'Pancake' hints at how cosmos grew lumpy.

It's a puzzle that has left astrophysicists in the dark for more than two decades: How did the universe, which apparently began as a generally smooth broth of matter and energy, evolve into the current, lumpy collection of stars, galaxies and galaxy clusters?

Since the early 1970s, researchers have proposed two main theories to explain the enigma. One model, the bottom-up scenario, assumes that small lumps in the generally smooth texture of the early universe acted as seeds for groups of galaxies that formed later in time. A rival, top-down theory takes a constrasting view: Random lumps quickly led to the creation of giant, pancake-shaped gas clusters, which eventually fragmented into galaxies or clusters of galaxies.

The absence of direct evidence for giant pancake clusters in the very early universe left researchers to debate the two models. But now the discovery of a distant cloud of atomic hydrogen, made during a radio survey of vast regions of the heavens, might tip the balance in favor of the top-down model.

Using the Very Large Array radio telescope near Socorro, N.M., a trio of astronomers has found the most massive blob of atomic hydrogen gas ever detected. More than 100 trillion times the mass of the sun, this clump resides some 3 billion light-years from Earth. From their radio measurements, the astronomers infer that the blob has the pancake shape predicted by Soviet physicist Yakov B. Zel'dovich and his colleagues, who first proposed the top-down theory in the early 1970s.

After searching a volume of sky greater than [10.sup.25] cubic light-years during a four-year hunt for cosmic pancakes, Juan M. Uson, Durgadas S. Bagri and Timothy J. Cornwell of the National Radio Astronomy Observatory in Socorro found tell-tale radio emissions of a massive clump of atomic hydrogen last spring. The diffuse source, which stretches across a span of about 5 million light-years, likely represents a giant pancake cluster about to break into galaxies, Uson says. He and his colleagues report their results in the Dec. 9 PHYSICAL REVIEW LETTERS.

The competing, bottom-up theory could also account for a few massive clusters, Uson says, but a discovery of several more of these blobs could spell trouble for bottom-up supporters.

Uson says his team has now found a second candidate pancake. Detecting more will prove challenging, he adds, because although the universe may have formed many such objects, each might last only a few hundred million years before splitting into galaxies.

Though he calls the new finding important, P. J. E. Peebles of Princeton University, who helped develop the bottom-up theory, says "it's not likely that one new observation is going to finally reveal the [correct theory]."

Peebles acknowledges that the newly discovered gas cloud "certainly looks like the predicted Zel'dovich pancake." However, he expressed skepticism "about accepting this [as conclusive] evidence" for the top-down theory because "there are arguments against the pancake picture that seem to me to be strong."

Peebles points to a young, developing cluster comprising several ancient galaxies--including the Milky Way--as one of several indications that the universe evolves from little to big, not the other way around.

However, he observes, the new finding may help further refute a particular version of the bottom-up theory, which invokes a hypothetical, hidden type of material known as cold dark matter. Such matter would interact weakly with photons, allowing primordial lumps in the cosmos to grow relatively rapidly. The cold dark matter scenario has come under increasing attack because it still cannot explain a variety of recent observations about the structure of the universe (SN: 1/26/91, p.52).

Concludes Peebles, "This [new result] could be one more nail in a coffin [for cold dark matter] that's already studded with nails."