The universe was static, that was the credo of all scientists and philosophers until three decades into the twentieth century when the expansion was discovered.


Three-quarters of a century later, the notion of a simple expansion of the universe was found to be wrong: the universe is accelerating. One implication is that before 2000, we believed that our successors, billions of years hence would see an ever larger space of galaxies, more numerous than grains of sand on a vast beach. Today with our telescopes we can see billions of galaxies: in the far future, there would be, we once believed, uncountable billions to study and even search for signs of life. But with the discovery of accelerating space, our horizon has shrunk immensely. The distant galaxies are racing away from us at ever faster velocities. In a hundred and fifty billion years time, our Milky Way will be the only galaxy left in the observable universe. This lecture will explore the origin of this paradigm shift in our cosmic horizon, and discuss the origin of the acceleration as a phenomenon that we call dark energy.