Dr Saul Perlmutter, a professor of physics at the University of California, Berkeley, a senior scientist at Lawrence Berkeley National Laboratory (LBNL), and a 2011 Nobel laureate in physics, discusses the discovery of the accelerating expansion of the Universe and the essential role of computing in this field of research.
Could you explain what it means that the Universe is expanding?
The most immediate question that many people have as soon as they hear about the expansion of the Universe is how the Universe can expand: after all, one may think that the Universe is everything, so there is nothing for it to expand into. But let us picture at the moment that the Universe happens to be infinite. This means that we live in a galaxy that is set in a sea of galaxies that extends infinitely in all directions around us. So if we look up with a big telescope, we see galaxy after galaxy; if we could look down through the floor, or off to the side, we would also see galaxy after galaxy. In this infinite sea of galaxies, one could talk about a typical average distance between galaxies: that is, it could on average take a certain number of millions of years to travel in a given direction to go from galaxy to galaxy. When we say that the Universe is expanding, we mean that all of those distances between every one of those galaxies gets a little bit bigger over time, and it would take a little longer on average to get from galaxy to galaxy. So, if you ask, what is expanding into what? Well, nothing is expanding into anything else: it’s just that we apparently have more distance between every two points in the Universe, between every two galaxies. It’s almost as if we are puffing space in between all of the points. This is mind-boggling to think about, but this is apparently the way the Universe behaves. This also means that, if we go backwards in time, we are sucking space out from in between all of the galaxies and points, and things get closer and closer to each other. In other words, the Universe becomes more and more dense, but it’s still infinite! And if we go really far back in time, we get to the point where everything is on top of each other: it’s still infinite, but it’s now a dense, hot soup — we usually call that dense-hot-soup stage the ‘Big Bang.’ Then we have to think about what could have been before that, and we really don’t know.