Both Murchison and Karoo are radio-quiet enough to have satisfied the baseline science requirements for the SKA, but some interference is unavoidable. GPS satellites, commercial aircraft and omnipresent human transmissions all contribute radio pollution to the airwaves. Measurements of the sites' levels of radio interference and atmospheric disturbances are ongoing, but any number of other factors will come into play. Which site has the best infrastructure? Which political climate is more inviting? Where will the project deliver the most cultural benefits? "It's a whole long list of criteria," says Cornell's James Cordes, chair of the U.S. SKA consortium, a group of U.S. institutions that have been working to develop the SKA.
Australia has the advantage of a long legacy of leadership in radio astronomy. After World War II, when radio astronomy began in earnest, one of the leading teams in the field was a CSIRO group led by John G. Bolton and J. L. Pawsey. The 2000 film The Dish, about the Parkes radio telescope, is a major cultural touchstone. But South Africa has been steadily investing in radio astronomy, and Jonas says that the nation is no longer an underdog facing doubts about technical expertise. "I think we're well beyond that now," he says.
Roy Booth, an astronomer at the Hartebeesthoek Radio Astronomy Observatory in South Africa who serves as associate director for science operations on MeerKAT, says that bringing a world-class science project to Africa would have great cultural benefits. "The stars, of course, are something you see whether you’re rich or poor," Booth says. "If we could set up the SKA as sort of a World Cup for science, we might interest the people in the villages."
Pulsar populations and the deep universe
Cordes's interest in the SKA stems partly from the array's capacity for finding pulsars, highly magnetized stellar remnants that spin extremely rapidly. Some can complete hundreds of revolutions in a single second. Pulsars emit a beam of radiation; as a pulsar rotates its beam sweeps across Earth, like a lighthouse panning its light beam across a ship. But no lighthouse can compete with a pulsar's reliability: The beams of some pulsars sweep through space with a regularity on par with the ticking of an atomic clock.
Pulsars—dense, massive objects with the capacity to serve as a precision clock—are ideal natural laboratories for testing the predictions of general relativity, Albert Einstein's theory of gravitation. Russell Hulse and Joseph Taylor earned the 1993 Nobel Prize in Physics for their discovery of a pulsar orbiting another massive object in 1974, when both were at the University of Massachusetts Amherst. Their so-called binary pulsar allowed for the indirect confirmation of the existence of gravitational waves, ripples in the fabric of space and time that emanate from the movement of massive bodies. Hulse and Taylor found their binary pulsar, PSR 1913+16, using the 305-meter Arecibo observatory in Puerto Rico, currently the largest radio dish in existence.
But Arecibo is a fairly noisy site, as far as radio interference is concerned, and a full SKA would have much more collecting area—and hence more sensitivity to faint sources of radio waves. "With the SKA, we want to do a full galactic census of pulsars," Cordes says, "and that means try to detect every damn one of them in the galaxy." Astronomers have already located about 2,000 pulsars, but there could easily be 10 times that many in just the Milky Way. Although 2,000 pulsars makes for a sizable sample, the most interesting pulsars—those in binary pairings such as PSR 1913+16 and the rapid spinners known as millisecond pulsars—are relatively rare. A full census of pulsars might even reveal a few oddballs that could reveal gaps in our understanding of gravity. "We would love to find a pulsar orbiting around a black hole," CSIRO's Diamond says. "That is probably the regime in which general relativity will begin to break down."
With a powerful enough telescope, irregularities in the measured arrival time of pulses from millisecond pulsars could also mark the presence of gravitational waves passing through the local universe. Those gravitational waves compress and stretch space, and the radio waves passing through it, throwing off the pulsar's usually predictable ticking. "These waves should be all over the place," Cordes says. "They affect every line of sight that you look at."
By collecting long-wavelength radio waves, the SKA might also be able to see deep into the early universe, when the first galaxies and stars appeared. (Because of light's finite speed, astronomers can see the universe as it existed long ago by looking at ancient, distant objects in the sky whose light was emitted billions of years ago but is only now reaching Earth.) Neutral hydrogen, which dominated the first few hundred million years following the big bang, emits photons at a telltale wavelength in the microwave band; those microwave photons are stretched to longer radio wavelengths in their journey across a vast, expanding universe. SKA could chart the early universe's neutral hydrogen to peer into the cosmic dark ages, the time before luminous stars and galaxies emerged and ionized the intergalactic hydrogen.
That would give astronomers a glimpse at a mostly inaccessible stage of structure formation early in the universe's evolution. "I would like to see radio images of the first galaxies being born after the big bang," Terzian says. "I can tell you, this would be thrilling."
Just how large the telescope array will become—and just how revolutionary its science—depends on financing. En route to the Murchison site, Boyle, a tireless pitchman for his project, smiles at the mention of the full-scale, 3,000-dish SKA. He acknowledges that the project could stall at partial build-out, that it may take awhile to move beyond a few hundred telescopes.
But even a stunted SKA would still be the largest instrument of its kind. It could easily dwarf the Very Large Array in New Mexico, one of the premier radio telescope arrays on Earth, which features 27 dishes, each 25 meters across. And it would have more collecting area than Arecibo, the biggest single dish on the planet. "It would pass Arecibo at considerably less than full build-out," Cordes notes. With only about 400 dishes, each 15 meters in diameter, the SKA would boast more collecting area than Arecibo with considerably less radio interference.
On April 2, the vision of the radio telescopes now dotting the Australian and South African landscapes someday giving way to a mammoth cluster of 3,000 dishes began to look a little untenable. A coalition of nine countries' national science agencies formally signed on to the Square Kilometer Array, establishing a founding board and a project office at Jodrell Bank Observatory in England. But the list of signatories did not include the U.S. National Science Foundation (NSF), which had been expected to contribute one third of the funds for the project. Another third would have come from Europe—and indeed France, Germany, Italy, the Netherlands and the U.K. did sign on—with the final third coming from the rest of the international partners.
"NSF did not sign on to the founding board agreement because this included a pledge to participate in the next phase at a significantly enhanced funding level," James Ulvestad, the NSF division director for astronomical sciences, said in an e-mail. A 2010 report from the National Research Council, intended to guide U.S. astronomy priorities in the coming decade, had concluded that participating in the SKA was not possible in the current fiscal environment unless the country shut down other "highly productive facilities"; Ulvestad said that the NSF had accepted that conclusion. He noted that the U.S. has already made investments in the SKA, such as supporting technology development for the project, and "will continue to make such investments as funds permit, if independent reviews give them very high priority."
Before that decision was announced, Cordes had stressed that the SKA would be a modular instrument—it would work great with 3,000 dishes, sure, but even one dish can do science. "I think we need to break through some of these perceptions," he said, that the SKA is "this big behemoth with a price tag in the billions."