Although well motivated from theoretical arguments, the cosmological constant \emph{ 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, \emph{ phantom} dark energy or \emph{ superquintessence} has been proposed in which the usual imposition $\omega \geq -1$ is relaxed. Here, we study possible observational limits to the \emph{ 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, Type Ia supernova data and CMB measurements. We find that there is much \emph{ observationally} acceptable parameter space beyond the $\Lambda$ \emph{ barrier}, thus opening the possibility of existence of more exotic forms of energy in the Universe.
Categories: astro-ph gr-qc hep-th