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Distinguishing between w<-1 Dark Energy Models
Abstract
Recent data and new data analysis methods show that most probably the
parameter w in the equation of state of the dark energy is smaller than
-1 at low redshifts. We briefly review some of the models with such a
property and without violating null energy condition. We investigate the
difference between the observables and predictions of these models, and
how they can be explored to single out or constrain the origin of dark
energy and its properties.
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We propose a nonparametric method to determine the sign of γ — the redshift evolution index of dark energy. This is important for distinguishing between positive energy models, a cosmological constant, and what is generally called ghost models. Our method is based on geometrical properties and is more tolerant to uncertainties of other cosmological parameters than fitting methods in what concerns the sign of γ. The same parametrization can also be used for determining γ and its redshift dependence by fitting. We apply this method to SNLS supernovae and to gold sample of re-analyzed supernovae data from Riess et al. Both datasets show strong indication of a negative γ. If this result is confirmed by more extended and precise data, many of the dark energy models, including simple cosmological constant, standard quintessence models without interaction between quintessence scalar field(s) and matter, and scaling models are ruled out. We have also applied this method to Gurzadyan–Xue models with varying fundamental constants to demonstrate the possibility of using it to test other cosmologies.
In cosmological context, classical scalar fields are important ingredients for inflation models, many candidate models of dark energy, symmetry breaking and phase transition epochs, and their consequences such as baryo and lepto-genesis. We investigate the formation of these fields by studying the production of a light quantum scalar field during the decay of a heavy particle. For simplicity it is assumed to be a scalar too. We discuss the effects of the decay mode, the thermodynamical state of the decaying field, boundary conditions, and related physical parameters on the production and evolution of a condensate. For a simplified version of this model we calculate the asymptotic behaviour of the condensate and conditions for its contribution to the dark energy with an equation of state close to a cosmological constant. We also discus the role of the back-reaction from interactions with other fields and expansion of the Universe on the evolution of the condensate. Comment: 19 pages, no figures,v2: a reference is added,v3: text improved
We investigate the possibility of replacing the cosmological constant with gradual condensation of a scalar field produced during the decay of a superheavy dark matter. The advantage of this class of models to the ordinary quintessence is that the evolution of the dark energy and the dark energy are correlated and cosmological coincidence problem is solved. This model does not need a special form for the quitessence potential and even a simple theory or an axion like scalar is enough to explain the existence of the Dark Energy. We show that the model has an intrinsic feedback between energy density of the dark matter and the scalar field such that for a large volume of the parameter space the equation of state of the scalar field from very early in the history of the Universe is very close to a cosmological constant. Other aspects of this model are consistent with recent CMB and LSS observations. Comment: 14 pages, 4 figures
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