Complexity and Simplicity of Self–Gravitating Fluids

We review a recently proposed definition of complexity of the structure of self–gravitating fluids [54], and the criterium to define the simplest mode of their evolution [55, 60]. We analyze the origin of these concepts and their possible applications in the study of gravitational collapse. We start by considering the static spherically symmetric case, extending next the study to static axially symmetric case [58]. Afterward we consider the non–static spherically symmetric case. Two possible modes of evolution are proposed to be the simplest one. One is the homologous condition as defined in [55], however, as was shown later on in [60], it may be useful to relax this last condition to enlarge the set of possible solutions, by adopting the so-called quasi-homologous condition. As another example of symmetry, we consider fluids endowed with hyperbolical symmetry. Exact solutions for static fluid distributions satisfying the condition of minimal complexity are presented [61]. An extension of the complexity factor as defined in [54], to the vacuum solutions of the Einstein equations represented by the Bondi metric is discussed [59]. A complexity hierarchy is established in this case, ranging from the Minkowski spacetime (the simplest one) to gravitationally radiating systems (the most complex). Finally we propose a list of questions which, we believe, deserve to be treated in the future.