\ddot{a}}{a}$. The model parameters are constrained by $ χ^2$ minimization techniques. Using 35 CC measurements we obtain present day expansion rate $H_0= 65.715 Km/S/Mpc$ and the value of $ n= 1.3024$. Further to estimate the local uncertainties of the best fit parameter, we calculate covariance from the Jacobin of the normalized residuals. The corresponding 1 $σ$ uncertainties are $ H_0=65.715 \pm 2.392 Km/S/Mpc$ and $n= 1.3024 \pm 0.0842$. The physical and dynamical behaviour of present model has been discussed by a graphical representation of cosmological parameters. The observational constraints on the expansion history are found to be consistent with a present accelerated phase and a transition from deceleration to acceleration at an intermediate redshift. The corresponding evolution of the derived cosmological quantities is also investigated within the adopted model.

physics.gen-ph↗

arXiv2026

Comparative Study of Early-Universe Epochs in an $f(R,L_m)$ Gravity Model with Effective Curvature--Matter Interaction and $Λ$CDM Cosmology

We investigate a specific gravity model of the form $f(R, L_m) = αR + L_m^β + γ$, where the nonlinear dependence on the matter Lagrangian $L_m$ introduces an effective curvature-matter interaction, leading to the non-conservation of the energy-momentum tensor. Using distance modulus data, we constrain the parameters through $χ^2$ minimization and Bayesian MCMC analysis, obtaining statistically robust best-fit values: $H_0 = 73.75 \pm 0.16~\mathrm{km\,s^{-1}\,Mpc^{-1}}$, $λ= 0.262 \pm 0.007$, and $w = -0.005 \pm 0.001$. This study presents a comprehensive and statistically rigorous comparison of three key early-Universe epochs: structure formation, recombination, and matter-radiation equality between the $f(R,L_m)$ model and the standard $Λ$CDM cosmology. The model predicts an earlier onset of nonlinear structure formation ($z_c^{f(R,L_m)} \approx 25.6$) and a higher matter-radiation equality redshift ($z_{\mathrm{eq}}^{f(R,L_m)} \approx 4203$) compared to $Λ$CDM ($z_{\mathrm{eq}}^{Λ\mathrm{CDM}} \approx 2779$), while maintaining consistency with the observed recombination redshift ($z_{\mathrm{rec}} \approx 1092$). The recombination visibility function, derived using standard microphysical expressions with the modified expansion history, exhibits a slightly broader full width at half maximum, suggesting an extended photon decoupling period.

astro-ph.CO↗

arXiv2026

Constraining a $f(R, L_m)$ Gravity Cosmological Model with Observational Data

We investigate a spatially flat FLRW cosmological model in the framework of modified gravity described by the function $ f(R, L_m) = αR + L_m^β+ γ$, where $ L_m $ is the matter Lagrangian density. The modified Friedmann equations yield the Hubble parameter as $ H(z) = H_0 \sqrt{(1 - λ) + λ(1 + z)^{3(1 + w)}},$ with the parameters $ λ= \fracγ{6αH_0^2} + 1 $ and $ w = \frac{β(n - 2) + 1}{2β- 1} $. Using a Bayesian Markov Chain Monte Carlo (MCMC) approach, we constrain the model parameters with recent observational data, including cosmic chronometers, the Pantheon+ Supernovae dataset, Baryon Acoustic Oscillations (BAO), and Cosmic Microwave Background (CMB) shift parameters. The best-fit values are found to be $ H_0 = 72.773^{+0.148}_{-0.152} $ km/s/Mpc, $ λ= 0.289^{+0.007}_{-0.007} $, and $ w = -0.002^{+0.002}_{-0.002} $, all quoted at the 1$σ$ confidence level.This model predicts a transition redshift of $ z_t \approx 0.76 $ for the onset of cosmic acceleration and an estimated universe age of 13.21 Gyr. The higher inferred value of $ H_0 $ compared to the Planck 2018 result offers a potential resolution to the Hubble tension. Additionally, using $ ρ_0 = 0.534 \times 10^{-30} \, \text{g/cm}^3 $ and assuming $ n = 1 $, we derive the model constants as $ β= 1.00201 $, $ α= 512247 $, and $ γ= -1.215 \times 10^{-29} $. We also evaluate the Bayesian Information Criterion (BIC) to compare the model's performance with that of the standard $Λ$CDM model. The small BIC difference ($ Δ\text{BIC} = 0.16 $) indicates comparable statistical support for both models. Thus, the $ f(R, L_m) $ gravity scenario serves as a consistent and viable alternative to $Λ$CDM, potentially addressing open questions in late-time cosmology.

