Quintessence Behavior of an Anisotropic Bulk Viscous Cosmological Model in Modified f(Q)-Gravity

In this article, we consider an anisotropic viscous cosmological model having LRS Bianchi type I spacetime with f(Q) gravity. We investigate the modified f(Q) gravity with form f(Q)=αQ2+β, where Q is the non-metricity scalar and α, β are the positive constants. From the modified Einstein’s field equation having the viscosity coefficient ξ(t)=ξ0H, the scale factor is derived as a(t)=2sinhm+26ξ0α(2m+1)t. We apply the observational constraints on the apparent magnitude m(z) using the χ2 test formula with the observational data set such as JLA, Union 2.1 compilation and obtained the best approximate values of the model parameters m,α,H0,ξ0. We find a transit universe which is accelerating at late times. We also examined the bulk viscosity equation of state (EoS) parameter ωv and derived its current value satisfying ωv<−1/3, which shows the dark energy dominating universe evolution having a cosmological constant, phantom, and super-phantom evolution stages. It tends to the Λ cold dark matter (ΛCDM) value (ωv=−1) at late times. We also estimate the current age of the universe as t0≈13.6 Gyrs and analyze the statefinder parameters with (s,r)→(0,1) as t→∞.

Observational Constraints and Some Toy Models in f(Q) Gravity with Bulk Viscous Fluid

The standard formulation of general relativity fails to describe some recent interests in the universe. It impels us to go beyond the standard formulation of gravity. The f(Q) gravity theory is an interesting modified theory of gravity, where the gravitational interaction is driven by the nonmetricity Q. This study aims to examine the cosmological models with the presence of bulk viscosity effect in the cosmological fluid within the framework of f(Q) gravity. We construct three bulk viscous fluid models, i.e., (i) for the first model, we assuming the Lagrangian f(Q) as linear dependence on Q, (ii) for the second model the Lagrangian f(Q) as a polynomial functional form, and (iii) the Lagrangian f(Q) as a logarithmic dependence on Q. Furthermore, we use 57 points of Hubble data and 1048 Pantheon dataset to constrain the model parameters. Then, we discuss all the energy conditions for each model, which helps us to test the self-consistency of our models. Finally, we present the profiles of the equation of state parameters to test the models’ present status.

Metric-Affine Myrzakulov Gravity Theories

In this paper, we review the so-called Myrzakulov Gravity models (MG-N, with N = I, II, …, VIII) and derive their respective metric-affine generalizations (MAMG-N), discussing also their particular sub-cases. The field equations of the theories are obtained by regarding the metric tensor and the general affine connection as independent variables. We then focus on the case in which the function characterizing the aforementioned metric-affine models is linear and consider a Friedmann-Lemaître–Robertson–Walker background to study cosmological aspects and applications. Historical motivation for this research is thoroughly reviewed and specific physical motivations are provided for the aforementioned family of alternative theories of gravity.

Wormhole Solutions in Symmetric Teleparallel Gravity with Noncommutative Geometry

This article describes the study of wormhole solutions in f(Q) gravity with noncommutative geometry. Here, we considered two different f(Q) models—a linear model f(Q)=αQ and an exponential model f(Q)=Q−α1−e−Q, where Q is the non-metricity and α is the model parameter. In addition, we discussed the existence of wormhole solutions with the help of the Gaussian and Lorentzian distributions of these linear and exponential models. We investigated the feasible solutions and graphically analyzed the different properties of these models by taking appropriate values for the parameter. Moreover, we used the Tolman–Oppenheimer–Volkov (TOV) equation to check the stability of the wormhole solutions that we obtained. Hence, we found that the wormhole solutions obtained with our models are physically capable and stable.

Accidental Gauge Symmetries of Minkowski Spacetime in Teleparallel Theories

In this paper, we provide a general framework for the construction of the Einstein frame within non-linear extensions of the teleparallel equivalents of General Relativity. These include the metric teleparallel and the symmetric teleparallel, but also the general teleparallel theories. We write the actions in a form where we separate the Einstein–Hilbert term, the conformal mode due to the non-linear nature of the theories (which is analogous to the extra degree of freedom in f(R) theories), and the sector that manifestly shows the dynamics arising from the breaking of local symmetries. This frame is then used to study the theories around the Minkowski background, and we show how all the non-linear extensions share the same quadratic action around Minkowski. As a matter of fact, we find that the gauge symmetries that are lost by going to the non-linear generalisations of the teleparallel General Relativity equivalents arise as accidental symmetries in the linear theory around Minkowski. Remarkably, we also find that the conformal mode can be absorbed into a Weyl rescaling of the metric at this order and, consequently, it disappears from the linear spectrum so only the usual massless spin 2 perturbation propagates. These findings unify in a common framework the known fact that no additional modes propagate on Minkowski backgrounds, and we can trace it back to the existence of accidental gauge symmetries of such a background.