A femtosecond frequency comb is a simple and compact tool that allows the phase coherent connection of the radio frequency domain with the optical domain. It greatly simplified high precision optical frequency measurements of lasers and provides the long awaited clockwork mechanism for an all-optical atomic clock. Another emerging application of frequency combs is the calibration of conventional spectrographs for high precision measurements that can not be performed by direct laser excitation.

Maybe the most prominent examples are in astronomy where tiny Doppler shifts of stars are used to detect the recoil motion caused by planets in orbits. The current measurement accuracy is sufficient to detect “hot Jupiters” but fails for Earth like planets. Very sensitive velocity measurements are also required to directly probe the accelerating expansion of the Universe. Observing the acceleration directly rather than derive it from other observations assuming the validity of General Relativity can be help to learn more about the nature and the existence of dark energy.