Related papers: Can we be tricked into thinking that w is less tha…
Traditionally, inflationary models are analyzed in terms of parameters such as the scalar spectral index ns and the tensor to scalar ratio r, while dark energy models are studied in terms of the equation of state parameter w. Motivated by…
Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The effective equation of state can be w < -1 as well as w > -1. In the former case the expansion of the…
We report the constraints on the dark energy equation-of-state w(z) using the latest 'Constitution' SNe sample combined with the WMAP5 and SDSS data. Based on the localized principal component analysis and the model selection criteria, we…
Recent observations and theoretical considerations have motivated the study of models for dark energy with equation of state characterized by a parameter $w=p/\rho<-1$. Such models, however, are usually believed to be inviable due to their…
We outline a dynamical dark energy scenario whose signatures may be simultaneously tested by astronomical observations and laboratory experiments. The dark energy is a field with slightly sub-gravitational couplings to matter, a logarithmic…
We recently presented a series of dark energy theorems that place constraints on the equation of state of dark energy ($\wdark$), the ime-variation of Newton's constant ($\dot G$), and the violation of energy conditions in theories with…
Important observables to reveal the nature of dark energy are the equation of state $w$ and its time derivative in units of the Hubble time $w'$. Recently, it is shown that the simplest scalar field models of dark energy (quintessence)…
Cosmological observations strongly suggest the presence of dark energy which comprises the majority of the current energy density of the universe. The equation of state relating the pressure and energy density of this dark energy, p = w…
Many ambitious experiments have been proposed to constrain dark energy and detect its evolution. At present, observational constraints are consistent with a cosmological constant and there is no firm evidence for any evolution in the dark…
The equivalence between f(R) gravity and scalar-tensor theories is invoked to study the null, strong, weak and dominant energy conditions in Brans-Dicke theory. We consider the validity of the energy conditions in Brans-Dicke theory by…
We demonstrate that cosmic microwave background observations consistent with a cosmological constant universe predict in a well-defined sense that lower redshift measures will nearly automatically deliver w=-1 for the dark energy equation…
We present a nonparametric method to determine the sign of $w+1$ in the equation of state of dark energy. It is based on geometrical behaviour and is more tolerant to uncertainties of other cosmological parameters than fitting methods. It…
A new single-dynamical-scalar-field model of dark energy is proposed, in which either higher derivative terms nor structures of extra dimension are needed. With the help of a fixed background vector field, the parameter for the effective…
We extend previous work showing that violation of the null energy condition implies instability in a broad class of models, including gauge theories with scalar and fermionic matter as well as any perfect fluid. Simple examples are given to…
We focus on uncertainties in supernova measurements, in particular of individual magnitudes and redshifts, to review to what extent supernovae measurements of the expansion history of the universe are likely to allow us to constrain a…
I review the observational prospects to constrain the equation of state parameter of dark energy and I discuss the potential of future imaging and redshift surveys. Bayesian model selection is used to address the question of the level of…
Recently, the observed equation of state for dark energy appears to favor values below $-1$. The tendency implies that the nature of dark energy may be quite different from that of the cosmological constant. In view of the adjustment on the…
Recent determinations of the equation of state of dark energy hint that this may well be of the phantom type, i.e., $w_{de} < -1$. If confirmed by future experiments, this would strongly point to the existence of fields that violate the…
We review the Equation of State (EoS) approach to dark sector perturbations and apply it to $f(\mathcal{R})$ gravity models of dark energy. We show that the EoS approach is numerically stable and use it to set observational constraints on…
In this brief review, we examine the theoretical consistency and viability of phantom dark energy. Almost all data sets from cosmological probes are compatible with dark energy of the phantom variety (i.e., equation-of-state parameter…