No abstract

No abstract

No abstract

In this work it is investigated if fermionic sources could be responsible for accelerated periods during the evolution of a universe where a matter field would answer for the decelerated period. The self-interaction potential of the fermionic field is considered as a function of the scalar and pseudoscalar invariants. Irreversible processes of energy transfer between the matter and gravitational fields are also considered. It is shown that the fermionic field could behave like an inflaton field in the early universe and as dark energy for an old universe.PACS numbers: 98.80.-k, 98.80.Cq

The Universe is modeled as a binary mixture whose constituents are described by a van der Waals fluid and by a dark energy density. The dark energy density is considered either as quintessence or as the Chaplygin gas. The irreversible processes concerning the energy transfer between the van der Waals fluid and the gravitational field are taken into account. This model can simulate ͑a͒ an inflationary period where the acceleration grows exponentially and the van der Waals fluid behaves like an inflaton, ͑b͒ an accelerated period where the acceleration is positive but it decreases and tends to zero whereas the energy density of the van der Waals fluid decays, ͑c͒ a decelerated period which corresponds to a matter dominated period with a non-negative pressure, and ͑d͒ a present accelerated period where the dark energy density outweighs the energy density of the van der Waals fluid.

Abstract. A cosmological model where a fermion field is non-minimally coupled with the gravitational field is studied. By applying Noether symmetry the possible functions for the potential density of the fermion field and for the coupling are determined. Cosmological solutions are found showing that the non-minimally coupled fermion field behaves as an inflaton describing an accelerated inflationary scenario, whereas the minimally coupled fermion field describes a decelerated period, behaving as a standard matter field.PACS numbers: 98.80.Cq, 95.35.+d ‡ Dedicated to Professor Luis P. Chimento on the occasion of his sixtieth birthday.

No 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 *