Cosmology is the branch of astrophysics that studies the origin, nature, structure and evolution of the Universe.
History of scientific cosmologies
The scientific cosmology established at any given time depends directly on what we know about the universe. Before the 19th century, the known universe was essentially reduced to the solar system alone, and cosmology therefore focused only on the formation of this one. It was not until the first half of the 19th century that the distance to nearby stars began to be known or estimated relatively realistically (from 1838 thanks to Friedrich Wilhelm Bessel). The study of the spatial distribution of stars within our galaxy was then carried out until the beginning of the 20th century. Finally, in the 1920s the extragalactic nature of what were then called “nebulae” (today galaxies) was discovered by Edwin Hubble. Shortly after, Georges Lemaître also discovered the expansion of the Universe, that is to say the fact that the galaxies of the universe are moving away from each other, and all the more quickly that they are far. Cosmology as we understand it today is therefore the study of the structure, history and evolution of a universe filled with galaxies as far as the eye can see.
A few orders of magnitude
Earth is a relatively small planet (about 6,370 km in radius), orbiting a Main Sequence star, the Sun. By definition, the Earth-Sun distance is used to define the astronomical unit, approximately 150 million kilometers. Other planets orbit the Sun. The farthest planet from the Sun (not counting the dwarf planets) is Neptune, about 4.5 billion kilometers from the Sun, or thirty times the Earth-Sun distance.
The solar system is itself linked to a structure, the Galaxy (in the case of our galaxy: the Milky Way), comprising several hundred billion stars. The closest star to the Sun, Proxima Centauri, is located just over 4 light years away, or ± 38,000 billion kilometers from the Sun, or 260,000 times more than the Sun-Earth distance (AU or the astronomical unit). Most of the stars visible to the naked eye in the night sky are several tens or even hundreds of light years away. The Sun is located on the outskirts of the Galaxy. It is about 25,000 light years away; the Galaxy has a radius approximately twice as large as this distance, for a diameter of approximately 100,000 light years. These dimensions make it a typical galaxy in the universe.
If we except the dwarf galaxies which exist in number in the vicinity of our galaxy, the massive galaxy closest to us is the Andromeda galaxy, whose distance is slightly more than 2 million light years. Our galaxy and that of Andromeda are the two most massive representatives of a group of galaxies linked by gravity and called the Local Group, a few million light years wide. There are other larger structures in the universe called galaxy clusters and superclusters. The cluster closest to the Local Group is the Virgo cluster, itself located near the center of the Virgo supercluster. Superclusters are the largest structures in the universe, however their size does not exceed 200 to 300 million light years. These structures are organized in filaments of higher densities which surround almost empty spaces with a size of the order of hundreds of millions of light years (the stars entering them being slowed down by the gravitational effect of the filaments made up of superclusters).
(Map of the cosmological diffuse background, an important object of study of modern cosmology, obtained by reconstruction of observations of the WMAP satellite.)
The limit of the observable universe is estimated at 45 billion light years, which corresponds to the distance from the cosmological horizon under the standard model of cosmology.
It is estimated at:
- 10 million the number of superclusters located in the observable universe;
- 25 billion the number of clusters of galaxies located in the observable universe;
- 350 billion the number of massive galaxies (greater than or on the order of that of our galaxy) located in the observable universe;
- 30 trillion billion (3 × 1022) the number of stars located in the observable universe.
The density of the Earth is about 5 tons per cubic meter. Given its size, its mass is approximately 6 × 1024 kg. The Sun, which is a typical star, is about 300,000 times as massive, or 2 × 1030 kg. For larger objects (galaxies, clusters of galaxies), it is customary to use the solar mass as the unit of mass, the kilogram becoming too small a unit given the numbers in play.
Observations indicate that galaxies are significantly more massive than the stars that compose them. We are pretty sure today that in addition to the ordinary matter of which we are made, there is another form of matter, currently unknown in the laboratory, called dark matter. Unlike ordinary matter, this dark matter does not interact with light and is therefore invisible. In addition, it does not form compact structures like stars, planets or asteroids, but has a much more diffuse distribution within galaxies. The mass of dark matter in galaxies (and the entire universe) is about six times that of ordinary matter. The mass of our galaxy is therefore a little over a trillion solar masses.
The estimated mass of the superclusters is around 1015 solar masses. Compared to their size, superclusters are extremely thin objects: only a few dozen atoms per cubic meter. The mass of the observable universe is estimated at 1.4 × 1024 solar masses.
The period of revolution of the Earth around the Sun is one year (actually a tropical year). The farther the planets are from the Sun, the greater their period of revolution, a consequence of Kepler’s third law. Thus, Neptune has a period of 165 years.
