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In this paper, we study an isotropic flat FLRW-model in scale-covariant theory of gravity f γ δ (ϕ) 1 which is explained in terms of ordinary and covariant differentiation of scalar field ϕ. As we know, the deceleration parameter (DP) is time-dependent, so we consider the DP q as the function of t. Using this methodology, we find all the important cosmological factors in terms of a hyperbolic function of the cosmic time t. In turn, we create the model having the behavior of the late-time universe, which is ever accelerated expanding and faces a Big Freeze at the end. The model shows the quintessence dark energy model from early to late times. We compute the constrained values of Hubble parameter H 0 = 7 0. 9 7 9 − 0. 0 0 4 3 + 0. 0 2 1 and the model parameter n = 1. 2 4 0 7 9 − 0. 0 0 0 7 9 + 0. 0 0 0 1 5 using joint analysis of the OHD data of 77-points and Pantheon bin data. The model exhibits point-type singularity as it begins with a point of zero volume, infinite energy density and temperature. Furthermore, we obtain the present value of DP (q 0) ≈ − 0. 5 4. Also, we examine the ultimate behavior of our model by the proper analysis of energy conditions (ECs), cosmographical parameters and Statefinder diagnostic. Finally, the proposed model behaves like a quintessence dark energy model. [ABSTRACT FROM AUTHOR]

This work explores the dark energy nature of logarithmic f (R , L m) -gravity models with observational constraints, where L m represents the matter Lagrangian for perfect fluid and R is the Ricci-scalar curvature. With a matter Lagrangian L m = ρ , a flat FLRW space–time metric and an arbitrary function f (R , L m) = R 2 + L m − μ ln L m , where μ is a positive model parameter, we have derived the field equations of this model. By using the Hubble function, we have been able to solve the energy conservation equation and create a relationship between Ω m 0 , Ω q 0 and Ω μ 0 . Using the most recent two observational datasets as 32 H (z) and 1048 Pantheon SNe Ia datasets, we conducted MCMC analysis. We have obtained best fit values of model parameters with 1 − σ , 2 − σ , 3 − σ errors and using these values, we have explored the cosmological properties of the model. We have performed om diagnostic analysis for the model and estimated the present age of the universe. Thus our derived model presents a transit phase decelerating to accelerating model without dark energy term Λ. We found that the effective equation of state parameter is in the range − 1 ≤ ω ≤ 0 over − 1 ≤ z < ∞ with present value ω ≈ − 0. 6 9 7 1 , − 0. 7 1 2 0. [ABSTRACT FROM AUTHOR]

In this investigation, we have observed an anisotropic Bianchi type-I dark energy universe model in f (R , L m) gravity with observational constraints, where f (R , L m) is an arbitrary function of Ricci-scalar curvature R and matter Lagrangian L m . For our investigation, we have considered a specific form of this function as f (R , L m) = R 2 + α L m n − Λ with α , n as model free parameters and Λ is the "cosmological constant". We have solved the field equations analytically and established a relationship between energy parameters Ω m , Ω k and Ω Λ and obtained Hubble function H (z) in terms of redshift z. We have compared this Hubble function with two observational datasets Union 2.1 plus bined and H (z) for obtaining the best fit values of model parameters and energy parameters, by applying χ 2 -test. We have analyzed and discussed the results using these best fit values throughout the paper. We have found a accelerating transit phase phantom dark energy model which tends to Λ CDM model at late-time universe. [ABSTRACT FROM AUTHOR]

We present Bianchi type-I cosmological models in the presence of generalized Chaplygin gas and perfect fluid for early and late time epochs. Exact solutions of Einstein’s field equations for this model are obtained. The general solutions of gravitational field equations are expressed in an exact parametric form, with average scale factor as parameter. In the limiting cases of small and large values of the average scale factor, the solutions of the field equations are expressed in exact analytic forms. Moreover, this model predicts that the expansion of Universe is accelerating for the late times. The physical and geometrical properties of the corresponding cosmological models are discussed. [ABSTRACT FROM AUTHOR]

In this paper we solve the field equations for Scale covariant theory of gravitation which was introduced by Caunato et al. [1], for Bianchi V line element in the presence of perfect fluid medium. Here the deceleration parameter is considered to be time dependent which gives the average scale factor a ( t ) = [ sinh ⁡ ( β t ) ] 1 / n , where n and β are positive constants. This value of average scale factor is the key expression for solving the field equations. Using the recent observational value of q 0 = - 0.52 - 0.04 + 0.08 and H 0 = 69.2 ± 1.2 derived from BAO/CMB and H(z) data by Santos et al. (2016) [46], we have evaluated three different pairs of ( n , β ) . We observe that the model represents a phase transition from early deceleration to a present accelerating phase for a particular choice of the pair ( n = 2 , β = 92.75 ) . Applying some recently developed diagnostic tools like jerk parameter and statefinders, we find that the derived model is exactly in accordance with standard ΛCDM model. Along with these, many physical, geometric and kinematic properties of the model are thoroughly studied and found consistent with recent observations., (© 2020 Published by Elsevier Ltd.)