6The Universe Through Gravity’s Lens
The dark matter problem is one of the most important outstanding questions in cosmology today, because the precise composition and the amount of dark matter determine the ultimate fate of our Universe-whether we continue to expand, begin to contract or start to oscillate. The standard framework of modern cosmology revolves around a small set of defining parameters that need to be determined observationally in order to obtain a complete description of the underlying cosmological model of the Universe. These three key cosmological parameters are the Hubble parameter (or Hubble constant) H0, the mass density parameterΩ (the total matter content of the Universe, counting both the luminous and dark matter contributions) and the value of the cosmological constant∧. These parameters together define the physical nature and the basic geometry of the Universe we inhabit.