III

Gravitation and Cosmology

General Relativity (GR) is considered by many the major triumph of science. The elegance of its concepts and equations contains precise descriptions of simple phenomena, such as the fall of an apple to earth, and also very complex ones, as the dynamics of binary pulsars and the universe itself. GR has passed all the observational tests up to date with flying colours. However, many of the consequences of the theory (e.g gravitational waves) have not been observed yet. Moreover, there seem to exist some critical situations in which GR may not be applicable; some amendment or generalization is needed. In the following we list these situations, along with a short description.

The final stage of some evolution in stars

It is known that starts with masses larger than a certain critical mass will inevitably collapse, giving way to a black hole, a region from where nothing can escape. According to GR, inside this objects lurks a singularity, where all our physical laws are useless. Black holes emit radiation of quantum origin, and this evaporation mines their energy. Little is known about the final stage of this process. Wormholes are another kind of objects that display unusual features (such as fast interstellar travel or closed timelike curves), based on speculative physics.

The Early Universe

The early universe was, only some decades ago, an extremely speculative field of research. This situation has changed a lot in the last years. The abundance of light elements allows the testing of cosmological models up to times t » 10^{- 4}s. The analysis of the anisotropies in the cosmic background radiation opens a window to very early epochs, through which we may be able to say something about the origin of the primordial fluctuations that gave rise to large scale structures in the universe. There are some importants flaws in the standard cosmological model that can be solved by the study of the primitive universe. These are:

The existence of an initial singularity
The initial conditions of the universe

The singularity problem was considered in the 70?s as the main problem in cosmology. The existence of such a singularity, a finite time away from today, has an unpleasant consequence: we cannot know the initial data of the Universe. This fact would limit the chance of getting a closed cosmological description. The existence of the singularity was considered an indisputable fact at first, thanks to the status of a series of theorems. These assumed that GR was the good theory of gravity, and demonstrated that nonsingular configurations were almost impossible in the realm of classical physics. This was the main driving force for the studies of quantum aspects in cosmology. During the 80?s, different approaches, involving various couplings between matter and gravity produced alternatives to the no-go theorems mentioned above. These results launched a new line of research focused in the properties of nonsingular Universes in their extremely condensed phase.

The agreement between the high degree of isotropy inferred from the cosmic background radiation, and the causal properties in a Friedmann-like universe was hard to explain. In the 70?s, the Russian school (led by Lifshitz, Khalatnikov, Belinski and others) started a program that tackled this issue. In spite of their important contributions, they could not solve the problem in a satisfactory way. In the 80?s, a different proposal was suggested: the inflationary universe. After the dissapearence of the initial wave of optimism (and after the publication of many articles on the subject) it was realized that inflation is an incomplete propposal. The features of a possible initial anisotropic phase of the universe are still being investigated. One of the hardest problems that researchers in the field have to deal with is that of the compatibility between the Friedmann universe (with homogeneous spatial section) and the evidence that shows the existence of different scales of inhomogeneity in the Universe. Some models for structure formation need small perturbations in a primordial phase of the universe, during which the spacetime structure is controlled either by radiation in thermal equilibrium or by exotic matter. This happens for instance in the inflationary universe, in which the initial perturbations are amplified thanks to gravitational instabilities, to generate the structures we see today. These perturbations would be determined - according to different models - by high energy, short distance interactions. In this way, a strong liason is established between Quantum Field Theory and High Energy Physics, and Gravitation. In the last two decades, the precision in the extragalactic astronomical data has greatly increased. The analysis of the production of these structures, from the initial data furnished by the perturbations, was brought to the forefront of the research. Consequently high energy phenomena - and the theories that describe them - have underwent a new analysis in order to compare their predictions with such observations. To deal with these issues, it is mandatory to have a model that can describe the initial stages of the Universe.