 In the past two decades astronomers have made a truly revolutionary discovery that the cosmos is not only expanding but is doing so at an ever faster rate. The discovery of the accelerated expansion of the universe was awarded the 2011 Nobel Prize in Physics. This is the ESOcast, cutting-edge science and life behind the scenes of ESO, the European Southern Observatory, exploring the ultimate frontier with our host Dr J, a.k.a. Dr Joe Liske. Hello and welcome to the ESOcast. In this episode we're going to find out how astronomers learned that the expansion of the universe is speeding up and why this finding is so important not only for understanding of the cosmos but in fact for all of physics. Now this discovery was awarded the 2011 Nobel Prize in Physics and observations from ESOS telescopes in Chile played a significant role in this breakthrough. The universe we live in was created in the Big Bang some 13.7 billion years ago. Ever since then the universe has been expanding and for decades astronomers have wanted to learn more about the nature of this expansion. For a long time there were two main ideas. Either that the expansion would gradually slow down and would ultimately come to a halt after which the universe would start to contract towards a big crunch or that the cosmos would continue to expand forever. But how could astronomers find out which of these models of the universe is the correct one? Well one of the simplest ways of doing this is to accurately measure distances to very far away galaxies and then to compare these measurements with the predictions from these models for these particular galaxies. Now the comparison between the measurements and the predictions tells us which of the models is the right one. But how does this work? How can astronomers precisely determine these huge distances across the cosmos? Well stellar explosions of supernovae play a key role here. Supernovae are rare cosmic events. They are exploding stars. There is a certain type of explosion known as a Type 1A supernova which is ideal for measuring distances in the cosmos. These supernovae are very bright which means they can be seen even in distant galaxies and what's more their intrinsic brightnesses are always the same meaning that their distances can be inferred from how bright they appear to us from Earth. By the 1990s two separate research teams had begun to carefully observe these exploding stars. For their studies astronomers partly used telescopes at ESO's La Silla Observatory in Chile. Observing extremely distant supernovae in the mid 1990s was extremely challenging and exciting. We at ESO used the 3.6 meter, the entity and the 1.5 meter telescopes to observe these high-rich supernovae discovered at the nearby Tololo Observatory. In those days 15 years ago we were actually counting literally every single photon. This was a beautiful experiment to be part of because it was extremely challenging. The critical component of all of this is of course to realize that we did not set out to find the accelerating universe so watching a new paradigm in physics establish itself has of course been very interesting and it's been great fun. Once we had established that the distant supernovae were too far away for a universe that was dominated by gravity we had to go back and measure this again. So the accelerated expansion that we measured with the first set of supernovae which then was translated very quickly into a new component for cosmology, dark energy we had to confirm that result. And what we did we asked for VLT time like other groups as well several other groups did the same thing to confirm what we had measured to get better data with a bigger telescope and to get a better sampling of the supernovae themselves. The discovery of the accelerating expansion of the universe was one of the most unexpected and important of the last decades. It was so unexpected because up until that point everyone believed that the expansion of the universe should be slowed down by the attractive force of gravity exerted by all of the matter in the universe but as it turns out the universe is in fact much more interesting than that. But why is this acceleration so important? Well as far as we know there are two possible explanations for the acceleration the number one explanation is that nearly three quarters of the universe consist of some form of this mysterious dark energy. Now dark energy is so mysterious because it exerts negative pressure. That's pretty exotic stuff. The number two explanation is that there's something wrong with our understanding of gravity in other words that Einstein's theory of general relativity is not quite correct. Now in either of these cases we are confronted with completely new physics that's why this is so important and why this discovery was awarded the 2011 Nobel Prize in Physics. This is Dr. J signing off for the ESOcast. Join me again next time for another cosmic adventure.