Search Results (21)

We investigate the dynamics of quantum coherence in an anisotropically expanding emergent universe, modeled by a Bianchi type I spacetime. In particular, our findings suggest that the presence of small anisotropic perturbations introduces a directional dependence in the behavior of quantum coherence. Notably, we identify the emergence of frozen coherence regimes when the $\{a_0,H_0,m,k\}$ parameters lie within particular ranges. The physical origin of these frozen regimes can be attributed to the suppression of mode mixing, which consequently leads to reduced particle creation. Full article

We analyze the anisotropic Bianchi models, and in particular the Bianchi Type IX known as the Mixmaster universe, where the Misner anisotropic variables obey Deformed Commutation Relations inspired by Quantum Gravity theories. We consider three different deformations, two of which have been able to remove the initial singularity similarly to Loop Quantum Cosmology when implemented in the single-volume variable. Here, the two-dimensional algebras naturally implement a form of Non-Commutativity between the space variables that affects the dynamics of the anisotropies. In particular, we implement the modifications in their classical limit, where the Deformed Commutators become Deformed Poisson Brackets. We derive the modified Belinskii–Khalatnikov–Lifshitz map in all the three cases, and we study the fate of the chaotic behavior that the model classically presents. Depending on the sign of the deformation, the dynamics will either settle into oscillations between two almost-constant angles, or stop reflecting after a finite number of iterations and reach the singularity as one last simple Kasner solution. In either case, chaos is removed. Full article

An exact non-perturbative model of a gravitational wave with pure radiation is constructed. It is shown that the presence of dust matter in this model contradicts Einstein’s field equations. The exact solution to Einstein’s equations for gravitational wave and pure radiation is obtained. The trajectories of propagation and the characteristics of radiation are found. For the considered exact model of a gravitational wave, a retarded time equation for radiation is obtained. The obtained results are used to construct an exact model of gravitational wave and pure radiation for the Bianchi type IV universe. Full article

I deduce an exact and analytic Bianchi type I solution of Einstein’s field equations, which generalizes the isotropic ΛCDM universe model to a corresponding model with anisotropic expansion. The main point of the article is to present the anisotropic generalization of the ΛCDM universe model in a way suitable for investigating how anisotropic expansion modifies observable properties of the ΛCDM universe model. Although such generalizations of the isotropic ΛCDM universe model have been considered earlier, they have never been presented in this form before. Several physical properties of the model are pointed out and compared with properties of special cases, such as the isotropic ΛCDM universe model. The solution is then used to investigate the Hubble tension. It has recently been suggested that the cosmic large-scale anisotropy may solve the Hubble tension. I consider those earlier suggestions and find that the formulae of these papers lead to the result that the anisotropy of the cosmic expansion is too small to solve the Hubble tension. Then, I investigate the problem in a new way, using the exact solution of the field equations. This gives the result that the cosmic expansion anisotropy is still too small to solve the Hubble tension in the general Bianchi type I universe with dust and LIVE (Lorentz Invariant Vacuum Energy with a constant energy density, which is represented by the cosmological constant) and anisotropic expansion in all three directions—even if one neglects the constraints coming from the requirement that the anisotropy should be sufficiently small so that it does not have any significant effect upon the results coming from the calculations of the comic nucleosynthesis during the first ten minutes of the universe. If this constraint is taken into account, the cosmic expansion anisotropy is much too small to solve the Hubble tension. Full article

