Статті в журналах: "Friedmann-Robertson-Walker cosmologies"

1

Visser, Matt. "Conformally Friedmann–Lemaître–Robertson–Walker cosmologies." Classical and Quantum Gravity 32, no. 13 (June 4, 2015): 135007. http://dx.doi.org/10.1088/0264-9381/32/13/135007.

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2

Bikwa, Ojeh, Fulvio Melia, and Andrew Shevchuk. "Photon geodesics in Friedmann-Robertson-Walker cosmologies." Monthly Notices of the Royal Astronomical Society 421, no. 4 (February 21, 2012): 3356–61. http://dx.doi.org/10.1111/j.1365-2966.2012.20560.x.

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3

Esposito, Giampiero, Gennaro Miele, Luigi Rosa, and Pietro Santorelli. "Quantum effects in Friedmann - Robertson - Walker cosmologies." Classical and Quantum Gravity 12, no. 12 (December 1, 1995): 2995–3005. http://dx.doi.org/10.1088/0264-9381/12/12/015.

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4

Mak, M. K., and T. Harko. "Exact causal bulk viscous stiff cosmologies." Australian Journal of Physics 53, no. 2 (2000): 241. http://dx.doi.org/10.1071/ph99067.

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Анотація:

An exact solution of the gravitational field equations is presented for a homogeneous flat Friedmann-Robertson-Walker universe filled with a causal bulk viscous fluid obeying the Zeldovich stiff equation of state and having bulk viscosity coefficient proportional to the fourth root of the energy density.

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5

Dyer, Charles C., and Chris Oliwa. "Matching of Friedmann-Lemaître-Robertson-Walker and Kasner cosmologies." Classical and Quantum Gravity 18, no. 14 (June 29, 2001): 2719–29. http://dx.doi.org/10.1088/0264-9381/18/14/309.

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6

Dyer, Charles C., Sylvie Landry, and Eric G. Shaver. "Matching of Friedmann-Lemaître-Robertson-Walker and Kasner cosmologies." Physical Review D 47, no. 4 (February 15, 1993): 1404–6. http://dx.doi.org/10.1103/physrevd.47.1404.

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7

Melia, F. "The lapse function in Friedmann—Lemaître–Robertson–Walker cosmologies." Annals of Physics 411 (December 2019): 167997. http://dx.doi.org/10.1016/j.aop.2019.167997.

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8

Singh, T., R. Chaubey, and Ashutosh Singh. "Bouncing cosmologies in Brans–Dicke theory." Canadian Journal of Physics 94, no. 7 (July 2016): 623–27. http://dx.doi.org/10.1139/cjp-2016-0081.

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In this paper it is shown that in Brans–Dicke theory, if one considers a non-minimal coupling between the matter and scalar field, can give rise to a bouncing universe (i.e., an expanding universe preceded by a contracting universe). Two examples of such universes have been considered in a spatially flat Friedmann–Robertson–Walker universe and their physical properties have been studied.

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9

MAIER, R., I. DAMIÃO SOARES, and E. V. TONINI. "NONLINEAR RESONANCE IN BRANEWORLD COSMOLOGIES." International Journal of Modern Physics A 24, no. 08n09 (April 10, 2009): 1510–13. http://dx.doi.org/10.1142/s0217751x09044905.

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The phase space dynamics of preinflationary closed Friedmann-Robertson-Walker (FRW) models is examined in a string-inspired bulk-brane scenario. Friedmann equations on the brane contain correction terms arising from the brane-bulk interaction that implement nonsingular metastable bounces in the early evolution of the universe. The matter content of the model is a massive inflaton field plus dust and/or radiation. In narrow windows of the parameter space, labeled by an integer n ≥ 2, nonlinear resonance phenomena destroy KAM tori that trap the inflaton, leading to a graceful entrance to inflation. As a consequence nonlinear resonance imposes constraints on the physical parameters, and in the initial configurations of the models so that inflation may be realized.

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10

Forte, Mónica. "Kinematic equivalence between models driven by DBI field with constant γ and exotic holographic quintessence cosmological models". Modern Physics Letters A 32, № 26 (11 серпня 2017): 1750140. http://dx.doi.org/10.1142/s0217732317501401.

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We show the kinematic equivalence between cosmological models driven by Dirac–Born–Infeld (DBI) fields [Formula: see text] with constant proper velocity of the brane and exponential potential [Formula: see text], and interactive cosmological systems with modified holographic Ricci type fluids as dark energy (DE) in flat Friedmann–Robertson–Walker (FRW) cosmologies.

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11

Coley, A. A., and B. O. J. Tupper. "Two-fluid Friedmann–Robertson–Walker cosmologies and their numerical predictions." Canadian Journal of Physics 64, no. 2 (February 1, 1986): 204–9. http://dx.doi.org/10.1139/p86-036.

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Friedmann–Robertson–Walker models satisfying the Einstein field equations for a combination of two fluids, one of which is a comoving perfect fluid with the radiation equation of state [Formula: see text], representing the cosmic microwave background, are discussed. Existing models, in which the second fluid is a comoving perfect fluid, are reviewed and their numerical predictions calculated. These models are generalized by considering the case in which the second fluid is an imperfect fluid. This fluid is necessarily noncomoving, the tilt representing the motion of the local supercluster of galaxies relative to the cosmic microwave background. The numerical predictions of one such model are calculated and are found to be in excellent agreement with observation.

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12

CATALDO, MAURICIO, and LUIS P. CHIMENTO. "FORM INVARIANT TRANSFORMATIONS BETWEEN n- AND m-DIMENSIONAL FLAT FRIEDMANN–ROBERTSON–WALKER COSMOLOGIES." International Journal of Modern Physics D 17, no. 11 (October 2008): 1981–89. http://dx.doi.org/10.1142/s0218271808013790.

