Let’s remember: In late 2015, the scientific community celebrated the 100th anniversary of Einstein’s formulation of his theory of gravitation. What some researchers already knew, but kept secret, was that recently the ligo gravitational-wave detectors in the United States had directly detected the passage of a gravitational-wave stream on Earth for the first time. They warp the geometry of spacetime as a wave would on the surface of water or in an elastic medium like the interior of our blue planet as seismic waves pass.
Finally, early the following year, the noosphere learned that the era of gravitational-wave astronomy had begun on September 14, 2015 with the discovery on Earth of gravitational-wave source GW150914. Signal analysis was performed by members of the Ligo collaboration, but also by Virgo in Europe. The signal detected by Ligo came from the recent events in which two stellar-mass black holes forming a binary pair spiral together and then merge into a single compact star (see videos below). It should be remembered that France and Italy are both theoretically and observationally engaged in the study of gravitational waves with the Virgo detector, as evidenced by the award of one of the CNRS gold medals to Alain Brillet and Thibault Damour, as well as the existence of large teams, who work in laboratories such as the LAPP (Annecy Laboratory for Particle Physics) or the Observatory of the Côte d’Azur (OCA) in Nice, Artemis.
A video presentation of Virgo and the Chase of Gravitational Waves. © CNRS
From Ligo to Kagra to the future eLisa
Futura took the opportunity to interview several times one of the members of Artemis, the astrophysicist Olivier Minazzoli, who also works at the Monaco Science Center. For example, he had given us several explanations on the possibility of testing the theory of superstrings with Virgo and Ligo, and on the possibility of obtaining extremely convincing evidence for the existence of black holes by emphasizing the influence of what they call quasi – mark holes. Normal modes in the gravitational wave spectrum resulting from the merger of two black holes.
The founders of Ligo in the USA, the American physicists Rainer Weiss, Kip S. Thorne and Barry C. Barish, received the Nobel Prize in Physics shortly afterwards – in the two following videos they can be seen presenting their discoveries and achievements. Ligo and Virgo aimed to detect multiple collisions of gravitational-wave sources, mergers of two black holes, collisions between two neutron stars, and more rarely collisions between black holes and neutron stars in the following years. These are not supermassive or intermediate-mass black hole events yet, we have to wait for the 2030s horizon with the eLisa detector in space.
Between each search campaign, the detectors are upgraded to become more sensitive, so they are able to detect more distant events and derive more precise information about those events, such as the masses, the actual spin moments of the black holes about themselves, and finally pose new limitations to relativistic theories of gravity as alternatives to Einstein’s theory.
These upgrades require breaks and due to the Covid-19 pandemic, Ligo, Virgo and also their Japanese cousin Kagra haven’t seen anything since 2020. Virgo will be a little behind on his schedule as he won’t be joining Ligo on his research until late summer or even early fall of 2023. Kagra also resumed its hunt on the same day as Ligo, that is May 24, 2023.
Ligo and the Chase of Gravitational Waves I. For a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose French. © LIGO Lab Caltech: MIT
Mergers of black holes, but also supernovae
Therefore, Ligo’s sensitivity has been increased, so the machines must be able to detect black hole collisions every two to three days on average, instead of once a week.
Researchers are now hoping to detect the gravitational waves emitted when a star begins the collapse that causes it to explode as a supernova. This evidence is currently limited to the Milky Way, but we have good hopes that, with any luck, we may be able to spot such an event before 2050.
The study program also includes the detection of waves emitted by rotating neutron stars due to the presence of “mountains” on their surface. According to Einstein’s theory, a perfect sphere could not emit these waves.
These recent advances were made possible in particular by the use of laser beams with “compressed” electromagnetic waves. These are techniques for preparing a laser beam from photons, limiting as much as possible the quantum blur resulting in particular from the famous Heisenberg inequalities.
Ligo and the Chase of Gravitational Waves II. For a reasonably accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose French. © LIGO Lab Caltech: MIT
To end this article, let us remember a book for children about gravitational wave astronomy. We owe this to two renowned specialists in this new science: Tania Regimbau and Mairi Sakellariadou. Here is her brief presentation of the book.
“The idea for this book was born on a beach in Nice in September 2017, two years after the first discovery of gravitational waves. This discovery shook the scientific world because it represented the ultimate confirmation of Albert Einstein’s theory, but also because it marked the beginning of a new era in astrophysics. The detection of gravitational waves was awarded the Nobel Prize in Physics in 2017.
Since we were both members of the collaboration that made this discovery, we felt the desire to pass it on to children. Then we imagined a space story about two twin stars who loved each other so much that they merged and stayed together forever.
From this came the first story of this book, which marked the beginning of a new adventure for us, as three more stories followed…”.