gr-qc↗

arXiv2026

Scale-Invariant Bounce Cosmology in Weyl f(Q) Gravity with Quintom Signature

We investigate a bouncing cosmological model within the Weyl-type $f(Q)$ gravity framework, employing a power-law form of the non-metricity scalar $Q$. The model successfully resolves the initial singularity problem by demonstrating a nonsingular bounce, where the universe transitions from a contracting phase $ \dot{a}(t)<0 $ to an expanding phase ($ \dot{a}(t)>0 $) at the bouncing point $t \approx 0.$ Key features include the violation of the null energy condition (NEC) near the bounce and the crossing of the phantom divide line ($ω=-1$) by the equation of state (EoS) parameter, indicating quintom-like behavior. The model exhibits accelerated expansion post-bounce, suggesting an inflationary phase. Stability analysis via the adiabatic index reveals instability near the bouncing point, while energy conditions highlight the dominance of dark energy. Additionally, the study explores scalar fields, showing that quintessence-like kinetic energy becomes negative and phantom-like kinetic energy peaks positively near the bounce, aligning with dark energy dynamics. The Hubble parameter, deceleration parameter, and Hubble radius further validate the bouncing scenario, with the latter displaying symmetric behaviour around the bounce. These results underscore the viability of Weyl-type $f(Q)$ gravity as a framework for nonsingular bouncing cosmologies, offering insights into early universe dynamics and dark energy behaviour.

gr-qc↗

arXiv2025

Growth Rate Analysis in $f(R,L_m)$ Gravity: A Comparative Study with \boldmath$Λ$CDM Cosmology

We investigate the evolution of cosmic structures within the framework of modified gravity, specifically focusing on theories described by the function $f(R, L_m)$, where $R$ is the Ricci scalar and $L_m$ is the matter Lagrangian. This class of models introduces a non-minimal coupling between geometry and matter, leading to modifications in the dynamics of density perturbations. We derive the linear growth equation and compute the observable growth rate $fσ_8(z)$, which is directly accessible from redshift-space distortion (RSD) data. Using recent observational constraints from galaxy surveys such as eBOSS and DESI, we perform a comparative analysis between predictions from $f(R, L_m)$ gravity and the standard $Λ$CDM model. Our results indicate that while $Λ$CDM remains broadly consistent with current data, the $f(R, L_m)$ framework can accommodate subtle deviations in structure growth, offering a possible resolution to existing tensions in large-scale structure observations. We also outline the implications of our findings for future surveys, including Euclid and LSST.

gr-qc↗

arXiv2025

Observational constraints in late time for an axially symmetric transitioning model with bulk viscous fluid

In this paper, we explore an axially symmetric Bianchi type-I model of the universe with bulk viscous fluid as a source of gravitational field under the framework of Einstein's field equations by assuming barotropic bulk viscous pressure as $-3ζH^2$. The model parameters have been estimated with the help of four data sets: The Hubble 46 data set describes Hubble parameter values at various redshifts, Union 2.1 compilation data sets comprise a distance modulus of 580 SNIa supernovae at different redshifts, the Pantheon data set contains Apparent magnitudes of 1048 SNIa supernovae at various redshifts and finally BAO data set of volume averaged distances at 5 redshifts. The observational data is analyzed using the traditional Bayesian methodology, and the posterior distributions of the parameters are obtained using the Markov Chain Monte Carlo (MCMC) technique. To get the best-fit values for the model parameters for MCMC analysis, we use the $ emcee $ package. For parameter estimation, we have also employed the minimizing $χ^{2}$ function. We also tried to achieve these values statistically using combined data sets from the four described earlier. The OHD+BAO~and~OHD+Pan+BAO+Union combined data sets provide the best fit Hubble parameter value $H_0$ as $66.912 ^{+0.497}_{-0.501})$ Km/s/Mpc and $74.216 ^{+0.150}_{-0.148}$ Km/s/Mpc respectively. We have performed state finder diagnostics to discuss the nature of dark energy. Some other geometrical parameters like the Jerk parameter and the Om diagnostic are also being discussed to clarify the nature of the dark energy model. The study reveals that the model behaves like a quintessence in late time and approaches the $Λ$ CDM model.

