Showing papers on "Deceleration parameter published in 2023"

Cosmological tests of the osculating Barthel–Kropina dark energy model

Abstract: Abstract We further investigate the dark energy model based on the Finsler geometry inspired osculating Barthel–Kropina cosmology. The Barthel–Kropina cosmological approach is based on the introduction of a Barthel connection in an osculating Finsler geometry, with the connection having the property that it is the Levi-Civita connection of a Riemannian metric. From the generalized Friedmann equations of the Barthel–Kropina model, obtained by assuming that the background Riemannian metric is of the Friedmann–Lemaitre–Robertson–Walker type, an effective geometric dark energy component can be generated, with the effective, geometric type pressure, satisfying a linear barotropic type equation of state. The cosmological tests, and comparisons with observational data of this dark energy model are considered in detail. To constrain the Barthel–Kropina model parameters, and the parameter of the equation of state, we use 57 Hubble data points, and the Pantheon Supernovae Type Ia data sample. The st statistical analysis is performed by using Markov Chain Monte Carlo (MCMC) simulations. A detailed comparison with the standard $$\Lambda $$ Λ CDM model is also performed, with the Akaike information criterion (AIC), and the Bayesian information criterion (BIC) used as the two model selection tools. The statefinder diagnostics consisting of jerk and snap parameters, and the Om ( z ) diagnostics are also considered for the comparative study of the Barthel–Kropina and $$\Lambda $$ Λ CDM cosmologies. Our results indicate that the Barthel–Kropina dark energy model gives a good description of the observational data, and thus it can be considered a viable alternative of the $$\Lambda $$ Λ CDM model.

Bouncing cosmology in modified gravity with higher-order curvature terms

Abstract: A bstract A bouncing scenario of a flat homogeneous and isotropic universe is explored by using the reconstruction technique for the power-law parametrization of the Hubble parameter in a modified gravity theory with higher-order curvature and trace of the energy-momentum tensor terms. It is demonstrated that bouncing criteria are satisfied so that the cosmological initial singularity can be avoided. In addition, it is shown that the equation of state parameter crosses the line of the phantom divide. In the present scenario, the universe is filled with perfect fluid around the bouncing point, in which the universe becomes highly unstable and a big bounce can be realized. Furthermore, it is found that extremal acceleration occurs at the bouncing point.

Observational constraints on two cosmological models of f(Q) theory

Abstract: In the past few years, $f(Q)$ theories have drawn a lot of research attention in replacing Einstein's theory of gravity successfully. The current study examines the novel cosmological possibilities emerging from two specific classes of $f(Q)$ models using the parametrization form of the equation of state (EoS) parameter as $\omega \left( z\right) =-\frac{1}{1+3\beta \left( 1+z\right) ^{3}}$, which displays quintessence behavior with the evolution of the Universe. We do statistical analyses using the Markov chain Monte Carlo (MCMC) method and background datasets like Type Ia Supernovae (SNe Ia) luminosities and direct Hubble datasets (from cosmic clocks), and Baryon Acoustic Oscillations (BAO) datasets. This lets us compare these new ideas about the Universe to the $\Lambda $CDM model in a number of different possible ways. We have come to the conclusion that, at the current level of accuracy, the values of their specific parameters are the best fits for our $f(Q)$ models. To conclude the accelerating behavior of the Universe, we further study the evolution of energy density, pressure, and deceleration parameter for these $f(Q)$ models.

Modified cosmology through Barrow entropy

Abstract: We investigate the cosmological consequences of the modified Friedmann equations when the entropy associated with the apparent horizon, given by Barrow entropy, $S\sim A^{1+\delta/2}$, where $0\leq\delta\leq1$, represents the amount of the quantum-gravitational deformation of the horizon. We study implications of this model in a flat Friedmann-Robertson-Walker (FRW) universe with/without cosmological constant. Taking the cosmological constant into account, this model can describe the current accelerated expansion, although the transition from deceleration phase to the acceleration phase takes place in the lower redshifts. We investigate the evolution of the scale factor and show that with increasing $\delta$, the value of the scale factor increases as well. We also estimate the age of the universe in Barrow cosmology which is smaller than the age of the universe in standard cosmology.