The orders of magnitude increase considerably if we look at the period of revolution of the Sun around the galactic center: it is about 200 million years. The stars are not immutable objects. They form, start to glow, then go out for lack of nuclear fuel in them. The age of the Sun is approximately 4.5 billion years. The oldest stars in our galaxy are around 10 billion years old. It is also the age of our galaxy. Galaxies too are born from immense clouds of gas. The universe itself as we know it is not eternal. It originated from an extremely dense and hot phase, the Big Bang, which occurred approximately 13.819 billion years ago.
Contributions of general relativity
The goal of cosmology is to describe the universe and its formation, which can initially be represented by a relatively uniform (large-scale) distribution of matter. It turns out that Newtonian mechanics is unable to describe a uniform and infinite distribution of matter. To describe the universe, it is essential to appeal to general relativity, discovered by Albert Einstein in 1915. Einstein is also the first to publish a modern cosmological model, solution of his newly discovered theory, but describing a homogeneous universe finished and static. This model is essentially motivated by philosophical as well as physical considerations, but introduces an extremely ingenious (and somewhat hazardous at the time) idea, the cosmological principle.
The discovery a few years later of the expansion of the Universe by Edwin Hubble calls into question Einstein’s static universe model and ends up laying the foundations of modern cosmology: the universe (or in any case the region accessible to observations) is expanding, and described by general relativity. Its evolution is determined by this theory, as well as by the physical properties of the forms of matter present in the universe. It is essentially according to the latter that the various cosmological theories will emerge.
Observations indicate that the universe is expanding. It was denser and warmer in the past. This is the founding idea of the Big Bang, the model of which emerged in the middle of the 20th century. It indicates that the universe as we know it arose from a dense and hot phase (without claiming to know what happened at the very beginning of this phase), at the end of which it was in a extremely homogeneous state, that is to say without astrophysical objects (stars, galaxies …). These objects were subsequently formed by a mechanism called gravitational instability. As astrophysical objects form, the physical conditions in the universe change, ultimately producing the universe as we know it. The detail of these processes depends on many parameters, such as the age of the universe, its density, and the properties of the different forms of matter that coexist in the universe.
In practice, researchers are developing cosmological models, that is to say a kind of scenario describing here the different phases through which the universe has passed since and possibly during the Big Bang. In the 1990s, the standard model of cosmology finally emerged, which represents the simplest model capable of explaining all cosmological observations.
Standard model of cosmology
General relativity, quantum mechanics and field theory, coupled with numerous astronomical observations today allow us to sketch a relatively reliable scenario of the history of the universe over the last 13 or 14 billion years. It is now customary to speak of a standard model of cosmology, like the standard model in particle physics, although the latter is quantitatively better tested and better constrained. The Standard Model of Cosmology is based on the concept of the expansion of the Universe, and the fact that it has been denser and hotter in the past (hence the term Hot Big Bang). Its description is based on the use of general relativity to describe the dynamics of its expansion, and the data of its material content determined in part by direct observation, in part by a set of theoretical and observational elements. We consider today that the universe is homogeneous and isotropic (that is to say that it always has the same aspect whatever the place from which it is observed and the direction in which it is viewed. observes), that its spatial curvature is zero (that is, the large-scale geometry corresponds to the usual geometry in space), and that it is filled with a number of forms of matter, to know :
- Ordinary matter (atoms, molecules, electrons, etc.), also called baryonic matter, making up about 5% of the composition of the universe.
- Another form of matter called dark matter, of non-baryonic origin, composed of massive particles not detected to date, entering for about 25% of the total composition.
- Another form of energy whose nature is poorly known, but which could be a cosmological constant, and generically called dark energy, entering for 70% in the composition of the material content of the universe.
To this is added electromagnetic radiation, mainly in the form of a homogeneous background of photons from the dense and hot phase of the history of the universe, the cosmic diffuse background. There is also a cosmological background of neutrinos, not detected to date, but whose existence has been proven by a certain number of indirect observations, as well as a cosmological background of waves. gravitational, also not detected, directly or indirectly.
It is likely that in the past the material content has been different. For example, there is no, or only very little, antimatter in the universe, however it is believed that in the past matter and antimatter existed in equal amounts, but a surplus of ordinary matter has grown. formed during a process, still poorly understood, called baryogenesis. At present, only the earliest eras of the expansion phase of the universe are poorly understood. One of the reasons for this is that it is not possible to directly observe these eras, the most distant radiation detectable today (the cosmic diffuse background) having been emitted about 380,000 years later. A number of scenarios describing some of the earlier eras exist, of which the most popular is that of cosmic inflation.
The fate of the Universe is not, at the present time, either known with certainty, but a large number of elements suggest that the expansion of the universe will continue indefinitely. Another unresolved question is that of the topology of the universe, that is, its structure on a very large scale, where various ideas have been proposed.