Gravity finally observes the precession of Schwarzschild

Almost five years ago we celebrated centenary of the discovery of Einstein's theory of general relativity. For decades we have sought to refute it in favor of other theories of gravitation which, if they keep thespace-time curveEinstein, use others equations to govern the metric of the gravitational field and possibly introduce additional fields, often scalar like the one associated with the famous Brout-Englert-Higgs boson.

One of the strategies to try to decide between these competing theories is to study what they predict with regard to the movement of celestial bodies and the behavior of electromagnetic waves, via their frequencies and the trajectories of the associated light rays in a gravitational field. For that, it is necessary to find a solution of the equations of the gravitation which describe the space-time around a celestial body like, for example, the Sun. In the case of Einstein's theory such a solution was found to describe a star without rotation and strictly spherical, or at least which can be considered in practice as such to a good approximation in the situation studied.

This is the famous Schwarzschild solution, which was also described in its full version a black hole and this time rigorously. Now it turns out that we know of the existence of a supermassive black hole containing approximately 4 million masses at the heart of the Milky Way. So we have at our disposal an excellent laboratory to Review the theory of general relativity as well as black hole theory. Researchers have been working on it for decades by studying in particular the movement of stars around this black hole associated with a source. radio and which is called Sgr A *. Pending its first images that the members of the collaboration should provide Event Horizon Telescope, Reinhard Genzel, director of the Max-Planck Institute dedicated to Physical extraterrestrial (MPE) in Garching, Germany, has just published an article in Astronomy & Astrophysics, available on arXiv, in which he presents with his colleagues the latest results of the studies concerning Sgr A * which he has been pursuing for almost 30 years, with in particular the instruments of the Very Large Telescope ((VLT) of the'ESO.

The first measure of Schwarzschild's precession for a black hole

In this case, these are always measurements made with the one named Gravity and which ABSMARTHEALTH had already reported in the previous articles below. In an ESO press release, Reinhard Genzel presents the results in these terms: "Einstein's Theory of General Relativity predicts that the linked orbits of one object around another are not closed – contrary to what the Newtonian Gravitation theory predicts, but precede forward in the plane movement. This famous effect – observed for the first time in theorbit that describes the planet Mercury around the Sun – was the very first proof of the validity of the theory of General Relativity. A hundred years later, we have just detected the same characteristic within the movement of a star orbiting the compact radio source Sagittarius A *, located in the center of the Milky Way. This observational result reinforces the idea that Sagittarius A * constitutes a supermassive black hole whose mass is close to 4 million solar masses. ".

The star Reinhard Genzel is talking about is famous under the name of S2. It is located at 26,000 light years of Solar system and in 16 years it orbits around the supermassive black hole of the Milky Way. According to Kepler's laws, it moves on an elliptical orbit, which precedes because of the equations of general relativity, and its speed is most important when it passes as close as possible to the black hole at a distance of less than 20 billion km, which is 120 times the distance from Earth to Sun. This speed is then dizzying because it is around 3% of the speed of light.

Soon a measurement of the rotation of the black hole Sgr A *?

The astronomers measure them movements of S2 for 27 years, which means that they saw it initiate a second orbit around Sgr A * and that they can therefore effectively observe the effect they call the precession for the first time for a star orbiting a supermassive black hole. But there is better, as still explained in an ESO press release Guy Perrin and Karine Perraut, the French astronomers responsible for the project as part of an international team led by Franck Einsenhauer from the MEP and which enabled achieve this spectacular result: "The measurements made on S2 are so perfectly in agreement with the theory of General Relativity that we can estimate the quantity of matter invisible, such as the distribution of black matter or the possible presence of smaller black holes around Sagittarius A *. This field of investigation is of definite interest for our understanding of the formation and evolution of supermassive black holes. ".

Paulo Garcia, researcher at the Center forastrophysics and gravitation of Portugal, another of the scientific leaders of the project, recalls a result already obtained and of which ABSMARTHEALTH had reported in one of the articles below: “Our previous result showed that the light emitted by the star is affected by General Relativity. Now we demonstrate that the star itself feels the effects of General Relativity. ".

