Theoretical Challenges in Acceleration and Transport of Ultra High Energy Cosmic Rays: A Review

TL;DR

This review discusses the current experimental data on ultra high energy cosmic rays, highlighting the theoretical challenges in understanding their origins, acceleration mechanisms, and the transition from galactic to extragalactic sources.

Contribution

It provides a comprehensive overview of the theoretical issues related to cosmic ray acceleration and origin in light of recent observational data.

Findings

Contradictory experimental results challenge existing models.

The transition from galactic to extragalactic cosmic rays remains unresolved.

Theories on acceleration mechanisms in relativistic shocks are still developing.

Abstract

The wealth of data collected in the last few years thanks to the Pierre Auger Observatory and recently to the Telescope Array made the problem of the origin of ultra high energy cosmic rays a genuinely experimental/observational one. The apparently contradictory results provided by these experiments in terms of spectrum, chemical composition and anisotropies do not allow to reach any final conclusions as yet. Here I will discuss some of the theoretical challenges imposed by these data: in particular I will discuss some issues related to the transition from Galactic to extragalactic cosmic rays and how the different models confront our understanding of Galactic cosmic rays in terms of supernova remnants paradigm. I will also discuss the status of theories aiming at describing acceleration of cosmic rays to the highest energies in relativistic shocks and unipolar inductors.