Homogeneous Isotropic Cosmological Models Research Articles

Possible Modification of the Standard Cosmological Model to Resolve a Tension with Hubble Constant Values

The tensions concerning the values of Hubble constant obtained from the early and the late Universe data pose a significant challenge to modern cosmology. Possible modifications of the flat homogeneous isotropic cosmological ΛCDM model are considered, in which the Universe contains the dark energy, cold baryonic matter, and dark matter. They are based on general relativity and satisfy two requirements: (1) the value of the Hubble constant calculated from the value of the Hubble parameter at the recombination by formulas of the flat ΛCDM model, should be equal to 92% of the one based on low-redshift observations; (2) deviations from the ΛCDM model should not lead to effects that contradict astronomical observations and estimations obtained thereof. The analysis showed that there are few opportunities for the choice. Either we should consider DM with negative pressure −pdmc2 ≪ pdm < 0, which weakly affects the evolution of the Universe and the observed manifestations of DM, or we should admit the mechanism of generation of new matter, for example, by the dark energy decay.

Is the Axionic Dark Matter an Equilibrium System?

We consider an axionic dark matter model with a modified periodic potential for the pseudoscalar field in the framework of the axionic extension of the Einstein-aether theory. The modified potential is assumed to be equipped by the guiding function, which depends on the expansion scalar constructed as the trace of the covariant derivative of the aether velocity four-vector. The equilibrium state of the axion field is defined as the state, for which the modified potential itself and its first derivative with respect to the pseudoscalar field are equal to zero. We apply the developed formalism to the homogeneous isotropic cosmological model, and find the basic function, which describes the equilibrium state of the axionic dark matter in the expanding Universe.

Thermodynamical aspects of relativistic hydrodynamics in f(R,G) gravity

We present investigation devoted to the dynamical study of relativistic hydrodynamics with some thermodynamical characteristics in [Formula: see text] gravity towards spatially homogeneous isotropic cosmological model filled with isotropic fluid. We govern the features of the derived cosmological model by considering the power-law inflation for the average scale factor. The temperature and entropy density of the proposed model are positive definite. We also discuss the energy conditions to our solutions. The strong energy condition violated, which indicates the accelerated expansion of the proposed model.

Axionic extension of the Einstein-aether theory: How does dynamic aether regulate the state of axionic dark matter?

In the framework of axionic extension of the Einstein-aether theory we establish the model, which describes a stiff regulation of the behavior of axionic dark matter by the dynamic aether. The aether realizes this procedure via the modified Higgs potential, designed for modeling of nonlinear self-interaction of pseudoscalar (axion) field; the modification of this potential is that its minima are not fixed, and their positions and depths depend now on the square of the covariant derivative of the aether velocity four-vector. Exact solutions to the master equations, modified correspondingly, are obtained in the framework of homogeneous isotropic cosmological model. The effective equation of state for axionic dark matter is of the stiff type. Homogeneous perturbations of the pseudoscalar (axion) field, of the Hubble function and of the scale factor are shown to fade out with cosmological time, there are no growing modes, the model of stiff regulation is stable.

Cosmological Density Perturbations in Newtonian- and MONDian Gravity Scenario: A Symmetry-Based Approach

We investigate the evolution of linear density contrasts obtained with respect to a homogeneous spatially flat Friedman-Lemaitre–Robertson–Walker (FLRW) background by solving the density contrast equations governed by Newtonian and MONDian force laws using symmetry-based approach. We find eight-parameter Lie group symmetries for the linear order density perturbation equation for the Newtonian case whereas the density contrast equation follows only one parameter Lie group symmetry in MONDian case. We use Lie symmetries to find the group invariant solutions from invariant curve condition. The physical features of the evolution for various mode of density contrast with respect to the global cosmic background density in homogeneous isotropic cosmological models have been investigated using analytical group invariant solutions along with their numerical solutions. An account for cosmological density contrast and mass fluctuation also have been provided. We also have shown that the MONDian force law generates higher amplitudes in the density fluctuation, results in a more rapid structure formation that cannot be possible under the Newtonian force law.

