imagining the heart of darkness

Astronomers are preparing Wednesday to unveil the first direct image of a black hole and surrounding whirlpool of white-hot gas and plasma, inexorably lured by gravity into its voracious mouth, with the light that's left behind. they generate.

The photo was captured by the Horizon Telescope Event (EHT), a network of eight radio telescopes around the world.

Paul McNamara, astrophysicist at the European Space Agency and scientist of the LISA project, responsible for tracking the huge mergers of black holes from space, helped AFP to contextualize what 's about. he called an "exceptional technical achievement".

How do we know that there are black holes?

"We think, of course, of a very dark black hole, but the mass in which it is sucked forms what is called an accretion disk that becomes so hot that it shines and emits light.

Over the years, we have accumulated other evidence of indirect observation – X-rays from objects, for example in other galaxies.

In September 2015, LIGO gravitational wave detectors in the United States measured two black holes that were shattering.

All the evidence we have around the universe – X-rays, radio waves, light – indicate very compact objects and gravitational waves have confirmed that it's been a good deal of black holes, even though we have never seen one. "

What is an "event horizon"?

"At the center of a black hole, there is something we call a" singularity "- a huge amount of mass is reduced to an infinitely small and zero dimension in space.

If you move away from this singularity, the escape velocity drops below the speed of light. It's the horizon of events.

This is not a physical barrier – you could not stick to that. If you are inside, you can not escape because you would need infinite energy. If you are on the other side, you could escape, in principle. "

What is the size of a black hole?

"The diameter of a black hole depends on its mass, but it's still twice what we call the Schwarzschild radius.

If the Sun were to be reduced to a singular point, the Schwarzschild radius would be three kilometers and its diameter six.

For Earth, the diameter would be 18 millimeters, about three-quarters of an inch. The horizon of events at the center of the Milky Way, Sagittarius A *, is approximately 24 million kilometers long.

Sagittarius A * (which is four million times the mass of the Sun) is one of the two black holes targeted by the ISE. The other, even bigger, is in the M87 galaxy. "

What will the image look like?

"The Horizon telescope of the event does not look at the black hole itself, but the material that it has captured.

It will not be a big record in high resolution as in the Hollywood film Interstellar. But we could see a black nucleus surrounded by a brilliant ring – the accretion disk.

The light behind the black hole is curved like a lens. Whatever the orientation of the disc, you will see it as a ring due to the gravity of the black hole.

Visually, it will look very much like an eclipse, although the mechanism is, of course, completely different. "

How is the image generated?

"The technical realization is remarkable. Rather than having a telescope 100 meters in diameter, they have many telescopes with an effective diameter of 12,000 kilometers, the diameter of the Earth.

The data is recorded with great precision, placed on hard disks and shipped to a central location where the image is digitally reconstructed.

It is a very, very, very long basic interferometry – all over the Earth's surface. "

A threat to general relativity?

"Einstein's theory of general relativity corresponds to all the observations made up to now on black holes.

The gravitational wave signature of the LIGO experiments, for example, is exactly what the theory might be expected to be.

But the black holes that LIGO measured were small and represented only 60 to 100 times the mass of the Sun. Maybe black holes millions of times more massive are different. We do not know yet.

We should see a ring. If we see something lying on an axis, then it can no longer be a singularity, it could be a violation of general relativity. "

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© 2019 AFP