gr-qc↗

arXiv2025

An Accelerating Flat FLRW Model with Observation Constraints and Dynamic $Λ$

In this paper, we explore power law solution of FLRW universe model that is associated with a variable cosmological term $Λ(t)$ as a linear function of $\frac{\ddot{a}}{a}, (\frac{\dot{a}}{a})^2$ and $ρ$. The model parameters were estimated on the basis of the four data sets: The Hubble 46 data, the Union 2.1 compilation data sets comprising of distance modulus of 580 SNIa supernovae at different redshifts, the Pantheon data set which contains Apparent magnitudes of 1048 SNIa supernovae at various redshifts and finally BAO data set of volume averaged distances at 5 redshifts. We employ the conventional Bayesian methodology to analyze the observational data and also the Markov Chain Monte Carlo (MCMC) technique to derive the posterior distributions of the parameters. The best fit values of Hubble parameter $H_0$ as per the four data sets are found as $61.53^{+0.453}_{-0.456}$, $ 69.270^{+0.229}_{-0.228}$, $78.116^ {+0.480}_{-0.479}$, and $ 71.318 ^{+2.473}_{-2.283}$ respectively. Off late the present value of Hubble parameters $H_0$ were empirically given as 73 and 67.7 (km/s)/Mpc using distance ladder techniques and measurements of the cosmic microwave background. The OHD+BAO+Union and ~OHD+Pan+BAO+Union combined data sets provide the best fit Hubble parameter value $H_0$ as $67.427^{+0.197}_{-0.199}$ and $74.997^{+0.143}_{-0.145}$ respectively. The various geometrical and physical properties of the model were also investigated and were found in good agreements with observations.

physics.gen-ph↗

arXiv2024

Power law cosmology in Gauss-Bonnet gravity with pragmatic analysis

In this study, we present an approach $ f(R, G) $ gravity incorporating power law in $ G $. To study the cosmic evolution of the universe given by the reconstruction of the Hubble parameter given by $ E(z) = \bigg( 1+\frac{z(α+(1+z)^β)}{2 β+ 1} \bigg)^{\frac{3}{2 β}} $. Subsequently, we use various recent observational datasets of OHD, Pantheon, and BAO to estimate the model parameters $ H_0,~α$, and $ β$ applying the Markov Chain Monte Carlo (MCMC) technique in the emcee package to establish the validity of the model. In our findings, we observe that our model shows consistency with standard $ Λ$CDM, transits from deceleration to acceleration, and enters the quintessence region in late times. The cosmological model satisfies necessary energy constraints, simultaneously violating the strong energy condition (SEC), indicating a repulsive nature and consistent with accelerated expansion. The cosmic evolution of the Hawking temperature and the total entropy for the various observational datasets also show the validity of the model. Thus, our established model demonstrates sufficient potential for explicitly describing cosmological models.

gr-qc↗

arXiv2023

An FLRW accelerating universe model in Weyl type $f(Q)$ gravity and Observational Constraints

We propose to develop a cosmological model of the universe based on Weyl type $ f(Q) $ gravity which shows the transition from decelerating in the past to acceleration at present by considering a particular functional form of $ f(Q) $ gravity as $ f(Q) = ({H_0}^2) (α_1 + α_2 \hskip0.05in log ({H_0^{-2}} Q)) $. We have solved Weyl type $ f(Q) $ gravity field equations numerically and have obtained numerical solutions to the Hubble and deceleration parameters, distance modulus, and apparent magnitudes of stellar objects like SNIa Supernovae. We have also obtained numerical solutions for the Weyl vector $ w $, non-metricity scalar $ Q $, and the Lagrangian multiplier $ λ$ appearing in the action of $ f(Q) $ gravity. We have compared our theoretical solutions with the error bar plots of the Observed Hubble data set of $ 77 $ points, $ 580 $ distance modulus SNIa data set, and $ 1048 $ supernova Pantheon data sets of apparent magnitudes. It is found that our results fit well with the observed data set points. \bf{The model envisages a unique feature that although the universe is filled with perfect fluid as dust whose pressure is zero, still the weyl vector dominance f(Q) creates acceleration in it. }

