Ten years of gravitational waves

On September 14th 2015, ten years ago, a signal arrived on Earth that was generated by a pair of black holes. After spiralling around each other at impressive speeds, they had merged, releasing an enormous amount of energy. To reach us, the signal had travelled some 1.3 billion years at the speed of light, but it was not a light signal, it was a tremor of space-time called a gravitational wave, first theorised by Albert Einstein 100 years earlier. That first direct detection of gravitational waves by the twin LIGO detectors in the US will only be announced to the world by the LIGO and Virgo Collaborations, after many months of analysis and verification, in February 2016.

The following year it led to the awarding of the Nobel Prize in Physics to three of LIGO’s founders: Rainer Weiss, professor emeritus of physics at MIT (who recently passed away at the age of 92), Barry Barish and Kip Thorne of Caltech.

“September 14th 2015 marked the beginning of gravitational astronomy with the first direct observation of gravitational waves, GW150914. – said Gianluca Gemme, Virgo Collaboration spokesperson and researcher at INFN (Istituto Nazionale di Fisica Nucleare) – The Virgo collaboration played a key role in validating the signal and defining joint analysis strategies. Over the next ten years, Virgo has contributed substantially to the detection of hundreds of gravitational events, progressively improving sensitivity and source localisation capability. Looking ahead, Virgo continues to be a pillar of the detector network, and its contribution will be essential in paving the way for the next generation of observatories, such as the Einstein Telescope.”

“This is an amazing time for gravitational wave research: thanks to instruments such as Virgo, LIGO and KAGRA, we can explore a dark universe that was previously completely inaccessible. – said Massimo Carpinelli, director of the European Gravitational Observatory and professor at University of Milano Bicocca– The scientific achievements of these 10 years are triggering a real revolution in our view of the Universe. We are already preparing a new generation of detectors such as the Einstein Telescope in Europe and Cosmic Explorer in the US, as well as the LISA space interferometer, which will take us even further into space and back in time. In the coming years, we will certainly be able to tackle these extraordinary challenges thanks to increasingly broad and solid cooperation between scientists, different countries and institutions, both at European and global level.”

Ten years after the first detection of gravitational waves, the LIGO-Virgo-KAGRA Scientific Collaboration has detected a total of about 300 black hole mergers, some of which have been confirmed while others await further analysis. The study of these signals has led to a series of significant scientific results, opening a new window for observing the cosmos.

Black holes or neutron stars?

By studying the waveform of the gravitational wave signal, we can estimate the masses of the objects that produced it and thus understand whether they are black holes or neutron stars, and this sometimes leads to surprises. The heaviest neutron star ever observed through light (electromagnetic radiation) has a mass 2.5 times that of the Sun, while the lightest black hole has a mass 5 times that of the Sun. This leaves a ‘hole’ in mass in between (called a mass gap). LIGO and Virgo have observed at least one object in this mass gap, so we do not know whether it is a black hole or a neutron star, or why it has this mass. This result has challenged theoretical models of stellar evolution and is an excellent example of how theoretical and experimental science influence each other.

More massive black holes

The “mass gap” between black holes and neutron stars is not the only case in which data from gravitational wave detectors has led to a revision of current theoretical models. The chemical theory of stellar evolution does not predict the possibility of the direct formation of black holes with a mass greater than 50 solar masses from the collapse of a single star. However, LIGO and Virgo have observed stellar black holes with masses greater than this. Either there may be something to revise in the theories of stellar evolution, or these black holes may have formed from the merger of other smaller black holes.

Multimessenger astronomy

On August 17, 2017, LIGO and Virgo detected a signal that would change the history of astronomy: GW170817. For the first time, it was the coalescence of two neutron stars, and not two black holes as previously observed signals. Neutron star mergers have something that black hole mergers do not: the emission of electromagnetic radiation (light) across its entire spectrum. Thanks to the localization of the source by LIGO and Virgo and NASA’s Fermi gamma-ray space telescope, this event was not only observed through gravitational waves, but was also seen in the electromagnetic spectrum by 70 observatories on Earth and in space. Thus, multi-messenger astronomy with gravitational waves was born.