Growth of perturbations using Lambert W equation of state

Recently, a novel equation of state (EoS) parameter for dark energy has been introduced which deals with a special mathematical function, known as the Lambert [Formula: see text] function. In this paper, we study the effect on the growth of perturbations for the Lambert [Formula: see text] dark energy model. We perform the analysis for two different approaches. In the first case, we consider the universe to be filled with two different fluid components, namely, the baryonic matter component and the Lambert [Formula: see text] dark energy component, while in the second case, we consider that there is a single fluid component in the universe whose equation of state parameter is described by the Lambert [Formula: see text] function. We then compare the growth rates of Lambert [Formula: see text] model with that for a standard [Formula: see text]CDM model as well as the CPL model. Our results indicate that the presence of Lambert [Formula: see text] dynamical dark energy sector changes the growth rate and affects the matter fluctuations in the universe to a great extent.

Accretions of Tsallis, Rényi and Sharma–Mittal dark energies onto higher-dimensional Schwarzschild black hole and Morris–Thorne wormhole

In this work, we study the dark energy accretion phenomena onto [Formula: see text]-dimensional Schwarzschild black hole and [Formula: see text]-dimensional Morris–Thorne wormhole. We obtain the [Formula: see text]-dimensional Schwarzschild black hole mass and [Formula: see text]-dimensional Morris–Thorne wormhole mass and their rate of change of masses due to accretion. For the dark energy component, we consider Tsallis, modified Rényi and “modified” Sharma–Mittal holographic dark energy (HDE) and new agegraphic dark energy (NADE). We also find the black hole mass and the wormhole mass in terms of redshift when cold dark matter and the specified forms of dark energies accrete onto them. In most cases, the black hole mass increases, and wormhole mass decreases for HDE and NADE accretions. The only exception is the Sharma–Mittal NADE, where the black hole mass decreases and wormhole mass increases during the evolution of the Universe. However, the slope of increasing/decreasing mass significantly depends on the dimension in almost all cases.

Extending Friedmann Equations Using Fractional Derivatives Using a Last Step Modification Technique: The Case of a Matter Dominated Accelerated Expanding Universe

We present a toy model for extending the Friedmann equations of relativistic cosmology using fractional derivatives. We do this by replacing the integer derivatives, in a few well-known cosmological results with fractional derivatives leaving their order as a free parameter. All this with the intention to explain the current observed acceleration of the Universe. We apply the Last Step Modification technique of fractional calculus to construct some useful fractional equations of cosmology. The fits of the unknown fractional derivative order and the fractional cosmographic parameters to SN Ia data shows that this simple construction can explain the current accelerated expansion of the Universe without the use of a dark energy component with a MOND-like behaviour using Milgrom’s acceleration constant which sheds light into to the non-necessity of a dark matter component as well.

The stability analysis of brane-induced gravity with quintessence field on the brane with a Gaussian potential

In this paper, we consider a normal branch of the DGP cosmological model with a quintessence scalar field on the brane as the dark energy component. Using the dynamical system approach, we study the stability properties of the model. We find that [Formula: see text], as one of our new dimensionless variables which is defined in terms of the quintessence potential, has a crucial role in the history of the universe. We divide our discussion into two parts: a constant [Formula: see text] and a varying [Formula: see text]. In the case of a constant [Formula: see text] we calculate all the critical points of the model even those at infinity and then assume all of them as instantaneous critical points in the varying [Formula: see text] situation which is the main part of this paper. We find that the effect of the extra dimension in such a model is independent of the value of [Formula: see text]. Then, we consider a Gaussian potential for which [Formula: see text] is not constant but varies from zero to infinity. We discuss the evolution of the dynamical variables of the model and conclude that their asymptotic behaviors follow the trajectories of the moving critical points. Also, we find two different possible fates for the universe. In one of them, it could experience an accelerated expansion, but then enters a decelerating phase and finally reaches a stable matter-dominated solution. In the other scenario, the universe could approach the matter-dominated critical point without experiencing any accelerated expansion. We argue that the first scenario is more compatible with observations.

