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 [1]. 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 ef f and on the "adiabatic index" γ. Comparing our relation with the data on the CMB temperature as a function of the redshift obtained from Sunyaev-Zel'dovich observations and at higher redshift from quasar absorption line spectra, we find w ef f = −0.97 ± 0.034, adopting for the adiabatic index γ = 4/3, in good agreement with current estimates and still compatible with w ef f = −1, implying that the dark energy content being constant in time.

The possibility to detect cosmological signals from the post{recombination Universe is one of the main aims of modern cosmology. In a previous paper we emphasized the role that elastic resonant scattering through LiH molecules can have in dumping primary CBR anisotropies and raising secondary signals. Here we extend our analysis to all the evolutionary stages of a primordial cloud, starting with the linear phase, through the turn{around and to the non linear collapse. We have done calculations for proto{clouds in a CDM scenario and, more generally, for a set of clouds with various masses and various turn{around redshifts, in this case without referring to any particular structure formation scenario. We found that the rst phase of collapse, for t=t free fall = 0:05 0:2 is the best one for simultaneous detection of the rst two LiH rotational lines. The observational frequency falls between 30 and 250 GHz and the line width is between 10 5 and 10 4 . As far as we know this is the most favourable process to detect primordial clouds before they start star formation processes.

Context. Primordial molecules were formed during the Dark Ages, i.e. the time between recombination and reionization in the early Universe. They were the constituents of the first proto-stellar clouds. Standard Big Bang nucleosynthesis predicts the abundances of hydrogen, helium, lithium, beryllium, and their isotopes in the early Universe. Heavier nuclei such as carbon, nitrogen, or oxygen are only formed in trace amounts. In nonstandard Big Bang nucleosynthesis models, it is possible to synthesize greater quantities of these heavier elements. The latter are interesting because they can form molecules with a high electric dipole moment which can increase the cooling in collapsing protostellar structures. Aims. The purpose of this article is to analyze the formation of primordial molecules based on heavy elements during the Dark Ages, with elemental abundances taken from different nucleosynthesis models. Methods. We present calculations of the full nonlinear equation set governing the primordial chemistry. We considered the evolution of 45 chemical species and used an implicit multistep method of variable order of precision with an adaptive stepsize control. Results. The cosmological recombination of heavy elements is presented for the first time. We find that the most abundant Dark Age molecules based on heavy elements are CH and OH. When considering initial conditions given by the standard Big Bang nucleosynthesis model, we obtain relative abundances [CH] = n CH /n b = 6.2 × 10 −21 and [OH] = n OH /n b = 1.2 × 10 −23 at z = 10, where n b is the total number density. But nonstandard nucleosynthesis can lead to higher heavy element abundances, while still satisfying the observed primordial light abundances. In that case, we show that the abundances of molecular species based on C, N, O, and F can be enhanced by two orders of magnitude, leading to a CH relative abundance higher than that of HD + or H 2 D + .