Flat Friedmann Lemaitre Robertson Walker Universe Research Articles

Generalized ghost dark energy model in Brans–Dicke theory with logarithmic scalar field

We study generalized ghost dark energy model in Brans–Dicke theory within the framework of flat Friedmann–Lemaitre–Robertson–Walker Universe. We assume the well-motivated logarithmic form of Brans–Dicke scalar field in terms of the scale factor to find the observational parameters such as equation of state parameter [Formula: see text] and deceleration parameter q. It is observed that the equation of state parameter crosses the phantom divide line [Formula: see text] for a suitable range of model parameters. We observe that the deceleration parameter is time-dependent which shows the recent phase transition from decelerated to accelerated expansion of the Universe. Moreover, the model shows that the Universe will go through two future phase transitions, recent accelerated expansion to decelerated expansion and again decelerated expansion to accelerated expansion. We have also plotted the trajectories of the model in the [Formula: see text] plane for different values of parameters and explored the freezing and thawing regions. Further, we apply the sound squared speed method to check the stability of the model and found that the model is stable for suitable range of parameters.

Cosmology with non-minimal coupled gravity: dynamical study of the inflationary universe in the deformed phase space scenario

In this work, we consider a noncommutative (NC) inflationary model with a homogeneous scalar field non-minimally coupled to gravity. We propose to use a canonical deformation between momenta in a spatially flat Friedmann–Le maitre–Robertson–Walker universe. This particular choice of noncommutativity allows interesting dynamics that other NC models seem not to allow. In this approach, the dimensional parameter $$\theta $$ , which considered as the length of Planck, presents the quantum regime. But also, it is crucial to recover the standard results by taking the appropriate limits of this parameter. To note that the $$1\mathrm{st}$$ Friedmann equation remains unaffected while both the Friedmann acceleration and the Klein-Gordon equations are affected by an additional term linear in the NC parameter.

Evolution of FLRW universe in Brans-Dicke gravity theory

This study aims to examine the dynamics of flat Friedmann-Lemaitre-Robertson-Walker (FLRW) model of universe with time varying cosmological constant $\Lambda (t)$ in Brans-Dicke theory of gravity. The authors find a solution of the field equations using simple parametrization of scale factor $a(t)$ i.e. $a(t)=\exp \{(\alpha t+\beta )^{p}\}$ , and power law relation between $a(t)$ and Brans-Dicke scalar field $\phi $ . The derived cosmological model shows transition from early cosmic deceleration to present cosmic acceleration. Such feature of cosmic expansion is in agreement with recent empirical observations. The behaviour of cosmographic parameters such as Hubble parameter, deceleration parameter, jerk, snap and lerk parameters is examined graphically. It is observed that model behaves like $\Lambda CDM$ model in late cosmic evolution. We also use statefinder and Om diagnostic tools to differentiate various dark energy models from $\Lambda CDM$ model. Moreover, model parameter $p$ and present value of deceleration parameter $q_{0}$ is also estimated using Hubble data set and Pantheon Type 1a supernova data set.

Understanding the phenomenology of interacting dark energy scenarios and their theoretical bounds

Non-gravitational interaction between dark matter and dark energy has been considered in a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) universe. The interaction rate is assumed to be linear in the energy densities of dark matter and dark energy and it is also proportional to the Hubble rate of the FLRW universe. This kind of interaction model leads to an autonomous linear dynamical system, and depending on the coupling parameters, could be solved analytically by calculating the exponential of the matrix, defining the system. We show here that such interaction rate has a very deep connection with some well known cosmological theories. We then investigate the theoretical bounds on the coupling parameters of the interaction rate in order that the energy densities of the dark sector remain positive throughout the evolution of the universe and asymptotically converge to zero at very late times. Our analyses also point out that such linear interacting model may encounter with finite time future singularities depending on the coupling parameters as well as the dark energy state parameter.

The Lambert W Function: A Newcomer in the Cosmology Class?

Abstract We propose a novel equation of state (EoS) which explains the evolutionary history of a flat Friedmann–Lemaitre–Robertson–Walker universe. The uniqueness of this EoS lies in the fact that it incorporates the Lambert W function in a special fashion. It is explicitly demonstrated that with observationally relevant values of the unknown parameters ϑ 1 and ϑ 2, all the evolutionary phases of the universe can be reproduced. Moreover, it also shows that the initial singularity is unavoidable and asserts that the late-time acceleration of the universe would continue forever.

Note on bouncing backgrounds

The theory of inflation is one of the fundamental and revolutionary developments of modern cosmology that became able to explain many issues of early universe in the context of the standard cosmological model (SCM). However, the initial singularity of the universe, where physics is indefinite, is still obscure in the combined `SCM+inflation' scenario. An alternative to `SCM+inflation' without the initial singularity is thus always welcome, and bouncing cosmology is an attempt of that. The current work is thus motivated to investigate the bouncing solutions in modified gravity theories when the background universe is described by the spatially flat Friedmann-Lemaitre-Robertson-Walker (FLRW) geometry. We show that the simplest way to obtain the bouncing cosmologies in such spacetime is to consider some kind of Lagrangians whose gravitational sector depends only on the square of the Hubble parameter of the FLRW universe. For these modified Lagrangians, the corresponding Friedmann equation, a constraint in the dynamics of the universe, depicts a curve in the phase space $(H,rho)$, where $H$ is the Hubble parameter and $rho$ being the energy density of the universe. As a consequence, a bouncing cosmology is obtained when this curve is closed and crosses the axis $H = 0$ at least twice, and whose simplest particular example is the ellipse depicting the well-known holonomy corrected Friedmann equation in Loop Quantum Cosmology (LQC). Sometimes, a crucial point in such theories is the appearance of the Ostrogradski instability at the perturbative level, however, fortunately enough, in the present work as long as the linear level of perturbations is concerned, this instability does not appear but such instability may appear at the higher-order of perturbations.