In the near future, researchers expect to go even further by flushing out less bright and closer stars.horizon of events from Sgr A *. This should be possible with theExtremely Large Telescope from ESO and we should have access to relativistic effects produced by the rotation of the black hole, effect stemming from another solution of Einstein's equations, that discovered in 1963 by the mathematician Roy Kerr and that door her name. Not only would we then have access to a new estimate of the mass of the black hole but also, and for the first time, to the value of its cinematic moment if thestar compact is well rotating, which is very credible.

Gravity confirms the existence of a supermassive black hole in the heart of the Milky Way

The existence of real black holes – in particular those described by the famous Kerr solution when they are in rotation – has just received additional confirmation by observing, thanks to the Eso Gravity instrument, Sagittarius A *, our supposed supermassive galactic black hole. This confirmation seems solid, even if the last word on this has not yet been said.

We were waiting for the end of this year to reveal an image associated with the supermassive black hole of the Milky Way thanks to theEvent Horizon Telescope (EHT). Image able to verify, or not, the validity of the theory of general relativity and its consequences, in particular the very existence of black holes with a horizon of events. But a few weeks ago, the astrophysicist Sara Issaoun revealed in an article on the website ofIstituto nazionale di astrofisica Italian, in short Inaf, for National Institute of Astrophysics, which will have to wait until 2024.

However, an international team of European researchers led by Reinhard Genzel – the Max-Planck Institute dedicated to Extraterrestrial Physics (MPE), the Paris Observatory, the Grenoble-Alpes University, CNRS, the Max-Planck Institute dedicated to Astronomy, the University of Cologne, Portuguese Center for Astrophysics and Gravitation (Centra), all members of ESO – has just announced that it has already achieved a spectacular result. This result confirms the existence of black holes and in particular those in rotation described by the famous Kerr metric (a discovery of the New Zealand mathematician Roy Kerr, in 1963) and which provide the bases of the explanations of the nature of quasars.

The researchers explain, in an article available on Astronomy & Astrophysics (A&A), that they used the Gravity instrument equipping theInterferometer of Very Large Telescope (VLT) from ESO to observe broadcasts of radiation infrared polarized from accretion disc that surrounds Sagittarius A * at the heart of our Galaxy. let's remember that Gravity allows to combine the light coming from the four telescopes of the VLT to create a virtual telescope 130 meters in diameter.

As ABSMARTHEALTH had explained in a previous article (see below), the VLT and Gravity had already made it possible to pass remarkable tests to the theory of general relativity and to certain alternative theories of relativistic gravitation, in particular by studying the close movements of certain stars around Sagittarius A *. Most recently, it is the relativistic shift towards red of the light emitted by the star S2 in the gravitational field of Sagittarius A * – an object which we know, without a shadow of a doubt, that it contains about four million solar masses – which had been highlighted with Gravity, confirming Einstein's theory.

A limit for the minimum size of a stable orbit around a black hole

Today, everything suggests that Gravity observed bursts of infrared radiation from equivalents of solar flares but located at hot spots in the plasma at high temperatures orbiting very close to the supposed horizon of the Kerr's black hole, Believed to be the Sagittarius A * radio source. However, if this is indeed the case, these hot spots would be in matter moving at about 30% of the speed of light, in about 45 minutes, in a circular orbit very close to that which the astrophysicists relativists call the innermost stable circular orbit (Innermost stable circular orbit or Isco). The stable relativistic circular orbit as close as possible to a black hole in this case, and which depends on the value of the angular momentum of the rotating Kerr black hole in addition to its mass. Below this orbit, it is the fall towards the black hole – a prediction characteristic of general relativity with this type of compact star – and which is not found in the theory of gravitation of Newton.

The characteristics of this orbit and the radiation from hot spots were discovered in part by serendipity while astrophysicists were above all occupied with observing the star S2 (indeed, the existence of these packages of plasma and their potential for the study of black holes had already been studied theoretically for years by Avery Broderick, now at the Perimeter Institute for Theoretical Physics and the University of Waterloo in Canada, and Avi Loeb from Harvard University). This discovery seems to be in perfect agreement with predictions deduced from the existence of a black hole with a space-time described by Kerr's metric, solution of Einstein's equations of general relativity. We can deduce the mass of the Sagittarius A * black hole, which is also compatible with that already estimated, and in theory, but not yet in practice, the value of its angular momentum (it will be necessary to accumulate other observations eruptions in the accretion disk around the black hole).

