The nature of the dark energy is still a mystery and several models have been proposed to explain it. Here we consider a phenomenological model for dark energy decay into photons and particles as proposed by Lima (J. Lima, Phys. Rev. D 54, 2571 (1996
)). He studied the thermodynamic aspects of decaying dark energy models in particular in the case of a continuous photon creation and/or disruption. Following his approach, we derive a temperature redshift relation for the CMB which depends on the effective equation of state $w_{eff}$ and on the adiabatic index $gamma$. Comparing our relation with the data on the CMB temperature as a function of the redshift obtained from Sunyaev-Zeldovich observations and at higher redshift from quasar absorption line spectra, we find $w_{eff}=-0.97 pm 0.034$, adopting for the adiabatic index $gamma=4/3$, in good agreement with current estimates and still compatible with $w_{eff}=-1$, implying that the dark energy content being constant in time.
Some models of Big Bang nucleosynthesis suggest that very high baryon density regions were formed in the early Universe, and generated the production of heavy elements other than lithium such as fluorine F. We present a comprehensive chemistry of flu
orine in the post-recombination epoch. Calculation of F, F- and HF abundances, as a function of redshift z, are carried out. The main result is that the chemical conditions in the early Universe can lead to the formation of HF. The final abundance of the diatomic molecule HF is predicted to be close to 3.75 10(-17) when the initial abundance of neutral fluorine F is 10(-15). These results indicate that molecules of fluorine HF were already present during the dark age. This could have implications on the evolution of proto-objects and on the anisotropies of cosmic microwave background radiation. Hydride of fluorine HF may affect enhancement of the emission line intensity from the proto-objects and could produce spectral-spatial fluctuations.
