Measurement of Cosmological Parameters via Gravitational Lensing
Quasars are the furthest objects visible in the universe: they can be seen at truly cosmological distances of billions of light years away. Gravitational lensing occurs when an intermediate large mass (the lens) distorts the quasar image into multiple images. This allows us to look for dark matter as invisible lenses and to measure Hubbles constant without the uncertainties of the infamous "Distance Ladder". The geometry and probability of a lensing event provides a measurement of the geometry of space-time that is then sensitive to the cosmological parameters such as the mass density, the cosmological constant, and possibly the equation of state of "Dark Energy" required by an accelerating expansion of the universe.