WHEREAS astrologers search the stars for hints at human destiny, astronomers are hunting for hints at the universe's. In 1998, the oracles were in a tizzy. Studies of supernovae in distant galaxies showed that the universe's expansion was accelerating. The discovery was baffling: after the initial Big Bang had propelled matter outwards, the attractive force of gravity should gradually have braked the universe's sprawl. Either the theory of gravity—and, with it, all its predictions—was flawed, or some unknown physical force was driving matter out at ever-faster speeds. Physicists dodged the first bullet, and instead bit the second, dubbing this mysterious force “dark energy”. And having named it, they want to measure it.
At a conference of the American Physical Society held a few days ago in Albuquerque, New Mexico, two teams of researchers presented plans for telescopes which could do just that. Though very different in approach, both projects would work by studying the past formation of galaxy clusters—collections of hundreds of individual galaxies that are among the largest structures in the universe.
Cluster-bombing the problem
Initial calculations suggest that the previously unsuspected dark energy actually accounts for two-thirds of the substance of the universe. Surprisingly, that helps. Before 1998, most physicists believed that the universe contained far too little matter and energy. The more awkward question is whether the amount of dark energy is changing. If dark energy has always been as prevalent as it is now, then the universe will probably expand forever. If, on the other hand, it has gradually dissipated over the aeons, the universe's expansion could eventually slow to a halt, or even go into reverse.
Measuring any change in dark energy requires creative thinking, which is where the clusters come in. The rate at which galactic clusters form depends, in part, on the amount of dark energy around at the time. Because it opposes the attractive force of gravity, dark energy tends to inhibit cluster formation. So if dark energy has declined as the universe has aged, clusters would have appeared more slowly in the distant past than in the recent past. If, however, it has remained constant, the rate of cluster formation should have remained constant, too.
John Carlstrom, of the University of Illinois at Chicago, and his colleagues suggested to the conference that this might be detected by examining the cosmic microwave background (CMB), the hum of radiation that has blanketed space since the Big Bang. As galaxy clusters form, they create clouds of hot electrons. These electrons deflect a small proportion, about 1%, of the background microwaves. Whereas the CMB normally gives the universe a temperature of about 3° above absolute zero, a hot, gaseous galaxy cluster would turn up as a region of sky about a thousandth of a degree cooler, because of the microwaves that have been diverted away from it, a phenomenon known as the Sunyaev-Zel'dovich effect (SZE). A radio telescope listening to the CMB will “hear” this cold spot in the background.
Dr Carlstrom says this method may be particularly useful because the SZE is detectable over much longer distances than light or X-rays. Light becomes fainter as it travels through space, but distortions produced by the SZE will persist indefinitely. This means that a telescope tuned to detect cold spots could see much older clusters than one that is trained only on distant sources of light.
Dr Carlstrom and his colleagues propose building an eight-metre-diameter radio telescope at the South Pole to search for such cold spots. The altitude, low temperatures and lack of sunlight (at least during the Antarctic winter) make the atmosphere at the pole ideal for star-gazing. The South Pole Telescope could thus see 10 billion years into the past, and might turn up thousands of clusters—more than enough to study the evolution of dark energy.
Another group of researchers wants to study galaxy clusters using a phenomenon called gravitational lensing. Because gravity pulls on light as well as matter, when light passes by a heavy object it bends as though it is passing through a lens. Tony Tyson and David Wittman, who work at Lucent Technologies' Bell Laboratories, in New Jersey, and their collaborators have proposed building a telescope that estimates the distribution of mass in the past by analysing these deflected light-waves, a technique known as three-dimensional mass tomography.
Light deflects around invisible matter in the same way that water ripples over hidden stones. Studying the ripples allows the mass of the deflecting objects to be calculated. If those objects are galaxy clusters, the growth-rates of such clusters at different times can then be worked out, and inferences drawn about the history of dark energy. Dr Tyson and his colleagues have recently finished making a preliminary design for such a device, which has been named the Large-aperture Synoptic Survey Telescope (LSST).
Physicists, and even more importantly their paymasters, must now weigh the merits of the two proposals. Because the LSST relies on studying distortions of light, it cannot see more than about 8 billion years into the past. But Dr Wittman argues that the LSST, unlike the South Pole Telescope, will be able to detect anything massive, not just stuff that once held hot gas. Moreover, he says that the LSST will have other uses, such as discovering more supernovae. That would provide yet another way to measure dark energy, as well as yielding results in other areas of astronomy. On the other hand, the LSST project is far more expensive. The $20m bill for the South Pole Telescope project would pay only for the camera on the LSST; the total bill for the latter comes to $110m.
Astrophysicists, though, are a co-operative bunch. Given the different strengths of the two telescopes it would be good, say the scientists involved, if both were built. That way, any conclusions about dark energy could be checked against the results of an independent investigation. “No one is going to overturn his theory of the universe based on just one set of measurements,” says Dr Wittman. Among astrophysicists, the hunt for darkness is all sweetness and light.