Cosmology with x-matter

Motivated by the possibility of H0t0 > 1 where H0 and t0 are the Hubble parameter and the age of the universe, respectively, we investigate the cosmology including xmatter. x-matter is expressed by the equation of state px = w0ρx0 + c 2 s (ρx − ρx0), where px, ρx and ρx0 are the pressure, the density of x-matter and the density at present, respectively. w0 and c 2 s are functions of ρx in general. x-matter has the most general form of the equation of state which is characterized by 1)violation of strong energy condition at present for w0 < −1/3 ; 2) locally stable (i.e. c 2 s ≥ 0); 3)causality is guaranteed (cs ≤ 1). Considering the case that w0 and c 2 s are constants, we find that there is a large parameter space of (w0, c 2 s ,Ωx0) in which the model universe is consistent with the age of the universe and the observations of distant Type I supernovae.