With the advent of string theory, new cosmological models have emerged alongside the development of a new cosmology:string cosmology.
Using specific objects from string theory, branes , physicists have been able to construct innovative models describing the origin and evolution of the universe in what is now called "brane cosmology".
The beginnings of brane cosmology emerged in the late 1990s under the impetus of physicists such as N. Arkhani-Hamed, R. Sundrum, L. Randall, P. Steinhardt and N. Turok. The latter then seek to solve the problem of the hierarchy, that is to say to find a mechanism which would explain the reason why the intensity of gravity is so weak compared to the intensity of the other fundamental interactions. /P>
To do this, physicists use string theory and its extra dimensions. The universe has four dimensions (3+1) that are accessible to us and several other additional dimensions beyond our reach.
Electromagnetism and weak and strong nuclear interactions are confined within the four visible dimensions of our space-time while gravity, on the other hand, by its nature, propagates through all dimensions. Its intensity is thus "diluted" and we therefore only observe a small part of its real intensity in our four dimensions.
Such considerations, moreover completely legitimate with regard to string theory, have required the development of models of the universe integrating these parameters. In particular a model in which our four-dimensional universe would be contained in a dynamic object, a brane, itself contained in a universe of larger dimensions (N> 4), a kind of "hyper-universe" ("hyperspace" in English). Such a higher dimensional universe is called a "bulk".
The main element from string theory on which brane cosmology is based is the brane (the word "brane" comes from "membrane"). A brane is an object with certain physical characteristics (energy, mass, charge, etc.), propagating in space-time and being dynamic. This last point means that a brane is not a static object, it can change configuration and position in space-time. It is also an extended object, that is, it is dimensioned.
More precisely, we speak of p-brane. The ‘p’ indicating the number of spatial dimension(s) of the brane. So, for example, a 3-brane is a brane of 3 spatial dimensions and one temporal dimension. While a 0-brane is a point particle.
The second important element is the rope (not to be confused with cosmic ropes). In string theory, a string is a one-dimensional object propagating in space-time and characterized by its size, tension, vibration mode and energy. It is its vibratory mode that determines the type of particle generated. Depending on how the string vibrates, it will be an electron, a quark, etc. Finally, a chord can be open (it's a segment) or closed (it's a circle).
In brane cosmology, our universe is contained in a 3-brane, and more precisely a D3-brane. The 'D' stands for "Dirichlet" after the German mathematician Johann Dirichlet. Indeed, in this D3-brane, all the particles and interactions are open strings of which at least one end is fixed to the brane. And this prerequisite is explained by a particular mathematical condition called “Dirichlet boundary condition”. This is not trivial because in brane cosmology, it is the ends of the open strings that generate the particles.
Only gravity differs from other interactions in that it is the only closed string of brane models. This means that, since none of its ends are attached to the D3-brane, it is free to propagate in the extra dimensions of the bulk outside the brane. This phenomenon of multidimensional propagation thus offers a direct solution to the problem of hierarchy.
But that's not all. For if brane cosmology provides an explanation of the problem of hierarchy, it also sheds light on another fundamental gray area of modern cosmology:the origin and evolution of the universe.
In the early 2000s, physicists N. Turok, P. Steinhardt and B. Ovrut developed a cosmological brane model explaining the origin of the universe:the ekpyrotic model. The term "ekpyrotic" comes from the Greek "ekpyrosis" meaning conflagration. In ancient Greece, under the impetus of thinkers like Plutarch, the idea emerged that the universe began with a great explosion of fire, followed by destruction, then by rebirth, and so on.
It is with this idea that physicists have built their theory. Within the framework of the ekpyrotic model, our universe is contained in a D3-brane "floating" in a universe of higher dimensions (at least more than 3 spatial dimensions) called bulk. In this bulk float other branes of various sizes. Some of these branes are empty, while others contain universes (or just matter and/or energy). Our D3-brane therefore spreads in the bulk alongside many other branes.
To answer the question of the origin of the universe, the ekpyrotic model advances the hypothesis that two empty branes would have collided in the bulk. A brane initially possessing a certain amount of energy, the collision of two branes would have resulted in a violent exchange of kinetic energy between them. Immediately, part of this energy would have transformed into a primordial soup of matter-energy, more precisely a quark-gluon plasma, and the other would have given the impetus for the expansion of the universe. This model, while respecting the Big Bang theory, is an alternative to inflation .
Such bran collisions are not necessarily rare and can occur randomly in the bulk. Therefore, the ekpyrotic model can be described as a cyclical model, because its authors specify that it is possible that our current universe is neither the first nor the last, our precious 3D-brane not being immune to 'an (other?) future collision.
Moreover, without considering potential collisions, some versions of the ekpyrotic model naturally predict a Big Bounce, therefore the alternation of the Big Bang and the Big Crunch.
The ekpyrotic model offers the possibility of being experimentally tested by observable predictions. Thus, as an alternative to the mechanism of inflation, the latter predicts for example the absence of particular polarization modes of the primordial gravitational waves, the B modes. If the observations indeed showed such an absence, the model would therefore find comfort in it.