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We illustrate how a group of symmetry transformations, which preserve the form of the n-dimensional flat Friedmann–Robertson–Walker cosmologies satisfying Einstein's equations, act in any dimension. This group relates the energy density and the isotropic pressure of the cosmic fluid to the expansion rate. The freedom associated with the dimension of the space–time yields assisted inflation even when the energy density of the fluid is a dimensional invariant and enriches the set of duality transformations leading to phantom cosmologies.

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13

Yılmaz, Nejat Tevfik. "Effective fluid FLRW cosmologies of minimal massive gravity." Modern Physics Letters A 30, no. 18 (May 25, 2015): 1550087. http://dx.doi.org/10.1142/s021773231550087x.

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By using a solution ansatz we partially decouple the metric and the Stückelberg sectors of the minimal massive gravity (MMGR). In this scheme for a diagonal physical metric we find the general solutions for the scalars of the theory and the particular fiducial (background) metric which leads to these solutions. Then we adopt this general formalism to construct the derivation of new Friedmann–Lemaitre–Robertson–Walker (FLRW) cosmologies of the theory in the presence of a so-called effective ideal fluid which arises from our solution ansatz as a modifying, non-physical source for the Einstein and the corresponding Friedmann equations.

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14

Singh, T., R. Chaubey, and Ashutosh Singh. "k-essence cosmologies in Kantowski–Sachs and Bianchi space–times." Canadian Journal of Physics 93, no. 11 (November 2015): 1319–23. http://dx.doi.org/10.1139/cjp-2015-0001.

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We investigate Kantowski–Sachs, locally rotationally symmetric Bianchi-I and Bianchi-III cosmology with k-essence and found a set of models that dissipate the initial anisotropy. We obtain the conditions leading to a regular bounce of the average geometry. We show that the linear k-field and polynomial kinetic function models evolve asymptotically to Friedmann–Robertson–Walker cosmologies. For linear k-essence we find the general solution in these cosmologies, when the k-field is driven by an inverse scalar potential.

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15

Li, Jibin, and Tonghua Zhang. "Bifurcations and Exact Solutions of the Equation of Barotropic FRW Cosmologies." International Journal of Bifurcation and Chaos 27, no. 05 (May 2017): 1750080. http://dx.doi.org/10.1142/s0218127417500808.

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In this paper, we study the equation of barotropic Friedmann–Robertson–Walker cosmologies. By using the method of dynamical systems, we obtain bifurcations of the phase portraits of the corresponding planar dynamical system. Corresponding to different level curves, we derive exact explicit parametric representations of bounded and unbounded solutions, such as periodic solutions, periodic peakon solutions, homoclinic and heteroclinic solutions and compacton solutions.

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16

PIMENTEL, LUIS O., and LUZ M. DIAZ-RIVERA. "COASTING COSMOLOGIES WITH TIME DEPENDENT COSMOLOGICAL CONSTANT." International Journal of Modern Physics A 14, no. 10 (April 20, 1999): 1523–29. http://dx.doi.org/10.1142/s0217751x99000762.

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The effect of a time dependent cosmological constant is considered in a family of scalar-tensor theories. Friedmann–Robertson–Walker cosmological models for vacuum and perfect fluid matter are found. They have a linear expansion factor, the so-called coasting cosmology, the gravitational "constant" decreases inversely with time; that is these models satisfy the Dirac Hypotheses. The cosmological "constant" decreases inversely with the square of time, therefore we can have a very small value for it at present time.

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17

Barzegar, Hamed, and David Fajman. "Stable cosmologies with collisionless charged matter." Journal of Hyperbolic Differential Equations 19, no. 04 (December 2022): 587–634. http://dx.doi.org/10.1142/s0219891622500175.

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It is shown that Milne models (a subclass of Friedmann–Lematre–Robertson–Walker (FLRW) spacetimes with negative spatial curvature) are nonlinearly stable in the set of solutions to the Einstein–Vlasov–Maxwell system, describing universes with ensembles of collisionless self-gravitating, charged particles. The system contains various slowly decaying borderline terms in the mutually coupled equations describing the propagation of particles and Maxwell fields. The effects of those terms are controlled using a suitable hierarchy based on the energy density of the matter fields.

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18

Chaubey, R. "General class of anisotropic cosmological models with dilaton and magnetic fields." Canadian Journal of Physics 92, no. 4 (April 2014): 289–94. http://dx.doi.org/10.1139/cjp-2013-0510.

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The Einstein–Maxwell dilaton field equations are considered for four-dimensional anisotropic Bianchi models. The general solutions have been obtained and their properties have been discussed. The exact solutions to the corresponding field equations are obtained for two different physical viable cosmologies. The cosmological parameters have been discussed in detail and it is also shown that the solutions tend asymptotically to isotropic Friedmann–Robertson–Walker cosmological model.

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19

MAK, M. K., and T. HARKO. "FULL CAUSAL DISSIPATIVE COSMOLOGIES WITH STIFF MATTER." International Journal of Modern Physics D 13, no. 02 (February 2004): 273–80. http://dx.doi.org/10.1142/s0218271804004645.

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Анотація:

The general solution of the gravitational field equations for a full causal bulk viscous stiff cosmological fluid, with bulk viscosity coefficient proportional to the energy density to the power 1/4, is obtained in the flat Friedmann–Robertson–Walker geometry. The solution describes a non-inflationary Universe, which starts its evolution from a singular state. The time variation of the scale factor, deceleration parameter, viscous pressure, viscous pressure-thermodynamic pressure ratio, co-moving entropy and Ricci and Kretschmann invariants is considered in detail.

