A Theory with Strings Attached

By MICHAEL D. LEMONICK

Astronomers have struggled for years, and in vain, to answer a nagging, fundamental question: Why is the universe lumpy? Some regions of the cosmos are crowded with giant clusters of galaxies, millions of light-years across. Other, even vaster spaces seem to be largely empty of matter. Scientists have assumed that this unevenness resulted from irregularities in the big bang that began the universe between 10 billion and 20 billion years ago. But that greatest of all explosions was almost perfectly uniform, as evidenced by its leftover radiation, which radio telescopes can detect in every part of the sky. Then how did such a smooth start result in a chunky cosmos?

The answer may be provided by a fantastic new theory reported last week in the Dutch journal Physics Letters B. In their report, two renowned Princeton scientists and a graduate student suggest that the pressure of electromagnetic radiation, emanating from dense "threads" of pure energy called cosmic strings, could have been responsible for making the universe lumpy. That pressure, the theory holds, pushed matter outward, piling it into thin shells and leaving huge voids in the cosmos. "If this theory is correct," says Astrophysicist Jeremiah Ostriker, the theory's co-author, "our views about cosmic-scale structure will be radically changed."

Indeed, the theory could explain not only the large-scale structure of the universe but also the origin of galaxies and other puzzling celestial phenomena. It combines some of the most advanced ideas in astrophysics and elementary-particle physics, and joins the independent research of Ostriker and Physicist Edward Witten. The unifying element: the cosmic strings -- bizarre, hypothetical entities that are thinner than an atomic nucleus, as long as the universe is wide, and so dense that a mile-long segment would weigh as much as the earth.

Strings first popped up unexpectedly in the mathematical models of particle physicists. Right after the big bang, the mathematics suggests, the rapidly cooling cosmos underwent a phase transition, roughly analogous to the sharp change that occurs when water turns into ice. And just as an ice cube is marred by cracks and lines left over from uneven freezing, the universe too might have developed flaws -- in the form of strings. These strings would try to straighten and contract, often whipping about and getting tangled in the process. When a string crossed itself, the resulting loop would pinch off from the main string and go vibrating away on its own.

Ostriker was thinking not about strings but about the structure of the universe when in 1981 he co-authored a theory on the formation of new galaxies, which are huge islands of billions of stars. Aware that entire galaxies can erupt in explosive bursts of energy, Ostriker had suggested that the force of such mighty blasts would drive away surrounding dust and gases, leaving vast regions of the sky virtually empty and compressing the expelled gases and dust into shells around the voids. It was from this compressed matter, he said, that new galaxies might form.

But one problem remained. While Ostriker's calculations showed that the forces released by an exploding galaxy could condense gas and dust enough to begin the formation of new galaxies, they were not powerful enough to sweep clean any regions as large as the largest observed in the sky. Then what mysterious agent had created the giant voids?*

Cut to Edward Witten, who was studying the properties of cosmic strings. "I was looking at ways you might be able to observe these very thin objects at cosmological distances," he recalls, "and I discovered that they could be superconductors -- they would conduct electric current forever." If that was true, he figured, electric currents as large as 100 quintillion (100 followed by 18 zeros) amperes could be induced in the strings. These currents could in turn produce intense magnetic fields around the strings, and particles, like electrons, caught in the fields would glow. In fact, a radio- telescope image of the center of the Milky Way, taken last spring by Mark Morris of UCLA and Farhad Yusef-Zadeh of Columbia University, shows threads that some astronomers think could indeed be glowing strings.

With his conclusions, Witten had inadvertently provided Ostriker with the agent he needed to produce the giant voids. "When I first saw Ed's paper, in 1985," Ostriker says, "it blew my mind." Reason: a vibrating, current- carrying loop is a radio transmitter, and if the current is large enough, the ultralow-frequency radio waves it emits will be incredibly powerful -- strong enough to push surrounding gases and dust incredible distances away from the loop. With Witten and Graduate Student Chris Thompson, Ostriker went to work calculating the effects of the waves. "Again and again," he says, "we thought we had found a fatal flaw, that the whole thing was crackers. But we haven't so far." A string loop, it seemed, could theoretically produce an awesome void.

While the theory looks promising, Ostriker shows proper scientific restraint. "We still don't know that there are such things as cosmic strings," he says, "or that they are necessarily superconductors or will in fact carry large currents. But all these things are quite possible. Within a few years, superconducting strings will have either transformed our view of the large-scale universe -- or be entirely forgotten."

FOOTNOTE: *Early in 1986, a team at Harvard announced a survey showing that these enormous bubble-like voids appear all over the universe. It also showed that galaxies seem to lie on the surfaces of the voids, results that the Princeton scientists call "primary motivators" in the construction of their theory.