Challenges and opportunities of gravitational-wave searches at MHz to GHz frequencies

The first direct measurement of gravitational waves by the LIGO and Virgo collaborations has opened up new avenues to explore our Universe. This white paper outlines challenges gains expected in wave searches at frequencies above LIGO/Virgo band, with a particular focus on Ultra High-Frequency Gravitational Waves (UHF-GWs), covering MHz GHz range. absence known astrophysical sources this frequency range provides unique opportunity discover physics beyond Standard Model operating both early late Universe, we highlight some most promising sources. We review several detector concepts which have been proposed take challenge, compare their sensitivity signal strength predicted various models. report is summary workshop "Challenges opportunities high-frequency detection" held ICTP Trieste, Italy October 2019, that set stage for recently launched Ultra-High-Frequency Wave (UHF-GW) initiative.