Every once in a while, a cosmic ray—a subatomic particle from outer space—strikes the atmosphere with an energy 10 million times higher than a humanmade particle accelerator has ever achieved. Physicists don't know where such mind-bogglingly energetic particles come from, but they could be closing in on an answer thanks to the expansion of one of the world's biggest cosmic ray experiments.
Japan will spend $3.7 million to nearly quadruple the size of the Telescope Array (TA), which currently consists of 507 particle detectors spread across 700 square kilometers of Utah desert. The detectors sense the avalanche of particles, or what physicists call an "extensive air shower," triggered when a ray hits the atmosphere. Physicists will deploy 400 more loosely spaced detectors to stretch TA's area to about 2500 square kilometers—twice the area of New York City—says Yoshiki Tsunesada, a physicist and TA team member at the Tokyo Institute of Technology. From the size and direction of an air shower, physicists can deduce the energy and direction of the original ray. Researchers hope to complete the expansion in 2017. Japan paid two-thirds of the current array's $25 million cost.
The expansion, known simply as TAx4 or "TA times four," could help researchers pin down the origins of the highest energy rays, in which a single subatomic particle can carry as much energy as a golf ball plunging to the green. Physicists have yet to find the sources of the rays. However, last July TA researchers reported an excess of rays with an energy above 60 exa–electron volts (EeV) coming from the general direction of the constellation Ursa Major, which includes the Big Dipper. "We've got about 20 events in a cluster with a width of about 20 degrees," says Hiroyuki Sagawa, a physicist at the University of Tokyo and co-spokesperson for the TA team. If the rays come equally from everywhere, then such a circle ought to contain about five rays, Sagawa says. "If we obtain more data we may observe structure within the hotspot," he says.
The expansion will also make TA almost as big as its rival, the Pierre Auger Observatory in Argentina, which has 1600 particle detectors of a different design spread over 3000 square kilometers. In 2007, the Auger collaboration reported that the highest energy cosmic rays appear to come from the fiery hearts of certain galaxies. However, that correlation has not held up as Auger has continued to collect more data. Auger commenced taking data in 2005, and TA in 2008, and over the years the teams have disagreed on several key results. For example, TA physicists argue that—as most physicists expected—the highest energy rays are protons, whereas Auger physicists argue they may include heavier atomic nuclei.
Years ago, Auger physicists had argued for building a twin version of their array in the Northern Hemisphere. Now, with its expansion, TA will effectively play that role. "With two [equal-sized] observatories we can see the whole sky," Sagawa says. "That's very important," he says, as the hemispheres may look different in cosmic rays.
Like Auger, TA also features batteries of specialized telescopes that on clear, moonless nights can detect the faint light, or fluorescence, produced by an extensive air shower. Such telescope observations provide a better measure of the energy of the shower and are key for calibrating the array of surface detectors: By comparing the readout of the fluorescence telescopes and surface detectors on the same events, physicists can figure out how to better estimate a shower's energy from the surface detector alone. TA currently has three batteries of telescopes and researchers are hoping the U.S. National Science Foundation (NSF) will pay for two more, says Douglas Bergman, a physicist at the University of Utah in Salt Lake City.
The TA team is hoping NSF will spend about $1 million mainly for building the new batteries, Bergman says. Researchers already have the telescopes themselves from a previous experiment, he says. Physicists applied to NSF last fall and will do so again this fall, Bergman says. "I think the prospects would be better in the sense of there being this exciting news from Japan," Bergman says.
Even some TA physicists caution that with more data the hotspot may not hold up. "I personally am still in doubt whether it is real," Tsunesada says. In the past, many tantalizing cosmic ray results have failed to pan out, he notes. In fact, Tsunesada says, if the expanded TA and Auger don't detect sources of the highest energy rays, the search could come to an end. "This could be the last chance for us air-shower researchers," Tsunesada says. Others are more optimistic. For example, Sagawa notes, it may be possible to study air showers over a vastly larger area using space-borne fluorescence telescopes.