Narrow Results

This work examines whether evolving wormhole solution is possible or not in $f(R,T)$ modified gravity theory. In the background of inhomogeneous FLRW-type wormhole configuration, the field equations are investigated for different choices of scale factors and shape functions. For the power law and exponential choice of the scale factor from cosmological context and decoupled power law of $f(R,T)$ in each variable, wormhole configuration has been examined for two viable choices of shape function. Energy conditions are examined graphically for a range of values of the parameters involved. Finally, the possibility of emergent scenario at early cosmic evolution has been examined.

In this work we have investigated the emergent scenario of the Universe described by loop quantum cosmology model, DGP brane model and Kaluza–Klein cosmology. Scalar field along with barotropic fluid as normal matter is considered as the matter content of the Universe. In loop quantum cosmology it is found that the emergent scenario is realized with the imposition of some conditions on the value of the density of normal matter in case of normal and phantom scalar field. This is a surprising result indeed considering the fact that scalar field is the dominating matter component! In case of tachyonic field, emergent scenario is realized with some constraints on the value of ρ₁ for both normal and phantom tachyon. In case of DGP brane-world realization of an emergent scenario is possible almost unconditionally for normal and phantom fields. Plots and table have been generated to testify this fact. In case of tachyonic field emergent scenario is realized with some constraints on . In Kaluza–Klein cosmology emergent scenario is possible only for a closed Universe in case of normal and phantom scalar field. For a tachyonic field, realization of emergent Universe is possible for all models (closed, open and flat).

In this paper, we study the emergent universe (EU) with interacting fluids in the background of Bianchi type I (BI) universe model. For this purpose, we consider polytropic equation of state (EoS) which constitutes three non-interacting fluids. In order to check the viability of the cosmological models, we take a two-fluid model interacting with dust fluid and a three-fluid model in which each fluid has nonlinear EoS interacting at $t\ge t_0$. It turns out that both models are realistic cosmological viable. We also check the validity of the generalized second law of thermodynamics (GSLT) for EU with interacting fluids. Finally, we study its validity in the framework of nonlinear electrodynamics (NLED) on apparent horizon.

We show that in the isentropic scenario, the first-order thermodynamical particle creation model gives an emergent universe solution even when the chemical potential is nonzero. However, there exists no emergent universe scenario in the second-order non-equilibrium theory for the particle creation model. We then point out a correspondence between the particle creation model with barotropic equation of state and the equation of state giving rise to an emergent universe without particle creation in spatially flat FRW cosmology.

A cosmological scenario where dark matter interacts with a variable vacuum energy for a spatially flat Friedmann–Robertson–Walker (FRW) spacetime is proposed and analyzed to show that with a linear equation of state and a particular interaction in the dark sector it is possible to get a model of an Emergent Universe. In addition, the viability of two particular models is studied by taking into account the recent observations. The updated observational Hubble data and the JLA supernovae data are used in order to constraint the cosmological parameters of the models and estimate the amount of dark energy in the radiation era. It is shown that the two models fulfil the severe bounds of ${\mathrm{\Omega }}_x(z\simeq 1100)<0.009$ at the 2$\sigma$ level of Planck.

Emergent universe model is presented in general theory of relativity with isotropic fluid in addition to viscosity. We obtain cosmological solutions that permit emergent universe scenario in the presence of bulk viscosity that are described by either Eckart theory or Truncated Israel Stewart (TIS) theory. The stability of the solutions are also studied. In this case, the emergent universe (EU) model is analyzed with observational data. In the presence of viscosity, one obtains emergent universe scenario, which however is not permitted in the absence of viscosity. The EU model is compatible with cosmological observations.

In this study, we have examined the evolving wormhole solution within Einstein-massive gravity, considering traceless, barotropic and anisotropic pressure fluids. We have conducted a comprehensive analysis of the constraints imposed by the constants derived from the wormhole solution. It is found that the wormhole throat, situated between two asymptotic universes, undergoes simultaneous expansion with acceleration. A detailed investigation of the energy conditions for traceless, barotropic, and anisotropic fluids suggests a wide range of possibilities for evolving wormhole configurations with nonexotic matter at the throat. The dependency of this feature on the various parameters arising from the study has also been examined.

In this work, we consider an emergent Universe in generalized gravity theories like the chameleon, f(R) and f(T) gravities. We reconstruct the potential of the chameleon field under the emergent scenario of the Universe and observe its increasing nature with the evolution of the Universe. We reveal that in the emergent Universe scenario, the equation-of-state parameter behaves like quintessence in the case of f(R) gravity and like phantom in the case of f(T) gravity.

