understand everything about the mysteries of dark matter and dark energy, which the European Euclid mission is going to study

This is one of the great challenges of modern physics. The European Euclid space telescope is due to take off on Saturday July 1 with a SpaceX Falcon 9 rocket to try to unlock the secrets of dark matter and dark energy.

>> Follow the launch of the Euclid mission in our live

What is it exactly? We still know almost nothing about it. And yet, dark matter and dark energy make up, according to scientists’ estimates, 95% of the universe. Ordinary matter, that which we know, which is found in the stars, in the planets, and of which we are made up, represents only 5% of the whole. The task promises to be enormous, and the mission led by the European Space Agency (ESA) could mark the history of cosmology.

What is the Euclid mission?

The space telescope will scan a third of the sky for at least six years, from a point – called Lagrange point 2 – located 1.5 million km from Earth. His mission: to create a three-dimensional map of the universe. Its field of observation is particularly wide. To visualize the vast area covered by Euclid, ESA has produced this infographic showing (in grey) the areas that will be observed.

With its two instruments (a 1.2 m diameter telescope in the visible and an infrared spectrometer), Euclid will scan 12 billion celestial objects in order to identify massive structures. In the universe, matter is not evenly distributed. It is organized around a cosmic web, a network of filaments of gas and dark matter, which “occupies the entire volume of the universe”, summarizes the Commissariat for Atomic Energy and Alternative Energies (CEA). Its structure can be reminiscent of that of a sponge or a neural network, notes astronomer Bruno Altieri, scientific manager on the Euclid mission at ESA.

To try to get an idea of ​​the size of this canvas, you have to project yourself to gigantic scales. In the canvas, intersections of filaments and emptier areas coexist. At the intersections are clusters and superclusters of galaxies, the most massive sets in the universe. For example, the Milky Way, our galaxy, is part of a supercluster called Laniakea, which sits at a crossroads of filaments. “If the Earth is your apartment, the solar system is your city, the Milky Way your region and Laniakea your continent”illustrates the Autour du ciel blog.

>> What is Laniakea, the celestial supercontinent where our galaxy orbits?

Euclid’s measurements should make it possible to understand how the cosmic web has evolved over time under the effects of dark matter and dark energy. Indeed, the telescope will observe very far, and therefore go back in time, observing up to 10 billion years ago (the Big Bang having occurred 13.7 billion years ago).

Dark matter and dark energy, what are they?

The classical matter, which we know and which constitutes us, occupies only 5% of the whole of the universe. Dark matter occupies 25% and dark energy 70%.

Dark matter. Although five times more present in the universe than classical matter, dark matter is invisible and undetectable. What scientists have perceived since the 1930s are its effects. They find that stars and galaxies are moving much faster than models predict. To explain what they are measuring, they estimate that invisible mass comes into play. This matter, which escapes our observation instruments, is what is called dark matter.

Dark energy. It was highlighted in 1998 when scientists discovered that the expansion of the universe was accelerating. A surprise, at the time, because the scientific world did not anticipate such a result at all. To explain this unexpected observation, a new parameter then had to be introduced, which the scientists named dark energy (or dark energy), to which the acceleration of the expansion of the universe is attributed. The three astrophysicists who carried out this work received the Nobel Prize in Physics in 2011.

How do dark matter and dark energy work?

Do not confuse dark matter with dark energy. The two are not part of the same family, and are even “antagonists”, according to Bruno Altieri. Dark matter has an attractive effect: its mass creates the force of attraction. On the scale of the universe, it should act as a brake on its expansion.

Dark energy has a “repellent effect”. Because of her, celestial objects repel each other, move away from each other. In the expansion of the universe, if dark matter is likened to a brake, dark energy acts as an accelerator. And it is she who “dominates the expansion of the universe for a few billion years”explains Bruno Altieri.

The names “dark matter” and “dark energy” do not mean that the two notions are linked. They are so named for lack of a better one. The qualifier “black” in fact refers to the thick mystery that still surrounds them since we know neither their origin nor their nature.

“When we talk about ‘dark matter’ or ‘dark energy’, we hide our ignorance with the word ‘dark’.”

Bruno Altieri, astronomer

at franceinfo

We really don’t know anything about them?

Scientists are not completely at a standstill. They throw out ideas, formulate hypotheses. But none stands out and manages to reach a consensus.

Dark matter. It still retains all its secrets despite intense efforts by scientists around the world. Where is she from ? “We don’t know”responds tit for tat Françoise Combes. “We don’t even know what it is”loose the astrophysicist bluntly. Indeed, the particles that could constitute it remain unknown. A certainty: dark matter “does not radiate or interact with light”notes Françoise Combes.

“Dark matter is transparent and it can be around us without us seeing it.”

Françoise Combes, astrophysicist

at franceinfo

Dark energy. Its recent discovery and its elusive side mean that it has still been little studied. “Energy is present everywhere, diffusely”says Françoise Combes. “No one knows what it’s made of, or even if it’s a form of energy!” exclaims the ESA.

How will the Euclid mission help to better understand them?

Dark matter can be observed indirectly, through its effects. A high density of matter can deflect light: we speak of “gravitational lensing”. This phenomenon distorts what we perceive of certain objects. It is visible in the first published image from the James Webb telescope: galaxies appear flattened, frayed, taking on an arc shape, due to the immense density of the whitish cluster of galaxies in the center of the image.

With Euclid, “we are going to study weak deformations and establish statistics on billions of galaxies”, explains Bruno Altieri, making it possible to map dark matter. By analyzing the evolution of the cosmic web, scientists will also be able to estimate the period when the expansion of the universe began to accelerate. “We will have the answers to questions like: ‘Is dark energy constant over time? or ‘Does it vary?'” specifies Françoise Combes. According to the astrophysicist, the first question should be answered in “five to ten years”.

If we don’t know anything about what makes up 95% of the universe, does that mean we have it all wrong?

The repercussions of the possible discoveries of the Euclid mission could call into question the standard cosmological model and Einstein’s theory of relativity which continues to be a reference. “Our understanding of gravity may not be perfect”, assures Bruno Altieri. And imagine: “The same way Einstein’s physics wrapped Newton’s physics into a bigger theory, maybe we’ve misunderstood something and need to develop a bigger theory.”

“Either we will discover new particles [pour la matière noire], or we will change the laws of physics. It’s still fundamental.”

Françoise Combes, astrophysicist

at franceinfo

From the first year, the Euclid readings will form a mass of data as important as all the scientific missions of the ESA for twenty-five years, underlines Bruno Altieri. To unravel the mysteries of these phenomena, scientists will therefore have to use their gray matter and perhaps spend a few sleepless nights there.