George Ellis One of the major discoveries of the past century was finding that the universe is expanding and changing with time. Indeed it has evolved from a very hot early state, when it consisted of nothing but hot gas, to the state we see today with numerous clusters of galaxies each containing billions of stars receding ever further from each other. The major discovery of the past few years is that the universe is not just expanding, it is accelerating the rate of expansion is increasing. The discovery has come about as a result of the development of telescopes and their instrumentation in the past decades. In particular, we have been able to put the Hubble space telescope into orbit around the Earth, getting rid of atmospheric interference and producing vastly more detailed images. Equally important, incredibly sensitive detectors have transformed astronomy; in particular, CCDs (charge coupled devices, which underlie the technology of digital cameras) have provided much greater sensitivity and accuracy of measurement. These technical developments have enabled an extraordinary observational feat.
Supernovae are huge explosions occurring at the end of the lifetimes of huge stars, when a single dying star can appear brighter than an entire galaxy. Astronomers have established a useful fact: the peak brightness of a particular type of supernova is directly related to the rate at which its light decays. This means that if we catch it early enough in its death throes, by detecting a distant luminous object in a distant galaxy where nothing was seen the last time we looked, we can monitor the way the light emitted by this huge explosion decreases with time as it decays after the peak of brightness. Thus can we determine its distance because we know its intrinsic luminosity from its decay rate, and we can measure its peak brightness. The ratio of the two tells us its distance. That is important because we know of nothing else out there at vast distances whose intrinsic brightness is so well understood. These are the only objects we can use to establish distances of very distant galaxies accurately.
Two teams of astronomers have set out on the extraordinary task of monitoring millions of galaxies to find those few where we can see a supernova explosion flare up. As soon as one is discovered, systematic observations of brightness accurately determine the light decay curve. Examining the spectra the way the energy received from the supernova varies with colour enables detection of characteristic lines emitted by elements in its atmosphere. These lines will be red shifted because of the expansion of the universe; that is, they will look redder than if produced by the same elements in a laboratory on Earth. The amount of red shifting tells us the rate at which the emitter is moving away from us. Thus with distance and velocity data for each supernova, we can plot a curve of velocity against distance (a”Hubble diagram”) for these galaxies. We expected this curve to bend down, because the matter in the universe is slowing its expansion, but both teams find that it bends up. That is the evidence for the accelerating expansion of the universe. The leader of one of the teams, Brian Schmidt of Australia, visited Cape Town earlier this year to take part in discussions on present-day cosmology hosted by the University of Cape Town’s mathematics department, and presented the latest such data which is regarded as wholly convincing by the astronomical community. The implication is that the dynamics of the present-day universe are dominated by a cosmological repulsive force quite unlike the effect of ordinary matter perhaps the cosmological constant postulated by Einstein in 1917 (for the wrong reason he wanted to show it could lead to a static, unchanging universe). This is a great puzzle to quantum physicists, because the existence of such a force is not understood from a present-day physics viewpoint. Studies of the quantum vacuum suggest this force should be much larger than the measured amount, or perhaps zero, but not the size it is measured to be. This remains one of the major unresolved problems in theoretical physics today. One possibility is that the force is not constant but may be changing with time in that case it is due to some cosmic field that has been called”quintessence”.
The fact that the universe is accelerating makes it very likely that it will expand forever, with all the matter and radiation cooling indefinitely, all life dying out, and even the matter eventually decaying away. It seems that the alternative to a fiery end to the universe as it recollapses into a vast fireball in the future is not to be. Our ultimate future fate is emptiness, coldness, and lifelessness. But it will take quite a while to get there. We do not understand the physical nature of whatever field is causing this acceleration, or why it should presently dominate the expansion of the universe. But the evidence is there. Astronomers are testing this conclusion to see whether any alternative could explain the observations for example, if supernovae were different in the distant past. Physicists are exploring alternative theories that might explain its existence for instance, could it be due to our universe being embedded in a higher dimensional spacetime? Whatever the outcome of these explorations, this is undoubtedly one of the major discoveries concerning the nature of the mysterious and beautiful universe in which we live. And, as is often the case, it is a surprise. George Ellis is a professor in the mathematics department at the University of Cape Town