Interdisciplinary Aspects of High-Energy Astrophysics

arXiv:1102.1436v1 [astro-ph.HE] 7 Feb 2011 Interdisciplinary Aspects of High-Energy Astrophysics G¨unter Sigl Abstract Modern astrophysics, especially at GeV energy scales and above is a typ- ical example where several disciplines meet: The location and distribution of the sources is the domain of astronomy. At distances corresponding to significant red- shift cosmological aspects such as the expansion history come into play. Finally, the emission mechanisms and subsequent propagation of produced high energy parti- cles is at least partly the domain of particle physics, in particular if new phenom- ena beyond the Standard Model are probed that require base lines and/or energies unattained in the laboratory. In this contribution we focus on three examples: High- est energy cosmic rays, tests of the Lorentz symmetry and the search for new light photon-like states in the spectra of active galaxies. 1 Introduction High energy astrophysics is nowadays a very interdisciplinary research field which either uses input from or provides new output to other fields including astronomy, cosmology, particle physics and even philosophy and (astro)biology. Examples very this becomes especially obvious includes the use of active galactic nuclei to probe the formation of structure at very high redshift of order ten, high energy cosmic rays as probes for the annihilation or decay of dark matter and the use of “standard candles” such as exploding white dwarfs and (more recently) gamma-ray bursts to probe the expansion history of the Universe. A particular problem that sometimes occurs at these intersections are differ- ent languages spoken by the different communities. In general, however, a lot of progress has been made in that respect. This is the case in particular in astroparticle physics, a still young but meanwhile well established research discipline in its own G¨unter Sigl II. Institut f¨ur theoretische Physik, Universit¨at Hamburg, Luruper Chaussee 149, D-22761 Ham- burg, Germany e-mail: guenter.sigl@desy.de 1 2 G¨unter Sigl right. This can be seen not least from the fact that funding agencies in most countries have developed programs and instruments aiming in specifically at this field. The present paper can naturally cover at most a tiny fraction of interesting exam- ples for such interfaces between neighboring research fields. We specifically focus on three topics at the interface beween astronomy, high energy astrophysics and particle physics: First, ultra-high energy cosmic rays, traditionally understood as particles with energies above 1018eV, have been observed with energies up to a few times 1020eV, which is a macroscopic energy of about 50 Joules, presumably of just one elementary particle. Therefore, very likely, the sources of these ultra-energetic particles have to be exceptionally powerful and visible in other wavelengths and channels. The search of these sources has thus a strong relation to astronomy. Second, the macroscopic energies of these particles makes them natural test beams for particle physics at energies that cannot be achieved in the laboratory in the foreseeable future. In particular, tiny violations of fundamental symmetries of Nature, such as the Lorentz symmetry, may become magnified at large energies. We are still lacking a description of gravity that is consistent with quantum mechanics and the way gravity unifies with the electromagnetic, weak and strong interactions may only manifest itself at energies approaching the Planck scale. In this case, high energy astrophysics may be an indispensable tool for the phenomenology of quan- tum gravity. Finally, at the opposite, low energy end, new physics may also exist in the form of very light particles that may morph into photons and vice versa. The strongest constraints on such possibilities that are often motivated by models of fundamental physics such as string theory and loop quantum gravity often come from astrophys- ical and cosmological observations which offer the largest baselines and the highest energies. 2 Astronomy with the Highest Energy Particles of Nature ? The research field of ultra-high energy cosmic rays started in 1938 when Pierre Auger proved the existence of extensive air showers (EAS) caused by primary par- ticles with energies above 1015eV by simultaneously observing the arrival of sec- ondary particles in Geiger counters many meters apart [1]. Since that time, ultra- high energy cosmic rays (UHECRs) have challenged the imagination of physicists and astrophysicists alike. The first cosmic ray with energy above 1020eV was dis- covered by John Lindsley in 1963 at the Volcano Ranch Observatory [2]. The record holder is probably still the famous “Fly’s Eye event” of ≃3 × 1020eV [3] and quickly, scientists were looking for astronomical sources [4]. Around the same time, the Akeno Giant Air Shower Array (AGASA) caused excitement because it ob- served an UHECR spectrum continuing seemingly as a power law around 1020eV. This was contrar

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