Chaplygin gas, which occurs in certain theories of cosmology, is a hypothetical substance that satisfies an exotic equation of state in the form p = − A / ρ α {\displaystyle p=-A/\rho ^{\alpha }}, where p {\displaystyle p} is the pressure, ρ {\displaystyle \rho } is the density, with α = 1 {\displaystyle \alpha =1} and A {\displaystyle A} a positive constant. Wikipedia