On Saturday the 4th July, the GEO 600 gravitational-wave detector in Hannover, Germany held its final public open day ahead of its closure at the end of this year. As a student of both Glasgow and Cardiff universities, who played major roles in the research conducted with GEO 600, and as a member of the GEO collaboration, this detector is very close to my heart. So I made the journey to Hannover to pay my respects and say goodbye from all of us at Glasgow (dressed in black, of course!).

The Open Day

The final public open day was a fitting farewell to the detector. I was pleasantly surprised by the crowds of people that had made their way to GEO 600. We were greeted by scientists explaining all aspects of gravitational-wave research from the Einstein@Home project to future space-based missions such as LISA and I picked up my fair share of postcards and stickers to display in the office. The highlight of the open day, however, was the tour that took us onto a viewing platform above the detector where we could see into the heart of the interferometer. Here, the tour guide talked us through the path of the laser and showed us one of the mirrors. We finished the day with a talk held in the control room before we walked down all 600 m of the east arm, following the laser down to the end station and bidding our final goodbyes to GEO 600.

GEO 600

GEO 600 was born when researchers from Germany and the UK decided to join forces to build a gravitational-wave detector in 1989. Originally GEO was planned to be a 3 km interferometer, similar to Virgo in Italy, but due to funding issues, it was downsized to 600 m instead. Construction began in September of 1995, 20 km south of Hannover, and was completed in 2001. While intended to be the case, GEO 600’s arms are not truly perpendicular to each other, instead they are slightly less than 90 degrees apart. To be perpendicular, the east arm needed to pass through a neighboring field but the owner would not give permission for their land to be used. A similar story nearly happened with the north arm but a gift of a bottle of fine Schnapps saved the day!

Once construction was completed, GEO 600 joined the LIGO Hanford and Livingston detectors during their first science runs from 2002-2009. GEO 600 has also participated in all four of the LIGO-Virgo-KAGRA observing runs and was operated alongside the Japanese KAGRA detector for two weeks in 2020 during the O3GK observing run. GEO 600 has been a part of gravitational-wave detection from the very beginning, and while it has never detected a gravitational wave itself, it has played a very important role in their detection.

Due to its shorter arm length, GEO 600 would never be as sensitive as its larger American, Italian and Japanese cousins in the frequency range we typically use to search for gravitational waves (more on this later) but its part in gravitational-wave detection has been invaluable, serving as a test-bed of new technologies that live inside the LIGO and Virgo detectors today. It was in GEO 600 that optical squeezing was first performed, a method to surpass the fundamental sensitivity limit nature puts on gravitational-wave detectors due to the quantum nature of light. Another key technology that started life at GEO 600 is signal recycling: adding a recycling mirror between the beam splitter and output of the interferometer, further improving the sensitivity of the detector and allowing us to tune it to specific frequencies of gravitational waves.

Since 2025, GEO 600 has continued the search for gravitational waves, but turning its focus to very high-frequency waves. Previously, detectors have searched within a frequency range of 10–6000 Hz but using the tuning provided by the signal recycling mirror and with a new laser amplifier and upgraded data acquisition systems, GEO 600 is looking for waves at frequencies of up to 2 MHz (2,000,000 Hz), something that simply cannot be done with the current set up of the LIGO, Virgo or KAGRA detectors. Researchers believe that these very high-frequency waves could be emitted from sources we have yet to detect such as sub-solar-mass compact objects or possibly mergers of light black holes that were formed in the very early universe, known as primordial black holes or even the result of a merger, as was looked for after GW170817. GEO 600 will continue searching for these high-frequency waves until the 31^st of December of this year before it is shut down for the last time. 

I think a brand-new discovery would be a very fitting swan song for GEO 600 in its final moments, so I’m keeping my fingers crossed. Either way GEO 600’s legacy and the impact it has had on gravitational-wave astronomy will not be forgotten. 

We will miss you GEO 600.

Happy classifying,

Elizabeth 🕵️‍♀️

On the 26^th May, the LIGO-Virgo-KAGRA Collaboration (LVK) released the fifth update to the Gravitational-Wave Transient Catalogue (GWTC). This update introduces 161 new candidate signals, bringing the total number of probable gravitational-wave candidates to almost 400. Within the catalogue were extra special events: from the loudest gravitational-wave signal detected to date to the best localised event to date and an event that allowed the first informative tests of a technique known as astrophysical calibration, where we can use gravitational-wave signals to tune our detectors.

GWTC-5.0 introduces events detected during the second part of the fourth observing run (04b) which took place for almost 9 months between April 2024 and January 2025. Rejoining the LIGO detectors for O4b was Virgo, after mitigating the instabilities and noise introduced by the installation of a signal recycling mirror during O4a, increasing the bandwidth of the Virgo detector. Between O4a and O4b, the LIGO detectors also underwent upgrades: LIGO Hanford’s coupling of nonlinear noise was reduced by the recommissioning of the main feedback control loop that controls the differential arm length and output loss was reduced by swapping the output mode cleaner with a spare. LIGO Livingston’s end test masses were cleaned, leading to a 15% increase in the gain generated by power-recycling and a 1.6 dB reduction in quantum noise was achieved through adjusting alignment controls. Both LIGO detectors did face problems during the run, with the unexplained glitching of Hanford’s laser and the shutter to protect Livingston’s output mode cleaner failing, both leading to multiple locklosses. These issues were swiftly resolved, however, by the teams at Hanford and Livingston. The upgrades applied to the detectors have led to increases in their sensitivity, and we are now starting to see events with more clarity. 

