Cosmic Cat Scan

In the beginning, the universe was a void full of energy but without form. And so it remained for many millions of years–exactly how long is still a major mystery of cosmology–until the first stars condensed from the fog of matter and lit up with a blue nuclear glow.

Telescopes are just like time machines: the farther out in space they look, the further back into the past they peer. But even the best optical telescopes cannot make out what the universe was like at an age of less than one billion years. Before that time, a haze of neutral hydrogen gas shrouded these first beacons in the infant cosmos.

A new radio observatory under construction on the high plateau of Ulastai in remote western China may soon yield images of this formative epoch, however–and for a bargain price, too, because the sprawling instrument is built almost entirely from parts that one could buy at RadioShack. Even though it will cost just $3 million, the Primeval Structure Telescope (PaST) is one of China’s largest investments so far in experimental astronomy. The project was launched in 2003 by Xiang-Ping Wu of the Chinese Academy of Sciences in Beijing, Jeffrey B. Peterson of Carnegie Mellon University in Pittsburgh and Ue-Li Pen of the Canadian Institute for Theoretical Astrophysics in Toronto.

Though formally a telescope, PaST is better thought of as an experiment. “We’ll get enough data from it to answer our principal questions within a couple weeks of turning it on” next year, Peterson says. (Analyzing those data may take years, however.) That is because the instrument is essentially a giant, incredibly sensitive television receiver.

PaST will combine radio signals picked up by 10,000 high-gain antennas arranged in lines up to three kilometers long. The log-periodic antennas, similar to those sold by the millions for rooftops, cost just $20 each. Household coaxial cable splitters, installed backwards, combine the signals from multiple antennas and feed them into a bank of 320 ordinary Pentium PCs, running free Linux software. The computers merge the data to produce a high-resolution picture of a 10-degree patch of sky centered near the North Star.

Actually “picture” is not the right word, because PaST will record thousands of simultaneous signals within a broad swath of the VHF spectrum. The scientists are writing software to sift out uninteresting signals–such as those from television stations, meteors and black holes at the centers of distant galaxies–to reveal a kind of three-dimensional CAT scan of the early universe that theorists predict lies buried within the noise.

As the first stars flickered on, their ultraviolet light excited neutral hydrogen atoms around them, causing the gas to emit a faint radio signal at 1,420 megahertz. As the starshine intensified, it eventually stripped electrons from the hydrogens, ionizing the atoms.

But over time the expansion of the universe stretched the ancient radio waves, lowering their frequencies by an amount proportional to their age. Astronomers can thus see a particular moment in time and location in space by “tuning” their receiver to the appropriate frequency. “It’s a bit like archaeology,” says Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics. “We can slice the universe and see more and more ancient layers as we go deeper.”

Astronomers expect that PaST will reveal a uniform haze of bright neutral hydrogen at about 200 million years after the big bang that became increasingly punctuated by bubbles of ionized–and thus dark–hydrogen surrounding the first stars. Simulations suggest that these shells then connected, like the voids in a Swiss cheese, to form tunnels. The neutral hydrogen fog gradually dissipated into stray wisps and vanished forever within the first billion years, leaving us with the transparent space we see today. This story will remain fuzzy until PaST or a competing observatory delivers more clarity. Let there be light.