Speaker
Description
Einstein Telescope is being designed for gravitational-wave observations in the audio band, where the dominant astrophysical event rates are expected. At frequencies above about 10 kHz, however, hypothetical cosmological sources may also produce detectable signals. Despite relevant previous research by other authors, it is not widely known that laser-interferometric GW observatories are not limited to low frequencies, but instead feature a comb of high optical sensitivity extending to high MHz and even GHz frequencies [1]. Here we examine this effect in the context of ET and show that high sensitivity is expected at integer multiples of the free spectral ranges of its resonators. We explain the physical mechanism behind this resonant response and discuss what would be required to access it in ET in practice, including the key technical limitations and readout challenges.
[1] Schnabel, R., Korobko, M. Optical sensitivities of current gravitational wave observatories at higher kHz, MHz and GHz frequencies. Sci Rep 15, 25733 (2025). https://doi.org/10.1038/s41598-025-08668-x