Friedman-Robertson-Walker Cosmology Research Articles

Noether symmetries and boundary terms in extended Teleparallel gravity cosmology

We discuss an extended Teleparallel gravity models comprising functions of scalar invariants constructed by torsion, torsion Gauss–Bonnet and boundary terms. We adopt the Noether symmetry approach to select the functional forms, the first integrals and, eventually, the exact solutions of the dynamics in the context of flat Friedman–Robertson–Walker cosmology. Standard perfect fluid matter, minimally coupled to geometry, is considered. Several physically consistent models are derived with related exact solutions.

Emergent dark energy via decoherence in quantum interactions

In this work we consider a recent proposal that gravitational interactions are mediated via classical information and apply it to a relativistic context. We study a toy model of a quantized Friedman–Robertson–Walker (FRW) universe with the assumption that any test particles must feel a classical metric. We show that such a model results in decoherence in the FRW state that manifests itself as a dark energy fluid that fills the spacetime. Analysis of the resulting fluid, shows the equation of state asymptotically oscillates around the value w = −1/3, regardless of the spatial curvature, which provides the bound between accelerating and decelerating expanding FRW cosmologies. Motivated with quantum-classical interactions this model is yet another example of theories with violation of energy-momentum conservation whose signature could have significant consequences for the observable universe.

Turbulence accelerating cosmology from an inhomogeneous dark fluid

Specific dark energy models with a linear inhomogeneous time-dependent equation of state, within the framework of 4d Friedman-Robertson-Walker (FRW) cosmology, are investigated. It is demonstrated that such 4d inhomogeneous fluid models may lead to a turbulence FRW cosmology. Both one-component and two-component models from 4d inhomogeneous dark fluid models are considered. In the one-component model the universe may develop from a viscous era with, for instance, a constant bulk viscosity, into a turbulent era. In the two-component model the fluid can be decomposed into two components, one non-turbulent (ideal) and another turbulent part, obeying two different equations of state. Conditions for the appearance of the turbulent dark energy universe in terms of the parameters in the equation of state (EoS) without introducing the turbulence concept explicitly are are obtained. An equivalent description in terms of an inhomogeneous fluid for the viscous Little Rip (LR) cosmology is also developed.

Rip brane cosmology from 4d inhomogeneous dark fluid universe

Specific dark energy models with linear inhomogeneous time-dependent equation of state, within the framework of 4d Friedman-Robertson-Walker (FRW) cosmology, are investigated. It is demonstrated that the choice of such 4d inhomogeneous fluid models may lead to a brane FRW cosmology without any explicit account of higher dimensions at all. Effectively, we thus obtain a brane dark energy universe without introducing the brane concept explicitly. Several examples of brane Rip cosmology arising from 4d inhomogeneous dark fluid models are given.

Deformation of Two Coupled Scalar Fields with Cosmological Solution

We know that the cosmic acceleration in modern cosmology are discussed in Refs. [1–3]. The amount of dark energy is almost 70% of total energy of the universe. This energy causes the present cosmic acceleration. The dark energy is a hypothetical form of energy which permeates the whole space and tends to increase rate of expansion of the universe. The existence of dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. The standard Friedman–Robertson–Walker (FRW) model with the real scalar fields can be a candidate to describe the dark energy. Also the presence of acceleration in cosmology leads us to consider scalar fields. In order to investigate the rate of expansion we obtain first-order differential equation which solves the corresponding equation of motion. Evidently, the presence of first-order equation eases the process of solving specific models and also can be used to fully understand the related cosmic evolution. The key motivation for the present work has appeared after the work [4] which deals with FRW cosmology driven by real scalar field. As we know, if the scale factor a(t) is known, the potential V (t) and ∗corresponding author; e-mail: pouriya@ipm.ir

Solutions of higher dimensional Gauss–Bonnet FRW cosmology

We examine the effect on cosmological evolution of adding a Gauss–Bonnet term to the standard Einstein–Hilbert action for a (1 + 3) + d dimensional Friedman–Robertson–Walker (FRW) metric. By assuming that the additional dimensions compactify as a power law as the usual 3 spatial dimensions expand, we solve the resulting dynamical equations and find that the solution may be of either de Sitter or Kasner form depending upon whether the Gauss–Bonnet term or the Einstein term dominates.

Erratum: “Euclidean Friedman–Robertson–Walker cosmology in Brans–Dicke-like theories of gravity” [J. Math. Phys. 43, 1487 (2002)

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Euclidean Friedman–Robertson–Walker cosmology in Brans–Dicke-like theories of gravity

An explicit and tractable form of the complete solution of Brans–Dicke Euclidean Friedman–Robertson–Walker cosmology, in the matter dominated area, is proposed. In this derivation, the (generally imposed) constraint on the Brans–Dicke parameter ω>−3/2 is relaxed. An isolated power-law solution is exhibited, which turns out to describe an accelerated expansion of the universe, without recourse to any cosmological constant or quintessence matter. It appears that this solution can be generalized for all Brans–Dicke-like theories of gravity.

Cumulative gravitational lensing in Newtonian perturbations of Friedman—Robertson—Walker cosmologies

It is a common assumption amongst astronomers that, in the determination of the distances of remote sources from their apparent brightness, the cumulative gravitational lensing due to the matter in all the galaxies is the same, on average, as if the matter were uniformly distributed throughout the cosmos. The validity of this assumption is considered here by way of general Newtonian perturbations of Friedman-Robertson-Walker (FRW) cosmologies. The analysis is carried out in synchronous gauge, with particular attention to an additional gauge condition that must be imposed. The mean correction to the apparent magnitude-redshift relation is obtained for an arbitrary mean density perturbation. In the case of a zero mean density perturbation, when the intergalactic matter has a dust equation of state, then there is indeed a zero-mean first order correction to the apparent magnitude-redshift relation for all redshifts. Point particle and Swiss cheese models are considered as particular cases.

Relativistic Self-Similar Flows

A mathematical framework analyzing a class of spherical self-similar hydrodynamical flows on a class of homothetic spherically symmetric background geometries is presented. The analysis covers relativistic flows propagating in the following class of spacetimes: (i) Minkowski spacetime, (ii) the homothetic Bondi–Tolman spacetime, (iii) the spatially flat Friedman–Robertson–Walker cosmologies. After the introduction of the mathematical framework, there follows a detailed investigation of special relativistic flows with the pressurePand densityρobeyingP=kρ,ka constant 0⩽k⩽1. An extensive analysis of the singular and critical points of the differential equations determining the flow is presented. By a combination of analytical and numerical techniques several classes of flows have been constructed. We have found flows describing an oscillation of the medium, as well as flows representing relativistic detonation waves propagating on a medium at rest. In addition, two classes of analytical closed form solutions are given. By invoking the relativistic Rankine–Hugoniot conditions we briefly discuss a way by which two self-similar solutions can be joined across an expanding (or contracting) shock wave.