Measuring energy production in the Universe over all wavelengths and all
  time
  The study of the extragalactic background light (EBL) is undergoing a
renaissance. New results from very high energy experiments and deep space
missions have broken the deadlock between the contradictory measurements in the
optical and near-IR arising from direct versus discrete source estimates. We
are also seeing advances in our ability to model the EBL from gamma-ray to
radio wavelengths with improved dust models and AGN handling. With the advent
of deep and wide spectroscopic and photometric redshift surveys, we can now
subdivide the EBL into redshift intervals. This allows for the recovery of the
Cosmic Spectral Energy Distribution (CSED), or emissivity of a representative
portion of the Universe, at any time. With new facilities coming online, and
more unified studies underway from gamma-ray to radio wavelengths, it will soon
be possible to measure the EBL to within 1 per cent accuracy. At this level
correct modelling of reionisation, awareness of missing populations or light,
radiation from the intra-cluster and halo gas, and any signal from decaying
dark-matter all become important. In due course, the goal is to measure and
explain the origin of all photons incident on the Earth's surface from the
extragalactic domain, and within which is encoded the entire history of energy
production in our Universe.
