scholarly journals Compact object with a local dark energy shell

2021 ◽  
pp. 2150039
Author(s):  
L. S. M. Veneroni ◽  
A. Braz ◽  
M. F. A. da Silva
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Density Profile ◽  
Compact Object ◽  
Radial Pressure ◽  
Compact Objects ◽  
The Other ◽  
Relativity Theory ◽  
Energy Conditions ◽  
Density Profiles

We investigate some models of compact objects in the general relativity theory with cosmological constant [Formula: see text], based on two density profiles, one of them attributed to Stewart and the other one to Durgapal and Bannerji, proposed in the literature to model “neutron stars”. For them, a nonlocal equation of state with cosmological constant is obtained as a consequence of the chosen metric. In another direction, we obtain a solution for configurations with null radial pressure. The first model (based on Stewart’s density profile) turned out to be the most interesting, since surprisingly it admits the presence of dark energy in the interior of the star, in the outermost layers, for a certain range of mass–radius ratio [Formula: see text]. This dark energy is independent of the cosmological constant, since it is a consequence of the tangential pressure of the fluid be sufficiently negative. Still in this case, for other values of [Formula: see text], all the energy conditions are satisfied. Another advantage of this model, as well as that based on the density profile of Durgapal and Bannerji, is the existence of intervals of [Formula: see text] compatible with physically acceptable models for [Formula: see text], [Formula: see text] and [Formula: see text], which also allowed us to analyze the influence of [Formula: see text] on the behavior of the fluid with respect to the energy conditions. The other configuration studied here, [Formula: see text], only allows solutions for [Formula: see text], in order to ensure a positive mass for the object and to satisfy all the energy conditions in a specific range of [Formula: see text].

The possibility of a stable flat dark energy-dominated Swiss-cheese brane-world universe

2020 ◽  
Vol 17 (05) ◽  
pp. 2050075
Author(s):  
Nasr Ahmed ◽  
Kazuharu Bamba ◽  
F. Salama
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Cosmological Models ◽  
Brane World ◽  
Fine Tuning ◽  
Energy Conditions ◽  
Late Time ◽  
Swiss Cheese ◽  
Black Strings ◽  
The Stability

In this paper, we study the possibility of obtaining a stable flat dark energy-dominated universe in a good agreement with observations in the framework of Swiss-cheese brane-world cosmology. Two different brane-world cosmologies with black strings have been introduced for any cosmological constant [Formula: see text] using two empirical forms of the scale factor. In both models, we have performed a fine-tuning between the brane tension and the cosmological constant so that the Equation of state (EoS) parameter [Formula: see text] for the current epoch, where the redshift [Formula: see text]. We then used these fine–tuned values to calculate and plot all parameters and energy conditions. The deceleration–acceleration cosmic transition is allowed in both models, and the jerk parameter [Formula: see text] at late-times. Both solutions predict a future dark energy-dominated universe in which [Formula: see text] with no crossing to the phantom divide line. While the pressure in the first solution is always negative, the second solution predicts a better behavior of cosmic pressure where the pressure is negative only in the late-time accelerating era but positive in the early-time decelerating era. Such a positive-to-negative transition in the evolution of pressure helps to explain the cosmic deceleration–acceleration transition. Since black strings have been proved to be unstable by some authors, this instability can actually reflect doubts on the stability of cosmological models with black strings (Swiss-cheese type brane-worlds cosmological models). For this reason, we have carefully investigated the stability through energy conditions and sound speed. Because of the presence of quadratic energy terms in Swiss-cheese type brane-world cosmology, we have tested the new nonlinear energy conditions in addition to the classical energy conditions. We have also found that a negative tension brane is not allowed in both models of the current work as the energy density will no longer be well defined.

scholarly journals On the universality of void density profiles

2014 ◽  
Vol 11 (S308) ◽  
pp. 551-554
Author(s):  
E. Ricciardelli ◽  
V. Quilis ◽  
J. Varela
Keyword(s):  
Dark Energy ◽  
Internal Structure ◽  
Density Profile ◽  
Matter Distribution ◽  
Density Profiles ◽  
Massive Galaxies ◽  
The Galaxy ◽  
Galaxy Sample ◽  
Low Mass ◽  
Void Radius

AbstractThe massive exploitation of cosmic voids for precision cosmology in the upcoming dark energy experiments, requires a robust understanding of their internal structure, particularly of their density profile. We show that the void density profile is insensitive to the void radius both in a catalogue of observed voids and in voids from a large cosmological simulation. However, the observed and simulated voids display remarkably different profile shapes, with the former having much steeper profiles than the latter. We ascribe such difference to the dependence of the observed profiles on the galaxy sample used to trace the matter distribution. Samples including low-mass galaxies lead to shallower profiles with respect to the samples where only massive galaxies are used, as faint galaxies live closer to the void centre. We argue that galaxies are biased tracers when used to probe the matter distribution within voids.

