Modelling Newtonian noise of vibroacoustic origin in the Virgo gravitational wave detector

With ~100 detections by the LIGO-Virgo-KAGRA network, gravitational astronomy has definitely begun. The Virgo detector is a modified Michelson interferometer with two arms of 3km each, made of optical resonator housing ~100 kW laser beams. It can measure a gravitational strain (ΔL/L) of ~10^(-21). The detector is susceptible to various sources of noise within specific frequency bands. Among these, the "Newtonian" or "gravity-gradient" noise (seismic and atmospheric/acoustic) could limit the low-frequency sensitivity (below a few tens of Hz) of LIGO and Virgo as they reach their full sensitivity, as well as that of next-generation detectors such as the European Einstein Telescope and the US Cosmic Explorer. This paper focuses on modeling Newtonian noise of acoustic origin, which refers to the small fluctuations in the gravity field resulting from the acoustic field present in the experimental halls. The fluctuating gravitational field directly causes random forces on sensitive optical elements, such as the interferometer test masses. The induced noise is quantified using a numerical acoustic model of the experimental room when it is excited by the air conditioning system.