An international team of researchers has just announced the discovery of no less than 25 new gravitational waves, three of which originate from a neutron star and a black hole.
Gravitational waves are incredible phenomena. Observed for the first time in 2015, these waves of a new kind had been predicted in 1905 by the French Poincaré and confirmed in Albert Einstein’s theory of general relativity. They are often the result of a collision between two behemoths in space, usually black holes, but they can also be neutron stars depending on the situation. Or a mixed collision between a black hole and a neutron star. This wave then propagates through space-time, the best way to imagine it and imagine a stone falling into a puddle.
The shock wave which extends to the surface of the water is a beautiful representation of what a gravitational wave is, on the scale of the Cosmos. With the difference that the latter are invisible to the naked eye, and that very precise instruments, and specifically designed for their research are necessary to find them. It is this difficulty of analysis that has delayed so much the first “direct” discovery of a gravitational wave.
But since 2015, and the establishment of the three main observatories on this subject, LIGO (in the United States), VIRGO (in Italy), and KAGRA (in Japan), the discoveries have multiplied and this week it is a “tsunami” which has invaded the scientific world. No less than 35 new waves were detected, bringing the total number of discoveries to 90. While the majority of them came from fusions of black holes of various sizes, some of these disturbances in space-time have attracted the attention of scientists.
3 neutron stars and 32 black hole mergers
Thus, among the 35 gravitational waves identified by the observatories during a joint study carried out between November 2019 and March 2020, only three were due to a collision between a neutron star and a black hole. This is for example the case of GW191219_163120 which is the result of a collision between a black hole and a relatively weak neutron star, only 1.2 solar masses, where the average weight for a star of this type is generally of 1.4 solar mass or more. The other collisions between neutron stars and black holes occurred between stars of respectively 1.4 and 2.8 solar masses.
For the latter, scientists have not succeeded in knowing whether it was indeed a surprisingly heavy neutron star or a black hole which would be very light. As for the 32 black hole mergers, the heaviest reaches 145 solar masses, while the lightest is barely at 18. Although discovered very recently, gravitational waves have been theorized for decades, and the scientific consensus was acquired at the Chapel Hill Conference in 1957.
Our new @LIGO @ego_virgo @KAGRA_PR catalog #GWTC3 includes all the #GravitationalWaves events detected during our first 3 Observing Runs #O1 # O2 and #O3. We love this new poster, from @ARC_OzGRav‘s Carl Knox, which shows all of our events! https://t.co/43rLJ247SR pic.twitter.com/hK09zvE0MM
– LIGO (@LIGO) November 8, 2021
Study them as a population and not as events
This discovery remains a major advance in the world of cosmology, it is indeed possible for scientists to study very many cases of encounters between neutron stars and black holes, or simply between two black holes. The idea will now be to go back through the history of these two stars to find out when and how they were born. A research which promises to be long, but which could give very interesting answers on the origins of black holes or on their evolution during their life.
More interesting still, this discovery of a very large quantity of gravitational waves should allow researchers to study this phenomenon as a whole and not as an isolated case. “There are features we see in these distributions that we cannot yet explain, opening up some exciting research questions to explore in the future.”, assures Shanika Galaudage, researcher at Monach University in Australia.