Cosmology with x-matter

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Abstract
Motivated by the possibility of H0t0H0 and t0 are the Hubble parameter and the age of the Universe, respectively), we investigate cosmology including x- matter. x-matter is expressed by the equation of state px=w0ρx0 + c2s(ρx − ρ ρx0) where px, ρx and ρx0 are the pressure and the density of x-matter and the density at present, respectively. w0 and c2s are functions of ρx in general, x-matter has the most general form of the equation of state which is characterized by (1) the violation of the strong energy condition at present for w0 < − 1/3; (2) being locally stable (i.e. c2 ≥ 0); and (3) having a guaranteed causality (cs ≤ 1). Considering the case in which w0 and c2s are constants, we find that there is a large parameter space of (ϖ0, c2s, Ωx0) in which the model universe is consistent with the age of the Universe and the observations of distant Type I supernovae.

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