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20

Harko, T., and M. K. Mak. "Iterative solutions for relativistic dissipative cosmologies." Canadian Journal of Physics 80, no. 5 (May 1, 2002): 591–98. http://dx.doi.org/10.1139/p01-148.

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Iterative solutions of the gravitational field equations for a homogeneous flat FriedmannRobertsonWalker Universe filled with a causal bulk viscous fluid in the framework of the full IsraelStewartHiscock theory are presented. The general solution of the field equations are presented in a parametric form in the zeroth-, first-, second-, and mth-order approximation by the method of iteration. The time evolution of the scale factor, energy density, deceleration parameter, entropy, Kretschmann scalar, and bulk viscous pressure-thermodynamic pressure ratio for a Zeldovich fluid-filled space-time are discussed. PACS Nos.: 98.80-k, 04.20Jb

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21

Holden, Damien J., and David Wands. "Phase-plane analysis of Friedmann-Robertson-Walker cosmologies in Brans-Dicke gravity." Classical and Quantum Gravity 15, no. 10 (October 1, 1998): 3271–90. http://dx.doi.org/10.1088/0264-9381/15/10/027.

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22

Rubio, Carlos A., Felipe A. Asenjo, and Sergio A. Hojman. "Quantum Cosmologies Under Geometrical Unification of Gravity and Dark Energy." Symmetry 11, no. 7 (July 2, 2019): 860. http://dx.doi.org/10.3390/sym11070860.

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A Friedmann–Robertson–Walker Universe was studied with a dark energy component represented by a quintessence field. The Lagrangian for this system, hereafter called the Friedmann–Robertson–Walker–quintessence (FRWq) system, was presented. It was shown that the classical Lagrangian reproduces the usual two (second order) dynamical equations for the radius of the Universe and for the quintessence scalar field, as well as a (first order) constraint equation. Our approach naturally unified gravity and dark energy, as it was obtained that the Lagrangian and the equations of motion are those of a relativistic particle moving on a two-dimensional, conformally flat spacetime. The conformal metric factor was related to the dark energy scalar field potential. We proceeded to quantize the system in three different schemes. First, we assumed the Universe was a spinless particle (as it is common in literature), obtaining a quantum theory for a Universe described by the Klein–Gordon equation. Second, we pushed the quantization scheme further, assuming the Universe as a Dirac particle, and therefore constructing its corresponding Dirac and Majorana theories. With the different theories, we calculated the expected values for the scale factor of the Universe. They depend on the type of quantization scheme used. The differences between the Dirac and Majorana schemes are highlighted here. The implications of the different quantization procedures are discussed. Finally, the possible consequences for a multiverse theory of the Dirac and Majorana quantized Universe are briefly considered.

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23

MELIA, FULVIO, and MAJD ABDELQADER. "THE COSMOLOGICAL SPACETIME." International Journal of Modern Physics D 18, no. 12 (November 30, 2009): 1889–901. http://dx.doi.org/10.1142/s0218271809015746.

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We present here the transformations required to recast the Robertson–Walker metric and Friedmann–Robertson–Walker equations in terms of observer-dependent coordinates for several commonly assumed cosmologies. The overriding motivation is the derivation of explicit expressions for the radius Rh of our cosmic horizon in terms of measurable quantities for each of the cases we consider. We show that the cosmological time dt diverges for any finite interval ds associated with a process at R → Rh, which therefore represents a physical limit to our observations. This is a key component required for a complete interpretation of the data, particularly as they pertain to the nature of dark energy. With these results, we affirm the conclusion drawn in our earlier work that the identification of dark energy as a cosmological constant does not appear to be consistent with the data.

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24

NASCIMENTO, RICARDO B., CALISTRATO S. CÂMARA, and JOEL C. CARVALHO. "HOW MATTER IS CREATED IN VSL THEORY?" International Journal of Modern Physics D 13, no. 07 (August 2004): 1437–40. http://dx.doi.org/10.1142/s0218271804005651.

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In this work we investigate matter creation in the context of two types of varying speed of light (VSL) cosmologies. We write the energy conservation law arising from Einstein equations for a Friedmann–Robertson–Walker (FRW) line element in a flat universe, solve the field equations and study how particles are created as c changes with cosmic epoch. We calculate the "adiabatic" particle creation rate, the total number of particle as a function of time and find the constrains imposed by the second law of thermodynamics upon the models.

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25

FIORINI, FRANCO, and RAFAEL FERRARO. "COSMOLOGICAL FRAMES FOR THEORIES WITH ABSOLUTE PARALLELISM." International Journal of Modern Physics: Conference Series 03 (January 2011): 227–37. http://dx.doi.org/10.1142/s2010194511001310.

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The vierbein (tetrad) fields for closed and open Friedmann-Robertson-Walker cosmologies are hard to work out in most of the theories featuring absolute parallelism. The difficulty is traced in the fact that these theories are not invariant under local Lorentz transformations of the vierbein. We illustrate this issue in the framework of f(T) theories and Born-Infeld determinantal gravity. In particular, we show that the early Universe as described by the Born-Infeld scheme is singularity free and naturally inflationary as a consequence of the very nature of Born-Infeld gravitational action.

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26

Abebe, Amare, Peter K. S. Dunsby, and Deon Solomons. "Integrability conditions of quasi-Newtonian cosmologies in modified gravity." International Journal of Modern Physics D 26, no. 06 (November 22, 2016): 1750054. http://dx.doi.org/10.1142/s0218271817500547.

