Astronomers are attempting to look back to when the first stars and galaxies lit up and changed the universe forever
They may be the strangest tele- scopes on Earth. They have no domes, no giant mirrors, no steer- able radio dishes—just scattered arrays of simple antennas, some on poles as tall as a person, others re- sembling robot spiders or bizarre garden furniture. These antenna arrays—one in Northern Europe, one in South Africa, a third in Australia— can’t point at particular heavenly targets. Instead, they passively take in whatever sig-
nals come their way and feed them to dis- tant supercomputers where the real work of detection is done.
The otherworldly instruments have an otherworldly target. They are probing a time so far back in the universe’s history that there was very little to see: just a few of the very earliest stars and galaxies. And their quarry is not the scattered points of light at that early epoch, but the diffuse ocean of gas between them, where a pro- found change was taking place.
By some 400,000 years after the big bang, the expansion of the universe had cooled the maelstrom of particles and energy formed in the instant of creation. The result was a dark fog of gas, mostly hydrogen. The universe’s “dark ages” had begun. It took many mil- lions of years for the gas, which was cool and electrically neutral, to slowly swirl together to form stars and galaxies—and when it did, the gas itself was transformed.
The most distant galaxies astronomers can now see, about a billion years after the
big bang, live in a universe full of ionized hydrogen—bare protons with their elec- trons stripped away. Just as the lights came on, something must have ionized all the universe’s hydrogen. The most likely cul- prits are the early stars and galaxies them- selves, but to do this they would have had to be very different from the stars and gal- axies we can see today: bigger, more violent, more exotic. Astronomers are desperate to know more—but not much can be gleaned from scattered lights in a fog more than 13 billion light-years away.
In 1997, however, British astronomer Martin Rees and colleagues Piero Madau and Avery Meiksin suggested that astronomers look for a signal from the early neutral hy- drogen itself. In a hydrogen atom, the central proton and the orbiting electron normally have opposite magnetic orientations. When some energy source flips them into the same orientation, the atom quickly relaxes back into its ground state and emits a microwave photon, at a wavelength of 21 centimeters.
Unlike the neutral gas, ionized hydrogen emits no such radiation. Rees et al. sug- gested that if astronomers could detect the 21-centimeter radiation from the so-called epoch of reionization (EoR), they might see radiation-free “bubbles” of ionized hydrogen around whatever was ionizing the gas. The size and distribution of those bubbles could provide information about the nature of the sources and the timing of reionization.
Astronomers began thinking about what it would take to detect such a signal. As 21- cm radiation from the EoR travels across the universe, cosmic expansion stretches its wavelength to about 2 meters. Conven- tional radio telescopes are mostly blind to such long wavelengths, and a purpose-built dish would be impractically large. But there was another way: an array of simple anten- nas and some heavy-duty number crunch- ing. As astrophysicist Don Backer of the University of California (UC), Berkeley, said at the time: “All you need is paperclips and a supercomputer.”
Now, several of these paperclips-and- supercomputer telescopes are in hot pur- suit of the first detection of the EoR signal. They hope to glimpse something within the next year or two—and the stakes could be enormous. Scientists say the 21-cm radia- tion could open up a floodgate of informa- tion about the astrophysics and cosmology of this unstudied part of the universe’s his- tory, perhaps comparable to the discoveries that have flowed from studying the cosmic
microwave background left over from the big bang. But detecting the primordial ra- dio signal amid the cacophony of other ra- dio sources, earthbound and astronomical, is akin to hearing a whisper amid a crowd of cheering sports fans. “We’re learning all the lessons,” says Judd Bowman of Arizona State University, Tempe, chief scientist of one of the new instruments, the Murchison Widefield Array (MWA) in Australia. “We’re hopeful and eager.”
in and around the peat bog at Exloo has 24 clusters, each containing more than 850 antennas, spanning a 4-kilometer- wide area; another 14 clusters are scattered around the Netherlands, plus another five in Germany and one each in France, Swe- den, and the United Kingdom. (More are under construction in Germany and Po- land.) Widely spaced stations give the in- terferometer finer resolution, enabling it to zoom in on smaller patches of sky.
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