Ten years ago, we discovered that the expansion of the universe is accelerating. To explain this, cosmologists have introduced a new form of energy, dark energy. But it remains enigmatic. Should deep cosmological models?

Despite tremendous progress over the past 20 years, cosmologists have had to face the fact that there are gaps in our understanding of the universe are far from being met. Some properties of the universe, or expansion of its age and its average density, are quite well known, but its content remains an enigma.

This dark energy has negative pressure and it acts as a kind of antigravity that powers the cosmic expansion, while the gravity of the matter the brakes. While baryonic matter and dark matter are diluted with the expansion of space, the density of dark energy remains almost constant.

What is the true nature of dark energy? Why this mysterious form of energy is indispensable in the standard cosmological model? Could we do without? We will try to answer these questions.

From a pragmatic perspective, the dark energy is just an additional quantity introduced in the standard cosmological model to fit the astronomical observations. Einstein had already introduced in 1916 an ad hoc scale, the “cosmological constant” in his equations. He wanted to establish a static model of the universe, according to his convictions. For this it was necessary that the gravitational pull of matter is compensated by a repulsive force. The cosmological constant was exactly this repulsion. However, Edwin Hubble discovered in the 1920s that galaxies are receding from each other, indicating that the universe is expanding. At the same time, Alexander Friedmann and Abbé Georges Lemaitre independently showed that the equations of general relativity of Einstein, in the absence of cosmological constant, leading to models of expanding universe. Einstein thus rejected his initial idea …