Low frequency gravitational waves give access to heavier, and also more red-shifted systems, of great interest from the general relativity and astrophysics points of view.
Low frequency signals are plagued with several noise sources. Instruments are far from the limit given by thermal and mirror motion from seismic noise. Other mirror degrees of freedom contribute, either directly or by coupling to error signals used in controls, either position or angular.
Another source of noise comes from diffused light recombining in a coherent way with the main beam bringing phase and amplitude noise in photodiodes.
Sensitivity of the interferometer to these noise sources has to be reduced with a critical analysis of the light circulating in the interferometer, of the immunity of the error signals, of the amount of spurious light present.
Seismic noise analysis deserves special attention, as subject to be studied using modern computing methods, as those arising from machine learning. Applications to the reduction of local gravity fluctuations can extend the lower frequency detection limit, making accessible heavier, or also more red-shifted systems to ground based interferometers. Application to early earthquake warning is an important result that can come from these studies.
Mirror coatings are the dominant noise source over most of the gw spectrum accessible to Virgo. A more systematic exploration from a condensed matter point of view of the properties of the coating materials used and of the deposition process can only give better insight of the dissipation mechanisms and suggest how to lower them.
Indirect but basic diagnostic techniques, to be correlated with dissipation measurements, can speed up the experimental iterations in search of new materials, guided by molecular dynamics simulations.