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We investigate the integrability conditions of a class of shear-free perfect-fluid cosmological models within the framework of anisotropic fluid sources, applying our results to [Formula: see text] dark energy models. Generalizing earlier general relativistic results for timelike geodesics, we extend the potential and acceleration terms of the quasi-Newtonian formulation of integrable dust cosmological models about a linearized Friedmann–Lemaître–Robertson–Walker background and derive the equations that describe their dynamical evolutions. We show that in general, models with an anisotropic fluid source are not consistent, but because of the particular form the anisotropic stress [Formula: see text] takes in [Formula: see text] gravity, the general integrability conditions in this case are satisfied.

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27

Debnath, Partha Sarathi. "Observational constraints of bulk viscous Friedmann–Robertson–Walker cosmology with hybrid expansion law." International Journal of Modern Physics A 35, no. 27 (September 30, 2020): 2050173. http://dx.doi.org/10.1142/s0217751x20501730.

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In this work, we study bulk viscous Friedmann–Robertson–Walker cosmologies with hybrid expansion law. The bulk viscous theory of dissipative effects described by Eckart theory, truncated Israel–Stewart theory and full Israel–Stewart theory are implemented here. The hybrid expansion law model of scale factor is a general analytic type of evolution from which one can recover power-law and exponential expansion as a special cases. Hybrid expansion law model are applied to describe the present accelerating phase and early phases of evolution. We have determined the cosmological parameters such as Hubble parameter, deceleration parameter, jerk parameter, energy density, bulk viscous pressure and coefficient of bulk viscosity of the universe to construct physically acceptable cosmological model. We have considered both flat and closed space–time of Friedmann–Robertson–Walker cosmology to implement hybrid expansion law with dissipative effect. The variations of the coefficient of bulk viscosity with cosmic evolution are studied here in Eckart, truncated and full Israel–Stewart theory for flat and closed space–time to obtain physically acceptable hybrid expansion models driven by viscosity. We have also estimated observational constraints of the parameters of hybrid expansion law model by considering recent observational data set. We further reveal possible connections of our models with [Formula: see text] tension problem.

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28

Saharian, A. A., and A. L. Mkhitaryan. "Vacuum fluctuations and topological Casimir effect in Friedmann–Robertson–Walker cosmologies with compact dimensions." European Physical Journal C 66, no. 1-2 (January 29, 2010): 295–306. http://dx.doi.org/10.1140/epjc/s10052-010-1247-0.

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29

BELINCHÓN, JOSÉ ANTONIO, and INDRAJIT CHAKRABARTY. "FULL CAUSAL BULK VISCOUS COSMOLOGIES WITH TIME-VARYING CONSTANTS." International Journal of Modern Physics D 12, no. 05 (May 2003): 861–83. http://dx.doi.org/10.1142/s0218271803003402.

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Анотація:

We study the evolution of a flat Friedmann–Robertson–Walker Universe, filled with a bulk viscous cosmological fluid, in the presence of time varying "constants." The dimensional analysis of the model suggests a proportionality between the bulk viscous pressure of the dissipative fluid and the energy density. Using this assumption and with the choice of the standard equations of state for the bulk viscosity coefficient, temperature and relaxation time, the general solution of the field equations can be obtained, with all physical parameters having a power-law time dependence. The symmetry analysis of this model, performed using Lie group techniques, confirms the uniqueness of the solution for this functional form of the bulk viscous pressure. In order to find another possible solution we relax the hypotheses and assume a concrete functional dependence for the "constants."

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30

Basilakos, Spyros. "Cosmic expansion and structure formation in running vacuum cosmologies." Modern Physics Letters A 30, no. 22 (July 3, 2015): 1540031. http://dx.doi.org/10.1142/s0217732315400313.

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We investigate the dynamics of the Friedmann–Lemaître–Robertson–Walker (FLRW) flat cosmological models in which the vacuum energy varies with redshift. A particularly well-motivated model of this type is the so-called quantum field vacuum, in which both kind of terms [Formula: see text] and constant appear in the effective dark energy (DE) density affecting the evolution of the main cosmological functions at the background and perturbation levels. Specifically, it turns out that the functional form of the quantum vacuum endows the vacuum energy of a mild dynamical evolution which could be observed nowadays and appears as dynamical DE. Interestingly, the low-energy behavior is very close to the usual Lambda cold dark matter (ΛCDM) model, but it is by no means identical. Finally, within the framework of the quantum field vacuum we generalize the large scale structure properties, namely growth of matter perturbations, cluster number counts and spherical collapse model.

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31

Angit, S., Rakesh Raushan, and R. Chaubey. "Universe with wet dark fluid: A dynamical systems approach." International Journal of Geometric Methods in Modern Physics 16, no. 08 (August 2019): 1950127. http://dx.doi.org/10.1142/s0219887819501275.

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Анотація:

We investigate the dynamical systems analysis of Friedmann–Robertson–Walker (FRW) cosmological model with wet dark fluid. A new equation of state for the dark energy component of the Universe has been used. It is modeled by the equation of state [Formula: see text], which can describe a liquid, e.g. water. To analyze the evolution equations, we have introduced suitable transformation of variables. The evolution of corresponding solutions is presented by curves in the phase–plane diagram. The nature of critical points are analyzed and stable attractors are examined for our cosmological model. We determine the classical stabilities of these cosmologies. We also examine the transition of early decelerating stage of the Universe to present the accelerating stage of the Universe.