gr-qc↗

arXiv2022

Dynamics of a parametrized dark energy model in $f(R,T)$ gravity

We investigate a flat FLRW-model in $f(R,T)$-gravity, which includes the quadratic variation in scalar curvature $R$ and the linear term of the trace of the stress-energy tensor $T$. In turn, we establish the model has the behaviour of the late time Universe, which is accelerated expanding and ends up in a big rip. Using the parametrization of scale factor $a(t)$, we propose a model, which begins with point-type singularity, i.e., the model starts with a point of zero volume, infinite energy density and infinite temperature. The model's behaviour is accelerated expanding at present and $Λ$CDM in late times. Finally, the proposed model behaves like a quintessence dark energy model in the present time and is consistent with standard cosmology $Λ$CDM in late times.

gr-qc↗

arXiv2022

Curvature dominance DE-model in $f(R)$-gravity

We have probed a cosmological model in $f(R)$-gravity, which is a cubic equation in scalar curvature $R$. The terms arise due to nonlinear $f(R)$ function are treated as energy due to curvature inspired geometry. As a result, we find accelerating expansion in the universe, which creates an anti-gravitating negative pressure in it. Some of the physical parameters are solved using numerical methods. The evolution of the model are examined by the latest observational Hubble data (46-data points) and Pantheon data (the latest compilation of SNIa with 40 binned in the redshift range $0.014 \leqslant z \leqslant1.62$). Some important features of the model have been discussed by analyzing the plots of various dynamical parameters. The plots of deceleration parameter $q$ and the Hubble parameter $H$ describe the accelerating expansion in the evolution of the Universe at the present epoch. The transition from deceleration to acceleration for our model is obtained at redshift $z_{tr} \simeq 0.694069$, which is in good agreement with $Λ$CDM. We have also carried out state finder analysis for our model. The analysis of specific features of the model confirms that our model is consistent with $Λ$CDM in late times.

gr-qc↗

arXiv2022

An Axially Symmetric Transitioning models with Observational Constraints

In this study, we have demonstrated the expansion history of an axially symmetric Bianchi type-I model of the universe. Our model as of now presents an accelerating universe, which had been in the decelerating phase in the past. Roles of the two crucial Hubble~$H(z)$ and deceleration~$q(z)$ parameters are examined. The energy parameters of the universe are estimated with the help of the latest observational Hubble data (46-data points) and Pantheon data (the latest compilation of SNIa with 40 binned in the redshift range $0.014 \leq z \leq 1.62)$. We also discuss the stability analysis of the model by state finder diagnosis. The analysis reveals that in late time, the model is a quintessence type and points towards the $Λ$CDM model. Our developed model agrees with observational findings in a proper way. We have discussed some of the physical aspects of the model.

gr-qc↗

arXiv2021

The simplest parametrization of equation of state parameter in the scalar field Universe

In this paper, we have investigated a scalar field cosmological model of accelerating Universe with the simplest parametrization of equation of state parameter of the scalar field. We used $H(z)$ data, pantheon compilation of SN Ia data and BAO data to constrained the model parameters using $χ^{2}$ minimization technique. We obtain the present values of Hubble constant $H_{0}$ as $66.2^{+1.42}_{-1.34}$, $70.7^{+0.32}_{-0.31}$ and $67.74^{+1.24}_{-1.04}$ for $H(z)$, $H(z)$ + Pantheon and $H(z)$ + BAO respectively. Also, we have estimated the present age of the Universe in derived model $t_{0} = 14.38^{+0.63}_{-0.64}$ for joint $H(z)$ and pantheon compilation of SN Ia data which has only $0.88~σ$ tension with its empirical value obtained in Plank collaboration \cite{Ade/2016}. Moreover, the present values of the deceleration parameter $q_{0}$ come out to be $-0.55^{+0.031}_{-0.038}$, $-0.61^{+0.030}_{-0.021}$ and $-0.627^{+0.022}_{-0.025}$ by bounding the Universe in derived model with $H(z)$, $H(z)$ + Pantheon compilation of SN Ia and $H(z)$ + BAO data sets respectively. We also have performed the state-finder diagnostics to discover the nature of dark energy.

