Abstract

Evidence from a number of astronomical observations appear to indicate that over 95% of the energy density making up our Universe is dark, we can not see it but can only infer its existence from observations of things we can see, namely the remaining 5% or so that make up the baryons. The 95% is split up into 25% dark matter which behaves like ordinary matter in that it acts like a pressureless dust, and 70% dark energy, an unusual form of matter which has a negative pressure. This dark energy is causing the Universe to accelerate today. In the talk we will discuss the evidence for dark energy, the theoretical proposals there are to explain what it is and the possible ways we may in the future be able to discriminate between the various proposals. Along the way, we will come across the cosmological constant, dynamical models of dark energy and modifications of Einstein’s gravity—all in all we will see there really is currently no natural explanation for this fantastic observation.