Historical research guide · 0478 · 2021
Challenges and opportunities of gravitational-wave searches at MHz to GHz frequencies
Nancy Aggarwal · Odylio D. Aguiar · Andreas Bauswein · Giancarlo Cella · Sebastian Clesse · Adrian Michael Cruise · Valerie Domcke · Daniel G. Figueroa · Andrew Geraci · Maxim Goryachev · Hartmut Grote · Mark Hindmarsh · Francesco Muia · Nikhil Mukund · David Ottaway · Marco Peloso · Fernando Quevedo · Angelo Ricciardone · Jessica Steinlechner · Sebastian Steinlechner · Sichun Sun · Michael E. Tobar · Francisco Torrenti · Caner Ünal · Graham White
In one minute
Twenty-five authors, writing up the ICTP Trieste workshop, ask what we would hear if we listened for gravitational waves far above the band LIGO and Virgo cover — megahertz to gigahertz rather than hundreds of hertz. Their answer is that nothing in the known astrophysical catalogue is small and dense enough to ring up there, so anything found would be new physics: primordial black holes, exotic compact objects, axion clouds around black holes, or the early universe itself at energies near the grand-unification scale. The second half is a hardware catalogue. It runs through levitated nanoparticles, quartz acoustic resonators, microwave cavities, superconducting rings, ferromagnetic crystals whose spin waves a passing wave can excite, and above all magnetic conversion — the inverse Gertsenshtein effect, in which a gravitational wave crossing a strong magnetic field turns into a photon. The authors are frank about the distance still to cover: the best proposals fall about six orders of magnitude short. A century ago the gap was sixteen.
Connection
Half of this paper is a catalogue of ways to turn a gravitational wave into an electrical signal and back — magnetic conversion, polarisation rotation in a waveguide ring, superconducting mirrors, spin waves in a ferrite — which is exactly the electromagnetic-to-gravitational coupling that chapters 10 and 11 are built on, written out with real magnets, real quality factors and real sensitivities. It is also the honest ledger the evidence ladder needs: who has hardware running, who has only a design, and how far each still is from the number that would settle it.
From the source
There are no known astrophysical objects small and dense enough to emit gravitational waves beyond 10 kHz, so any discovery of gravitational waves at higher frequencies would indicate new physics beyond the Standard Model of particle physics.Sect. 1 Introduction, third paragraph; Sect. 3 opening; Sect. 5 Discussion and conclusions
The inverse Gertsenshtein effect converts gravitational waves into photons in a static magnetic field; re-interpreting data already taken by axion-search experiments has set the first upper limits on gravitational waves at optical and X-ray frequencies, and a 100-metre conversion path in a uniform 5.6 tesla field with electromagnetic detectors at a thermal noise equivalent of 0.1 kelvin is projected to reach a characteristic strain sensitivity of about 10 to the minus 26.Sect. 4.2.2, Inverse Gertsenshtein effect
Cosmic magnetic fields coherent over kiloparsecs or megaparsecs act as an enormous conversion volume, so radio data from ARCADE 2 and EDGES can be recast as bounds on the stochastic gravitational-wave strain in the 3 to 30 gigahertz range and near 78 gigahertz.Sect. 4.2.2, closing paragraph
A gravitational wave passing through a ferromagnetic insulator can resonantly excite magnons, the collective excitations of electron spins, read out by placing the sample inside a microwave cavity; the projected strain sensitivity is about 7.6 times 10 to the minus 22 per root hertz at 14 gigahertz and about 1.2 times 10 to the minus 20 per root hertz at 8.2 gigahertz.Sect. 4.2.9, Graviton-magnon resonance; Table 1
Quartz bulk acoustic-wave devices reach quality factors up to 8 times 10 to the ninth at cryogenic temperatures across 5 to 700 megahertz with more than 100 sensitive modes in a single device, giving an estimated strain sensitivity of about 2 times 10 to the minus 22 per root hertz; a search using one such device and two modes at 4 kelvin has been running at the University of Western Australia since November 2018.Sect. 4.2.6, Bulk acoustic-wave devices
None of the proposals listed currently reaches the sensitivity needed to probe the new physics outlined, the best being at least six orders of magnitude short — but one hundred years ago the technological gap in both the LIGO and LISA bands was about sixteen orders of magnitude, and detectors based on magnetic conversion or on the deformation of microwave cavities seem the most promising avenues.Sect. 5 Discussion and conclusions, paragraphs 5–7
The way in
https://doi.org/10.1007/s41114-021-00032-5How to cite it
Nancy Aggarwal, Odylio D. Aguiar, Andreas Bauswein, Giancarlo Cella, Sebastian Clesse, Adrian Michael Cruise, Valerie Domcke, Daniel G. Figueroa, Andrew Geraci, Maxim Goryachev, Hartmut Grote, Mark Hindmarsh, Francesco Muia, Nikhil Mukund, David Ottaway, Marco Peloso, Fernando Quevedo, Angelo Ricciardone, Jessica Steinlechner, Sebastian Steinlechner, Sichun Sun, Michael E. Tobar, Francisco Torrenti, Caner Ünal, Graham White (2021) Challenges and opportunities of gravitational-wave searches at MHz to GHz frequencies doi:10.1007/s41114-021-00032-5