The Simplest Parametrization of the Equation of State Parameter in the Scalar Field Universe

Abstract: In this paper, we investigate a scalar field cosmological model of accelerating Universe with the simplest parametrization of the equation of state parameter of the scalar field. We use H(z) data, pantheon compilation of SN Ia data and BAO data to constrain the model parameters using the χ2 minimization technique. We obtain the present values of Hubble constant H0 as 66.2−1.34+1.42, 70.7−0.31+0.32 and 67.74−1.04+1.24 for H(z), H(z) + Pantheon and H(z) + BAO respectively. In addition, we estimate the present age of the Universe in a derived model t0=14.38−0.64+0.63 for joint H(z) and pantheon compilation of SN Ia data which has only 0.88σ tension with its empirical value obtained in Plank collaboration. Moreover, the present values of the deceleration parameter q0 come out to be −0.55−0.038+0.031, −0.61−0.021+0.030 and −0.627−0.025+0.022 by bounding the Universe in the 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.

The constrained cosmological model in Lyra geometry

Abstract: In this paper, we study a flat homogeneous FLRW model in Lyra geometry which is described by a time-dependent displacement vector. We consider an appropriate parametrization of the energy density of scalar field [Formula: see text] in terms of the cosmic scale factor. The result shows two transitions from deceleration to acceleration. Furthermore, we constrain the model parameter [Formula: see text] and the displacement field vector [Formula: see text] using the recent supernovae data, Hubble dataset of 77 points and their joint data which predict the accelerated expanding phase of the universe in late times. The effective equation of state parameter [Formula: see text] speculates [Formula: see text]CDM in late times. Finally, we use the statefinder diagnostic to differentiate our model from the various dark energy models.

Observational constraints in accelerated emergent f(Q) gravity model

Abstract: In this study, the non-metricity scalar Q, which characterizes the gravitational interaction, is used to analyze the Universe’s rapid expansion within the context of the f(Q) gravity theory. We suggest an emergent scale factor, which produces the deceleration parameter in redshift form which determines the solution of the field equations in the FLRW Universe. We evaluate the appropriate values of the model parameters by considering SNIa from Pantheon, CMB from Planck 2018, BAO, and 36 data points from Hubble datasets using Markov Chain Monte Carlo approach. The deceleration parameter’s development suggests that the Universe is moving from its deceleration phase into its acceleration phase. Furthermore, we analyze the statefinder (r, s) diagnostic parameter. We also use some different forms of f(Q) gravity models to study some other cosmological parameters, i.e. f(Q)=α1Q , f(Q)=α2Qm and f(Q)=α3Qm+Q , where α 1, α 2, α 3 and m all are free model parameters. Finally, we concluded that all f(Q) models predict that at z = 0, Universe is in the phase of accelerating and behaves like the quintessence models and at z=−1 , approaches to ΛCDM models.

Studying the Optical Depth behaviour of Parametrized Deceleration Parameter in non-flat Universe

Abstract: In this work, we have assumed the non-flat FRW model of the universe. We probed the optical depth behavior of a few cosmological models, including the deceleration parameter’s parametrized form. We have considered ten such models and carried out a qualitative analysis graphically. We found that these particular lensing phenomena depend greatly on the various parametrization forms of deceleration parameter in the cosmological models. Then we compared these models to each other as well as with [Formula: see text]CDM model.

f(R,T) Gravity and Constant Jerk Parameter in FLRW Spacetime †

Abstract: It is well known that the universe is undergoing accelerated expansion during recent times and that it underwent a decelerated expansion in early times. The deceleration parameter, essentially the second derivative of the scale factor, can be used to describe these eras, with a negative parameter for acceleration and a positive parameter for deceleration. Apart from the standard ΛCDM model in general relativity, there are many cosmological models in various other theories of gravity. In order to describe these models, especially the deviation from general relativity, the jerk parameter was introduced, which is basically the third derivative of the scale factor. In the ΛCDM model in general relativity, the jerk parameter j is constant, and j = 1. The constant jerk parameter, j = 1, leads to two different scale factor solutions, one power law and the other exponential. The power-law solution corresponds to a model in which our universe expands with deceleration, while the exponential solution corresponds to a model in which it expands by accelerating. In this study, the cosmological consequences of such a selection of the jerk parameter on the non-minimally coupled f (R, T) theory of gravity (where R is the Ricci scalar, and T is the trace of the energy–momentum tensor) and the dynamic properties of these models are investigated on a flat Friedmann–Lemaitre–Robertson–Walker backgfround.

