Leveling up GEO600

The detector is being upgraded to observe gravitational waves at very high frequencies.

April 25, 2025

The light from the new laser system recently installed at GEO600 shines through four cubes made of a special crystalline material. Inside these cubes, the properties of a part of the laser light are slightly modified to operate the gravitational-wave detector. The near-infrared (and therefore invisible) laser light appears violet in the image because the infrared filter was removed from the camera used to take the picture.

M. Weinert / Max Planck Institute for Gravitational Physics

The light from the new laser system recently installed at GEO600 shines through four cubes made of a special crystalline material. Inside these cubes, the properties of a part of the laser light are slightly modified to operate the gravitational-wave detector. The near-infrared (and therefore invisible) laser light appears violet in the image because the infrared filter was removed from the camera used to take the picture.

M. Weinert / Max Planck Institute for Gravitational Physics

To the point

Towards entirely new observations: The gravitational-wave detector GEO600 is being upgraded to enable gravitational-wave observations at very high frequencies. The calibration of the detector and first test runs are expected in 2025.
New insights: The detection of high-frequency gravitational waves could shed light on dark matter, exotic physics, and the early Universe.
Ready for the future: The upgrades at the cutting edge of science will maintain GEO600's status as a pioneer in gravitational-wave research.

Gravitational waves

Gravitational waves are ripples in space-time that travel at the speed of light. They are caused by the accelerated motion of masses, such as pairs of black holes orbiting each other. The rhythm of this motion determines the rhythm, or frequency, of the gravitational waves sent out into space: The faster the orbital motion, the higher the frequency of the gravitational waves.

Current instruments such as LIGO, Virgo, and KAGRA observe gravitational waves in a frequency window from 10 hertz to 6000 hertz, where hertz measures oscillations per second. In this window, mergers of black holes and / or neutron stars are routinely observed, with more than 90 confirmed signals and more than 200 signal candidates as of today. Signals from rotating isolated neutron stars and stellar explosions are also expected in this frequency range.

Insights into dark matter, exotic physics, and the early Universe

“Our Universe may harbor exotic objects that emit gravitational waves at very high frequencies – up to a million hertz and beyond,” says James Lough, the GEO600 lead scientist. “Searching for, finding and also failing to find these rapidly oscillating space-time ripples will deepen our understanding of cold dark matter, exotic physics, and the early Universe.”

This upgrade will keep the GEO600 at the cutting edge of science and will maintain its status as a pioneer in gravitational-wave research. This will be our first deep look into a largely unexplored part of the gravitational-wave spectrum.

Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), and director of the Institute for Gravitational Physics at Leibniz University Hannover

Upgrading a gravitational-wave detector

The international network of gravitational-wave detectors has observed more than 90 signals from mergers of black holes and / or neutron stars, such as GW230529, an inspiral of a lower mass-gap black hole (dark gray surface) and a neutron star (orange sphere). In this numerical-relativity simulation, the emitted gravitational waves are shown in colors from dark blue to cyan.

© I. Markin (Potsdam University), T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics), H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics)

The international network of gravitational-wave detectors has observed more than 90 signals from mergers of black holes and / or neutron stars, such as GW230529, an inspiral of a lower mass-gap black hole (dark gray surface) and a neutron star (orange sphere). In this numerical-relativity simulation, the emitted gravitational waves are shown in colors from dark blue to cyan.

© I. Markin (Potsdam University), T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics), H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics)

There are two major upgrades at GEO600. They are related to the laser source used to measure the distance changes caused by gravitational waves, and to making and digitally storing these measurements.

The laser source at GEO600, which served as a “blueprint” for the laser systems used in other gravitational-wave detectors worldwide, has been upgraded. A new laser amplifier has been installed that will provide up to 70 watts of laser power to the detector and increase its measurement precision.

Additionally, the data acquisition system at GEO600 will be upgraded with new components. These will take measurement data 4 million times each second to enable the observation of high-frequency gravitational waves. Previously, GEO600 data were sampled 16 thousand times each second, far too slow for the planned new observations.

The path towards observing very high frequency gravitational waves

The new high-power laser amplifier has been successfully installed at GEO600 in August 2024. It has been running reliably since then. First data have been taken with the new high-speed acquisition system.

“Our next steps will be putting everything together to calibrate the detector, assess its sensitivity, hunt for noise sources and mitigate them to further improve sensitivity,” explains James Lough. “Once everything is in place, it's going to get exciting. Then we can start the first test observations and feed the new data into our data analysis pipeline.”

“This is a step into uncharted territory and a realm of new gravitational-wave sources. GEO600 scientists will look for signals that no one has been able to search for before,” adds Karsten Danzmann.

Background information

Sources of high-frequency gravitational waves

According to our current understanding, our Universe contains five times more dark matter than ordinary matter. The nature of this ubiquitous yet elusive form of matter is still unknown. One popular explanation is that it consists of an unknown elementary particle.

Certain types of the this dark matter, called “ultralight bosons“, could form clouds around black holes. Typical black holes spin rapidly with a lot of energy stored in their rotation. Ultralight bosons particles in these clouds can tap into this energy reservoir and convert it into high-frequency gravitational waves through a process known as “black hole superradiance”.

The gravitational waves generated by boson clouds around spinning black holes would be sent into space for very long times. Their frequency depends on the mass of the (unknown) ultra-light bosons and on the properties of the black hole they orbit. The search for high-frequency gravitational waves with GEO600 will target such clouds in which the bosons have masses in a previously unexplored range.

Other possible sources of high-frequency gravitational waves are

Mergers of very light black holes formed in the early Universe, called primordial black holes,
Mergers of exotic compact objects, which resemble neutron stars and black holes, but which are composed of yet unknown elementary particles,
Various processes in the early Universe.

Tuning the detector: scanning at high frequencies

Gravitational waves change the brightness of the laser beam leaving the interferometer that is at the heart of the detectors. Sending these brightness changes back into the interferometer with a carefully placed partially reflective mirror can increase the overall gravitational-wave sensitivity of the detector.

This method known as “signal recycling” can also be used to “detune” the detector, making it much more sensitive to a specific, rather narrow band of frequencies – similar to tuning an FM radio to a specific station. In gravitational-wave detectors this is done by adjusting the position of the partially reflective mirror by microscopic amounts. The GEO600 team has experience with this: They operated their detector in this detuned mode before to optimize the detector's sensitivity to gravitational waves.

Tuning the detector to gravitational waves of a certain frequency is not a free lunch. This makes the instrument less sensitive to frequencies outside the target frequency band. By repeatedly changing the detuning and observing at the best sensitivity, it is possible to scan a wide range of frequencies with sensitivities that would otherwise be unattainable.

For continuous gravitational waves at very high frequencies, which GEO600 will target, this detuning plays a key role in covering the wide range of possible signal frequencies and types.