The detection of gravitational waves (GWs) using infermometer experiments, like LIGO and Virgo interferometers marked the beginning of a new era in astronomy. Sources of those gravitational waves are typically merger events of black holes with large masses, e.g. formed in supernova compressions.

However, several models also predict significantly lighter black holes, which have been created shortly after the big bang. During the period of rapid expansion known as inflation, and the subsequent era dominated by radiation, certain regions of subatomic matter could have reached such extreme densities that they collapsed under their own gravity, yielding black holes. Given their origin in the primordial area of the universe, those objects are called primordial black holes (PBH). They hypothetical masses are far below solar masses, hence they are relatively light objects. The merger event of those PBHs would create gravitational waves in the MHz to GHz regime, starting with low frequencies and ending with very high frequencies, corresponding to the shrinking orbit of the two black holes during a merger event.

The resulting high frequency gravitational waves cannot be observed by interferometer experiments, however, they are believed to leave signatures in electromagnetic cavities which are placed in a strong magnetic field. The basic principle behind the cavity-based experiments is simple: a gravitational wave distorts the cavity’s shape, altering the magnetic flux through the cavity and generating an electric signal that can be detected. Additional, the GW couples directly to the EM field via the inverse Gertsenshtein effect. Hence, a gravitational wave that is passing through a cavity within a static magnetic field, creates an effective current in Maxwell’s equations, leading to an electromagnetic field that oscillates at the same frequency as the gravitational wave. The induced electromagnetic field can be resonantly enhanced using microwave cavities and the generated radio frequency power detected. This approach for the detection of HFGW is the basis of the GravNet Collaboration.