scholarly journals Cosmological Models with Two Fluids. I. Robertson?Walker Metric

1972 ◽  
Vol 25 (1) ◽  
pp. 75 ◽  
Author(s):  
CBG McIntosh
Keyword(s):  
Equation Of State ◽  
Cosmological Constant ◽  
Cosmological Models ◽  
Fluid Models ◽  
Elementary Functions ◽  
Anisotropic Models ◽  
Two Fluids ◽  
All Solutions ◽  
Walker Metric ◽  
Two Fluid

Exact solutions in terms of elementary functions are given for flat, homogeneous and isotropic, relativistic cosmological models which contain two fluids, each with an equation of state of the form where p is the pressure, ? is the density, is a constant, and For other forms of v1/v2, the relevant solution is given in terms of a hypergeometric function. The cases when one of the v?s is equal to 2/3 or 2 are analogous to models with a Robertson?Walker metric with k = � 1 and to anisotropic models of the type discussed by Jacobs respectively. All solutions for the two-fluid models can be written in terms of elementary functions when v1 = 0, which is analogous to a cosmological constant. The fact that all two-fluid solutions with v1 = 2/3 which can be written in terms of elementary functions are given means that all such one-fluid solutions with k = � 1 are given.

scholarly journals Cosmological Models with Two Fluids. II. Conformal and Conformally Flat Metrics

1972 ◽  
Vol 25 (1) ◽  
pp. 83 ◽  
Author(s):  
CBG McIntosh ◽  
JM Foyster
Keyword(s):  
Cosmological Models ◽  
Conformally Flat ◽  
Elementary Functions ◽  
Two Fluids ◽  
Usual Form ◽  
Walker Metric ◽  
Similar Solutions ◽  
Conformally Flat Metrics ◽  
Two Fluid

Similar solutions to those in Part I are given for two-fluid cosmological models when the Robertson?Walker metric in its usual form is replaced by "conformal" and "conformally flat" forms. In these cases the solutions can be written in terms of elementary functions when (3v1?2)/(3v2?2) = 1?m?1, m = 1,2,3,... The relation of these solutions to one-fluid solutions with k = � 1 is also discussed.

scholarly journals MODEL MATERI GELAP DUA FLUIDA STATIS DENGAN TAMBAHAN KONSTANTA KOSMOLOGI

2018 ◽  
Vol 3 (2) ◽  
pp. 127-132
Author(s):  
Izrul Supriyadi ◽  
Widya Sawitar ◽  
Esmar Budi ◽  
Riser Fahdiran
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Cosmological Constant ◽  
Tangential Velocity ◽  
Momentum Tensor ◽  
Geodesic Equation ◽  
Fluid Models ◽  
Characteristic Energy ◽  
Expansion Model ◽  
Two Fluid

Abstrak Pada persamaan medan gravitasi Einstein terdapat konstanta kosmologi sebagai konstanta alam yang menjelaskan model mengembangnya alam semesta dan yang paling dominan terdapat di jagad raya ini adalah dalam bentuk energi gelap (dark energy). Kami meninjau model objek dua fluida tidak terkopel, seperti layaknya materi gelap (dark matter) atau bintang yang memiliki karakteristik tensor energi-momentum dan kecepatan-4 nya yang berbeda serta bersifat anisotropik, kemudian disatukan sebagai model dua fluida untuk ditinjau persamaan TOV (Tolman-Oppenheimer-Volkoff) dan persamaan geodesiknya dalam menunjukkan sifat gerak dan model dua fluida tersebut. Hasil perhitungan menunjukkan bahwa model ini dapat menjelaskan persamaan potensial efektif dengan tambahan konstanta kosmologi sebagai karakteristik gerak dan kecepatan tangensial partikel uji dalam orbit lingkaran stabil. Kata-kata kunci: konstanta kosmologi, anisotropik, potensial efektif, kecepatan tangensial. Abstract In Einstein's gravitational field equation has been found the cosmological constant as the natural constant that describes the universe's expansion model and the most dominant in the universe is the dark energy form. We review the two objects of fluid models are not coupled, like dark matter or stars which has the different characteristic energy-momentum tensor and four velocities and anisotropic tend, then combined as two-fluid models for TOV (Tolman-Oppenheimer-Volkoff) equation and the geodesic equation to characterize the movement and the two fluid models. The calculation result shows that this model can explain the potential equation with an addition of an effective cosmological constant as the movement characteristic and tangential velocity of a tested particle in a stable circular orbit. Keywords: cosmological constant, anisotropic, effective potential, tangential velocity.

scholarly journals SCALAR FIELD COSMOLOGIES WITH PERFECT FLUID IN ROBERTSON-WALKER METRIC

1996 ◽  
Vol 05 (01) ◽  
pp. 71-84 ◽  
Author(s):  
LUIS P. CHIMENTO ◽  
ALEJANDRO S. JAKUBI
Keyword(s):  
Scalar Field ◽  
Cosmological Constant ◽  
Perfect Fluid ◽  
Cosmological Models ◽  
Asymptotically Stable ◽  
Fluid Source ◽  
Homogeneous Cosmological Models ◽  
Walker Metric ◽  
Stable Solutions

