Angular Control Noise in Advanced Virgo and Implications for the Einstein Telescope

With significantly improved sensitivity, the Einstein Telescope (ET), alongwith other upcoming gravitational wave detectors, will mark the beginning ofprecision gravitational wave astronomy. However, the pursuit of surpassingcurrent detector capabilities requires careful consideration of technicalconstraints inherent in existing designs. The significant improvement of ETlies in the low-frequency range, where it anticipates a one million-foldincrease in sensitivity compared to current detectors. Angular control noise isa primary limitation for LIGO detectors in this frequency range, originatingfrom the need to maintain optical alignment. Given the expected improvements inET's low-frequency range, precise assessment of angular control noise becomescrucial for achieving target sensitivity. To address this, we developed a modelof the angular control system of Advanced Virgo, closely matching experimentaldata and providing a robust foundation for modeling future-generationdetectors. Our model, for the first time, enables replication of the measuredcoupling level between angle and length. Additionally, our findings confirmthat Virgo, unlike LIGO, is not constrained by alignment control noise, evenunder full power operation.