Eddington’s Prediction for General Relativity Revisited

Eddington’s prediction for general relativity (GR) is exemplified by three cases: a static spherically symmetric gravitational field, radiation of gravitational waves, and a homogeneous isotropic cosmological model. The results of GR for a static field are obtained using only Poisson’s equation instead of the Einstein–Hilbert equations. Degeometrization of GR will allow the gravitational interaction to be unified with others described in the framework of the Standard Model of particle physics.

Field interpretation of General Relativity

A field interpretation of General Relativity, without resorting to Einstein-Hilbert’s equations, has been presented for three cases: a static centrally symmetric gravitational field, radiation of gravitational waves and a homogeneous isotropic cosmological model. The possibility of an effective Riemannian space being used in General relativity should not been ruled out.

To theory of asymptotically stable accelerating Universein Riemann-Cartan spacetime

Homogeneous isotropic cosmological models built in the framework of the Poincar'e gauge theory of gravity based on general expression of gravitational Lagrangian with indefinite parameters are analyzed. Special points of cosmological solutions for flat cosmological models at asymptotics and conditions of their stability in dependence of indefinite parameters are found. Procedure of numerical integration of the system of gravitational equations at asymptotics is considered. Numerical solution for accelerating Universe without dark energy is obtained.

ON THEORY OF REGULAR ACCELERATING UNIVERSE IN RIEMANN–CARTAN SPACETIME

Isotropic cosmology built in the Riemann–Cartan spacetime is investigated. Properties of homogeneous isotropic cosmological models filled with usual gravitating matter and scalar fields are studied in the beginning of cosmological expansion near the limiting energy density. It is shown that cosmological models are regular not only with respect to the Hubble parameter and the energy density but also with respect to the torsion and curvature tensors.

Exact solutions and particle creation for nonconformal scalar fields in homogeneous isotropic cosmological models

The problem is solved of describing scale factors of a homogeneous isotropic spaces-time such that the exact solution for the scalar field with a nonconformal coupling to curvature can be obtained from solutions for the conformally coupled field by redefining the mass and momentum. Explicit expressions for dependence of time from the large-scale factor are presented in the form of Abelian integrals in these cases. The exact solution for a scalar field with Gauss-Bonnet type coupling with curvature is received and it is shown that the corresponding nonconformal additions can dominate at the particles creation by gravitational field.

W-cosmological singularities

In this paper we characterize barotropic index singularities of homogeneous isotropic cosmological models [M. P. Da\ifmmode \mbox{\c{}}\else \c{}\fi{}browski and T. Denkiewicz, Phys. Rev. D 79, 063521 (2009).]. They are shown to appear in cosmologies for which the scale factor is analytical with a Taylor series in which the linear and quadratic terms are absent. Though the barotropic index of the perfect fluid is singular, the singularities are weak, as it happens for other models for which the density and the pressure are regular.

Zero mass scalar field with bulk viscous cosmological solutions in Lyra geometry

In this paper, we have investigated spatially homogeneous isotropic Friedmann cosmological model with bulk viscosity and zero-mass scalar field in Lyra manifold. The cosmological models are obtained with the help of the special law of variation for Hubble’s parameter proposed by Bermann (Nuovo Cimento 74B:182, 1983) and power law relation. Some physical properties of the models are discussed.

Regular accelerating universe without dark energy in Poincaré gauge theory of gravity

Homogeneous isotropic cosmological models with two torsion functions filled with scalar fields and usual gravitating matter are built and investigated in the framework of the Poincaré gauge theory of gravity. It is shown that by certain restrictions on indefinite parameters of gravitational Lagrangian the cosmological equations at asymptotics contain an effective cosmological constant that can explain observable acceleration of cosmological expansion. The behavior of inflationary cosmological solutions at extremely high energy densities is analyzed and regular bouncing solutions are obtained. The role of the spacetime torsion provoking the acceleration of cosmological expansion is discussed.

Hidden past of dark energy cosmological models

In this Letter we analyse the possibility of having homogeneous isotropic cosmological models with observers reaching t=∞ in finite proper time. It is shown that just observationally-suggested dark energy models with w∈(−5/3,−1) show this feature and that they are endowed with an exotic curvature singularity. Furthermore, it is shown that non-accelerated observers in these models may experience a duration of the universe as short as desired by increasing their linear momentum. A subdivision of phantom models in two families according to this behavior is suggested.