A plane symmetric Bianchi-I model filled with strange quark matter (SQM) was explored in $f(R,T)=R+2\lambda T$ gravity, where R is the Ricci scalar, T is the trace of the energy-momentum tensor, and $\lambda$ is an arbitrary constant. Three different types of solutions were obtained. In each model, comparisons of the outcomes in $f(R,T)$ gravity and bag constant were made to comprehend their roles. The first power-law solution was obtained by assuming that the expansion scalar is proportional to the shear scalar. This solution was compared with a similar one obtained earlier. The second solution was derived by assuming a constant deceleration parameter q. This led to two solutions: one power-law and the other exponential. Just as in the case of general relativity, we can obtain solutions for each of the different eras of the universe, but we cannot obtain a model which shows transitional behavior from deceleration to acceleration. However, the third solution is a hybrid solution, which shows the required transition. The models start off with anisotropy, but are shear free at late times. In general relativity, the effect of SQM is to accelerate the universe, so we expect the same in $f(R,T)$ gravity. Full article

Exact models of primordial gravitational waves in the Bianchi type-III universe were constructed on the basis of the quadratic theory of gravity with a scalar field and pure radiation in Shapovalov wave spacetimes of type II (subtype 2). Exact solutions of the field equations and scalar equation were obtained. The characteristics of pure radiation were determined. An explicit form of the scalar field functions included in the Lagrangian of the considered quadratic theory of gravity was found. The trajectories of the propagation of light rays in the considered gravitational wave models were obtained. Full article

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 $\left(q\right)$ has been suggested: $q=\alpha -\frac{\beta }{H^2}$. 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. Full article

In this paper, we have studied an anisotropic Bianchi-I cosmological model in $f(R,T)$ gravity. To obtain the exact solutions of the field equations, we have used the condition $\sigma /\theta$ to be a function of the scale factor (IJTP, 54, 2740-2757, 2015). Our model possesses an initial singularity. It initially exhibits decelerating expansion and transits to accelerating expansion at late times. We have also discussed the physical and geometrical properties of the model. Full article

Self-similar cosmological solutions correspond to spacetimes that admit a homothetic symmetry. The physical properties of self-similar solutions can describe important eras of the cosmological evolution. Recently, self-similar cosmological solutions were derived for symmetric teleparallel $f\left(Q\right)$-theory with different types of connections. In this work, we study the stability properties of the self-similar cosmological solutions in order to investigate the effects of the different connections on the stability properties of the cosmic history. For the background geometry, we consider the isotropic Friedmann–Lemaître–Robertson–Walker space and the anisotropic and homogeneous Bianchi I space, for which we investigate the stability properties of Kasner-like universes. Full article

As a follow-up of a recent article in which we investigated the cosmological background expansion history of the universe in Bianchi type-V cosmological models with bulk viscous fluid and evolving cosmological $\Lambda$ and Newtonian G parameters, we study the evolution of the cosmological perturbations in the current work. In particular, we analyse the evolution of the viscous matter over-density that leads to formation of large-scale structures in the Bianchi-V model, and compare the results with standard $\Lambda$CDM solutions. Our results suggest that introducing viscous fluid in the background described by Bianchi-V geometry with evolving $\Lambda$ and G amplifies the structure-growth rate. Full article

In this article, we consider an anisotropic viscous cosmological model having LRS Bianchi type I spacetime with $f\left(Q\right)$ gravity. We investigate the modified $f\left(Q\right)$ gravity with form $f\left(Q\right)=\alpha Q^2+\beta$, where Q is the non-metricity scalar and $\alpha$, $\beta$ are the positive constants. From the modified Einstein’s field equation having the viscosity coefficient $\xi \left(t\right)={\xi }_{0}H$, the scale factor is derived as $a\left(t\right)=2sinh\left(\frac{m+2}{6}\sqrt{\frac{{\xi }_0}{\alpha (2m+1)}}t\right)$. We apply the observational constraints on the apparent magnitude $m\left(z\right)$ using the ${\chi }^2$ test formula with the observational data set such as JLA, Union 2.1 compilation and obtained the best approximate values of the model parameters $m,\alpha ,H_0,{\xi }_0$. We find a transit universe which is accelerating at late times. We also examined the bulk viscosity equation of state (EoS) parameter ${\omega }_v$ and derived its current value satisfying ${\omega }_v<-1/3$, which shows the dark energy dominating universe evolution having a cosmological constant, phantom, and super-phantom evolution stages. It tends to the $\Lambda$ cold dark matter ($\Lambda$CDM) value (${\omega }_v=-1$) at late times. We also estimate the current age of the universe as $t_0\approx 13.6$ Gyrs and analyze the statefinder parameters with $(s,r)\to (0,1)$ as $t\to \infty$. Full article

