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 $({\omega }_x\equiv p_x/{\rho }_x)$ is $>~-1,$ 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 $\omega >~-1$ 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.