Gravitational waves

Learning from gravitational waves

Nikhef’s Gravitational Waves research programme brings together Dutch research into gravitational waves. Researchers analyse vibrations from colliding compact objects such as black holes in the universe using the Virgo, LIGO and KAGRA detectors, and are developing new techniques and observatories such as the Einstein Telescope.

What are gravitational waves?

Gravitational waves are vibrations of the universe itself, which can also be measured on Earth using sensitive instruments via subtle changes in length in detectors spanning kilometres.

Such vibrations of space-time arise when extremely compact objects such as black holes or neutron stars collide and devour one another. In 2015, a gravitational wave was detected for the first time using the two LIGO detectors in the US. Since then, hundreds of gravitational waves have been observed by LIGO and also by the European Virgo detector.

The field of research has since taken off, including for Nikhef. The detection of gravitational waves has opened a new window onto the universe, for both physicists and astronomers.

What is gravitational wave research?

On the one hand, gravitational wave research focuses on detecting and interpreting the measured spacetime waves and their sources in the cosmos. This offers new astrophysical insights into black holes and the universe, as well as opportunities to test the theory of relativity itself.

On the other hand, the development and improvement of measurement and analysis techniques and detectors is an important field of research in its own right. Gravitational waves are so subtle that they are barely detectable amidst the natural vibrations of the detectors and their surroundings.

The observatories search for subtle variations in distance. To this end, laser light is confined between mirrors in two perpendicular beams, causing them to precisely cancel each other’s light waves. A passing gravitational wave alters the path lengths of the beams, causing this cancellation to temporarily cease.

From the details of the changing light pattern, properties of the wave can be deduced, as well as properties of the colliding black holes that caused it, such as masses, spin and orientation.

Despite numerous technical measures, large optical installations such as those at LIGO or Virgo inevitably exhibit noise and vibrations, making it difficult to detect gravitational waves in the signals.

When analysing the measurement signals, researchers therefore look for patterns that are theoretically expected from distant colliding black holes. Such patterns follow from relativistic models of moving and colliding black holes.

By combining measurements from different observatories, the direction of the source can be determined. When a measurement is made, astronomical partners are alerted so that telescopes can scan the sky.

Building sensitive gravitational wave detectors is a major technical challenge and a field of research in its own right. The installations must be decoupled from the inevitable vibrations in the environment using mechanical suspension techniques. The laser beams and mirrors must be precisely stabilised. Only then can the sought-after length variations, which are smaller than the diameter of an atomic nucleus, be measured with certainty.
Nikhef and gravitational waves

Nikhef is a member of the international LIGO/Virgo/KAGRA collaboration, comprising a number of gravitational wave observatories. Nikhef’s gravitational wave group comprises around 75 researchers spread across three universities: Maastricht, Utrecht and VU Amsterdam. The researchers make intensive and international contributions to the analysis of the measurements taken by the various detectors.

Nikhef is one of the coordinating institutes for the Advanced Virgo detector near Pisa, where new optical and mechanical noise-reduction techniques have been supplied to increase sensitivity. Nikhef’s mechanical workshop is heavily involved in this, both in design and construction.

Nikhef is also spearheading the initiative to realise a new generation of gravitational wave detector in the border region of the Netherlands, Belgium and Germany: the Einstein Telescope. At European level, the institute is a partner in the ETO organisation surrounding this future Einstein Telescope, which will peer much deeper into the universe than current observatories. Other locations are also being studied.

Together with Maastricht University, Nikhef is working to realise the ETpathfinder: a test facility for new instruments and techniques for even more sensitive gravitational wave detectors such as the Einstein Telescope. Research is being conducted into, among other things, different mirrors, different wavelengths for the lasers used, and extreme cooling of mirrors to suppress noise.

In 2026, the government pledged additional support for a planned expansion of the ETpathfinder, which will then be able to test techniques for the Einstein Telescope on a full scale.

Through the Nikhef Detector R&D research group, Nikhef is also involved, via ESA, in the development of components for the LISA gravitational wave detector in space.
More information

The universities participating in the Nikhef research program Gravitational Waves are University of Amsterdam, Free University Amsterdam, University Utrecht, University Maastricht, University of Groningen and Radboud University (Nijmegen).

Virgo collaboration

Einstein Telescope on the Nikhef website

Website of the Einstein Telescope

ETpathfinder information