We analyze the interaction between Dark Energy and Dark Matter from a thermodynamical perspective. By assuming they have different temperatures, we study the possibility of occurring a decay from Dark Matter into Dark Energy, characterized by a negative parameter Q. We find that, if at least one of the fluids has non vanishing chemical potential, for instance µx < 0 and µ dm = 0 or µx = 0 and µ dm > 0, the decay is possible, where µx and µ dm are the chemical potentials of Dark Energy and Dark Matter, respectively. Using recent cosmological data, we find that, for a fairly simple interaction, the Dark Matter decay is favored with a probability of ∼ 93% over the Dark Energy decay. This result comes from a likelihood analysis where only background evolution has been considered.

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The influence of a possible non zero chemical potential µ on the nature of dark energy is investigated by assuming that the dark energy is a relativistic perfect simple fluid obeying the equation of state (EoS), p = ωρ (ω < 0, constant). The entropy condition, S ≥ 0, implies that the possible values of ω are heavily dependent on the magnitude, as well as on the sign of the chemical potential. For µ > 0, the ω-parameter must be greater than -1 (vacuum is forbidden) while for µ < 0 not only the vacuum but even a phantomlike behavior (ω < −1) is allowed. In any case, the ratio between the chemical potential and temperature remains constant, that is, µ/T = µ0/T0. Assuming that the dark energy constituents have either a bosonic or fermionic nature, the general form of the spectrum is also proposed. For bosons µ is always negative and the extended Wien's law allows only a dark component with ω < −1/2 which includes vacuum and the phantomlike cases. The same happens in the fermionic branch for µ < 0. However, fermionic particles with µ > 0 are permmited only if −1 < ω < −1/2. The thermodynamics and statistical arguments constrain the EoS parameter to be ω < −1/2, a result surprisingly close to the maximal value required to accelerate a FRW type universe dominated by matter and dark energy (ω −10/21). 95.36.+x

This paper aims to put constraints on the transition redshift zt, which determines the onset of cosmic acceleration, in cosmological-model independent frameworks. In order to do that, we use the non-parametric Gaussian Process method with H(z) and SNe Ia data. The deceleration parameter reconstruction from H(z) data yields zt=0.59+0.12−0.11. The reconstruction from SNe Ia data assumes spatial flatness and yields zt=0.683+0.11−0.082. These results were found with a Gaussian kernel and we show that they are consistent with two other kernel choices.

In this paper we construct the complete evolution of the universe driven by the mass dimension one dark spinor called Elko, starting with inflation, passing by the matter dominated era and finishing with the recent accelerated expansion. The dynamic of the fermionic Elko field with a symmetry breaking type potential can reproduce all phases of the universe in a natural and elegant way. The dynamical equations in general case and slow roll conditions in the limit H ≪ m pl are also presented for the Elko system. Numerical analysis for the number of e-foldings during inflation, energy density after inflation and for present time and also the actual size of the universe are in good agreement with the standard model of cosmology. An interpretation of the inflationary phase as a result of Pauli exclusion principle is also possible if the Elko field is treated as an average value of its quantum analogue.