Ten years of Gravitational Wave Astronomy

They were first observed on September 14, 2015: gravitational waves, which are created when two black holes merge and spread through the universe as distortions in space-time. Since these distortions are very (very!) small, the observation was a sensation that was recognized shortly thereafter—in 2017—with the Nobel Prize in Physics for leading scientists of the LIGO detector collaboration. Ten years later, gravitational wave astronomy has advanced further: in the current fourth observation run of the four detectors now working together, about a hundred events are reported each year.

An article by Jens Kube

The first “Wuiippp!”

In September 2015, it was the two LIGO detectors in Hanford and Livingston (both in the USA) that first detected (and heard) the characteristic signal of the increasingly rapid and intense gravitational distortion. We have described this first observation of GW150914 in detail here on Einstein-Online. Ten years later, the technical achievement of this first observation remains impressive: in the two L-shaped detectors, the length change of the four-kilometer-long L-arms was measured to be less than one-thousandth of the diameter of a nucleus of a hydrogen atom. The cosmic event that generated these waves was extremely powerful: two black holes with 29 and 36 solar masses merged, emitting the energy content of three solar masses in the form of gravitational waves. In the last 0.2 seconds of the merger process, this event released more than ten times as much energy as all the stars in the universe combined!

Black holes and neutron stars

The predecessor objects of such gravitational wave objects are the remnants of massive stars. In the case of GW150914, the predecessor stars were stellar giants with 40 to 100 times the mass of our sun, which had become black holes at the end of their fusion phase. Over the course of many billions of years, they approached each other by emitting very weak gravitational waves. Over billions of years, this spiraling became faster and more intense, and became observable in the final seconds. Since such binary black hole systems are not uncommon on a cosmic scale, another event was observed as early as December 2015.

However, in order to be able to observe the merger of less massive objects such as neutron double stars, the sensitivity of LIGO had to be further improved. In the second observation run of the detectors in the USA – supported at the end of the run by the European Virgo detector near Pisa – the signal from the merger of two neutron stars was actually detected in August 2017: GW170817. This signal was visible in the detectors for around 100 seconds – a stroke of luck, in a sense, because the objects were much closer to us than the black holes observed previously, and so the gravitational waves, which were much weaker at the source, arrived strong enough for the detectors on Earth.

What exactly generated the signal?

Compared to astronomy in the electromagnetic range (light, radio waves, X-rays), gravitational wave astronomy faces a crucial additional challenge: it does not know its observation objects in advance, and the observation itself lasts only a few seconds. Nevertheless, the gravitational wave detectors (LIGO Hanford, LIGO Livingston, Virgo near Pisa, KAGRA in Japan, and GEO600 near Hanover) generate enough data to reconstruct which objects have merged based on the shape of the gravitational waves.

Through more detailed analysis and comparison with complex model calculations, it is even possible to determine information about the internal structure of the merging neutron stars. In recent years, the physical understanding of these objects has been advanced with the help of gravitational wave astronomy.

Multi-messenger astronomy

Equally important for understanding the objects beyond mere identification is the immediate observation of the objects with other astrophysical sensors, i.e., conventional telescopes and particle detectors. Unlike large telescopes, which can only observe a very narrow region of the sky at any given time, gravitational wave detectors are always ready to receive signals from all directions – even through the entire Earth, because gravitational waves are not shielded by anything. For this reason, the processes for identifying gravitational waves have been automated to such an extent that immediately after a gravitational wave passes through the detectors, an alarm can be sent to other observatories, which can then observe the corresponding region of the sky as quickly as possible.

First catalogs of events

In the first two observation runs of LIGO (2015/16 and 2016/17) and Virgo since the second half of 2017, a total of 11 mergers of massive objects were observed: 10 times binary systems of two black holes, once of two neutron stars. Together with the third observation run (2019/20), which was supplemented in its final weeks by the Japanese KAGRA detector, the catalog was expanded to 90 gravitational wave signals in November 2021. This also includes some particularly unusual events, such as a signal in which a neutron star with only 1.17 times the mass of the Sun was swallowed by a black hole with 32 times the mass of the Sun.

The expanded observation capabilities were made possible by the installation of a special component that generates what is known as squeezed light. This development reduces the quantum noise of the required laser light and makes the detectors even more sensitive to weak signals.

The fourth observation run: improved squeezed light, AI, and a record

The fourth observation run of the LIGO, Virgo, and KAGRA collaboration has been underway since May 2023. It is scheduled to continue until November 18, 2025, and is being conducted with the highest sensitivity of the detectors involved to date. Not only have the detectors been serviced, cleaned, and readjusted for “O4,” they have also been improved: The squeezed light has been further enhanced with the aid of a 285-meter-long optical resonator, reducing the noise in the Virgo detector to around one-third of its previous value.

Artificial intelligence has also found its way into gravitational wave astronomy in the meantime. It helps to quickly identify the direction and source objects of gravitational waves and thus alert other observatories as quickly as possible so that they can also observe in many wavelength ranges of electromagnetic radiation. This is particularly helpful in the case of neutron star mergers: In just one second, an artificial neural network determines the mass and rotational speed of the objects as well as their position in the sky!

The LIGO-Virgo-KAGRA collaboration announced a record in July 2025: two black holes with 100 and 140 solar masses, respectively, were observed as they merged. The exciting thing is that, according to current models of star evolution, black holes with such masses should not exist! However, the gravitational waveforms of the GW231123 merger were so complex that this suggests a complicated past for the two black holes: they may themselves be the result of a collision between lighter black holes.

The future lies in space

The successes of Earth-based gravitational wave astronomy over the last ten years provided the decisive impetus for the construction of the European space-based gravitational wave observatory LISA, which has been in the planning stages for decades. With a planned launch date in 2037 and the start of observations in 2038, a whole new chapter in gravitational wave astronomy will begin in just over a decade: merging black holes, for example, will then be visible in the data not just for a few seconds, but for centuries in advance. In addition, the three LISA space probes should be able to observe completely different objects and events, such as compact binary stars or even residual signals from the Big Bang.

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