The problem of escape: a missing bit in the theory of the origin of cosmic rays

The escape of cosmic rays from their sources, as well as from the region surrounding a source, or from the galaxy hosting the sources, is a non-linear process that involves a complex chain of events, often overlooked. On the other hand, these phenomena are responsible for setting the maximum energy in accelerators, shaping the source spectra and determining the conditions for escape from the galaxy hosting the sources, a process that is usually modeled by imposing ad hoc boundary conditions in our equations. Here we discuss some of these phenomena and how they affect the spectra of cosmic rays measured at the Earth.