Ten years ago, on 14 September 2015, the ground-based gravitational wave detector LIGO started its first observing run. About 1.4 billion years earlier, two black holes had collided in a far-away galaxy, producing a powerful gravitational wave. Already then, it was determined that the wave would reach the Earth on precisely 14 September 2015. Yet for it to become the first gravitational wave measured by humanity, a lot had to happen.
But when the wave finally passed by Livingston in the United States, and seven milliseconds later through Hanford the LIGO detector was in place and operational right on time. From the time delay and the slightly different orientation of both detectors, astronomers figured out the direction of the wave. And from the frequency, amplitude and duration they deduced the source.
A 36 solar mass black hole had collided with a 29 solar mass black hole to create a 62 solar mass black hole. The missing three solar masses had turned into pure energy in the form of a ripple in spacetime during the final 0.2 seconds of the black holes spiraling into each other.
Since this first detection, over 200 more sources have been measured by LIGO and its European counterpart Virgo. In the meantime, a Dutch consortium is developing a large contribution to the first space-based successor to LIGO, called LISA. Three spacecraft will orbit the Sun in a triangle formation with arms spanning 2.5 million kilometers. SRON, Nikhef, TNO and the universities of Amsterdam, Groningen, Leiden, Maastricht, Utrecht and Radboud are working on hardware and software for LISA. This includes the photoreceivers that will detect the infrared laser beams in between the three spacecraft and the pointing mechanism that will precisely point the beams along the gigantic virtual arms. In 2023 the consortium received an NWO Roadmap grant, led by SRON.
LISA will be sensitive to gravitational waves with a frequency of 0.1 to 100 milliHertz. This is unchartered territory because ground-based detectors LIGO and Virgo can only measure frequencies between 10 and 10,000 Hertz. It allows LISA to be the first to hear collisions of supermassive black holes, with each other and with stars.
Also gravitational waves from the distant Universe have low frequencies, because their wavelengths are stretched out on their long journey through the ever expanding Universe. So LISA will also be the first to perform measurements on the period shortly after the Big Bang, when the Universe probably was expanding at unprecedented speeds.