Challenges and Opportunities of Gravitational Wave Searches at MHz to GHz Frequencies (2011.12414v2)

Abstract: 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, with a particular focus on Ultra High-Frequency Gravitational Waves (UHF-GWs), covering the MHz to GHz range. The absence of known astrophysical sources in 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 gravitational sources. We review several detector concepts which 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 the workshop "Challenges and opportunities of high-frequency gravitational wave detection" held at ICTP Trieste, Italy in October 2019, that set up the stage for the recently launched Ultra-High-Frequency Gravitational Wave (UHF-GW) initiative.

• The paper presents innovative detection proposals for ultra-high-frequency gravitational waves using strain sensitivities around 10⁻²² Hz⁻¹/².
• It identifies key technical challenges such as mechanical and quantum noise while outlining pathways to scale current detector technology.
• The study highlights potential breakthroughs in early Universe cosmology and high-energy physics, opening new avenues beyond the Standard Model.

Overview of High-Frequency Gravitational Wave Detection

Gravitational wave (GW) astronomy is at the forefront of modern astrophysics, opening up avenues for observing the universe through ripples in spacetime. This paper discusses the challenges and possible advantages in searching for gravitational waves at frequencies higher than those covered by current detectors such as LIGO and Virgo, which operate best at frequencies between 10 Hz and a few kHz. There is a particular emphasis on ultra-high-frequency gravitational waves (UHF-GWs) in the MHz to GHz range, an area less explored in modern astrophysics but full of potential for discoveries beyond the Standard Model of particle physics.

Theoretical Implications and Prospects

Detecting UHF-GWs is motivated by the absence of known astrophysical sources in this frequency range. Instead, any discovery at these frequencies could suggest new physics. For instance, phenomena such as primordial black holes, early Universe phase transitions, and cosmic strings can produce high-frequency GWs. These could potentially offer insights into moments shortly after the Big Bang, allowing us to probe energy scales far beyond the reach of particle accelerators. The detection of UHF-GWs could thereby provide unprecedented constraints on early Universe cosmology and high-energy physics.

Detection Proposals and Sensitivity Goals

Several innovative detector concepts target these high-frequency gravitational waves. Among them are optically levitated sensors, which can achieve strain sensitivities on the order of $10^{-22} \, \text{Hz}^{-1/2}$ over a wide frequency band. Other promising approaches include using the inverse Gertsenshtein effect, whereby cosmic magnetic fields convert GWs into photons, and employing high-quality superconducting rings and bulk acoustic wave devices. Each proposal offers unique advantages but also faces formidable technical challenges, particularly concerning the sensitivity required to detect the very minute GW signals anticipated at such high frequencies.

Current and Future Developments

Though no detector currently reaches the requisite sensitivity to probe the strength of these hypothetical UHF-GW sources, technological advancements continue to narrow the gap. For example, the planned Neutron Star Extreme Matter Observatory (NEMO) aims to achieve impressive sensitivity at frequencies around 2 kHz, providing proof of concept for scaling these technologies to much higher frequencies.

The development of high-frequency detectors requires overcoming challenges analogous to those faced by early laser interferometers—then considered impractical by many. These include addressing issues related to mechanical and quantum noise and enhancing coherence length measurement capabilities. Successful detection of high-frequency GWs might revolutionize our understanding of fundamental physics, much like radio and X-ray astronomy did over half a century ago.

Conclusion

UHF-GW astronomy remains a speculative yet potentially transformative frontier. By successfully implementing the various proposed detection schemes, the scientific community could probe phenomena from before cosmic recombination, offering insights into both the microphysics of the early universe and the macroscopic architecture of spacetime. As technology improves, the prospects for high-frequency GW detection could eventually bring pivotal new discoveries in cosmology and particle physics.