Resumen de A multisymplectic approach to gravitational theories
Jordi Gaset Rifá 
he theories of gravity are one of the most important topics in theoretical physics and mathematical physics nowadays. The classical formulation of gravity uses the Hilbert-Einstein Lagrangian, which is a singular second-order Lagrangian; hence it requires a geometric theory for second-order field theories which leads to several difficulties. Another standard formulation is the Einstein-Palatini or Metric-Affine, which uses a singular first order Lagrangian.
Much work has been done with the aim of establishing the suitable geometrical structures for describing classical field theories. In particular, the multisymplectic formulation is the most general of all of them and, in recent years, some works have considered a multisymplectic approach to gravity. This formulation allows us to study and better understand several inherent characteristics of the models of gravity.
The aim of this thesis is to use the multisymplectic formulation for first and second-order field theories in order to obtain a complete covariant description of the Lagrangian and Hamiltonian formalisms for the Einstein-Hilbert and the Metric-Affine models, and explain their characteristics; in particular: order reduction, constraints, symmetries and gauge freedom.
Some properties of multisymplectic field theories have been developed in order to study the models. We have established the constraints generated by the projectability of the Poincaré-Cartan form. These constraints are related to the fact that the higher order velocities are strong gauge vector fields. The concept of gauge freedom for field theories also has been analyzed. We propose to use the term "gauge'' to refer to the non-regularity of the Poincaré-Cartan form. Therefore, the multiple solutions are characterized by two sources: the gauge related one, arising from gauge symmetries and related to the non-regularity; and the non-gauge related one, which arises exclusively from field theories.
We studied in detail two models of gravity: the Einstein-Hilbert model and the Metric-Affine (or Einstein-Palatini) model. In both cases, a covariant Hamiltonian multisymplectic formalism has been presented. In every situation, we find the final submanifold where solutions exist, and we explicitly write all semi-holonomic multivector fields solution of the field equations. The natural Lagrangian symmetries are presented aswell. Furthermore, we emphasize different aspects in each model:
The Einstein-Hilbert model is a singular second order field theory which, as a consequence of its non-regularity, it is equivalent to a regular first order theory. For this model we have presented the unified Lagrangian-Hamiltonian formalism. We have also considered the presence of energy-matter sources and we show how some relevant geometrical and physical characteristics of the theory depend on the source's type.
The Metric-Affine model is a singular first order field theory which has a gauge symmetry. We recover and study this gauge symmetry, showing that there are no more. The constraints of the system are presented and analysed. Using the gauge freedom and the constraints, we establish the geometric relation between the Einstein-Palatini and the Einstein-Hilbert models, including the relation between the holonomic solutions in both formalisms. We also present a Hamiltonian model involving only the connection which is equivalent to the Hamiltonian Metric-Affine formalism.