It would therefore be, as the authors of the discovery insistently emphasize in conclusion of their article, confirmation of the validity of the black hole theory in addition to obtaining the most precise observations to date of the material orbiting as close to a black hole. This result will no doubt be consolidated in the near future as well thanks to the study of gravitational waves only by the results expected from the observations of the EHT, which will be complementary to those available with Gravity and which we will continue to collect. Conversely, these observations could show us that the theory of general relativity must be replaced by one of the many theories proposed for decades, to extend and unify the laws of physics, and even that black holes do not exist in not, even if it seems increasingly unlikely.

Gravity: Einstein's general relativity checked near our supermassive black hole

Article by Laurent Sacco published on 07/26/2018

Observations carried out using the instruments fitted to ESO's VLT in Chile led to the first successful Review of Einstein's theory of relativity near a supermassive black hole. In this case, it is a Review with the effect of the spectral shift towards the red of the star S2 orbiting the Milky Way around Sgr A *.

The Einstein's theory of general relativity is more than a century old. Much more than for its discoverer, it testifies to the mysterious capacity of the human spirit to anticipate the structure of reality, far from theuniverse daily life that has accompanied the evolution of brain ofHomo, based on mathematics not found there. Any confirmation of the predictions of general relativity can be seen as a triumph but also as a disaster because we are impatiently waiting to be able to survey and understand new deeper and larger aspects of the cosmos. This would be possible if Einstein's theory showed its limits.

Sagittarius A *, the supermassive laboratory black hole

As planned for the month of May 2018, one of the most famous stars orbiting the central black hole of the Milky Way found itself passing through periaster, i.e. at the point closest to Sgr A * on this orbit. Called S2, it found itself only about 16 light hours, 120 times the Earth-Sun distance or four times the Sun-Neptune distance from the supermassive black hole of four million solar masses that we think is around the center of our Galaxy. This also corresponds to a distance equivalent to almost 1,500 Schwarzschild rays of this black hole. S2 is found at this periosteum approximately every 16 years and at that time, it traversed a portion of its elliptical orbit at almost 2.7% of the speed of light or 8,000 km / s.

Relativistic astrophysicists eagerly awaited this event because, as ABSMARTHEALTH explained in a previous article (see below), one could expect effects from the gravitational field of the black hole Sagittarius A * which are not described by Newton's theory of gravitation, and perhaps even not completely by Einstein's theory, opening up a window on new physics.

The opportunity to also be able to probe the gravitational field of a black hole in a regime, where it is intense, could not be missed. This is why many researchers and engineers had seized it within the framework of the Gravity consortium led by the German Institute Max Planck for extraterrestrial physics (MPE) and involving the CNRS, the Paris Observatory – PSL, the Grenoble-Alpes University and several other French universities (as well as the University of Cologne and the Portuguese Center for Astrophysics and Gravitation ). It was about being able to combine by a method ofinterferometry infrared observations made by several of the telescopes in the Very Large Telescope (VLT) from ESO to make the equivalent of a telescope of more than 100 meters in diameter, while analyzing light using three instruments, Naco, Sinfoni, and Gravity. The challenge was to be able to observe and measure the hour-by-hour movements of S2, with an accuracy of 50 microseconds of angle, which amounts to observing a tennis ball placed on the Moon since the Earth.

The first measurement of the spectral shift effect of a black hole

ESO has just announced today, via a press conference, that the publication on arXiv of an article explaining the scientific results, that a culmination of 26 years of star observations around Sgr A * with its telescopes had been reached. Indeed the theory of general relativity implies that the gravitational field of a star produces a red shift of light that it can emit, all the more important that it is massive or dense, and this, according to a precise law.

This is indeed what was observed with S2, and just as it was the case almost a century ago with the deflection of the light rays of stars by the Sun observed and measured during the famous eclipse from 1919, the measured effects cannot be explained with Newton's theory of gravitation but are on the contrary in full agreement, with the precision of the measurements reached, with Einstein's theory.

It is the first time that this offset effect has been measured for the gravitational field of a black hole. We knew him before, especially with white dwarfs (the first solid detection dates from 1954 with 40 Eridani B), and it could be measured in the much weaker field of the Earth via the famous experiment of Pound and Rebka.

