APP Questions

Astroparticle Physics has evolved as a new interdisciplinary field at the intersection of particle physics, astronomy and cosmology.

It combines the experimental techniques and theoretical methods from both astronomy and particle physics. Particle physics is devoted to the intimate structure of matter and the laws that govern it. Cosmology addresses the large scale structure of the Universe and its evolution since the Big Bang. Astrophysics studies the physical processes at work in celestial objects. Most discoveries in particle physics have immediate consequences on the understanding of the Universe and, vice versa, discoveries in cosmology have fundamental impact on theories of the infinitely small.

Astroparticle physics addresses some of the most fundamental questions of contemporary physics (see also "Connecting Quarks with the Cosmos: Eleven Questions for the New Century", National Academies Press 2003). Achieving an answer to most of these questions would mark a major break-through in understanding our Universe and would open up entirely new fields of research.

What is the Universe made of?

 Only 4% of the Universe is made of ordinary matter. Following the latest measurements and cosmological models, 73% of the cosmic energy budget seems to consist of "dark energy" and 23% of dark matter. The nature of dark energy remains a mystery, probably intimately connected with the fundamental question of  the "cosmological constant problem".


Do protons have a finite lifetime?

Grand Unified Theories (GUTs) of particle physics predict that the proton has a finite lifetime.  Proton decay is one of the most generic and verifiable implications arising from GUTs.


What are the properties of neutrinos? What is their role in cosmic evolution?

Neutrinos have provided the first reliable evidence of phenomena beyond the Standard Model of particle physics. In the Standard Model, neutrinos have no mass. A major breakthrough of the past decade has been the discovery that neutrinos, on the contrary, are massive.


What do neutrinos tell us about the interior of Sun and Earth, and about Supernova explosions?

In 2002, Ray Davis and Masatoshi Koshiba were awarded the Nobel Prize in Physics for opening the neutrino window on the Universe, specifically for the detection of neutrinos from the Sun and a Supernova.  The observation that solar neutrinos change their identity on their way from the Sun to the Earth ("neutrino oscillations") has provided the first indications of massive neutrinos, i.e. of physics beyond the Standard Model of particle physics.


What's the origin of high energy cosmic rays?  What's the sky view at extreme energies?

Nearly a century ago, the Austrian physicist Victor Hess discovered cosmic rays, charged particles that hit our atmosphere like a steady rain from space. Later, it turned out that some of these particles have energies a hundred million times greater than that achievable by terrestrial accelerators.


Can we detect gravitational waves? What will they tell  us about violent cosmic processes and basic physics laws ?

Gravitation governs the large scale behaviour of the Universe. Weak compared to the other macroscopic force, the electromagnetic force, it is negligible at microscopic scales. The main prediction of a field theory is the emission of waves. For electromagnetism this has been established through the discovery of electromagnetic waves in 1888.


We note that not all of these questions are going to be answered exclusively by experiments belonging to the field we define as "astroparticle physics". Take dark matter searches as an example. First evidence for dark matter has been obtained from the kinematics of Galaxies as revealed by ground-based optical observations in the first third of the 20th century. Since then, dark matter has become a keystone of the standard cosmology model based on much wider evidence than optical astronomy alone, notably on radio-astronomy. The ultimate answer on the nature of dark matter will likely come from the observation of exotic particles constituting dark matter. These particles may be first observed in subterranean laboratories, by the planned detectors recording the nuclear recoils due to the impact of dark matter particles ("direct detection"). Alternatively, signs of dark matter particles may arise as products of their annihilation in celestial bodies and may be detected by gamma telescopes at ground level or in space, by neutrino telescopes deep underwater or ice, or by cosmic ray spectrometers in space ("indirect detection").  Last but not least, it may well be that the Large Hadron Collider provides first evidence for dark matter candidates through their production in accelerator based experiments. From an experimental point of view, optical and radio observations are assigned to the field of astronomy, accelerator research to that of particle physics. Direct and indirect detection make use of laboratories deep underground which is the traditional environment of astroparticle and non-accelerator particle physics. These techniques also use neutrino and gamma telescopes, whose techniques have evolved from particle physics. It is this part of the search for dark matter that we assign to the field of astroparticle physics.