we explain to you how science sees more clearly on black holes

They are among the most fascinating objects in the cosmos. Black holes are now gaining traction in the mainstream media, because “knowledge on the subject has been the subject of great progress in recent years”remarks to franceinfo the astrophysicist Françoise Combes.

Franceinfo explains how these advances were made on these mysterious objects so dense that not even light can escape.

By detecting gravitational waves (predicted by Einstein)

Progress is “essentially due to those of the instruments”sums up Jean-Pierre Luminet, emeritus research director at the CNRS. “The classical theory of black holes had been very well established for two decades”notes the author of The Universe in 40 questions (Librio editions). Nevertheless, “there was a lack of really convincing things about direct observation of black holes”he notes.

“Until gravitational waves, in 2015, we had only seen relatively indirect, but quite strong arguments for the existence of black holes.”

Jean-Pierre Luminet, astrophysicist

at franceinfo

>> Three questions to try to understand gravitational waves

What happened in 2015? Two measuring instruments have detected a gravitational wave directly for the first time. These waves are tiny ripples in space-time that propagate through the Universe at the speed of light. They are produced under the effect of the displacement of an object of very great mass. Albert Einstein had conceptualized them in 1916. Almost a century later, an American observatory succeeded in the first detection of such a wave. This was born during the last fraction of a second before the merger of two black holes, the most violent gravitational phenomenon in the Universe.

For this first, the three Americans Rainer Weiss, Barry Barish and Kip Thorne won the Nobel Prize in Physics in 2017. They were the ones who developed the detector that carried out the measurement: the Ligo instrument, to Laser Interferometer Gravitational-Wave Observatory.

>> What will the discovery of gravitational waves be used for?

Gravitational astronomy has found itself in full light. She then quickly continued on her way. The European Virgo detector in turn identified, in September 2017, for the first time, gravitational waves.

By discovering a new type of black hole

The exploitation of the data collected by the Ligo and Virgo instruments has enabled other important advances. Thanks to their observations, the family of black holes has grown, with those known as “intermediate mass”.

Scientists have known for many years about supermassive black holes, which are found in particular at the center of galaxies and reach millions or even billions of solar masses. At the other end of the spectrum, they have also known for a long time the so-called stellar black holes, which are a few solar masses. The existence of black holes of 30 to 40 solar masses was proven in 2015. But, in 2019, scientists highlighted the existence of black holes larger than these, with the discovery of a hole black of 142 solar masses. This caliber had never before been detected directly, the category of intermediate-mass black holes starting at 100 times the mass of our sun.

It’s the “missing link”then explained to franceinfo the physicist Marie-Anne Bizouard, researcher at the CNRS, evoking a “milestone”. “For me, the most striking recent discovery is that of the existence of intermediate-mass black holesconfirms Françoise Combes. These intermediate-mass black holes cannot come from the supernova explosion of a massive star at the end of its life, but only from the successive merger of increasingly massive black holes.

Using a virtual Earth-sized telescope (to get first images)

A black hole is extremely dense, as if all of Earth’s mass were packed into a nut. The density is so high that matter cannot escape, nor even light. In principle, it is therefore impossible to capture a black hole image. On the other hand, the effect he produces on his neighborhood betrays him.

This is what we see in the very first image of a black hole, which was published in April 2019. An event that marked the history of astronomy. The object in question is the supermassive black hole that sits at the center of the M87 galaxy, 50 million light-years from Earth.

To obtain this image, an international collaboration, the Event Horizon Telescope (EHT), was mobilized. It brings together a dozen radio telescopes and observatories spread around the Earth, from Europe to Antarctica, via Chile and Hawaii. By combining the instruments, as if they were the fragments of a giant mirror, astronomers manage to have a virtual telescope the size of the Earth. With such a diameter and such a power, it’s as if we could observe from New York a person on a terrace in Paris, and distinguish the bubbles in his glass.

The same international collaboration set the stage again and revealed, in May 2022, the first image of the black hole in our galaxy, the Milky Way: Sagittarius A*.

>> Why the release of the first image of the black hole at the center of our galaxy is historic for space research

What do these images show? Jean-Pierre Luminet, who carried out the first digital simulations of black holes in 1978, points out that the two images “look alike” while the orders of magnitude are not at all the same: the mass of the black hole at the center of the galaxy M87 is 1,500 times greater than that at the center of the Milky Way. “You find the same things on two different scales. This is a resounding confirmation of one aspect of black hole theory: a form of black hole uniqueness.”

By using a 3D map of the Milky Way (to spot the most discreet black holes)

Detecting black holes is not easy. They are usually identified by their X-ray emissions. This means that they must be in a relationship with at least one star and that this relationship is in a very particular period during which an exchange of matter takes place between the two objects, that is, when the black hole begins to “devour” the star. “This phase lasts perhaps 10,000 years. A nod to the cosmic scale”, comments Jean-Pierre Luminet. All of these conditions pose many obstacles to the detection of black holes. But they were lifted, in particular, thanks to the Gaïa project.

Launched in 2013, this project surveys the stars in our galaxy. It is a question of listing them, of noting their position, their displacement, their age. Objective: to create a three-dimensional map of the Milky Way. After observing some two billion stars, the Gaia teams published, in May 2022, a map with unprecedented precision.

Analysis of this data has detected black holes that had previously gone unnoticed, the European Space Agency (ESA) has announced. (in English), on March 30. The attention of scientists has been drawn to the very weak oscillation of certain stars. It turns out, on closer inspection, that it was caused by the presence of black holes in their vicinity.

These two black holes, called BH1 and BH2, are the closest to us that we know of. BH1 is 1,560 light-years away in the constellation Ophiuchus, and BH2 is 3,800 light-years away in the constellation Centaurus. On the scale of the Universe, it is as if they were “in our garden”underlines the ESA.

These two new black holes are not star eaters. They fall into the category of dormant black holes, meaning they interact very little with their environment.

“Thanks to new, extremely precise techniques, we are beginning to flush out silent black holes.”

Jean-Pierre Luminet, astrophysicist

at franceinfo

Researchers expect that objects of the same type will be discovered in large numbers in our galaxy, especially since the Gaia mission must continue until 2025.

Using the power of the James Webb Telescope

Long-awaited, the most powerful telescope ever sent into space has not disappointed since its launch in December 2021. The James Webb (JWST) was designed to observe the first moments of the Universe. And that’s what it did by detecting the oldest black hole ever discovered., NASA announced (in English) April 6. With its imposing size (20 million times the mass of the Sun), this black hole would have nothing to do there, according to the scenarios established so far.

What undermines all of our knowledge? Absolutely not, assures Jean-Pierre Luminet. Nevertheless, “this raises questions about the speed of formation of large structures”he notes.

“This revives a hypothesis 30 or 40 years old, and which had been left aside: that of a first generation of black holes. Primordial holes, which would have formed just after the Big Bang, in the seconds which followed.”

Jean-Pierre Luminet, astrophysicist

at franceinfo

“I was very skeptical about primordial black holes. I’m not anymore”slips with franceinfo Marta Volonteri, specialist in supermassive black holes at the CNRS, anticipating important announcements “in the coming months“. Jean-Pierre Luminet cautiously posits that primordial black holes (which could have been between 100,000 and 1 million times the mass of the Sun) may have served as “germs” to give birth to a mastodon such as the one discovered recently. A hypothesis which, like others, must be verified.

The study of black holes is booming. And the momentum is not going to stop: the European Euclid mission, which is to be launched in July, will study the speed of the expansion of the Universe, dark matter, dark energy, and further feed the work on black holes.