This new success of the theory of general relativity should soon be followed by another, very likely. Indeed, the observations in progress should make it possible to observe the relativistic component of the precession of the peri-sphere of S2, the equivalent of the famous relativistic precession of the perihelion of Mercury. 16 years ago, although we did not have as powerful instruments as today, a previous passage into the peri-sphere of S2 was observed, allowing a comparison in progress with that for 2025.

Astronomers hunt the fifth force around our galactic black hole

Gravity, pointed at the galactic black hole, ready to check general relativity

Article by Xavier Demeersman published on 25/06/2016

With its supermassive black hole of four million solar masses, the center of the Galaxy is, for astrophysicists, the ideal laboratory for testing Einstein's theory of general relativity. The VLT's new Gravity instrument, designed to listen to it, did not disappoint: it has just offered its first observations of a star moving very close to Sagittarius A *, the center of our galactic world. In 2018, it will be so close that the relativistic effects will be detectable directly.

In the summer of 2015, ten years after the launch of the project, an international team of astronomers and engineers installed the Gravity instrument in the tunnels set up under one of the largest observatories in the world, the VLT (Very Large Telescope), at the top of Mount Paranal in Chile. Working in interferometry, this system optoelectronics combines the light from four 1.8-meter VLT auxiliary telescopes (Auxiliary Telescopes, or AT) to create a mirror virtual 130 meters in diameter with the Very Large Telescope Interferometer (VLTI), which brings a resolution much more important.

The instrument completed its first observation campaign in January 2016. "Gravity will make it possible to extend optical interferometry to the observation of much less luminous objects, and will push well beyond current limits the sensitivity of astronomy at high angular resolution" commented then the director of operations, Franck Eisenhauer, of the Max Planck Institute.

Now the preliminaries are over and the tests carried out with the instrument coupled to the four giants, the units of 8.2 meters in diameter each (Unit Telescopes, or UT), are very promising, has just announced ESO, the European Southern Observatory. Compared with the observations of only one of these units, the gains in resolution and precision on the position of an object are by a factor of 15. It will soon reach, for example, centimetric precision for an object located on the Moon.

The theory of general relativity put to the Review of Gravity

The main objective of Gravity is the study of the intense gravitational field of black holes, and more particularly that of Sagittarius A *, which with 4 million solar masses, belongs to the category of supermassives. Invisible, these are betrayed by the dances of the stars trapped around them. This is how the position and mass of Sgr A *, lurking in the heart of our Galaxy, about 25,000 light years from Earth, could be inferred in 2002.

Also, by tracking with the greatest possible precision the movement of the stars which surround it, the researchers intend to learn more (rate of rotation, mass, electric charge) on this dark body. And above all, as the name of the project indicates, they will be able to compare the measurements with the predictions of the general relativity theory by Albert Einstein. Indeed, for physicists, the center of the Milky Way constitutes an ideal laboratory, in line with their expectations.

The piercing view of the instrument (10 microseconds of angle to determine the position of objects, and a resolution of four milliseconds of angle to image objects) recently made it possible to follow the star S2, a very close neighbor of Sgr A *, which turns around him in sixteen years. “When the light from the star first interfered, the team had a fantastic time, crowning eight years of hard work, says Franck Eisenhauer. At first, we stabilized theinterference on a close and bright star. Only a few minutes later, we were able to observe the interference generated by the weak star. brightness. " An achievement of which they are very proud.

Critical observations for 2025

The first results of Gravity, a very complex instrument to implement, are therefore very encouraging. So much the better, because for 2025, the star S2, which travels at 2.5% of the speed of light (30 million km / h), will reach the point of its orbit closest to supermassive black hole. It will only be 18 billion kilometers away, or around 17 light hours, or 4 times the distance between Neptune and the sun. The gravitational effects will therefore be strongly felt, which promises crucial observations.

For the very first time, specifies ESO, the team will be able to measure two relativistic effects caused by the rotation of a star around a black hole: the gravitational redshift, resulting from the loss ofenergy that undergoes the light of the star when it escapes from the intense gravitational field of the black hole, and the precession of the pericenter, an effect observed on a smaller scale with the orbit of Mercury around the Sun (with an intensity however 6,500 times lower than near the galactic black hole).