The study of the evolution of the universe has always been a very active subject of research. Whether about its past, present or future, these questions are at the heart of modern cosmology.
However, if theories like the Big Bang give scientists insight into the beginnings of the universe, are there, conversely, ways to know or determine the fate of the universe? Yes, it is possible, provided you consider a number of parameters including the expansion of the universe, its density or its curvature.
The work and observations of many scientists, including Vesto Slipher, Georges Lemaître, Edwin Hubble, Albert Einstein and Alexandre Friedmann, have enabled researchers, since the beginning of the XX th century, to understand that the universe was expanding. In other words, the galaxies are moving away from each other as the space between them expands. Physicists then sought to understand the dynamics of this expansion.
At the end of the 1920s, the physicist A. Friedmann found a solution to the equations of general relativity in the context of a homogeneous and isotropic universe:the Friedmann equations. These equations thus describe the dynamics of the universe and, more particularly, its expansion. Friedmann demonstrates that the expansion of the universe is a function of the properties (in particular the pressure) of the different forms of matter and energy that compose it, of its average energy density as well as of its spatial curvature.
These equations were further developed by physicists G. Lemaître, H. Robertson and A. Walker and today constitute the Friedmann-Lemaître-Robertson-Walker metric, a key mathematical tool for describing the evolution of our universe.
As early as 1917, A. Einstein tried to find solutions of general relativity to describe the universe. To do this, he postulates a homogeneous and isotropic universe, that is to say a universe in which all the zones of space have identical characteristics and for which the large-scale structure remains identical, independently of the direction of movement. 'observation. These two parameters constitute the cosmological principle.
Now, for the universe to present these two characteristics, it must necessarily have passed through a primitive phase of homogeneity and isotropy. It was not until 1979 that the physicist Alain Guth introduced the theory of inflation (1). This postulates that the observable universe has undergone an extremely violent and rapid expansion phase.
Inflation is included in the Big Bang model initially proposed by Friedmann and Lemaître. Today, the majority of cosmological models incorporate the Big Bang theory. As for the mechanism of inflation, in addition to giving a theoretical basis to the cosmological principle, it allows us to assume another key parameter in determining the fate of the universe:spatial curvature.
In 1998 (2), after measuring the luminosity distance of several type Ia supernovae, two international teams led by Adam Riess and Saul Perlmutter published their results concerning the expansion of the universe:it is accelerating. In other words, the speed at which galaxies move away increases with time. This result is unexpected because Friedmann's equations predict a deceleration of expansion under the effect of gravitation. Physicists therefore expected to observe evidence of this deceleration and not the opposite.
In order to explain the mechanism at the origin of the acceleration of the expansion, scientists postulate the existence of a repulsive energy at negative pressure:dark energy, the nature of which remains unknown. Several hypotheses have been formulated in recent years and the cosmological constant introduced by Einstein to balance his universe, which he will later describe as being "the greatest error of his existence", is coming back to the fore as a candidate. serious to dark energy (3).
Although dark energy was introduced to correspond to the observations made, it naturally solves a number of theoretical problems that arose previously. It thus offers, for example, an explanation for the current diameter of the observable universe, about 90 billion light-years, compared to the age of the universe itself. Furthermore, the presence of dark energy is consistent with the observed distribution of large-scale galaxies and with the fluctuations of the cosmic microwave background, observed by WMAP (4).
Dark energy is a key factor in determining the fate of the universe because the various observations carried out over the past twenty years have confirmed the acceleration of the expansion of the universe and currently tend to indicate that this expansion will continue. indefinitely. If in the past expansion did not always experience an acceleration phase (observations show that the universe has gone through phases of deceleration or plateau), today nothing indicates that this acceleration is intended to stop.
If the discovery of the expansion and its acceleration is a determining factor for building hypotheses on the future of our universe, it is however not sufficient. It is also important to know the universe itself. To do this, it is necessary to determine the content of the universe and the various factors that have acted on it during its history.
All these parameters are called “cosmological parameters”. The peculiarity of cosmological parameters lies in the fact that their values are either unknown or very poorly determined. This is why a viable cosmological model is a model that uses as few cosmological parameters as possible.
A cosmological model is a model built on a double base of theory and observations allowing to describe the history, the evolution and the structure of the universe. The majority of cosmological models are based on general relativity and the Big Bang theory (5).
The one that currently offers the most faithful description of the universe is the Standard Model of cosmology and is poetically called the “ΛCDM model”. The letter Λ (lambda) refers to the presence of a cosmological constant (it is thus noted in the modified equations of general relativity taking into account the acceleration of expansion).
As for the letters "CDM", these are the acronym for "Cold Dark Matter", literally "cold dark matter". Cold dark matter refers to a theory in which dark matter particles interact very little with baryonic matter (ordinary matter) and electromagnetic radiation, and possess a speed much lower than the speed of light. In this theory, the structures of the universe were formed from the smallest to the largest (by accretion, aggregation and fusion).
On the contrary, in the theory of hot dark matter, the particles move at a speed very close to that of light and the large structures were formed from the largest to the smallest (by splitting, bursting and disintegration). However, observations of the cosmic microwave background tend to favor the existence of cold dark matter, as hot dark matter cannot adequately explain the formation of structures from the Big Bang.
The ΛCDM model therefore contains certain cosmological parameters representing properties of the universe that cannot be determined correctly. This is for example the case of the content of the universe and therefore of its average density. Physicists consider that there are five forms of energy and matter:photons, neutrinos, baryonic matter, dark matter and dark energy.
The density of photons and neutrinos is calculated respectively from the cosmic microwave background and the diffuse neutrino background; these densities are known with precision and are therefore not cosmological parameters (6).
On the other hand, the density of baryonic matter is extremely complicated to define since it is currently impossible to know precisely the distribution of mass in a given cube of the universe, because of the large local inhomogeneities in the distribution of baryonic matter. . Only approximations are possible.
However, the average density is of paramount importance since it determines the curvature of the universe. The same is true for dark matter and dark energy densities. These three quantities are therefore cosmological parameters.
The other three cosmological parameters concern two quantities relating to inflation (the spectral index and the amplitude of density fluctuations) and a parameter describing the period of re-ionization during the formation of the first stars. The ΛCDM model therefore contains six cosmological parameters and requires the presence of dark matter, dark energy, and zero curvature (6).