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32

DOMAZET, SILVIJE, VOJA RADOVANOVIĆ, MARKO SIMONOVIĆ, and HRVOJE ŠTEFANČIĆ. "ON ANALYTICAL SOLUTIONS OF f(R) MODIFIED GRAVITY THEORIES IN FLRW COSMOLOGIES." International Journal of Modern Physics D 22, no. 02 (February 2013): 1350006. http://dx.doi.org/10.1142/s0218271813500065.

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Анотація:

A novel analytical method for f(R) modified theories without matter in Friedmann–Lemaitre–Robertson–Walker (FLRW) spacetimes is introduced. The equation of motion for the scale factor in terms of cosmic time is reduced to the equation for the evolution of the Ricci scalar R with the Hubble parameter H. The solution of equation of motion for actions of the form of power law in Ricci scalar R is presented with a detailed elaboration of the action quadratic in R. The reverse use of the introduced method is exemplified in finding functional forms f(R), which leads to specified scale factor functions. The analytical solutions are corroborated by numerical calculations with excellent agreement. Possible further applications to the phases of inflationary expansion and late-time acceleration as well as f(R) theories with radiation are outlined.

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33

Fermi, Davide, Massimo Gengo, and Livio Pizzocchero. "On the Necessity of Phantom Fields for Solving the Horizon Problem in Scalar Cosmologies." Universe 5, no. 3 (March 11, 2019): 76. http://dx.doi.org/10.3390/universe5030076.

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Анотація:

We discuss the particle horizon problem in the framework of spatially homogeneous and isotropic scalar cosmologies. To this purpose we consider a Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime with possibly non-zero spatial sectional curvature (and arbitrary dimension), and assume that the content of the universe is a family of perfect fluids, plus a scalar field that can be a quintessence or a phantom (depending on the sign of the kinetic part in its action functional). We show that the occurrence of a particle horizon is unavoidable if the field is a quintessence, the spatial curvature is non-positive and the usual energy conditions are fulfilled by the perfect fluids. As a partial converse, we present three solvable models where a phantom is present in addition to a perfect fluid, and no particle horizon appears.

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34

Imankul, M. P., and Sh R. Myrzakul. "f (R) HO’RAVA-LIFSHITZ COSMOLOGIES VIA NOETER’S SYMMETRIES." BULLETIN Series of Physics & Mathematical Sciences 70, no. 2 (June 30, 2020): 177–82. http://dx.doi.org/10.51889/2020-2.1728-7901.27.

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Анотація:

Wе іnvеstіgаtе thе gеnеrаl аpprоаch tо іndіng еxаct cоsmоlоgіcаl sоlutіоns in f (R) Hо’rаvа-Lіfshіtz grаvіtу, bаsеd оn Nоеthеrs thеоrеm. А fеаturе оf thіs аpprоаch іs thаt іt usеs thе bеhаvіоr оf аn еﬀеctіvе Lаgrаngіаn undеr іnіnіtеsіmаl trаnsfоrmаtіоns оf thе dеsіrеd sуmmеtrу, еxplіcіtlу dеtеrmіnіng thе fоrm f (R) fоr whіch such sуmmеtrіеs еxіst. Іt іs shоwn thаt thе dуnаmіcs оf thе scаlе fаctоr chаngеs аccоrdіng tо еіthеr а еxpоnеntіаl functіоn оf tіmе оr tо а pоwеr lаw. f-essence is one of generalized fermion fields. In this work, the dynamics of f-essence with a viscous fluid in the flat Friedmann-Robertson-Walker universe are studied. In addition, we analyzed various types of viscous fluids and investigated the possibility of reproducing the current accelerating expansion of the Universe. The cosmological parameters of this model are obtained. It shows that accelerated expansion can also be determined with a viscous fluid of the fermion field, as in the case of non-interacting perfect fluid considered in most modern models of accelerated expansion of the Universe.

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35

CHIMENTO, LUIS P., MARTÍN G. RICHARTE, and IVÁN E. SÁNCHEZ. "FORM INVARIANCE SYMMETRY GENERATES A LARGE SET OF FRW COSMOLOGIES." Modern Physics Letters A 28, no. 04 (January 29, 2013): 1250236. http://dx.doi.org/10.1142/s0217732312502367.

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We show that Einstein's field equations for spatially flat Friedmann–Robertson–Walker (FRW) spacetimes have a form invariance symmetry (FIS) realized by the form invariance transformations (FIT) which are indeed generated by an invertible function of the source energy density. These transformations act on the Hubble expansion rate, the energy density and pressure of the cosmic fluid; likewise such transformations are endowed with a Lie group structure. Each representation of this group is associated with a particular fluid and consequently a determined cosmology, so that, the FIS defines a set of equivalent cosmological models. We focus our seek in the FIT generated by a linear function because it provides a natural framework to express the duality and also produces large sets of cosmologies, starting from a seed one, in several contexts as for instance in the cases of a perfect fluid source and a scalar field driven by a potential depending linearly on the scalar field kinetic energy density.

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36

Martín-Benito, Mercedes, Daniel Martín-de Blas, and Guillermo A. Mena Marugán. "Approximation methods in loop quantum cosmology: from Gowdy cosmologies to inhomogeneous models in Friedmann–Robertson–Walker geometries." Classical and Quantum Gravity 31, no. 7 (March 20, 2014): 075022. http://dx.doi.org/10.1088/0264-9381/31/7/075022.

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37

MYUNG, Y. S. "STANDARD COSMOLOGY FROM THE BRANE COSMOLOGY WITH A LOCALIZED MATTER." Modern Physics Letters A 17, no. 29 (September 21, 2002): 1915–22. http://dx.doi.org/10.1142/s0217732302008319.