Observational constraints and stability in viscous gravity

In this paper, we study the [Formula: see text] gravity model in the presence of bulk viscosity by the flat Friedmann–Robertson–Walker metric. The field equation is obtained by teleparallel gravity with a tetrad field. The universe components are considered matter and dark energy, with the dark energy component associated with viscous [Formula: see text] gravity. After calculating the Friedmann equations, we obtain the energy density, pressure, and equation of state of dark energy in terms of the redshift parameter. Afterward, we plot the corresponding cosmological parameters versus the redshift parameter and examine the accelerated expansion of the universe. In the end, we explore the system stability using a function called the speed sound parameter.

Analysis of entropy corrected holographic and new agegraphic dark energy models in generalized Rastall gravity

This work deals with two fluid system in the framework of generalized Rastall gravity theory. One component represents dark energy whereas the other is dark matter. For the dark energy component, entropy corrected holographic and entropy corrected new agegraphic dark energy models in power-law and logarithmic versions are taken into account. For this study, we assume two classes of scale factors in which one corresponds to the future singularity and another corresponds to the initial singularity. For each of the entropy corrected dark energy models, the cosmological parameters such as Hubble parameter, deceleration parameter and equation of state parameter are calculated and their implications are established. Furthermore, to describe the stability analysis of the models, the behaviors of the squared speed of sound are analyzed graphically for each of these models. From the graphical analysis of [Formula: see text] plane, the thawing or freezing regions of all the models are determined.

Nonsingular black holes from charged dust collapse: A concrete mechanism to evade interior singularities in general relativity

In this paper, we examine the gravitational collapse of a nonrelativistic charged perfect fluid interacting with a dark energy component. Given a simple factor for the energy transfer, we obtain a nonsingular interior solution which naturally matches the Reissner–Nordström–de Sitter exterior geometry. We also show that the interacting parameter is proportional to the overall charge of the final black hole thus formed. For the case of quasi-extremal configurations, we propose a statistical model for the entropy of the collapsed matter. This entropy extends Bekenstein’s geometrical entropy by an additive constant proportional to the area of the extremal black hole.

A common explanation of the Hubble tension and anomalous cold spots in the CMB

ABSTRACT The standard cosmological paradigm narrates a reassuring story of a universe currently dominated by an enigmatic dark energy component. Disquietingly, its universal explaining power has recently been challenged by, above all, the ∼4σ tension in the values of the Hubble constant. Another, less studied anomaly is the repeated observation of integrated Sachs–Wolfe (ISW) imprints ∼5× stronger than expected in the Lambda cold dark matter (ΛCDM) model from $R_{\rm v}\gtrsim 100\,\rm {\mathit{ h}^{-1}Mpc }$ superstructures. Here, we show that the inhomogeneous AvERA (Average Expansion Rate Approximation) model of emerging curvature is capable of telling a plausible albeit radically different story that explains both observational anomalies without dark energy. We demonstrate that while stacked imprints of $R_{\rm v}\gtrsim 100\,\rm {\mathit{ h}^{-1}Mpc }$ supervoids in cosmic microwave background (CMB) temperature maps can discriminate between the AvERA and ΛCDM models, their characteristic differences may remain hidden using alternative void definitions and stacking methodologies. Testing the extremes, we then also show that the CMB Cold Spot can plausibly be explained in the AvERA model as an ISW imprint. The coldest spot in the AvERA map is aligned with multiple low-z supervoids with $R_{\rm v}\gtrsim 100\,\rm {\mathit{ h}^{-1}Mpc }$ and central underdensity δ0 ≈ −0.3, resembling the observed large-scale galaxy density field in the Cold Spot area. We hence conclude that the anomalous imprint of supervoids may well be the canary in the coal mine, and existing observational evidence for dark energy should be reinterpreted to further test alternative models.

Probing kinematics and fate of Bianchi type V Universe

In this paper, we have constrained the cosmological parameters of Bianchi type V universe by bounding the model under consideration with recent observational [Formula: see text] and Pantheon data. It is assumed that the energy conservation law holds separately for barotropic fluid and dark energy fluid i.e. dark energy component does not interact with barotropic matter. We obtain the present value of deceleration parameter and age of universe as [Formula: see text] and 13.329 Gyrs respectively. The analysis of [Formula: see text] and jerk parameters concur that the derived model approaches [Formula: see text]CDM universe at late time. We find that estimated values of Hubble’s constant and energy density parameters have pretty consistency with their corresponding values, obtained by recent observations of WMAP and PLANCK collaborations. It is also observed that the universe shows signature flipping from decelerating to accelerating phase for transitional red-shift value [Formula: see text] in past.