Hyperbolic Scenario of Accelerating Universe in Modified Gravity

Abstract: Throughout this study, locally rotationally symmetric (LRS) Bianchi type-V space-time is pondered with Tsallis holographic dark energy (THDE) with the Granda–Oliveros (GO) cut-off in the Sáez–Ballester (SB) theory of gravity. A parameterization of the deceleration parameter (q) has been suggested: q=α−βH2. The proposed deceleration parameterization demonstrates the Universe’s phase transition from early deceleration to current acceleration. Markov chain Monte Carlo (MCMC) was utilized to have the best-fit value for our model parameter and confirm that the model satisfies the recent observational data. Additional parameters such as deceleration parameter q with cosmographic parameters jerk, snap, and lerk have also been observed physically and graphically. The constructed model is differentiated from other dark energy models using statefinder pair analysis. Some important features of the model are discussed physically and geometrically.

Cosmological consequences of first-order general-relativistic viscous fluid dynamics

Abstract: We investigate the out-of-equilibrium dynamics of viscous fluids in a spatially flat Friedmann-Lema\^itre-Robertson-Walker cosmology using the most general causal and stable viscous energy-momentum tensor defined at first order in spacetime derivatives. In this new framework a pressureless viscous fluid having density $\rho$ can evolve to an asymptotic future solution in which the Hubble parameter approaches a constant while $\rho \rightarrow 0$, even in the absence of a cosmological constant (i.e., $\Lambda = 0$). Thus, while viscous effects in this model drive an accelerated expansion of the universe, the density of the viscous component itself vanishes, leaving behind only the acceleration. This behavior emerges as a consequence of causality in first-order theories of relativistic fluid dynamics and it is fully consistent with Einstein's equations.

Exact Solutions and Cosmological Constraints in Fractional Cosmology

Abstract: This paper investigates exact solutions of cosmological interest in fractional cosmology. Given μ, the order of Caputo’s fractional derivative, and w, the matter equation of state, we present specific exact power-law solutions. We discuss the exact general solution of the Riccati Equation, where the solution for the scale factor is a combination of power laws. Using cosmological data, we estimate the free parameters. An analysis of type Ia supernovae (SNe Ia) data and the observational Hubble parameter data (OHD), also known as cosmic chronometers, and a joint analysis with data from SNe Ia + OHD leads to best-fit values for the free parameters calculated at 1σ, 2σ and 3σ confidence levels (CLs). On the other hand, these best-fit values are used to calculate the age of the Universe, the current deceleration parameter (both at 3σ CL) and the current matter density parameter at 1σ CL. Finding a Universe roughly twice as old as the one of ΛCDM is a distinction of fractional cosmology. Focusing our analysis on these results, we can conclude that the region in which μ>2 is not ruled out by observations. This parameter region is relevant because fractional cosmology gives a power-law solution without matter, which is accelerated for μ>2. We present a fractional origin model that leads to an accelerated state without appealing to Λ or dark energy.

Evolution of Generalized Brans-Dicke Parameter within a Superbounce Scenario

Abstract: We studied a superbounce scenario in a set up of the Brans–Dicke (BD) theory. The BD parameter was considered to be time-dependent and was assumed to evolve with the Brans–Dicke scalar field. In the superbounce scenario, the model bounced at an epoch corresponding to a Big Crunch provided the ekpyrotic phase continued until that time. Within the given superbounce scenario, we investigated the evolution of the BD parameter for different equations of state. We chose an axially symmetric metric that has an axial symmetry along the x-axis. The metric was assumed to incorporate an anisotropic expansion effect. The effect of asymmetric expansion and the anisotropic parameter on the evolving and non-evolving parts of the BD parameter was investigated.

Observational Insights into the Accelerating Universe through Reconstruction of the Deceleration Parameter

Abstract: Recent developments in the exploration of the universe suggest its accelerated phase of expansion. In this regard, our manuscript aims to probe the current scenario of the universe with the aid of the reconstruction technique. The primary factor that describes cosmic evolution is the deceleration parameter. Here, we provide a physically plausible, newly defined, model-independent parametric form of the deceleration parameter. Further, we constrain the free parameters through statistical MCMC analysis for different datasets, including the most recent Pantheon+. With the statistically obtained results, we analyze the dynamics of the model through the phase transition, EoS parameter and energy conditions. Also, we make use of the tool Om diagnostic to test our model.