Changes in the role of energy conditions in f(R) cosmology by the acceleration of universe

2018 ◽  
Vol 15 (09) ◽  
pp. 1850148
Author(s):  
Alireza Sepehri ◽  
Somayeh Shoorvazi ◽  
Hossein Ghaforyan
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Energy Equation ◽  
The Other ◽  
Energy Conditions ◽  
Big Rip ◽  
Rindler Space ◽  
Phantom Phase ◽  
Two Universes

Recently, the role of energy conditions in [Formula: see text]-cosmology has been investigated. We generalize these results to BIonic systems in accelerating systems and show that the energy conditions can be changed in this system. We show that [Formula: see text]-gravity produces a wormhole between two universes and forms a BIonic system. On the other hand, the acceleration creates two regions in the Rindler space-time which BIon in each region acts reverse to other in another region. This means that by the expansion of universes of the BIon in one region, universes of another BIon in other regions contract. Also, in this model, by increasing the order of curvatures in [Formula: see text]-gravity, the energy and the entropy of system in one region increases and in other region decreases. Amount of this growth or decrease in the energy depends on the acceleration of universes. Finally, by calculating the dark energy equation of state, we observe that one universe enters to phantom phase and goes toward the big rip singularity and another goes out of phantom state.

N-DIMENSIONAL GRAVITATIONAL COLLAPSE WITH DARK ENERGY

2008 ◽  
Vol 17 (08) ◽  
pp. 1295-1309
Author(s):  
R. S. GONÇALVES ◽  
JAIME F. VILLAS DA ROCHA
Keyword(s):  
Dark Energy ◽  
Radial Pressure ◽  
The Self ◽  
Energy Conditions ◽  
Anisotropic Fluid ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Self Similarity ◽  
Naked Singularities ◽  
Self Similar

We study the evolution of an N-dimensional anisotropic fluid with kinematic self-similarity of the second kind and find a class of solutions to the Einstein field equations by assuming an equation of state where the radial pressure of the fluid is proportional to its energy density (pr= ωρ) and that the fluid moves along timelike geodesics. As in the four-dimensional case, the self-similarity requires ω = -1. The energy conditions and geometrical and physical properties of the solutions are studied. We find that, depending on the self-similar parameter α, they may represent black holes or naked singularities. We also study the presence of dark energy in some models, and find that their existence gives rise to some constraints on the dimensions of the space–times.

scholarly journals Conformally symmetric relativistic star

2017 ◽  
Vol 32 (08) ◽  
pp. 1750053 ◽  
Author(s):  
Farook Rahaman ◽  
Sunil D. Maharaj ◽  
Iftikar Hossain Sardar ◽  
Koushik Chakraborty
Keyword(s):  
Conformal Symmetry ◽  
Compact Object ◽  
Radial Pressure ◽  
Compact Stars ◽  
Energy Conditions ◽  
Anisotropic Pressure ◽  
Matter Distribution ◽  
Strong Energy ◽  
Principal Directions ◽  
The Masses

We investigate whether compact stars having Tolman-like interior geometry admit conformal symmetry. Taking anisotropic pressure along the two principal directions within the compact object, we obtain physically relevant quantities such as transverse and radial pressure, density and redshift function. We study the equation of state (EOS) for the matter distribution inside the star. From the relation between pressure and density function of the constituent matter, we explore the nature and properties of the interior matter. The redshift function and compactness parameter are found to be physically reasonable. The matter inside the star satisfies the null, weak and strong energy conditions. Finally, we compare the masses and radii predicted from the model with corresponding values in some observed stars.

scholarly journals ON THE BLACK HOLE INTERIOR SPACETIME

2009 ◽  
Vol 24 (08n09) ◽  
pp. 1593-1597 ◽  
Author(s):  
HRISTU CULETU
Keyword(s):  
Black Hole ◽  
Energy Density ◽  
Cosmological Constant ◽  
Bulk Viscosity ◽  
Radial Pressure ◽  
Time Dependent ◽  
Energy Conditions ◽  
Anisotropic Fluid ◽  
Viscosity Coefficients ◽  
Black Hole Interior

A spacetime endowed with an anisotropic fluid is proposed for the interior of a black hole. The geometry has an instantaneous Minkowski form and is a solution of Einstein's equations with a stress tensor on the r.h.s. obeying all the energy conditions. The interior fluid is compressible, with time dependent shear and bulk viscosity coefficients. The energy density ρ and the "radial" pressure p are proportional to 1/t2, with no pressures on θ- and ϕ- directions. The model leads to a time dependent cosmological constant.