Analysis of inflationary cosmological models in gauge theories of gravitation

Inflationary homogeneous isotropic cosmological models filled by scalar fields and ultrarelativistic matter are examined in the framework of gauge theories of gravitation. By using a quadratic scalar field potential, a numerical analysis of flat, open and closed models is carried out. Properties of cosmological models are investigated in dependence on an indefinite parameter of cosmological equations and initial conditions at a bounce. A fulfilled analysis demonstrates the regular character of all cosmological models.

Radon transform of the Wheeler-De Witt equation and tomography of quantum states of the universe

The notion of standard positive probability distribution function (tomogram) which describes the quantum state of universe alternatively to wave function or to density matrix is introduced. Connection of the tomographic probability distribution with the Wigner function of the universe and with the star-product (deformation) quantization procedure is established. Using the Radon transform the Wheeler-De Witt generic equation for the probability function is written in tomographic form. Some examples of the Wheeler-DeWitt equation in the minisuperspace are elaborated explicitly for a homogeneous isotropic cosmological models. Some interpretational aspects of the probability description of the quantum state are discussed.

Further results for astrophysics from the cosmology within the framework of the Projective Unified Field Theory

Recently the 5-dimensional Projective Unified Field Theory (PUFT) of the author (Schmutzer 1995a, Schmutzer 1995b) has been applied to a closed homogeneous isotropic cosmological model with the result of a cosmology without big bang (Schmutzer 1999a, Schmutzer 1999b). Continuing this approach, in this paper following subjects are treated: recalculation of numerical values of cosmological quantities, exact solution of the field equations to a point-like body, motion of a test body in such a field, definition of the empirical effective gravitational factor (“constant”), Einstein effects compared to the empirical situation, adiabatic approximation of the motion of an orbiting testbody under the influence of the expanding cosmos (transition of the ellipses to circles, decrease of the radius of the orbiting bodies, decrease of the excentricity, increase of the frequency of orbiting objects etc.), heat production in a moving body induced by the cosmological expansion with application to various cosmic objects.

Further results for astrophysics from the cosmology within the framework of the Projective Unified Field Theory

Recently the 5-dimensional Projective Unified Field Theory (PUFT) of the author (Schmutzer 1995a, Schmutzer 1995b) has been applied to a closed homogeneous isotropic cosmological model with the result of a cosmology without big bang (Schmutzer 1999a, Schmutzer 1999b). Continuing this approach, in this paper following subjects are treated: recalculation of numerical values of cosmological quantities, exact solution of the field equations to a point-like body, motion of a test body in such a field, definition of the empirical effective gravitational factor (“constant”), Einstein effects compared to the empirical situation, adiabatic approximation of the motion of an orbiting testbody under the influence of the expanding cosmos (transition of the ellipses to circles, decrease of the radius of the orbiting bodies, decrease of the excentricity, increase of the frequency of orbiting objects etc.), heat production in a moving body induced by the cosmological expansion with application to various cosmic objects.

Deceleration without dark matter

In homogeneous isotropic cosmological models the angular size theta of a standard measuring rod changes with redshift z in a manner that depends upon the parameters of the model. It has been argued that as a population ultracompact (milliarcsecond) radio sources measured by very long-baseline interferometry (VLBI) do not evolve with cosmic epoch, and thus comprise a set of standard objects, at least in a statistical sense. Here we examine the angular-size/redshift relation for 256 ultracompact sources with z in the range 0.5 to 3.8 for cosmological models with two degrees of freedom (Omega_0 and Lambda_0). The canonical inflationary cold dark matter model(Omega_0=1, Lambda_0=0) appears to be ruled out by the observed relationship, whereas low-density models with a cosmological constant of either sign are favoured.

4D Homogeneous Isotropic Cosmological Models Generated by the 5D Vacuum

4-dimensional homogeneous isotropic cosmological models obtained from solutions of vacuum 5-dimensional Einstein equations are considered. It is assumed, that the G(55)-component of the 5-d metric simulates matter in the comoving frame of reference. Observable 4-d metric is defined up to conformal transformations of the metric of 4-d section \tilde{g}, with a conformal factor as a function of the component G(55). It is demonstrated, that the form of this function determines the matter equation of state. Possible equations of state are analyzed separately for flat, open and close models.