Advanced Virgo+ status and future perspectives (Invited Paper)

The Virgo detector contributed to the observations in the O3 observing run and increased its sensitivity from the initial 46 up to 60 Mpc during the run. The detector has undergone to a series of improvements since the end of the O3 observing run in view of O4, as the implementation of an additional recycling cavity at the output of the interferometer – the Signal Recycling cavity (SRC) – to broaden the sensitivity band, the Frequency Dependent Squeezing (FDS) to reduce quantum noise at all frequencies, and a new higher power laser. Some criticality have emerged mainly due to the presence in Virgo of marginally stable recycling cavities with respect to the stable recycling cavities present in the LIGO detectors, which increases the difficulty in controlling the interferometer in presence of defects as those introduced by the high power on the mirrors. This resulted in a delayed joining the O4 run due to a longer than expected commissioning phase. At present the detector is running with a lower laser power (and a lower sensitivity w.r.t. the project design). A new stop of about 2 yr is planned between O4 and O5 starting in 2027, to implement new upgrades (phase II). To improve the stability of the interferometer, we are considering a large upgrade to introduce stable cavities and this will imply heavy infrastructural works also with the minimal design. The aim is to reach a Binary Neutron Star (BNS) sensitivity larger than 100 Mpc. Plans are being made for the post-O5 period as a bridge between 2nd and 3rd generation detectors and a new collaborative effort has born under the name of Virgo nEXT with the aim to keep and push the infrastructure and maintain alive the community.