We consider a nonsingular origin for the universe starting from an Einstein static universe, the so-called "emergent universe" scenario, in the framework of a theory which uses two volume elements and Φd⁴x, where Φ is a metric independent density, used as an additional measure of integration. Also curvature, curvature square terms and for scale invariance a dilaton field ϕ are considered in the action. The first-order formalism is applied. The integration of the equations of motion associated with the new measure gives rise to the spontaneous symmetry breaking (SSB) of scale invariance (SI). After SSB of SI, it is found that a nontrivial potential for the dilaton is generated. In the Einstein frame we also add a cosmological term that parametrizes the zero point fluctuations. The resulting effective potential for the dilaton contains two flat regions, for ϕ → ∞ relevant for the nonsingular origin of the universe, followed by an inflationary phase and ϕ → -∞, describing our present universe. The dynamics of the scalar field becomes nonlinear and these nonlinearities produce a nontrivial vacuum structure for the theory and are responsible for the stability of some of the emergent universe solutions, which exists for a parameter range of values of the vacuum energy in ϕ → -∞, which must be positive but not very big, avoiding the extreme fine tuning required to keep the vacuum energy density of the present universe small. The nontrivial vacuum structure is crucial to ensure the smooth transition from the emerging phase, to an inflationary phase and finally to the slowly accelerated universe now. Zero vacuum energy density for the present universe defines the threshold for the creation of the universe.

General relativity has been the dominant paradigm theory of gravity for the last century, it manages to explain a plethora of astronomical and cosmological observations with high accuracy. A major challenge faced by general relativity is, that it predicts a singularity as the beginning of our universe. In this paper, we shall explore a well-known idea in the literature that of the emergent universe and its viability according to Planck 2018 data and BBN constraints.

In the Emergent scenario, the Universe should evolve from a nonsingular state replacing the typical singularity of General Relativity, for any initial condition. For the scalar field model by Ellis and Maartens [Class. Quantum Grav. 21 (2003) 223] we show that only a set of measure zero of trajectories leads to emergence, either from a static state (an Einstein model), or from a de Sitter state. Assuming a scenario based on CDM interacting with a Dark Energy fluid, we show that in general flat and open models expand from a nonsingular unstable de Sitter state at high energies; for some closed models this state is a transition phase with a bounce, other closed models are cyclic. A subset of these models are qualitatively in agreement with the observable Universe, accelerating at high energies, going through a matter-dominated decelerated era, then accelerating toward a de Sitter phase.

Cosmological solutions in the Randall–Sundrum (RS) type II brane-world model including Gauss–Bonnet (GB) term, in the presence of a bulk viscous cosmological fluid are discussed. Transport equations described by Eckart and Israel Stewart are implemented here. Cosmological models admitting emergent universe (EU) of the early universe is explored here in the presence of imperfect fluid described by Eckart theory and Truncated Israel Stewart (TIS) theory and Full Israel Stewart (FIS) theory. We also study the permitted values of the EU model parameters by considering cosmological observational dataset. The stability analysis of the equilibrium points of the dynamical system associated with the evolution in the RS brane including GB term are studied in Eckart theory, TIS theory and FIS theory.

Emergent universe (EU) cosmological models with viscosity in a modified gravity which contains a general function $f(R,T)$, where $R$ and $T$ denote the curvature scalar and the trace of the energy–momentum tensor, respectively, are studied in a flat Friedmann–Robertson–Walker metric. Cosmological solutions are obtained in $f(R,T)$ theory of gravity, which represented as $f(R,T)=f(R)+f(T)$ with bulk viscosity that described by Eckart theory, truncated Israel Stewart theory and full Israel Stewart Theory. The physical and geometrical features of the EU models in $f(R,T)={\alpha }_{1}R+{\lambda }_{1}T$ gravity, where ${\alpha }_1$ and ${\lambda }_1$ are coupling parameters, with bulk viscosity are studied in details. Constraints of the EU models parameters in $f(R,T)$ gravity with bulk viscosity are estimated from observational data set. The stability analysis of the equilibrium points admitting cosmological solutions of the dynamical system associated with the evolution in the modified gravity is studied in Eckart theory, truncated Israel Stewart theory and full Israel Stewart theory.

The existence and stability of the Einstein static solution have been built in the Einstein-Cartan gravity. We show that this solution in the presence of perfect fluid with spin density satisfying the Weyssenhoff restriction is cyclically stable around a center equilibrium point. Thus, study of this solution is interesting because it supports non-singular emergent cosmological models in which the early universe oscillates indeterminately about an initial Einstein static solution and is thus past eternal.