« The multi-messenger spectrum of the universe »
Our understanding of the radiation record of the universe has advanced dramatically over the past decade. The brightness of the extragalactic sky is now measured in photons, neutrinos, and cosmic rays by observatories on the ground, in the depths of the sea and ice, by satellites orbiting the Earth, and by probes at the edge of the solar system. The combination of their measurements now allows us to estimate the energy density of most components of the extragalactic background with an uncertainty better than 30%. Such accuracy is critical for our understanding of extragalactic source populations and the origin of their power, whether from stellar evolution, black hole accretion, or ejection.
This seminar will focus on recent advances in two components of the extragalactic multi-messenger spectrum, which rely heavily on our understanding of astroparticle propagation on Mpc to Gpc scales. The most intense component, covering the optical and infrared domains, has been the subject of controversy over the past two decades. I will show that recent discoveries show a remarkable convergence between the three independent measurement methods, suggesting that we are on the way to obtaining a census of thermal emission since the end of the Dark Ages. The highest-energy component of the extragalactic spectrum, at frequencies 18 orders of magnitude above those of visible photons, is one of the least understood. The comparative study of large-scale surveys with anisotropies at the highest energies suggests that the discovery of the most extreme accelerators in the universe is within reach, especially as a fourth observational dimension opens up with transient factories.
One of the founding puzzles of modern cosmology is the darkness of the night sky. Its solution lies in the cosmic history of astrophysical emissions, whose hidden facets promise to be revealed by the incredible panchromatic and pan-messenger coverage now at our disposal.