In one of his chronicles, Alexandre Vialatte wrote: “The mind of man never stops dreaming. But also meditating, calculating, calculating. He wants to know and needs not to understand. In short, to know and to marvel. Contradictory necessities: one requires knowledge and the other requires ignorance. Hence science and poetry.” He was right. Because yes, what fascinates us, ultimately, are the puzzles. However, riddles, indeed, cosmology has never failed to produce them: was the universe really born? If yes, how ? What is it made of? What drives its development? Will it ever end? These mysteries could still thicken, because it happens that new discoveries increase… our ignorance! More precisely, the knowledge that we have of our ignorance: thanks to them, we suddenly discover that we didn’t know… that we didn’t know!
In cosmology, two problems have thus appeared and no one can predict what conceptual upheavals they could lead to. They both concern the inventory of the material and energetic content of the universe: it has been discovered that a large part is of a perfectly unknown nature. However, according to the theory of general relativity, it is the content of the universe that determines its evolution, that which it will have in the future, but also that which it has had in the past, including in its past. the most distant… So, for a few years, physicists, rather than seeing life in pink, sometimes give the impression of brooding: they speak of “dark matter” and “dark energy”… In the language current, the word black is used to say sometimes the obscure, the mysterious, the frightening, sometimes the hidden, the unknown, the opaque. But what do we mean when we say of a body, an environment, an object or a substance that they are black?
Let’s start with dark matter. For several decades, the observation of the dynamics of galaxies encourages the assumption that their visible part is enveloped by an enormous mass of invisible matter. There would be in short an “additional” matter which would act in a gravitational way but would not emit light. This material is called “black” in the sense that it remains mysterious, but it is by no means black in the physical sense of the term. Rather, it is a material that neither emits nor absorbs light, which is therefore transparent. What is it made of? Could it be made up of particles we already know? Physicists have long thought so, but they no longer think so. If it exists, then dark matter is made up of radically new particles. But which ones? Nobody knows.
Physicists have their work cut out
Let’s come to dark energy. Twenty years ago, astrophysicists realized that the expansion of the universe is in the acceleration phase for several billion years. What does that mean? In the process of expansion, gravitation, always attractive, acts as a brake: it tends to bring massive objects closer to each other, so that matter can only slow down expansion. But what the measurements show is that another process opposes it, playing an accelerating role. Everything happens as if a kind of “antigravity” had taken over, expanding space more and more and constantly increasing the speed of its expansion.
What is driving this acceleration? Several tracks are mentioned, but none is sure. Cautious, physicists speak of a mysterious so-called “dark” energy, on the one hand because we cannot see it, on the other hand because they are unaware of its nature. What is therefore now established is that visible, ordinary matter, made up of atoms, that which makes up our bodies, the planets and the stars, is in reality only a fringe of the content of the universe, its foam visible. It represents only three or four percent of the total, no more.
Physicists therefore have their work cut out for them. Fortunately, the ESA’s Euclid space telescope, launched in a few months, should allow them to progress by collecting valuable data. And, who knows, maybe they will soon be able to take on this fulgurance of Gaston Bachelard: “In the night of the material black flowers bloom. They already have their velvet and the formula of their perfume”.