Cosmological Models with Matter Creation in Open Thermodynamic Systems

Regarding the universe as an open thermodynamicsystem, the creation of matter/radiation particles outof gravitational energy is investigated. A new class ofFRW models with creation of matter is obtained and their properties are examined. A suitablechoice of the particle number density function n(t) =(A/t)^3/2 leads toinflationary solutions during the particle creationphase; subsequently the universe enters the Friedmann era. It is found that fora physically acceptable solution > 1. Acomparative study is made for = 4/3, 2, 8/3, and10/3 in order to find a viable model of theuniverse.     

Cosmological Model Universe Consisting of Two Forms of Dark Energy

Considering a spatially flat FRW metric we obtain a model universe consisting partly of quintessence form of dark energy and partly of cosmological constant form of dark energy; and after studying their physical,dynamical and kinematical properties it is found that our model is a new and viable form of model universe containing dark energy.     

Cosmological Parameters from the Thermodynamic Model of Gravity

Within our recent thermodynamic model of gravity the dark energy is identified with the energy of collective gravitational interactions of all particles in the universe, which is missing in the standard treatments. For a simple model universe composed of neutral and charged particles of identical mass we estimate the radiation, baryon and dark energy densities and obtain the values which are very close to the current cosmological observations.     

Cosmological Implications of Interacting Polytropic Gas Dark Energy Model in Non-flat Universe

The polytropic gas model is investigated as an interacting dark energy scenario. The cosmological implications of the model including the evolution of EoS parameter w _Λ, energy density Ω_Λ and deceleration parameter q are investigated. We show that, depending on the parameter of model, the interacting polytropic gas can behave as a quintessence or phantom dark energy. In this model, the phantom divide is crossed from below to up. The evolution of q in the context of polytropic gas dark energy model represents the decelerated phase at the early time and accelerated phase later. The singularity of this model is also discussed. Eventually, we establish the correspondence between interacting polytropic gas model with tachyon, K-essence and dilaton scalar fields. The potential and the dynamics of these scalar field models are reconstructed according to the evolution of interacting polytropic gas.     

A Quantum Universe and the Solution to the Cosmological Problems

We have found that the hierarchial problems appearing in cosmology are a manifestation of the quantum nature of the universe. The universe is still described by the same formulae that once hold at Planck's time. The universe is found to be governed by the Machian equation, GM = Rc ², where M and R are mass and radius of the universe. A Planck's constant xsfor different cosmic scales is provided. The status of the universe at different stages is shown to be described in terms of the fundamental constants (c, , G, , H) only. The concept of maximal (minimal) acceleration, power, temperature, etc., is introduced and justified.     

Inhomogeneous Universe Models with Varying Cosmological Term

The evolution of a class of inhomogeneous spherically symmetric universe models possessing a varying cosmological term and a material fluid, with an adiabatic index either constant or not, is studied.     

Thermodynamics of Universe with Time Varying Cosmological Term

The FRW-type cosmologies with time varying cosmological term is discussed within the frame work of a thermodynamic context. If at some cosmological time, the cosmological term begins increasing again, as presently observed, expansion will accelerate and matter and/or radiation will be transformed back into dark energy. It is shown that such accelerated expansion is a route towards a new kind of gravitational singular state, characterized by an empty, conformally transitive spacetime in which all energy is dark. We investigate whether dynamic dark energy cosmologies are compatible with the second law of thermodynamics. We examine also the total entropy evolution with time. We observed that the dynamic dark energy cosmology is less restricted with second law of thermodynamics. Some physical implications of these solutions are briefly discussed.     

Modern Cosmological Data and Rotation of the Universe

A new nonstationary rotating cosmological model is developed which describes the evolution of the observed Universe, since its properties are in good agreement with recent astronomical observations. It is demonstrated that the energy density of cosmological rotation can play a role of dark energy and induce the accelerated expansion of the Universe detected recently. In some situations when the causality parameter of the rotating cosmological model is negative, the cosmological rotational energy can be represented as phantom matter that violates the weak energy condition p + ε ≥ 0. The suggested cosmological model has no initial singularity, that is, the cosmological rotation can prevent the formation of the singularity.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 3–6, March, 2005.     

Early Universe with Variable Gravitational and Cosmological Constants

Einstein field equations are considered in zero-curvature Robertson–Walker (R–W) cosmology with perfect fluid source and time-dependent gravitational and cosmological “constants.” Exact solutions of the field equations are obtained by using the ’gamma-law' equation of state p = (γ − 1)ρ in which γ varies continuously with cosmological time. The functional form of γ (R) is used to analyze a wide range of cosmological solutions at early universe for two phases in cosmic history: inflationary phase and Radiation-dominated phase. The corresponding physical interpretations of the cosmological solutions are also discussed.     

Thermodynamic Analysis of Cosmological Black Hole

The objective of this paper is to investigate the thermodynamic properties of Mc Vittie universe. It turns out that the Einstein field equations are obtained by using Clausius equation near apparent horizon of Mc Vittie universe.For the existence of horizons, different conditions in terms of Mc Vittie parameters are discussed. Diverse cases of surface gravity are studied and conditions in terms of parameters for which surface gravity is positive, zero and negative are obtained and analyzed. It is shown that the generalized second law(GSL) holds near the apparent horizon if˙RA 0.Our results show that the heat capacity, CP, of the universe is always negative for matter dominated phase having P = 0. On the other hand, when P 0, the heat capacity CP 0 provided RA √2/√8πP- 3H2 and negative otherwise. Furthermore, when P 0, the temperature of the universe is minimum at Rmin =√2/√8πP- 3H2 while its heat capacity diverges.     

