Viscous cosmological models and accelerated universes

Abstract

It is shown that a present acceleration with a past deceleration is a possible solution of the Friedmann equation by considering the Universe as a mixture of a scalar with a matter field and by including a non-equilibrium pressure term in the energy-momentum tensor. The dark energy density decays more slowly with respect to the time than the matter energy density does. The inclusion of the non-equilibrium pressure leads to a less pronounced decay of the matter field with a shorter period of past deceleration. PACS: 98.80.CqAccording to recent cosmological observations there exists an evidence that the Universe is flat (see Sievers et al [1]) and expanding with a positive acceleration (see Perlmutter et al [2] and Riess et al [3]). The flatness of the Universe is connected with the total density parameter Ω tot which is the sum of the density parameters related to vacuum energy Ω Λ , cold dark matter Ω cdm and baryons Ω b , i. Phenomenological cosmologies play an important role in the understanding of the evolution of the Universe. A remarkable combination of general relativity and thermodynamics allow the description of different regimes in cosmological *

Irreversible processes and the accelerated-decelerated phases of the Universe

“…[4,9,10,11]). For the solution of the system of differential equations (9) and (10) we introduce the dimensionless quantities…”

“…[10,11]). The relationship between the cosmic scale factor and the energy density of the scalar field can be obtained from the integration of (6) by considering the equation of state given in (1), yielding…”

“…If the irreversible processes are taken into account the non-equilibrium pressure, in this case, is the responsible for the transfer of energy between the matter and gravitational fields. There exist at least two candidates in the literature for the dark energy density, namely the quintessence [15][16][17][18]10] and the Chaplygin gas (see e.g. [11] and the references therein).…”

“…the works [1][2][3]), whereas in extended (second-order or causal) thermodynamic theory the non-equilibrium pressure is supposed to obey an evolution equation (see e.g. the works [4][5][6][7][8][9][10][11][12]). These theories are known in the literature as viscous cosmological models.…”

Phase space analysis of interacting dark energy in f(T) cosmology

“…Some notable examples are (R) gravity [14][15][16][17][18][19][20][21][22][23][24][25][26][27], scalar-tensor gravity [28][29][30] and Lovelock gravity [31,32] and more recently (T ) gravity [33,34], to name a few. Other models where matter action is modified include the bulk viscous stress [35][36][37] and the anisotropic stress [38] or some exotic fluid like Chaplygin gas (CG) [39]. One of the crucial tests to check the viability of extended theories of gravity is the potential detection of gravitational waves [40][41][42].…”