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We discuss the brane cosmology in the 5D anti de Sitter Reissner–Nordstrom ( AdSRN 5) space–time. A brane with tension σ is defined as the edge of an AdSRN 5 space with mass M and charge Q. In this case we get the CFT-radiation term (ρ CFT ) from M and the charged dust [Formula: see text] from Q2 in the Friedmann-like equation. However, this equation is not justified because it contains [Formula: see text]-term with the negative sign. This is unconventional in view of the standard cosmology. In order to resolve this problem, we introduce a localized dust matter which satisfies Pdm = 0. If [Formula: see text], the unwanted [Formula: see text] is cancelled against [Formula: see text] and thus one recovers a standard Friedmann–Robertson–Walker universe with CFT-radiation and dust matter. For the stiff matter consideration, we can set ρcsti ~ Q2 with the negative sign. Here we introduce a massless scalar which plays the role of a stiff matter with Psca = ρsca to cancel -ρcsti. In this case, however, we find a mixed version of the standard and brane cosmologies.

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38

MAK, M. K., T. HARKO, and J. A. BELINCHÓN. "CAUSAL BULK VISCOUS DISSIPATIVE ISOTROPIC COSMOLOGIES WITH VARIABLE GRAVITATIONAL AND COSMOLOGICAL CONSTANTS." International Journal of Modern Physics D 11, no. 08 (September 2002): 1265–83. http://dx.doi.org/10.1142/s0218271802002232.

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We consider the evolution of a flat Friedmann–Robertson–Walker Universe, filled with a causal bulk viscous cosmological fluid, in the presence of variable gravitational and cosmological constants. The basic equation for the Hubble parameter, generalizing the evolution equation in the case of constant gravitational coupling and cosmological term is derived, under the supplementary assumption that the total energy of the Universe is conserved. By assuming that the cosmological constant is proportional to the square of the Hubble parameter and a power law dependence of the bulk viscosity coefficient, temperature and relaxation time on the energy density of the cosmological fluid, two classes of exact solutions of the field equations are obtained. In the first class of solutions the Universe ends in an inflationary era, while in the second class of solutions the expan-sion of the Universe is noninflationary for all times. In both models the cosmological "constant" is a decreasing function of time, while the gravitational "constant" increases in the early period of evolution of the Universe and tends in the large time limit to a constant value.

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39

SAAL, MARGUS, LAUR JÄRV, and PIRET KUUSK. "TIME EVOLUTION OF SCALAR-TENSOR COSMOLOGIES IN THE GENERAL RELATIVITY LIMIT." International Journal of Modern Physics: Conference Series 03 (January 2011): 238–45. http://dx.doi.org/10.1142/s2010194511001322.

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We consider Friedmann-Lemaître-Robertson-Walker flat cosmological models in the framework of general Jordan frame scalar-tensor theories of gravity in two different cases: in the dust matter dominated era and in the potential dominated era. Motivated by the local weak field constraints and by cosmological observations, we develop and use an approximation scheme for the regime close to the so-called limit of general relativity. The ensuing nonlinear approximate equations for the scalar field can be solved analytically in cosmological time in both cases. We find criteria for the functions ω and V characterizing a scalar-tensor theory, to determine whether the theory does or does not possess solutions converging to general relativity asymptotically in time. The converging solutions can be subsumed under two principal classes: exponentional or polynomial convergence, and damped oscillations around general relativity. The classes of scalar-tensor theories of gravity which contain these types of solutions and satisfy observational constraints, are candidates to explain possible deviations from the standard ΛCDM model. Finally, the effective equation of state parameter w eff is used to illustrate possible asymptotic cosmological dynamics.

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40

KRAUSE, AXEL, and SIEW-PHANG NG. "GHOST COSMOLOGY: EXACT SOLUTIONS, TRANSITIONS BETWEEN STANDARD COSMOLOGIES AND GHOST EVOLUTION." International Journal of Modern Physics A 21, no. 05 (February 20, 2006): 1091–122. http://dx.doi.org/10.1142/s0217751x0602516x.

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The recently proposed infrared modification of gravity through the introduction of a ghost scalar field results in a number of interesting cosmological and phenomenological implications. In this paper, we derive the exact cosmological solutions for a number of scenarios where at late stages, the ghost behaves like dark matter, or dark energy. The full solutions provide information about the nonlinear regime beyond the asymptotic analysis presented in the literature. The generic feature is that these ghost cosmologies give rise to smooth transitions between radiation dominated phases (or more general power-law expansions) at early epochs and ghost dark matter resp. ghost dark energy dominated late epochs. The current age of our universe places us right at the nonlinear transition phase. By studying the evolution backwards in time, we find that the dominance of the ghost over ordinary baryonic matter and radiative contributions persists back to the earliest times such that the Friedmann–Robertson–Walker geometry is dictated to a good approximation by the ghost alone. We also show that a Jeans instability arises in the ghost dark energy scenario at late times, while it is absent in the ghost dark matter scenario.

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41

García, Alberto A., Alberto García-Quiroz, Mauricio Cataldo, and Sergio del Campo. "Relationship between (2 + 1) and (3 + 1)-Friedmann–Robertson–Walker cosmologies; linear, non-linear, and polytropic state equations." General Relativity and Gravitation 37, no. 4 (April 2005): 685–711. http://dx.doi.org/10.1007/s10714-005-0056-5.

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42

El-Nabulsi, Rami Ahmad. "Asymptotically Static Universe Dominated by Phantom Energy." Zeitschrift für Naturforschung A 70, no. 2 (February 1, 2015): 101–8. http://dx.doi.org/10.1515/zna-2014-0242.