Cosmological Relativity: Determining the Universe by the Cosmological Redshift as Infinite and Curved

Using cosmological relativity theory, we derive the formula for the cosmological redshift written explicitly in terms of 1 – , where = /_c is the ratio of the average mass density to the critical closure density. Based on the present day data of observed redshifts, we conclude that < 1, which means the universe is infinite and curved, and expands forever.     

A Classical Cosmological Model for Triviality

The aim of this paper is to study the triviality of λ ϕ⁴ theory in a classical gravitational model. Starting from a conformal invariant scalar tensor theory with a self-interaction term λ ϕ⁴, we investigate the effect of a conformal symmetry breaking emerging from the gravitational coupling of the large-scale distribution of matter in the universe. Taking in this cosmological symmetry breaking phase the infinite limit of the maximal length (the size of the universe) and the zero limit of the minimal length (the Planck length) implies triviality, i.e. a vanishing coupling constant λ. It suggests that the activity of the self-interaction term λ ϕ⁴ in the cosmological context implies that the universe is finite and a minimal fundamental length exists.     

Thermodynamic Equilibrium in the Expanding Universe

We show that a relativistic gas may be at"global" equilibrium in the expanding universe for anyequation of state 0 < p /3, provided thatthe gas particles move under the influence of aself-interacting, efiective one-particle force in between elasticbinary collisions. In the force-free limit we recoverthe equilibrium conditions for ultrarelativistic matterwhich imply the existence of a conformal timelike Killing vector.     

A Cosmological Model with Static Extra Dimensions and Varying Cosmological Constant

A five dimensional cosmological model with FLRW type Kaluza-Klien metric has been investigated with static extra dimensions and varying cosmological constant. The field equations with static extra dimension are solved by considering the cosmological constant as a function of time for different cases. The effective pressure is considered as the difference of pressure corresponding to the extra dimension and the usual four dimensions. The conditions for acceleration of the universe are then discussed.     

A Cosmological Model of Holographic Brane Gravity

A cosmological scenario with two branes (A and B) moving in a 5-dimensional bulk is considered. As in the case of ecpyrotic and born-again braneworld models it is possible that the branes collide. The energy-momentum tensor is taken to describe a perfect barotropic fluid on the A-brane and a phenomenological time-dependent cosmological constant on the B-brane. The A-brane is identified with our Universe and its cosmological evolution in the approximation of a homogeneous and isotropic brane is analysed. The dynamics of the radion (a scalar field on the brane) contains information about the proper distance between the branes. It is demonstrated that the deSitter type solutions are obtained for late time evolution of the braneworld and accelerative behaviour is anticipated at the present time.     

Conformal Invariance, Accelerating Universe and the Cosmological Constant Problem

We investigate a conformal invariant gravitational model which is taken to hold at early universe. The conformal invariance allows us to make a dynamical distinction between the two unit systems (or conformal frames) usually used in cosmology and elementary particle physics. In this model we argue that when the universe suffers phase transition, the resulting mass scale introduced by particle physics should have a variable contribution to vacuum energy density. This variation is controlled by the conformal factor which is taken as a dynamical field. We then deal with the cosmological consequences of this model. In particular, we shall show that there is an inationary phase at early times. At late times, on the other hand, it provides a mechanism which makes a large effective cosmological constant relax to a sufficiently small value. Moreover, we shall show that the conformal factor acts as a quintessence field that leads the universe to accelerate at late times.     

The Thermodynamic Evolution of the Cosmological Event Horizon

By manipulating the integral expression for the proper radius R _e of the cosmological event horizon (CEH) in a Friedmann-Robertson-Walker (FRW) universe we obtain an analytical expression for the change δR _e in response to a uniform fluctuation δρ in the average cosmic background density ρ. We stipulate that the fluctuation arises within a vanishing interval of proper time, during which the CEH is approximately stationary, and evolves subsequently such that δρ/ρ is constant. The respective variations 2πR _e δR _e and δE _e in the horizon entropy S _e and enclosed energy E _e should be therefore related through the cosmological Clausius relation. In that manner we find that the temperature T _e of the CEH at an arbitrary time in a flat FRW universe is E _e /S _e , which recovers asymptotically the usual static de Sitter temperature. Furthermore it is proven that during radiation-dominance and in late times the CEH conforms to the fully dynamical First Law T _e dS _e =PdV _e −dE _e , where V _e is the enclosed volume and P is the average cosmic pressure.     

Analysing Hessence Intermediate and Logamediate Universe in Loop Quantum Cosmological Background

We have discussed here Hessence inflation in Loop Quantum Cosmological background. In this work, we have emphasized on late times, taking into account various slow-roll conditions. This model has been constructed taking intermediate and logamediate scale factors. In both cases the forms of hessence field, potential, number of e-folds, slow-roll parameters are manipulated by taking the dissipative co-efficient Γ =Γ₀, where Γ₀ > 0 is a constant, in accordance with second law of thermodynamics.