Determining the Cosmic Equation of State from Gravitational Lensing

An independent method is proposed to determine the equation of state, ω_x ≡ p_x/ρ_x, for the dark energy component in the universe. This method is based on gravitational lensing statistics. An analytically simple expression for the optical depth of gravitational lensing in general Friedmann-Robertson- Walker cosmologies with dark energy component is derived. It is shown that for a flat universe the optical depth depends sensitively on the redshift to the source ｚ_s and the equation of state ω_x which makes the method proposed here efficient and robust.