Do we understand the extremes of the Universe?

The challenge of astrophysics is to explain what we see in the sky using exactly the same physical laws that we can measure in the laboratory. Sometimes this challenge is one of implementation of well-understood physics in the face of a complicated dataset, but there are often much greater issues of principle. There where the observational constraints are few, such as in the case of gamma-ray bursts, the appropriate physics at work can be highly uncertain. Conversely, astrophysics also offers unique possibilities for probing fundamental physics beyond the level that can be explored in the laboratory. The examples are gravity in the strong-field regime, and particle physics at energies above the TeV scale.

These extreme applications of astrophysics deal with grand and general themes. At the greatest extreme of scale, we find questions of cosmology: How the Universe came to exist in its current form, and the nature of its contents. Here, astronomy has made what is indisputably its greatest contribution to physics by the detection of a non-zero vacuum density – the so-called dark energy. It is a common cliché to describe dark energy as the greatest unsolved problem in physics, but it is hard to disagree. There is a widespread feeling that the dark energy density should vary with time, possibly in a way that is detectable. It may also have been very much larger at very early times, causing a phase of inflation that started the current expansion. If so, fluctuations in density and radiation temperature are relics of this era, and have much to tell us about how it happened.

The origin of the Universe is arguably the greatest challenge in strong-field gravity, but it remains to verify classical strong gravity in the Universe today. Many candidate black holes exist, but so far there is no direct evidence for an event horizon. Equally, gravitational waves are inferred only indirectly from binary systems containing pulsars. One probe of the very centre of a black hole may very well come from the phenomena of jets and outflows; another may come from a better understanding of the most powerful celestial explosions: Supernovae and gamma-ray bursts. Finally, hyper-energetic particles may originate near black hole horizons, or from annihilation of dark matter particles. Therefore, the study of cosmic rays is included in the list of extreme areas of astronomy.

With this motivation, there are important questions to face:

• How did the Universe begin? What is dark matter and dark energy? 
• Can we observe strong gravity in action?
• How do supernovae and gamma-ray bursts work?
• How do black hole accretion, jets and outflows operate?
• What do we learn from energetic radiation and particles?