**Contents**


#### **Preface to "Exact Solutions in Classical Field Theory: Solitons, Black Holes and Boson Stars"**

Compact objects is the name given to solutions in General Relativity (or alternative models of gravity) that are typically so dense that the curvature of space–time around them has detectable effects. An extreme case exists, which is realised and observed in nature: black holes. There exists at least one alternative to black holes, albeit more exotic, given by compact objects made out of bosonic fields, the simplest example being the boson star made of a complex valued, massive scalar field.

The study of compact objects as dense as black holes and boson stars is interesting in its own right. It is also very important from another perspective: Since these objects create very strong gravitational fields, they are also an ideal testing ground for alternative models of gravity and/or for testing the limits of General Relativity. The latter theory works extremely well in the weak regime, but has not been explored in full detail in the very strong regime. This Special Issue is mainly devoted to exact solutions to Einstein–Yang–Mills-type theories and their various extensions. It is dedicated to Prof. Yves Brihaye on the occasion of his 65th birthday. Prof. Brihaye contributed significantly and is still contributing to this very active topic.

> **Nicolas Boulanger, Andrea Campoleoni** *Editors*
