Introducing the gravitational Universe
We have all been outside on a beautiful moonless night and we have marvelled at the display of the stars which appeared to speckle the sky with scintillation. After a few minute observation, we locate the opal band of the Milky Way, some constellations, we identify various shades of light, and make out a swarming in the background, guessing the presence of even more distant stars. The Universe is lying in front of our eyes, in all its glory and immutability. And we feel like specks of dust thrown among these grains of light, for an instant pathetically small compared to the cosmic ages. For centuries, poets, philosophers, artists, storytellers have dreamed, written, debated about this dizzying perspective that is offered to anyone raising the head to contemplate the starry night sky. All has been said about it. Yet…
Yet the physics of the last century has taught us that the observable Universe is much richer than what our eyes may reveal, and that this richness is accessible, here on Earth and now. Accessible through detecting means more powerful than our eyes, as we have known since the days of Galileo and his telescope. But also accessible through different tools, which do not probe light but other types of radiation. And we are only starting to understand the consequences of this, such as seeing the Universe differently or reaching to its more fundamental nature.
Let us return to our starry night. Each grain of light is colloquially called a star, yet we have known for a long time that some of them are the planets of our own solar system, reflecting the light of the Sun: Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. We have also known for a century that a few of these luminous dots are galaxies, enormous accumulations of stars. Our own star, the Sun, is part of such an ensemble, the Milky Way which is our own Galaxy. Because the solar system is somewhat at the outskirts of this galaxy, we see the Milky Way by the side. Hence its elongated shape across the sky. The milky aspect of this strip is due to the concentration of stars in this direction. We also see stars of the Milky Way in other directions, but they are more scattered. All the stars that we can identify in the sky are actually part of our own Galaxy. Other luminous dots are of extragalactic nature, which means that they are localized outside our Galaxy. They are in fact galaxies in their own right: they are located so far away from us that one cannot separate the individual stars that form them.
The world according to the astronomer William Herschel in 1785: the Milky Way
The discovery of the extragalactic nature of these light sources, in other words the discovery of other galaxies, in the 1920s came with an even more surprising finding: these galaxies move away from us. Thus the Universe, which had impressed us for its immutability, is on the contrary dynamical. And since all galaxies recede from us, one must deduce, if we are not in a privileged location, that it is the very structure of space-time which is dynamical: every galaxy moves away from every other galaxy. One translates this by saying that the Universe is in expansion.
It is the light that provides the proof of this expansion. Indeed, one may study the material elements present in each star by analysing the light they emit, in particular the colour of this light, associated with its frequency. And the works of Lemaître and Hubble in the 1920s showed that the light emitted by known elements in the cosmos is slightly shifted in frequency towards the red, an effect that would be attributed to their motion away from us. The distant galaxies that emit them are receding from us.
The Andromeda galaxy 2.5 million light-years from us
But light has another property than its color (frequency): it has a finite velocity. And this has rather remarkable consequences for our observer of a starry night. The light emitted by a celestial body takes a finite amount of time to reach us: 8.32 minutes from the Sun, 100.000 years from the borders of the Milky Way, 2.5 million years from the Andromeda galaxy. This means that, when we contemplate the sky, we are not watching the Universe as it is today, but rather a series of pictures which are all the more ancient that they are more distant. A sort of static movie unfolds in front of our eyes.
It is a formidable advantage for anyone interested in the past history of the Universe: this history is narrated for us, here and now. The most distant celestial bodies that we see (located at some 14 billion light years) are in the state that they were in at the beginning of the Universe.
But what is responsible for the expansion of the Universe? Where does the energy necessary to dilate these distances come from? The answer is given by Einstein. In 1915, one hundred years ago, he is looking for a system of equations that unifies the description of gravitational phenomena in the framework of his theory of special relativity, conceived ten years earlier. These will be the famous Einstein’s equations, which quantify the deformation of space-time under the effect of a mass, or more generally of a concentration of energy, and allow to compute the trajectory of an object close to a massive body. They form the heart of what is called general relativity.
Soon Einstein attempts to apply his equations, not just to objects (such as celestial objects) in gravitational interaction, but to the whole Universe. And precisely, solutions exist for which the Universe is in expansion. The origin of expansion is to be found in the gravitational force itself. In other terms, the Universe as a whole is run by gravitation! Paraphrasing Aristotle, according to whom God was the first engine of the Universe, we see that gravitation is the first engine of the Universe evolution.