Several isotropic, homogeneous cosmological models containing a self-interacting minimally coupled scalar field, a perfect fluid source and cosmological constant are solved. New exact, asymptotically stable solutions with an inflationary regime or a final Friedmann stage are found for some simple, interesting potentials. It is shown that the fluid and the curvature may determine how these models evolve for large times.

scholarly journals The Influence of Evolving Dark Energy on Cosmology

2005 ◽  
Vol 22 (4) ◽  
pp. 315-325 ◽  
Author(s):  
Luke Barnes ◽  
Matthew J. Francis ◽  
Geraint F. Lewis ◽  
Eric V. Linder
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Equation Of State ◽  
Cosmological Constant ◽  
Energy Exchange ◽  
Observational Evidence ◽  
Cosmological Models ◽  
Observational Constraints ◽  
Expansion Of The Universe ◽  
The Universe

AbstractObservational evidence indicating that the expansion of the universe is accelerating has surprised cosmologists in recent years. Cosmological models have sought to explain this acceleration by incorporating ‘dark energy’, of which the traditional cosmological constant is just one possible candidate. Several cosmological models involving an evolving equation of state of the dark energy have been proposed, as well as possible energy exchange to other components, such as dark matter. This paper summarizes the forms of the most prominent models and discusses their implications for cosmology and astrophysics. Finally, this paper examines the current and future observational constraints on the nature of dark energy.

Transit cosmological models in FRW universe under the two-fluid scenario

2019 ◽  
Vol 16 (01) ◽  
pp. 1950007 ◽  
Author(s):  
Pryanka Garg ◽  
Rashid Zia ◽  
Anirudh Pradhan
Keyword(s):  
Cosmological Models ◽  
Observational Cosmology ◽  
Accelerating Universe ◽  
Frw Universe ◽  
Two Fluids ◽  
Slowing Down ◽  
Two Fluid Scenario ◽  
The Universe ◽  
Two Fluid ◽  
Friedmann Robertson Walker

This paper is an attempt to revisit the Friedmann–Robertson–Walker (FRW) cosmological models under the new scenario of observational cosmology, which has established that the current universe is expanding with an increasing rate, in contrast to the earlier belief that the rate of expansion is constant or slowing down. This paper represents a model which encompasses both, earlier decelerating and the current accelerating universe, passing through a transition phase. The universe is assumed to be filled with two fluids, barotropic and dark energy. We have considered two cases; first, when these fluids are assumed to be non-interacting and second, when they interact with each other. Some physical, kinematic and geometric properties of the model are also discussed along with the acceptability and stability of the solution. The results found are very compatible with the established results as well as recent observations.

scholarly journals The I-Love-Q Relations for Superfluid Neutron Stars

Universe ◽  
2021 ◽  
Vol 7 (4) ◽  
pp. 111
Author(s):  
Cheung-Hei Yeung ◽  
Lap-Ming Lin ◽  
Nils Andersson ◽  
Greg Comer
Keyword(s):  
Fluid Dynamics ◽  
Equation Of State ◽  
Neutron Stars ◽  
Quadrupole Moment ◽  
Fluid Model ◽  
Normal Component ◽  
Approximate Equation ◽  
Mean Field Model ◽  
Polytropic Model ◽  
Two Fluid

The I-Love-Q relations are approximate equation-of-state independent relations that connect the moment of inertia, the spin-induced quadrupole moment, and the tidal deformability of neutron stars. In this paper, we study the I-Love-Q relations for superfluid neutron stars for a general relativistic two-fluid model: one fluid being the neutron superfluid and the other a conglomerate of all charged components. We study to what extent the two-fluid dynamics might affect the robustness of the I-Love-Q relations by using a simple two-component polytropic model and a relativistic mean field model with entrainment for the equation-of-state. Our results depend crucially on the spin ratio Ωn/Ωp between the angular velocities of the neutron superfluid and the normal component. We find that the I-Love-Q relations can still be satisfied to high accuracy for superfluid neutron stars as long as the two fluids are nearly co-rotating Ωn/Ωp≈1. However, the deviations from the I-Love-Q relations increase as the spin ratio deviates from unity. In particular, the deviation of the Q-Love relation can be as large as O(10%) if Ωn/Ωp differ from unity by a few tens of percent. As Ωn/Ωp≈1 is expected for realistic neutron stars, our results suggest that the two-fluid dynamics should not affect the accuracy of any gravitational waveform models for neutron star binaries that employ the relation to connect the spin-induced quadrupole moment and the tidal deformability.

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 New classes of anisotropic models with generalized polytropic equation of state

2018 ◽  
Vol 78 (6) ◽  
Author(s):  
S. A. Mardan ◽  
I. Noureen ◽  
M. Azam ◽  
M. A. Rehman ◽  
M. Hussan
Keyword(s):  
Equation Of State ◽  
Anisotropic Models ◽  
Polytropic Equation
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