Quasar lensing helps to solves astrophysical puzzles in various
ways, but one of its most beautiful applications is probably the
determination of the Hubble parameter H0. 3.1. The Time-delay
Method In 1964, the Norwegian astronomer Sjur Refsdal proposed
an original method (Refsdal 1964) to use gravitational lensing
as a tool to measure the size/age of the Universe. When photons
propagate from a distant source toward the observer, they are
under the effect of the gravity field of lenses along the line
of sight. They do not follow a straight line anymore, but their
trajectory is curved and longer than the original one. As a
consequence, it takes more time for the photons to travel from a
lensed source than from an unlensed one. The geometrical
difference introduced by the lens between the two, lensed and
unlensed paths, introduces a time-lag between the arrival times
of the (lensed and unlensed) photons at the position of the
observer. This time lag is called the geometrical "time-delay",
tgeom. While passing i