The expansion of the universe can be characterized essentially by one entity, called the Hubble constant. Traditionally it is calculated from the cosmological redshift of remote galaxies and their distance, where the distance determination is problematic.

Gravitational lensing allows to determine the Hubble constant directly. In this case the lens is a galaxy and the source a quasar, which should be mapped at least twice. When one of the images changes its brightness, the other one does, too - but not exactly at the same time. A delay is caused by two effects. One is due to the difference of the length of light-paths, and the other is due to the influence of the gravitational lens on the run-time of light-rays. If the size and the mass-distribution of the lens is known, one can determine from the measured delay of changes in brightness directly the total light-travel time from the quasar, hence its distance.

In this way, for the first time in 1995 the Hubble constant was determined - independent from previous methods - at the double quasar Q0957+561.