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AbstractIn this article, we investigated a generalised scalar field cosmology characterised by a time-dependent coupling function, a time-dependent cosmological constant, a chameleonic field, and a time-dependent equation of state parameter. Based on a particular choice of the Hubble parameter as function of the scalar field and its time derivative, we have investigated the dynamics of the Friedmann–Robertson–Walker flat universe. We have observed that for a particular choice of the free parameters in the theory, the universe accelerates with time and asymptotically tends toward a static universe. For specific values of the free parameters, the asymptotically static universe may be dominated by phantom energy or a cosmological constant without the need to implement in the theory multiple scalar fields or additional interactions. It is also pointed out that an asymptotically static universe is as well one special class of solutions for Horndeski and generalised galileons cosmologies. Special properties and features are discussed accordingly.

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43

CZINNER, VIKTOR G., and MÁTYÁS VASÚTH. "REVISITING ROTATIONAL PERTURBATIONS AND THE MICROWAVE BACKGROUND." International Journal of Modern Physics D 16, no. 11 (November 2007): 1715–23. http://dx.doi.org/10.1142/s0218271807011176.

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We consider general-relativistic rotational perturbations in homogeneous and isotropic Friedmann–Robertson–Walker (FRW) cosmologies. Taking linear perturbations of FRW models, the general solution to the field equations contains tensorial, vectorial and scalar functions. The vectorial terms are in connection with rotations in the given model and due to the Sachs–Wolfe effect they produce contributions to the temperature fluctuations of the cosmic microwave background radiation (CMBR). In this paper we obtain the analytic time dependence of these contributions in a spatially flat FRW model with pressureless ideal fluid, in the presence and the absence of a cosmological constant Λ. We find that the solution can be separated into an integrable and a nonintegrable part, as in the case of scalar perturbations. Analyzing the solutions and using the results of recent observations, we estimate the order of magnitude of the angular velocity corresponding to the rotation tensor at the time of decoupling and today.

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44

CÂMARA, C. S., J. C. CARVALHO, and M. R. DE GARCIA MAIA. "NONLINEARITY OF ELECTRODYNAMICS AS A SOURCE OF MATTER CREATION IN A FLAT FRW COSMOLOGY." International Journal of Modern Physics D 16, no. 02n03 (February 2007): 427–32. http://dx.doi.org/10.1142/s0218271807010213.

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The thermodynamics of cosmological matter creation has been extensively investigated in the literature. In the context of general relativity, the particle production in the cosmological models is due to mechanisms such as an imperfect fluid with bulk viscosity or the decaying vacuum. Another interesting proposal is matter creation in cosmologies with variation of fundamental constants. In this work, we study the nonlinearity of the electrodynamics as a source of matter creation in cosmological models with flat Friedmann–Robertson–Walker (FRW) line geometry. We write the energy conservation law arising from Einstein field equations with cosmological term Λ, solve the field equations, and study how particles are created as the magnetic field B changes with cosmic epoch. We obtain solutions for the adiabatic particle creation rate, the total number of particles, and the scale factor as a function of time in three cases: Λ = 0, Λ = constant and Λ ∝ H2 (cosmological term proportional to the Hubble parameter). We find the constraints imposed by the second law of thermodynamics upon the cosmological solutions.

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45

CÂMARA, C. S., R. B. NASCIMENTO, J. C. CARVALHO, and M. R. DE GARCIA MAIA. "SOLUTIONS OF COSMOLOGICAL MODELS WITH VARYING SPEED OF LIGHT." International Journal of Modern Physics D 16, no. 02n03 (February 2007): 433–37. http://dx.doi.org/10.1142/s0218271807010225.

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In recent years, there has been some renewed interest in cosmological models in which the constants of nature vary with time. In one of these, one supposes a dependence on the redshift of the fine structure constant as suggested by some astronomical observations of quasars. This has led, among others, to the proposal that the speed of light varies with the cosmological time. A Varying Speed of Light (VSL) would be able to solve some long standing problems of the standard cosmological model, without the need for inflation. In this work we study two models in which (1) c varies according to a power law of the scale factor c(t) ∝ a-r and (2) c is proportional to the Hubble parameter c(t) ∝ Hu. We present solutions for these two types of VSL cosmologies. We find that for a flat (k = 0) Friedmann–Robertson–Walker (FRW) universe, the variation of c is coupled to a cosmological Λ-term and both singular and non-singular solutions are possible.

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46

Pan, Supriya. "Exact solutions, finite time singularities and non-singular universe models from a variety of Λ(t) cosmologies". Modern Physics Letters A 33, № 01 (9 січня 2018): 1850003. http://dx.doi.org/10.1142/s0217732318500037.

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Cosmological models with time-dependent [Formula: see text] (read as [Formula: see text]) have been investigated widely in the literature. Models that solve background dynamics analytically are of special interest. Additionally, the allowance of past or future singularities at finite cosmic time in a specific model signals for a generic test on its viabilities with the current observations. Following these, in this work we consider a variety of [Formula: see text] models focusing on their evolutions and singular behavior. We found that a series of models in this class can be exactly solved when the background universe is described by a spatially flat Friedmann–Lemaître–Robertson–Walker (FLRW) line element. The solutions in terms of the scale factor of the FLRW universe offer different universe models, such as power-law expansion, oscillating, and the singularity free universe. However, we also noticed that a large number of the models in this series permit past or future cosmological singularities at finite cosmic time. At last we close the work with a note that the avoidance of future singularities is possible for certain models under some specific restrictions.

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47

Hossienkhani, H., H. Yousefi, and N. Azimi. "Anisotropic behavior of dark energy models in fractal cosmology." International Journal of Geometric Methods in Modern Physics 15, no. 12 (December 2018): 1850200. http://dx.doi.org/10.1142/s0219887818502006.

