Astroparticle physics was born at the intersection of astrophysics, particle physics, and cosmology. It had multiple origins: the movement towards underground laboratories to study the decay of the proton and neutrino properties, the first detection of high energy photons using particle physics methods, and the large surveys searching for astronomical dark matter.
Today it addresses fundamental questions: the physics of the primordial Universe, the nature of dark matter and dark energy; the eventual unification of fundamental interactions; the stability of protons; the properties of neutrinos and their role in cosmic evolution; the origin of cosmic rays; the nature of the Universe at extreme energies studied using multi-messenger probes including high-energy cosmic rays, photons, neutrinos and gravitational waves.
At least four Nobel prizes in recent years (supernova neutrino detection 2002, cosmological microwave background fluctuations 2006, dark energy 2011, and neutrino oscillation 2015) are related to the field. In particular the recent discoveries of the Higgs particle and neutrino mixing angles, and the precision measurements of Planck consolidate the above interdisciplinary links.
They give for the first time the possibility of formulating a consistent picture of fundamental physics covering a multitude of energy scales: from the electroweak symmetry breaking scale (Higgs scale) to that of inflation (possibly the scale of unification of interactions) passing through other postulated scales for instance the one responsible for dark matter particle phenomena or neutrino mass and matter-antimatter asymmetry.
There are two basic questions of astroparticle – and of fundamental physics more generally:
1. How many new physical scales exist between the electroweak scale and that of inflation?
This question is connected with the theories of inflation as well as dark matter and energy, but it is also connected with the better understanding of the neutrino sector and the possibilities of unification of interactions.
2. How do the particles and fields of these new energy scales influence the genesis, formation and destruction of cosmic structures?
This question is related to the multi-messenger studies of high energy photons, neutrinos, high-energy charged particles and gravitational waves.