The hunt for gravitational waves from supernovae resumes

Let’s remember, it was at the end of 2015 that the scientific community celebrated the 100th anniversary of Einstein’s formulation of his theory of gravitation. What some researchers already knew – but kept it a secret – was that shortly before, the Ligo gravitational wave detectors in the United States had made the first direct detection on Earth of the passage of a flow of gravitational waves. They deform the geometry of space-time like a wave would do on the surface of water or in an elastic medium, like that of the interior of our blue planet when seismic waves pass.

Early the following year, the noosphere finally learned that the era of gravitational wave astronomy had begun on September 14, 2015 with the detection on Earth of the gravitational wave source GW150914, GW standing for Gravitational Wave, in English. The signal analysis had been carried out by members of the Ligo collaboration, but also of Virgo in Europe. The signal detected by Ligo came from the latest events when two stellar-mass black holes, forming a binary couple, approach each other in a spiral, then merge into a single compact star (see the videos below). It should be remembered that France and Italy are very involved theoretically and observationally in the study of gravitational waves with the Virgo detector, as evidenced by the awarding of one of the CNRS gold medals to Alain Brillet and Thibault Damour, as well as as the existence of large teams working in laboratories such as the LAPP (Annecy Laboratory for Particle Physics) or that of the Observatory of the Côte d’Azur (OCA) in Nice, Artemis.

A video presentation of Virgo and the hunt for gravitational waves. © CNRS

From Ligo to Kagra and into the future eLisa

Futura took the opportunity to interview several times one of the members of Artemis, the astrophysicist Olivier Minazzoli, also working at the scientific center of Monaco. He had, for example, brought us several explanations concerning the possibility of testing the theory of superstrings with Virgo and Ligo as well as the possibility of obtaining extremely convincing proof of the existence of black holes by highlighting the influence of what are called their quasi-normal modes on the gravitational wave spectrum resulting from the merger of two black holes.

The originators of Ligo in the United States, the American physicists Rainer Weiss, Kip S. Thorne and Barry C. Barish received the Nobel Prize in Physics shortly afterwards – they can be seen presenting their discoveries and achievements in the two videos below. Ligo and Virgo were going, in the years which were going to follow, to detect several collisions sources of gravitational waves, mergers of two black holes, collisions between two neutron stars, and more rarely collisions between black holes and neutron stars. These are not yet events involving supermassive or intermediate-mass black holes, we will have to wait for the horizon of the 2030s with the detector in space eLisa.

Between each search campaign, the detectors are upgraded to become more sensitive, thus able to detect more distant events and to derive more precise information concerning these events such as the masses, the angular moments of rotation proper on themselves of the black holes. and finally pose new constraints on relativistic theories of gravitation alternatives to Einstein’s theory.

These upgrades require breaks and due to the Covid-19 pandemic, Ligo, Virgo and also their Japanese cousin, Kagra, had not seen anything since 2020. wave hunting. Virgo will be a little behind on his schedule since he will not join Ligo in his research until the end of the summer, or even the beginning of the fall of 2023. Kagra has also resumed his hunt, the same day as Ligo , i.e. this May 24, 2023.

Ligo and the hunt for gravitational waves I. To obtain 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

Ligo’s sensitivity has therefore been increased, so that the machines must be able to detect black hole collisions every two or three days on average instead of once a week.

Researchers now hope to detect the gravitational waves emitted when a star begins the collapse that will lead to it exploding as a supernova. This detection is currently limited to the Milky Way, but we have good hopes of detecting such an event probably before 2050, with a little luck.

The study program also includes the detection of waves emitted by rotating neutron stars due to the presence of “mountains” on their surface. A perfect sphere could not emit these waves according to Einstein’s theory.

These latest advances have been made possible in particular by using laser beams with “compressed” electromagnetic waves. These are techniques for preparing a laser beam of photons by limiting as much as possible the quantum blur resulting in particular from the famous Heisenberg inequalities.

Ligo and the hunt for gravitational waves II. To obtain 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

To conclude this article, let’s recall a book intended for children, which talks about the astronomy of gravitational waves. We owe it to two renowned specialists in this new science: Tania Regimbau and Mairi Sakellariadou. Here is their short presentation of the book.

« The idea for this book was born on a beach in Nice in September 2017, two years after the first detection of gravitational waves. This discovery shook the scientific world, being the ultimate confirmation of Albert Einstein’s theory but also because it marked the beginning of a new era in astrophysics. Gravitational wave detection was awarded the 2017 Nobel Prize in Physics.

As both members of the collaboration that made this discovery, we felt the desire to pass it on to children. We then imagined a space tale featuring two twin stars who loved each other so much that they merged to stay together forever.

From there was born the first story of this book which marked the beginning of a new adventure for us, since three other stories followed… ».

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