We obtain exact solutions to the field equations for five-dimensional locally rotationally symmetric (LRS) Bianchi type-I spacetime in the $f(R,T)$ theory of gravity, where specifically, the following three cases are considered: (i) $f(R,T)=\mu (R+T)$, (ii) $f(R,T)=R\mu +RT{\mu }^2$, and (iii) $f(R,T)=R+\mu R^2+\mu T$, where R and T, respectively, are the Ricci scalar and trace of the energy–momentum tensor. It is found that the equation of state (EOS) parameter w is governed by the parameter $\mu$ involved in the $f(R,T)$ expressions. We fine-tune the parameter $\mu$ to obtain the effect of phantom energy in the model. However, we also restrict this parameter to obtain a stable model of the universe. Full article

Quantum theory of a test field on a quantum cosmological spacetime may be viewed as a theory of the test field on an emergent classical background. In such a case, the resulting dressed metric for the field propagation is a function of the quantum fluctuations of the original geometry. When the backreaction is negligible, massive modes can experience an anisotropic Bianchi type I background. The field modes propagating on such a quantum-gravity-induced spacetime can then unveil interesting phenomenological consequences of the super-Planckian scales, such as gravitational particle production. The aim of this paper is to address the issue of gravitational particle production associated with the massive modes in such an anisotropic dressed spacetime. By imposing a suitable adiabatic condition on the vacuum state and computing the energy density of the created particles, the significance of the particle production on the dynamics of the universe in Planck era is discussed. Full article

Current observations indicate that, on a large enough scale, the universe is homogeneous and isotropic. However, this does not preclude the possibility of some anisotropy having occurred during the early stages of the evolution of the universe, which could then have been damped out later. This idea has aroused interest in the Bianchi models, which are homogeneous but anisotropic. Secondly, there is much interest in modified gravity these days due to the problems that the usual ΛCDM model faces in general relativity. Hence, in this paper, a study was conducted on the Bianchi type-I cosmological model in f(R,T)-modified gravity. Following some ideas from cosmography, a specific form of the deceleration parameter was assumed, leading to a model that exhibited a transition from early deceleration to late-time acceleration. The derived model approached isotropy at late times. The physical properties of the model were discussed, and expressions for the various parameters of the model were derived. It is also possible to make progress towards solving the cosmological constant problem, since in this model in f(R,T) gravity, a variable cosmological-type parameter arose, which was large early on but decreased to a constant value in later times. Full article

Although the present universe is believed to be homogeneous and isotropic on large scales, there is some evidence of some anisotropy at early times. Hence, there is interest in the Bianchi models, which are homogeneous but anisotropic. In this presentation, the Bianchi type–I space-time in the framework of the f(R,T) modified theory of gravity has been investigated for the specific choice of f(R,T) = R + 2f(T), where f(T) = −mT, m = constant. The solution of the modified gravity field equations has been generated by assuming that the deceleration parameter q is a function of the Hubble parameter H, i.e., q = b − n/H (where b and n are constants, and n > 0), which yields the scale factor a = k[exp(nt) − 1]^1/(1+b) (where k is a constant). The model exhibits deceleration at early times, and is currently accelerating. It is also seen that the model approaches isotropy at late times. Expressions for the Hubble parameter in terms of red-shift, luminosity distance, and state-finder parameter are derived and their significance is described in detail. The physical properties of the cosmological model are also discussed. An interesting feature of the model is that it has a dynamic cosmological parameter, which is large during the early universe, decreases with time, and approaches a constant at late times. This may help in solving the cosmological constant problem. Full article