Testing $\Lambda$CDM cosmology in a binned universe: anomalies in the deceleration parameter

Abstract: We study the reconstructed deceleration parameter splitting the data in different redshift bins, fitting both a cosmographic luminosity distance and also assuming a flat $\Lambda$CDM model, using the Pantheon+ sample of type Ia supernova data (SNIA). We observe tensions $\sim 2\sigma-3\sigma$ for different redshift and distance indicators if the full sample is used. However, those tensions disappear when the SNIA at $z<0.008$ are removed. If the data is splitted in 2 hemispheres according to our movement w.r.t CMB, a strange $3.8 \sigma$ tension appears in one of the samples between particular redshift bins. Finally, considering posterior distribution as Gaussian, general linear model prefers a positive slope for $q_0$ across redshift bins opposed to a zero slope expected in a $\Lambda$CDM universe. We discuss possible explanations for our results and the influence of lowest redshift SNIA data in cosmological analysis.

The reconstruction of constant jerk parameter with f(R, T) gravity in Bianchi-I spacetime

Abstract: We have developed a Bianchi I cosmological model of the universe in $f(R,T)$ gravity theory which fit good with the present day scenario of accelerating universe. The model displays transition from deceleration in the past to the acceleration at the present. As in the $\Lambda$CDM model, we have defined the three energy parameters $\Omega_m$, $\Omega_{\mu}$ and $\Omega_{\sigma}$ such that $\Omega_m$ + $\Omega_{\mu}$ + $\Omega_{\sigma}$ = 1. The parameter $\Omega_m$ is the matter energy density (baryons + dark matter), $\Omega_{\mu}$ is the energy density associated with the Ricci scalar $R$ and the trace $T$ of the energy momentum tensor and $\Omega_{\sigma}$ is the energy density associated with the anisotropy of the universe. We shall call $\Omega_{\mu}$ dominant over the other two due to its higher value. We find that the $\Omega_{\mu}$ and the other two in the ratio 3:1. 46 Hubble OHD data set is used to estimate present values of Hubble $H_0$, deceleration $q_0$ and jerk $j$ parameters. 1$\sigma$, 2$\sigma$ and 3$\sigma$ contour region plots for the estimated values of parameters are presented. 580 SNIa supernova distance modulus data set and 66 pantheon SNIa data which include high red shift data in the range $0\leq z\leq 2.36$ have been used to draw error bar plots and likelihood probability curves for distance modulus and apparent magnitude of SNIa supernova's. We have calculated the pressures and densities associated with the two matter densities, viz., $p_{\mu}$, $\rho_{\mu}$, $p_m$ and $\rho_m$, respectively. The present age of the universe as per our model is also evaluated and it is found at par with the present observed values.

Theoretical analysis on the Barrow holographic dark energy in the Finsler-Randers Cosmology

Abstract: Cosmological features of Barrow Holographic Dark Energy (BHDE), a recent generalization of original Holographic dark energy with a richer structure, are studied in the context of Finsler–Randers universe, where the Hubble horizon is considered as the IR cutoff. Following this setup, we derive the evolution equation for the Barrow holographic dark energy-density parameter, the equation-of-state (EoS) parameter and deceleration parameter. As a result of our study, it is obtained that the model is able to describe the currently accelerating universe in both noninteractive and interactive scenarios, and that the development of the deceleration phase over to the acceleration phase also occurs later in this case. Furthermore, we discuss the statefinder diagnosis of this model, while plotting [Formula: see text] and [Formula: see text] versus redshift [Formula: see text] and the evolutionary trajectories of [Formula: see text]. We find that the statefinder is not only superior in breaking the parsimony of the different coupling parameter values in this model, but also clearly distinguishes between the Barrow holographic dark energy model and the [Formula: see text] model. In addition, we find that the statefinder pair performs slightly worse than [Formula: see text] for both [Formula: see text] and [Formula: see text] in this model. Finally, we show that the distance modulus of the theoretical model is compared with the observed data of Ia supernovae, and it is found that the theoretical model is in good agreement with the observed data. Our model satisfactorily explains the present history of the universe, thus providing a good candidate for dark energy.