Testing dark energy beyond the cosmological constant barrier

Although well motivated from theoretical arguments, the cosmological constant barrier, i.e., the imposition that the equation-of-state parameter of dark energy (omega(x)equivalent top(x)/rho(x)) is greater than or equal to-1, seems to introduce bias in the parameter determination from statistical analyses of observational data. In this regard, phantom dark energy or superquintessence has been proposed in which the usual imposition omegagreater than or equal to-1 is relaxed. Here, we study possible observational limits to the phantom behavior of the dark energy from recent distance estimates of galaxy clusters obtained from interferometric measurements of the Sunyaev-Zel'dovich effect and x-ray observations, type Ia supernova data, and cosmic microwave background measurements. We find that there is much acceptable parameter space beyond the Lambda barrier, which opens, from a purely observational point of view, the possibility of the existence of more exotic forms of energy in the Universe.