Dark energy

The discovery that the Universe is speeding up and not slowing down was gre eted with open arms by theorists. First, because the dark energy powering t he acceleration provided the energy needed to make the Universe flat, in ac cord with a key prediction of inflation. Second, because theorists now have a new puzzle to solve, the nature of the mysterious dark energy. I have no doubt that the dark energy problem will be just as fundamental and just as interesting as the dark matter problem. Determining its nature will requir e the work of both astronomers and particle physics and will shed light on both fundamental physics and the fate of the Universe.