Due to these higher-sensitivity detectors, GWTC-5.0 introduces many loud events, with five of these with signal-to-noise ratios (SNRs) of over 30, louder than the first direct detection of gravitational-waves, GW150914 which was made with an SNR of ~24. The loudest event released in GWTC-5.0, and the loudest detected by the LVK to date, is GW250114, with an SNR of almost 80, almost twice as loud as the previous record-holder. GW250114 was detected almost 10 years after the first direct detection of gravitational waves, GW150914, and is a very similar signal. What’s most impressive, however, is how improvements to detector sensitivity through upgrades have reduced the amount of noise present in the detectors. By seeing events with this much clarity, we are able to perform tests of general relativity: investigating the nature of gravity itself! To do this, we can see if the modes in the ringdown stage of the black hole merger, similar to how a bell rings at a specific tone when it is hit, match up to those predicted by general relativity. While small differences between the observed and expected modes may be observed, they could have been made by deviations from general relativity or from noise such as glitches. With GW250114 being the clearest signal to date, we were able to constrain possible deviations with, in some cases, more precision than when combining 17 events from GWTC-4.0! No clear deviations from general relativity were spotted, so for now the theory holds true.

As well as seeing our signals more clearly, with upgrades and Virgo joining in the search once more, we have been able to narrow down the possible source locations of the gravitational-wave candidates better than ever before. Five of the events released as a part of GWTC-5.0 have sky localisations as least as narrow as the previous most localised event, GW170817. One of these events, GW240615_113620, is localised to just 6 square degrees, beating GW170817’s 16 square degrees by a wide margin, again showcasing the impressive upgrades the detectors have gone through and the importance of multiple gravitational-wave detectors.

Loud and clear signals help us improve our detections, and when they are not calibrated, we can also use them to help our detectors. GW240925_005809 (GW240925 hereafter) was detected when the LIGO Hanford detector wasn’t correctly calibrated. We can think of it as if the detector was out of tune. GW240925 was from a merger of black holes 9 and 7 times the mass of the Sun and it was very loud, with an SNR of over 30, which meant we could perform the first informative tests of a technique called astrophysical calibration: using this signal to tune the Hanford detector so that we can use detector data when they’re not correctly calibrated. Ensuring correct calibration means that we can use and trust data from our detectors when they are not perfectly tuned. While in this case, we were testing astrophysical calibration, and we didn’t need to use it to trust our Hanford data, in the case of GW250207_115645 (detected just after the conclusion of O4b and therefore releasing at a later date), the Hanford detector was so unsettled that without astrophysical calibration, we could not use its data at all! 

Another pair of exciting events gave us insights into the formation of binary black holes. GW241011_233834 and GW241110_124123 (GW241011 and GW241110 hereafter) were detected just under a month from each other and showed evidence for the heavier black hole involved in the merger to be highly spinning. When black holes merge, they leave behind a heavier black hole that is rapidly rotating, or spinning. For the heavier black hole in both GW241011 and GW241110 to be highly spinning suggests that they could have been the product of previous mergers, telling us about the history of the black hole pairs in the universe.

With this many detections, it is probably not a surprise that we saw a few coincident with glitches. Almost a quarter of the 161 new additions to the GWTC required glitch mitigation. The eagle-eyed among you that came across subject 103695457 may have spotted something chirp-like in the 4 s view of the glitch. This was, in fact, GW240930_035959 which appeared very close to the koi fish glitch that was uploaded to Gravity Spy.

GWTC-5.0’s 161 new additions showcase the clarity and precision of detections we are now able to achieve with our upgraded gravitational-wave detectors, allowing us to explore the physics behind these events. While GWTC-5.0 does give us plenty of data to play with for now, we can also start to look forward to GWTC-6.0 releasing in December, with yet more exciting events to come!

Happy classifying,

Elizabeth 🕵️‍♀️

The LVK will be hosting a webinar summarising the GWTC-5.0 introduction, methods and results papers on Thursday, July 9, 2026, at 10:00 AM Eastern US. Click here to register for the webinar and here for the recording when it becomes available.

Alongside the release of the O4a data on the 26^th of August 2025, the LIGO–Virgo–KARGA (LVK) Collaboration published the latest update to the Gravitational-Wave Transient Catalogue (GWTC), bringing the version from 3.0 to 4.0. This update added 128 probable gravitational-wave signals, more than doubling the number observed by the LVK.  

Within the 128 new additions came some LVK record-breakers: from likely the most massive binary system observed, GW231123_135430, to the loudest (signal-to-noise ratio of ~40) detection, GW230814_230901 (since beaten, but still very good). Also included were two candidates thought to be from neutron star-black hole binaries, GW230529_181500 and GW230518_125908, the latter detected during the pre-O4 engineering run. Perhaps the most exciting news for the Gravity Spy project is that of the 128 events, 37 required glitch mitigation, that’s almost 30%! In the case of GW230708_053705 we have a great example of a glitch overlapping in time with a gravitational-wave signal in the Hanford detector. 

Following the release of GWTC-4.0, the LVK published papers covering topics such as tests of Einstein’s theory of gravity, known as general relativity, and implications for cosmology, showcasing the science that can be performed with gravitational wave data.  

We have lots to look forwards to in future updates to the GWTC as there are many results from O4b, planned to be released in under two weeks, and O4c still to come.   

Happy classifying 🕵️‍♀️, 

Elizabeth 

For more information about the data release, take a look at this LIGO news article and this recent LVK webinar.