Scaling the Universe: Gravitational Lenses and the Hubble Constant

Gravitational lenses, besides being interesting in their own right, have been demonstrated to be suitable as "gravitational standard rulers" for the measurement of the rate of expansion of the Universe (H_o), as well as to constrain the values of the cosmological parameters such as Ω _o and Λ _o that control the evolution of the volume of the Universe with cosmic time.