The International School of Physics “Enrico Fermi” has been for several decades now one of the most prominent cultural initiatives promoted by the Italian Physical Society (SIF). The School was initiated in 1953 by the President of SIF at that time, Prof. Giovanni Polvani. That first edition focused on the contribution of cosmic ray physics to our knowledge of the laws of elementary particle physics. It was still a time in which many remarkable discoveries in particle physics originated from observations of cosmic rays, but it was also a period that marked the beginning of what we now call cosmic ray astrophysics. The results of the pioneering work of Enrico Fermi on particle acceleration were published in two articles in 1949 and 1954, respectively: for the first time, rather than wondering what one could learn on particle physics from these very energetic particles hitting the atmosphere of the Earth, scientists were wondering how could Nature produce those particles. Almost seventy years after that first School dedicated to cosmic rays, we decided to organise a school focused on the foundations of cosmic ray astrophysics. In this perspective, we selected some topics that, in our opinion, are more foundational than others and made an effort in having dedicated mini-courses covering these topics, with lectures delivered by known experts in the field. The lectures range from particle acceleration to cosmic ray transport, from radiation processes to feedback of cosmic rays on galaxy formation.

Being the energy density of cosmic rays, magnetic fields and gas in the Milky Way roughly the same, it is clear that these three components strongly feed back onto each other. This is true in the Galaxy at large and even more so in the proximity of acceleration regions, as well as in the large scale structure of the universe. Hence it is of crucial importance to achieve a satisfactory understanding of the interplay between these components, with cosmic rays generating magnetic fields and at the same time being affected by the presence of magnetic fields, and gas being moved around by the presence of cosmic rays and magnetic fields. The implications of this feedback are far-reaching especially in terms of star formation and the evolution of large scale structures in the universe.

We have made a special effort in providing the students with a proper description of the physics of cosmic ray transport in turbulent magnetic fields, and of the production of magnetic perturbations in the presence of cosmic ray gradients. Both these phenomena are essential in describing particle acceleration at shocks and transport of cosmic rays in the Galaxy. These topics are discussed from multiple points of view and in a number of sources, from shocks in supernova remnants to pulsar wind nebulae, from star clusters to reconnection layers in plasmas and in all these cases the role of radiative losses has received special attention. The transport of cosmic rays is also discussed in different environments, from the Milky Way to clusters of galaxies, and the universe at large, especially important for ultra high energy cosmic rays that, we now know, are mainly of extragalactic origin.

Although the main focus of the School is of theoretical type, we made sure that proper contact with current observations is provided, through dedicated lectures on the most recent developments in the measurement of the physical properties of both charged and neutral (gamma rays, neutrinos) cosmic rays. In particular, the accuracy of recent gamma-ray measurements over more than six decades in energy, from ∼100 MeV to ∼100 TeV and beyond, has reached a level that allows us to achieve a careful modelling of the acceleration and radiation processes inside or in the close proximity of cosmic ray factories.

There is no doubt that this field of investigation has been experiencing an extraordinary boost in recent times, both due to observations carried out with unprecedented accuracy, and to the possibility of testing new ideas and complex non-linear scenarios directly thanks to a leap in the development of numerical simulations. Dedicated lectures have covered these numerical challenges and the perspectives for their use to model sources and cosmic ray transport.

If the planning and construction of new experimental/observational facilities can be used as an indicator of the field dynamical state and its potential for future growth, the field of high energy astrophysics can look forward to decades of excitement, a fertile soil for todays students. We hope that this School may provide the necessary knowledge for them to play an important role in such exciting times.

Felix A. Aharonian, Elena Amato, Pasquale Blasi and Carmelo Evoli