Einstein’s theory of general relativity was formulated a little over 100 years ago now. But, from the 1920s to the 1950s, it only experienced significant developments in the hands of a handful of physicistsphysicists andastronomersastronomers exploring, such Georges LemaîtreGeorges Lemaîtrethe implications of the relativistic equations of gravitation in cosmology, and such EinsteinEinstein seeking to generalize his theory to incorporate the electromagnetic force and deduce the properties of the elementary particles then known. Vain attempts on these last points, so much so that the majority of physicists and scientists astrophysicistsastrophysicists of the time will mainly be concerned with developing the consequences of the discovery by Heisenberg and Schrödinger of the equations of quantum mechanics in the fields of physiquephysique atomic and nuclear, and to create the quantum and relativistic field theory involved therein.
Sagittarius A*, a laboratory for astrophysics
This year 2025 we will celebrate the centenary of Heisenberg’s discovery of these equations. But, incontestably, as the Nobel Prize winner in physics Subrahmanyan Chandrasekhar liked to remind us, “ the theory of general relativity is a theory of gravitation and like the Newtonian theory of gravitation, which it refines and broadens, its foyerfoyer natural is astronomy “, so that it experienced a renaissance from the 1960s with the discovery of quasarsquasarsof fossil radiationfossil radiation of Big BangBig Bang and finally, pulsarspulsars.
Jean-Pierre Luminet, research director at the CNRS, and Françoise Combes, professor at the Collège de France, tell us about black holes, in particular the large supermassive black holes in galaxies which are behind quasars and which impact the evolution of galaxies. © Hugot Foundation of the Collège de France
It is from these years, and especially during the 1970s, that we will intensively develop the physics of black holesblack holesof the gravitational wavesgravitational waves and also explore alternatives to Einstein’s theory of gravitation which, while assuming that there exists the same space-timespace-time curve, will postulate equations different from those of Einstein (we will test them during these years in the Solar systemSolar system and with pulsars binarybinary). Black holes will then become, in this context, theoretical laboratories making it possible to test both the most fundamental consequences of Einstein’s physics of strongly curved space-times and those of these alternatives. We will realize more and more that they are the key to the behavior of active galactic nucleiactive galactic nuclei and that they strongly influence the evolution of the latter. We will also understand that black holes must contain the keys to a quantum theory of gravitation, a probable key in turn to the birth ofUniverseUniverseof the mattermatter that it contains and the appearance of galaxies and the large structures that bring them together.
It turns out we think we’re lucky to have a black hole available to study via observations this time all these questions in our Milky WayMilky Way and it is supermassive, like at the heart of almost all other large galaxies, whether spiral or elliptical mainly. We initially unknowingly discovered it in the form of an intense radio source in the constellationconstellation of Sagittarius. It is referred to as Sagittarius A*Sagittarius A* (Sgr A*) and it is located approximately 27,000 light yearslight years of the Solar System.
From EHT to James-Webb
For decades, progress in its study will be made essentially by studying the movementsmovements of some starsstars loved ones around Sagittarius A*. These movements combined with other observations at various wavelengthswavelengths indicate that there is a very compact object that does not radiate like a star and therefore behaves, in many ways, like a true black hole from the point of view astrophysicsastrophysics. Studies on these movements were carried out mainly by the Nobel Prize winners in physics Reinhard Genzel and Andrea Ghez and they showed that the revealed compact object had a massemasse a little more than 4 million times that of SoleilSoleil.
If it is indeed a black hole, which implies that it has a event horizonevent horizon which defines a sort of closed membrane that can only be crossed in one direction – because it would be necessary to go beyond the speed of lightspeed of light to get out of it – we do not yet know very well whether it is described by the solution of Einstein’s equations for a black hole without rotation, the famous Schwarzschild solution, or in rotation as we think, which implies that space-time is that of the so-called Kerr solution.
More recently, as shown in the video at the very beginning of this article, these are the members of the collaboration Event Horizon Telescope who focused on the study of Sagittarius A* in the field of electromagnetic waves accessible to radio telescopes. But for the first time, these observations were supplemented by those made possible in the field ofinfraredinfrared means the instruments of télescope spatial James-Webbtélescope spatial James-Webbthe JWST.
-As demonstrated by a published article, a version of which is freely accessible on arXivan international team led by astronomers from the Harvard & Smithsonian Center for Astrophysics (CfA) has detected for the first time a mid-infrared flare in the accretion diskaccretion disk surrounding the black hole Sgr A* thanks to the JWST. Mid-infrared makes it possible to observe objects and phenomena, such as the equivalents of solar flaressolar flareswhich are often difficult to observe in other wavelengths due to impenetrable dust.
An analogue of magnetic solar flares
In a statement, Joseph Michail, one of the lead authors of the paper and a postdoctoral researcher at Harvard CfA, explains: “ The Sgr A* flare was evolving and changing rapidly, within a few hours, and not all of these changes are visible at all wavelengths. For over 20 years, we have known what was happening in the radio domain and what was happening in the near infrared, but the link between the two had never been clear or 100% certain. This new mid-infrared observation fills this gap and connects the two. »
We will thus be able to better test the digital modelsdigital models describing what is happening in the turbulent accretion disk of the supermassive black holesupermassive black hole and which predict eruptions, according to magnetohydrodynamic and plasma physics mechanisms found behind the better-known solar eruptions. Indeed, numerous simulations suggest that the Sgr A* eruptions are caused by the famous reconnection of the lines of magnetic fieldmagnetic field in the turbulent accretion disk. We thus observe in the case of the Sun that when two magnetic field lines approach each other, they can connect to each other and release a large quantity of their energyenergy feeding theemissionemission dite synchrotron d’electronselectrons moving at speeds close to the speed of light along magnetic field lines.
The new observations offered by the JWST are consistent with existing models and simulations, providing additional evidence to support the theory of what is behind the flares.
« Although our observations suggest that the mid-infrared emission of Sgr A* does indeed result from synchrotron emission from cooling electrons, much remains to be understood about the magnetic reconnectionmagnetic reconnection and the turbulenceturbulence in the Sgr A* accretion disk. This first mid-infrared detection and the variability observed with SMA not only filled a gap in our understanding of what causes Sgr A* flares, but also opened an important new avenue of research “, explains Sebastiano von Fellenberg, postdoctoral researcher at the Max-InstitutPlanckPlanck de radioastronomie (MPIfR) and main author of the new article.
The James-Webb observations were then completed simultaneously with the submillimeter array (SMA), at the summit of Mauna Kea/Hawaii), the NuSTAR telescope and the observatory at x-raysx-rays ChandraChandra.
The SMA thus showed that the observation of the eruption in millimeter waves was delayed by approximately 10 minutes compared to the eruption in the mid-infrared. There was, however, apparently not enough energy to produce detectable X-ray radiation.
