Challenges and Opportunities of Gravitational Wave Searches above 10 kHz

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 the challenges and gains expected in gravitational-wave searches at frequencies above the LIGO/Virgo band. The scarcity of possible astrophysical sources in most of this frequency range provides a unique opportunity to discover physics beyond the Standard Model operating both in the early and late Universe, and we highlight some of the most promising of these sources. We review several detector concepts that have been proposed to take up this challenge, and compare their expected sensitivity with the signal strength predicted in various models. This report is the summary of a series of workshops on the topic of high-frequency gravitational wave detection, held in 2019 (ICTP, Trieste, Italy), 2021 (online) and 2023 (CERN, Geneva, Switzerland).

💡 Research Summary

The white paper “Challenges and Opportunities of Gravitational Wave Searches above 10 kHz” provides a comprehensive overview of the scientific motivation, potential sources, and detector concepts for high‑frequency gravitational waves (HFGWs) in the 10 kHz–GHz band. It begins by emphasizing that, unlike electromagnetic radiation, gravitational waves can propagate freely from the earliest moments of the Universe, making them unique probes of physics at temperatures above 10¹⁰ GeV. Because conventional interferometers such as LIGO, Virgo, and KAGRA are limited to ≤ kHz frequencies, any detection above 10 kHz would almost certainly point to new physics beyond the Standard Model, whether in the form of exotic compact objects, primordial black‑hole evaporation, or early‑Universe phenomena like inflationary pre‑heating, first‑order phase transitions, oscillons, or topological defects.

The authors categorize sources into three classes—stochastic backgrounds, transient bursts, and persistent coherent signals—and discuss the expected strain amplitudes, spectral shapes, and characteristic frequencies for each. Late‑Universe candidates include light primordial black holes (which evaporate and emit high‑frequency bursts), memory effects from asymmetric mass ejection, exotic compact objects such as boson stars, and black‑hole superradiance. Early‑Universe mechanisms can generate broadband stochastic backgrounds with peaks in the tens of kHz to GHz range, especially if the source size is much smaller than the cosmological horizon at the time of production.

A major portion of the paper surveys more than twenty detector concepts, ranging from extensions of laser interferometers to resonant mass detectors, optically levitated sensors, bulk acoustic wave devices, microwave and millimeter‑wave cavities, low‑mass axion haloscopes, light‑shining‑through‑a‑wall experiments, and various electromagnetic‑gravitational conversion schemes. For each technology the authors present the target frequency band, projected strain sensitivity (typically 10⁻²⁰–10⁻²⁴ Hz⁻¹/²), current experimental status (conceptual, prototype, or first limits), and the dominant noise sources (thermal, quantum, seismic, electromagnetic interference). They highlight that most proposals are still at the proof‑of‑concept stage and that the projected sensitivities are generally several orders of magnitude above the predicted signal strengths.

The paper stresses the importance of cross‑correlation techniques between multiple detectors, introducing the overlap reduction function and discussing data‑acquisition challenges for long‑duration observations. It also outlines a roadmap for research and development: first, demonstrate low‑noise operation and quantify fundamental limits; second, improve coupling mechanisms (e.g., optomechanical transduction or electromagnetic conversion efficiency); third, scale up detector arrays and integrate them into a global network to enable stochastic‑background searches.

In the concluding section the authors argue that despite the technical challenges, the scientific payoff—potentially uncovering physics at the Grand‑Unification or even Planck scale—justifies a coordinated, interdisciplinary effort. They propose the establishment of the Ultra‑High‑Frequency Gravitational Wave (UHF‑GW) initiative to foster collaboration between theorists, experimentalists, and technology developers, and to support prototype construction, noise‑budget studies, and data‑analysis pipelines. The paper serves both as a status report of the field and as a call to action for the community to invest in the next generation of high‑frequency gravitational‑wave detectors.