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We study effects of anisotropy (although low) on the ghost and generalized ghost dark energy (DE) models in the framework of fractal cosmology. We obtain the equation of state parameter, [Formula: see text], the deceleration parameter, and the evolution equation of the ghost and generalized ghost dark energy. We find that, in both models, [Formula: see text] cannot cross the phantom line and eventually the universe approaches a de-Sitter phase of expansion. We show that the anisotropy effects on ghost and generalized ghost dark energy (GDE) in fractal cosmology correspond to [Formula: see text]CDM limit on the statefinder plane. We evaluate the anisotropy effects on both the linear perturbation and the spherical collapse from the DE models and compare them with the results of the DE of the Friedmann–Robertson–Walker and [Formula: see text]CDM models. We also show that in ghost and generalized ghost cosmologies, the growth factor [Formula: see text] rise front the values for an [Formula: see text]CDM universe. Finally, we constrain the model parameters by using the maximum likelihood analysis and a combined dataset of baryon acoustic oscillation (BAO) and OHD.

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48

Melia, F., and M. López-Corredoira. "Alcock–Paczyński test with model-independent BAO data." International Journal of Modern Physics D 26, no. 06 (November 23, 2016): 1750055. http://dx.doi.org/10.1142/s0218271817500559.

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Cosmological tests based on the statistical analysis of galaxy distributions usually depend on source evolution. An exception is the Alcock–Paczyński (AP) test, which is based on the changing ratio of angular to spatial/redshift size of (presumed) spherically-symmetric source distributions with distance. Intrinsic redshift distortions due to gravitational effects may also have an influence, but they can now be overcome with the inclusion of a sharp feature, such as the Baryonic Acoustic Oscillation (BAO) peak. Redshift distortions affect the amplitude of the peak, but impact its position only negligibly. As we shall show here, the use of this diagnostic, with new BAO peaks from SDSS-III/BOSS at average redshifts [Formula: see text], [Formula: see text] and [Formula: see text], disfavors the current concordance ([Formula: see text]CDM) model at [Formula: see text]. Within the context of expanding Friedmann–Robertson–Walker (FRW) cosmologies, these data instead favor the zero active mass equation-of-state, [Formula: see text], where [Formula: see text] and [Formula: see text] are, respectively, the total density and pressure of the cosmic fluid, the basis for the [Formula: see text] universe.

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49

Melia, Fulvio, Jin Qin, and Tong-Jie Zhang. "Assessing cosmic acceleration with the Alcock–Paczyński effect in the SDSS-IV quasar catalogue." Monthly Notices of the Royal Astronomical Society: Letters 499, no. 1 (January 20, 2020): L36—L40. http://dx.doi.org/10.1093/mnrasl/slaa153.

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ABSTRACT The geometry of the Universe may be probed using the Alcock–Paczyński (AP) effect, in which the observed redshift size of a spherical distribution of sources relative to its angular size varies according to the assumed cosmological model. Past applications of this effect have been limited, however, by a paucity of suitable sources and mitigating astrophysical factors, such as internal redshift-space distortions and poorly known source evolution. In this Letter, we introduce a new test based on the AP effect that avoids the use of spatially bound systems, relying instead on sub-samples of quasars at redshifts z ≲ 1.5 in the Sloan Digital Sky Survey IV, with a possible extension to higher redshifts and improved precision when this catalogue is expanded by upcoming surveys. We here use this method to probe the redshift-dependent expansion rate in three pertinent Friedmann–Lemaître–Robertson–Walker cosmologies: Λ cold dark matter (ΛCDM), which predicts a transition from deceleration to acceleration at z ∼ 0.7; Einstein–de Sitter, in which the Universe is always decelerating; and the Rh = ct universe, which expands at a constant rate. ΛCDM is consistent with these data, but Rh = ct is favoured overall.

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50

Aguilar-Pérez, Gilberto, Ana A. Avilez-López, and Miguel Cruz. "Qualitative analysis for viscous cosmologies in a non linear regime of the Israel-Stewart formalism." Journal of Cosmology and Astroparticle Physics 2022, no. 11 (November 1, 2022): 016. http://dx.doi.org/10.1088/1475-7516/2022/11/016.

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Abstract We explore the dynamical properties of a cosmological model that includes viscous effects in the dark matter sector of the fluid equations in a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime. The bulk viscous effects are described by a non linear extension of the full Israel-Stewart model, which is a fluid causal scheme. We allow the interchange of energy in the dark sector and describe this by means of the interaction term, namely Q. We establish the dynamical system corresponding to Friedmann and fluid set of equations associated to the model and study the linear stability of its critical points. From the exploration of the dynamical system, we show the appearance of a critical point characterizing a de Sitter universe within the non interacting and interacting dark sector. We focus our study to analyse the stability of this fixed point in a large region of parameter space and derive linearized solutions around it. These approximate and analytical solutions are potentially able to describe the expansion of the universe since they are close to a de Sitter stationary solution. Within this regime with Q ≠ 0, we realize the existence of regions in the space of parameters where this critical point is stable and describes the behavior of dark energy as quintessence, cosmological constant and phantom like fluids. We perform a comparison between numerical and linearized solutions nearby the critical points within the full non linear regimes and also contrast them against ΛCDM model as a fiducial model. We find that the fully non linear regime is favored by observations and closer to the concordance model due to the non-zero value of the parameter j, which controls the non linear effects of bulk viscosity. In fact, at low redshift values, the expansion rate associated to the full non linear regime is practically indistinguishable from the ΛCDM model. The deceleration parameter obtained in this regime exhibits a transition from decelerated to accelerated cosmic expansion.

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