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 $\geq -1$, 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 $\omega \geq -1$ 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/X-ray observations and Type Ia supernova data. We find that there is much observationally acceptable parameter space beyond the $\Lambda$ barrier, thus opening the possibility of existence of more exotic forms of energy in the Universe.