astro-ph.CO↗

arXiv2021

Accelerating Universe with binary mixture of bulk viscous fluid and dark energy

In this paper, we have proposed a model of accelerating Universe with binary mixture of bulk viscous fluid and dark energy. and probed the model parameters: present values of Hubble's constant $H_{0}$, Equation of state paper of dark energy $ω_{de}$ and density parameter of dark energy $(Ω_{de})_{0}$ with recent OHD as well as joint Pantheon compilation of SN Ia data and OHD. Using cosmic chronometric technique, we obtain $H_{0} = 69.80 \pm 1.64~km~s^{-1}Mpc^{-1}$ and $70.0258 \pm 1.72~km~s^{-1}Mpc^{-1}$ by restricting our derived model with recent OHD and joint Pantheon compilation SN Ia data and OHD respectively. The age of the Universe in derived model is estimated as $t_{0} = 13.82 \pm 0.33\; Gyrs$. Also, we observe that derived model represents a model of transitioning Universe with transition redshift $z_{t} = 0.7286$. We have constrained the present value of jerk parameter as $j_{0} = 0.969 \pm 0.0075$ with joint OHD and Pantheon data. From this analysis, we observed that the model of the Universe, presented in this paper shows a marginal departure from $Λ$CDM model.

gr-qc↗

arXiv2021

Modeling of accelerating Universe with bulk viscous fluid in Bianchi V space-time

In this paper, we have investigated a bulk viscous anisotropic Universe and constrained its model parameters with recent $H(z)$ and Pantheon compilation data. Using cosmic chronometric technique, we estimate the present value of Hubble's constant as $H_{0} = 69.39 \pm 1.54~km~s^{-1}Mpc^{-1}$, $70.016 \pm 1.65~km~s^{-1}Mpc^{-1}$ and $69.36 \pm 1.42~km~s^{-1}Mpc^{-1}$ by bounding our derived model with recent $H(z)$ data, Pantheon and joint $H(z)$ and Pantheon data respectively. The present age of the Universe is specified as $t_0= 0.9796H_0^{-1}\sim 13.79$ Gyrs. The model favours a transitioning Universe with the transition red-shift as $z_{t} = 0.73$. We have reconstructed the jerk parameter using the observational data sets. From the analysis of the jerk parameter, it is observed that, our derived model shows a marginal departure from the concordance $Λ$CDM model.

gr-qc↗

arXiv2020

Transitioning Universe with hybrid scalar field in Bianchi I space-time

In this paper we investigate a Bianchi type I transitioning Universe in Brans-Dicke theory. To get an explicit solution of the field equations, we assume scalar field as $ϕ= ϕ_{0}\left[t^αexp(βt)\right]^δ$ with $ϕ_{0}$, $α$, $β$ and $δ$ as constants. The values of $α$ and $β$ are obtained by probing the proposed model with recent observational Hubble data (OHD) points. The interacting and non-interacting scenarios between dark matter and dark energy of the derived Universe within the framework of Brans-Dicke gravity are investigated. The $om(z)$ analysis of the Universe in derived model shows that the Universe is filled with dynamical dark energy with its equation of state parameter $ω_{de} > -1$. Hence the Universe behaves like a quintessence model at present epoch. Some physical properties of the Universe are also discussed.

gr-qc↗

arXiv2020

An FLRW interacting dark energy model of the Universe

In this paper, we have presented an FLRW universe containing two-fluids (baryonic and dark energy) with a deceleration parameter (DP) having a transition from past decelerating to the present accelerating universe. In this model, dark energy (DE) interacts with dust to produce a new law for the density. As per our model, our universe is at present in a phantom phase after passing through a quintessence phase in the past. The physical importance of the two-fluid scenario is described in various aspects. The model is shown to satisfy current observational constraints such as recent Planck results. Various cosmological parameters relating to the history of the universe have been investigated.

physics.gen-ph↗