Singular Model of an Inflationary Universe

A cosmological model of an inflationary Universe is considered that is induced by the generalized form of the equation of state for a fluid in a spatially flat Friedmann–Lemaitre–Robertson–Walker Universe. A cosmological model of a fluid that describes an inflationary Universe with a type-IV singularity is investigated. Values of parameters in the equation of state of the fluid near singularities are found. Finally, we consider the correspondence of the theoretical values of the inflation parameters leading to singular inflation of type IV with the experimental data obtained from observations made by the Planck mission.

Cosmological Model with Dark Matter in a Dissipative Universe

A cosmological model is considered, in which two ideal fluids interact: dark energy and dark matter, in a homogeneous and isotropic flat Friedmann–Lemaitre–Robertson–Walker Universe with scattering. Scattering in this Universe is investigated in two ways: in terms of bulk viscosity and with the help of the entropic model. The interaction between dark energy and dark matter is described in terms of parameters of the equation of state for dark energy, which take account of dissipative properties of the Universe. In the example of the model of the Universe with a singularity known as the Little Rip, application of a formalism describing dissipative properties is demonstrated. An analytical representation of this model in terms of thermodynamic parameters of the equation of state for dark energy, taking dissipation into account, is obtained. In the expression for the coefficient of bulk viscosity, corrections are taken into account which are due to the interaction between dark energy and dark matter.

Spherical and nonspherical models of primordial black hole formation: exact solutions

We construct spacetimes which provide spherical and nonspherical models of black hole formation in the flat Friedmann--Lemaitre--Robertson--Walker (FLRW) universe with the Lemaitre--Tolman--Bondi solution and the Szekeres quasispherical solution, respectively. These dust solutions may contain both shell-crossing and shell-focusing naked singularities. These singularities can be physically regarded as the breakdown of dust description, where strong pressure gradient force plays a role. We adopt the simultaneous big bang condition to extract a growing mode of adiabatic perturbation in the flat FLRW universe. If the density perturbation has a sufficiently homogeneous central region and a sufficiently sharp transition to the background FLRW universe, its central shell-focusing singularity is globally covered. If the density concentration is sufficiently large, no shell-crossing singularity appears and a black hole is formed. If the density concentration is not sufficiently large, a shell-crossing singularity appears. In this case, a large dipole moment significantly advances shell-crossing singularities and they tend to appear before the black hole formation. In contrast, a shell-crossing singularity unavoidably appears in the spherical and nonspherical evolution of cosmological voids. The present analysis is general and applicable to cosmological nonlinear structure formation described by these dust solutions.

Time-dependent embedding of a spherically symmetric Rindler-like spacetime

An anisotropic cosmic fluid with radial heat flux which sources a time-dependent Rindler-like geometry is investigated. Even though its energy density ρ is positive, the radial and transversal pressures are negative and the strong energy condition is not satisfied. The congruence of ‘static’ observers is not geodesic and the heat flux is oriented outward. We computed the Misner–Sharp energy associated with the Rindler-type metric embedded in a spatially flat Friedmann–Lemaitre–Robertson–Walker universe and found that the Weyl energy is vanishing thanks to the conformally flat form of the spacetime. The null geodesic expansions are computed and one finds that only one of the two apparent horizons is located inside the event horizon. The properties of the Rindler-like geometry embedded in the conformally flat de Sitter spacetime are investigated.

The particle versus the future event horizon in an interacting holographic dark energymodel

Choosing the future event horizon as the horizon of the flat Friedmann–Lemaitre–Robertson–Walkeruniverse, we show that the interacting holographic dark energy model is ableto explain the phantom divide line crossing. We show that if we take theparticle event horizon as the horizon of the universe, besides describing theω = −1 crossing (based on astrophysical data), we are able to determine appropriately the ratio ofdark matter to dark energy density at the transition time. In this approach, after the firsttransition from the quintessence to the phantom phase, there is another transitionfrom the phantom to the quintessence phase which avoids the big rip singularity.

Generation of seed perturbations from quantum cosmology

The origin of seed perturbations in the Universe is studied within the framework of a specific minisuperspace model. It is shown that the `creation' of the Universe as a result of a quantum transition from a flat empty spacetime would lead to a flat FLRW (Friedmann Lema\^\i tre Robertson-Walker) Universe with weak inhomogeneous perturbations at large wavelengths. The power spectrum of these perturbations is found to be scale invariant at horizon crossing (i.e., the Harrison-Zeldovich spectrum). It is also recognised that the seed perturbations generated in our model would be generically of the isocurvature kind.