EFFECTS OF RAMPS IN THE NAGEL–SCHRECKENBERG TRAFFIC MODEL

2002 ◽  
Vol 13 (06) ◽  
pp. 739-749 ◽  
Author(s):  
DING-WEI HUANG
Keyword(s):  
Phase Diagram ◽  
Phase Separation ◽  
Density Profile ◽  
The Other ◽  
Traffic Model ◽  
Current Phase ◽  
Stochastic Noise ◽  
Density Profiles ◽  
Traffic Jams ◽  
Maximum Current

We study the effects of on-ramp and off-ramp to the traffic flow in a cellular automaton. The stochastic noise and the extended hopping are included. The phase diagram is obtained. Three distinct phases are observed. The density profiles are analyzed. In the maximum current phase, standing traffic jams emerge in the upstream of the on-ramp. A region of free flow is observed in the downstream of the off-ramp. In between the ramps, a scaled density profile is observed in one branch and a phase separation is observed in the other branch.

Cosmological constant

2018 ◽  
Author(s):  
Michael Kachelriess
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Accelerated Expansion ◽  
Vacuum Fluctuations ◽  
Present Epoch ◽  
Modify Gravity ◽  
Expansion Of The Universe ◽  
The Universe

The contribution of vacuum fluctuations to the cosmological constant is reconsidered studying the dependence on the used regularisation scheme. Then alternative explanations for the observed accelerated expansion of the universe in the present epoch are introduced which either modify gravity or add a new component of matter, dubbed dark energy. The chapter closes with some comments on attempts to quantise gravity.

scholarly journals Investigation of Ring and Star Polymers in Confined Geometries: Theory and Simulations

Entropy ◽  
2021 ◽  
Vol 23 (2) ◽  
pp. 242
Author(s):  
Joanna Halun ◽  
Pawel Karbowniczek ◽  
Piotr Kuterba ◽  
Zoriana Danel
Keyword(s):  
Star Polymers ◽  
Polymer Chains ◽  
The Other ◽  
Nano Particles ◽  
Dilute Solution ◽  
Confined Geometries ◽  
Density Profiles ◽  
Ideal Ring ◽  
Ring Polymer ◽  
Monomer Density

The calculations of the dimensionless layer monomer density profiles for a dilute solution of phantom ideal ring polymer chains and star polymers with f=4 arms in a Θ-solvent confined in a slit geometry of two parallel walls with repulsive surfaces and for the mixed case of one repulsive and the other inert surface were performed. Furthermore, taking into account the Derjaguin approximation, the dimensionless layer monomer density profiles for phantom ideal ring polymer chains and star polymers immersed in a solution of big colloidal particles with different adsorbing or repelling properties with respect to polymers were calculated. The density-force relation for the above-mentioned cases was analyzed, and the universal amplitude ratio B was obtained. Taking into account the small sphere expansion allowed obtaining the monomer density profiles for a dilute solution of phantom ideal ring polymers immersed in a solution of small spherical particles, or nano-particles of finite size, which are much smaller than the polymer size and the other characteristic mesoscopic length of the system. We performed molecular dynamics simulations of a dilute solution of linear, ring, and star-shaped polymers with N=300, 300 (360), and 1201 (4 × 300 + 1-star polymer with four arms) beads accordingly. The obtained analytical and numerical results for phantom ring and star polymers are compared with the results for linear polymer chains in confined geometries.

scholarly journals Testing the effect of H0 on fσ8 tension using a Gaussian Process method

2020 ◽  
Author(s):  
En-Kun Li ◽  
Minghui Du ◽  
Zhi-Huan Zhou ◽  
Hongchao Zhang ◽  
Lixin Xu
Keyword(s):  
Growth Rate ◽  
Dark Energy ◽  
Cosmological Constant ◽  
Gaussian Process ◽  
Dark Energy Model ◽  
Hubble Space Telescope ◽  
Λcdm Model ◽  
Space Distortion ◽  
Best Fit ◽  
Process Method

Abstract Using the fσ8(z) redshift space distortion (RSD) data, the $\sigma _8^0-\Omega _m^0$ tension is studied utilizing a parameterization of growth rate f(z) = Ωm(z)γ. Here, f(z) is derived from the expansion history H(z) which is reconstructed from the observational Hubble data applying the Gaussian Process method. It is found that different priors of H0 have great influences on the evolution curve of H(z) and the constraint of $\sigma _8^0-\Omega _m^0$. When using a larger H0 prior, the low redshifts H(z) deviate significantly from that of the ΛCDM model, which indicates that a dark energy model different from the cosmological constant can help to relax the H0 tension problem. The tension between our best-fit values of $\sigma _8^0-\Omega _m^0$ and that of the Planck 2018 ΛCDM (PLA) will disappear (less than 1σ) when taking a prior for H0 obtained from PLA. Moreover, the tension exceeds 2σ level when applying the prior H0 = 73.52 ± 1.62 km/s/Mpc resulted from the Hubble Space Telescope photometry. By comparing the $S_8 -\Omega _m^0$ planes of our method with the results from KV450+DES-Y1, we find that using our method and applying the RSD data may be helpful to break the parameter degeneracies.

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