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- Using the FLRW cosmological model, this paper explores the dynamics of perfect fluid as a source in the context of modified gravity, where the non-metricity Q, which causes the gravitational interaction, is represented by the arbitrary function Lagrangian as the trace of the non-metricity tensor Q, say f(Q) gravity. We govern the features of the derived cosmological model in view of the parameterization of Hubble’s parameter of the form, H(z)=H0(γ+1)(γ+(1+z)ξ). We have spoken about how the energy density, pressure, equation of state parameter, and skewness parameter in our model represent the physical behavior of the cosmos. In addition, we have looked at the kinematic parameters in our model that describe the cosmos, including the jerk, deceleration, and Hubble parameters. The universe’s phase transition from deceleration to acceleration is indicated by our model’s deceleration parameter, q(z). Furthermore, the deceleration parameter’s present value, q0 clearly aligns with the essential ΛCDM model. In order to determine the nature of the dark energy model, we also examined geometrical diagnostics such as the Statefinder pairs and Om(z) diagnostic. Additionally, we used the squared speed of sound test to examine the stability of the cosmos in our model. In the end, at present, the universe in our model is expanding, accelerating, and behaving in a manner consistent with a quintessential dark energy concept while at late the cosmos is dominated by ΛCDM.
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- 2023| American Astronomica...In previous works, we proposed to estimate cosmological parameters with an artificial neural network (ANN) and a mixture density network (MDN). In this work, we propose an improved method called a mixture neural network (MNN) to achieve parameter estimation by combining ANN and MDN, which can overcome shortcomings of the ANN and MDN methods. Besides, we propose sampling parameters in a hyperellipsoid for the generation of the training set, which makes the parameter estimation more efficient. A high-fidelity posterior distribution can be obtained using ( ) 102 forward simulation samples. In addition, we develop a code named CoLFI for parameter estimation, which incorporates the advantages of MNN, ANN, and MDN, and is suitable for any parameter estimation of complicated models in a wide range of scientific fields. CoLFI provides a more efficient way for parameter estimation, especially for cases where the likelihood function is intractable or cosmological models are complex and resource-consuming. It can learn the conditional probability density p(θ|d) using samples generated by models, and the posterior distribution p(θ|d0) can be obtained for a given observational data d0. We tested the MNN using power spectra of the cosmic microwave background and Type Ia supernovae and obtained almost the same result as the Markov Chain Monte Carlo method. The numerical difference only exists at the level of ( ) s - 10 2 . The method can be extended to higher-dimensional data.
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- We consider a flat Friedmann–Robertson–Walker (FRW) universe with expansion history based on the Hubble parameter. The Hubble parameter ansatz, by construction, yields the de Sitter like evolution at early and late-times. The phase transition from accelerated to decelerated era during early times and decelerated to accelerated era in the recent past provide the complete cosmological history of the universe in a natural way. In order to exhibit the dynamical evolution of universe, we explore the inflationary dynamics along with graceful exit, behaviors of cosmological parameters, energy conditions along with behavior in 𝜔 − 𝜔′ plane to identify the thawing and freezing regions. Statefinder diagnostic highlights that the universe will evolve to the 𝛬 cold dark matter model during late-times. We also find the scalar field description for the considered Hubble parameter ansatz.
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- Einstein's field equations with variable gravitational and cosmological parameters G and A in the presence of bulk viscosity are considered. We confine our attention to homogeneous and isotropic universe models. Power law and exponential solutions for the scale factor R are shown to exist in the Eckart theory and the truncated and full Israel-Stewart theories. In the Eckart theory we consider the energy-momentum tensor to be conserved, whereas in the other two theories we consider a modified energy-conservation law.
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