IPNO, Université Paris-Sud, Univ. Paris/Saclay, CNRS/IN2P3
Besides the classic heralds of astronomy, from radio waves to X-rays, the past two decades have seen the emergence of four fantastic messengers: astroparticles. Gravitational waves, GeV-TeV gamma rays, PeV neutrinos, and cosmic rays at EeV energies and above are all now, since 2015, informants on the conditions prevailing in the most extreme accelerators that our universe houses. In this talk, I will illustrate the first steps of multi-messenger astronomy with gamma-ray and cosmic-ray signals that have traveled to our ground-based detectors from cosmic scales, ranging from the innards of our super-cluster, Laniakea, to a time when the universe was only a fifth of its current age, beyond z ~ 2. Astroparticles from the most extreme extragalactic emitters not only enable the probe of particle acceleration at energies beyond the reach of human-made facilities, but can also be exploited as tracers of the electromagnetic content of the universe. Magnetic and photon fields populating the largest voids in the cosmos are starting to reveal their mysteries, setting the grounds for the newborn field of gamma-ray cosmology and long-awaited cosmic-ray astronomy. Major astroparticle observatories are under upgrade or construction and they hold a formidable scientific potential for current and next-generation researchers.