2003 **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.
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“…[4,9,10,11]). For the solution of the system of differential equations (9) and (10) we introduce the dimensionless quantities…”

confidence: 99%

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

confidence: 99%

“…[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…”

confidence: 99%

“…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].…”

confidence: 99%