Virgo’s Next Leap: How Upgrades After O4 Will Redefine Gravitational-Wave Detection

It isn’t often you hear silence praised so loudly—but for the Virgo interferometer in Italy, moments of quiet have become the groundwork for major leaps forward. After postponing its participation in the start of Observing Run 4 (O4) due to sensitivity issues, Virgo is now activating a sweeping set of upgrades under the AdV+ program. These changes are designed to push the boundaries of what detectors can hear in the universe, and what astronomers can learn from the ripples in space-time.

Why Virgo delayed and where things stood in O4

Originally, Virgo was expected to join the LVK (LIGO-Virgo-KAGRA) observing run in May 2023 for O4. But internal tests revealed that one of its mirrors and its “super-attenuator” suspension weren’t delivering the quiet stability needed, and the interferometer couldn’t reach its target sensitivity of 80–115 megaparsecs (Mpc) for detecting merging neutron stars. Instead, it only achieved around 45 Mpc, so the team decided to hold off, fix the mirror issues, and improve the optics before entering data-taking.

Virgo finally rejoined in the second part of O4 (O4b) in April 2024, reaching about 50–55 Mpc sensitivity. Across the full O4, the LVK network logged roughly 250 gravitational wave candidates—more than doubling the total number of signals from the prior three runs combined. Virgo’s contribution, while delayed, played a crucial role in this milestone. Sensitivity drops and noise had threatened to hold the project back, but the scientific return remained high overall.

The AdV+ upgrades: What’s being built behind the scenes

Virgo’s current upgrade strategy, dubbed AdV+ Phase I (and leading into Phase II), is an architectural boost spanning lasers, mirrors, optics, and vacuum systems. Key enhancements now in place include:

• A new 50 watt fiber laser—doubling the input power compared to previous setups—aimed at increasing photon numbers and reducing quantum statistical noise.
• Installation of a signal recycling mirror replacing the older fixed lens system. This transforms the output optics into a tunable cavity to better shape sensitivity across key frequency bands.
• Upgraded detection benches and photodetectors with higher tolerance to power and improved filtering, including absorbing plates (“baffles”) to cut down on stray light and scattered-light noise.
• Improvements to the central vacuum chamber sealing, and improvements in the input mode cleaner (IMC)—a 144-meter cavity that cleans the laser beam’s shape before it enters the core interferometer.
• Implementation of frequency-dependent squeezing via a 300 meter filter cavity—this helps reduce quantum noise across both high and low frequencies instead of only at the top end.

Together, these upgrades aim to reduce quantum noise, address coating and thermal issues in the optics, and overall improve the detector’s “duty cycle” (the fraction of time it’s observing stably).

Sensitivities, timelines, and what O5 might look like

With the AdV+ Phase I changes, Virgo has pulled its binary neutron star (BNS) detection range up to 55 Mpc as of early 2025. That’s a modest bump from the 45 Mpc seen at the delayed start of O4, but still well below the original target. LIGO meanwhile is detecting BNS events from 160–170 Mpc in the same timeframe. Virgo is working and planning toward O5, where Phase II of AdV+ will aim at reducing “coating Brownian thermal noise” and installing larger, better mirrors to improve sensitivity further, especially at mid and high frequencies.

O4 officially wrapped up in November 2025 for the LVK collaboration. In its wake is a planned period of maintenance and upgrades. As of now, a short observing run is tentatively planned for the late summer or early fall of 2026, with full participation varying by site. But most projections place the official start of O5 in late 2027. In that run, Virgo hopes its enhanced optics will push its reach closer to that of LIGO’s, expanding its capacity to detect fainter and more distant events on a daily basis.

Broader impact: science, community, and what’s at stake

Why all this effort? Because each increase in sensitivity means not just more detections, but better ones. More frequent detections sharpen statistics on population of black holes, clearer signals help test Einstein’s relativity in the most extreme conditions, and better sky localization enables electromagnetic follow-ups that connect gravitational waves with light and other messengers. For Virgo, bringing down sources of noise—quantum, thermal, acoustic, environmental—is what will unlock that next level of discovery.

This moment also highlights scientific patience. Virgo chose to hold off from O4 until key issues were addressed, even though that delayed its data collection. Fixing the input optics, vacuum leaks, stray light, and laser power fluctuations is messy, slow work—but it is essential. The upgrades underway now set the stage not only for O5, but for future projects like Virgo_nEXT, and third-generation detectors such as the Einstein Telescope and Cosmic Explorer.

Conclusion: Virgo is not just catching up—it’s gearing up.

Key takeaways

• Virgo delayed its entry into O4 due to sensitivity shortfalls, particularly three key mirrors and the interferometer optics structure.
• AdV+ Phase I upgrades have already doubled laser power and restructured signal output optics, aiming to reduce multiple kinds of noise.
• The BNS detection distance now sits around 55 Mpc; O5 is scheduled for late 2027, with Phase II pushing sensitivity higher.
• These technical advances could open the door to daily detections, more precise gravitational wave astronomy, and new multimessenger breakthroughs.

In the end, Virgo’s path forward is one of steady refinement. It’s not just about catching every gravitational whisper; it’s about making sure those whispers tell us something profound about the universe.