Seeing the Baryonic Forest for the Trees

Astronomers lost in an impenetrable forest may have finally found a way out. Two astrophysicists appear to have cleared up the puzzling distribution of matter that blocks certain wavelengths of light from brilliant, far-off objects called quasars, resulting in a "forest" of dark lines in their spectra as seen from Earth. The theory, published in the current issue of Astrophysical Journal, should transform these once-enigmatic lines into a powerful tool for gauging the amount of matter in the cosmos.

In the early 1970s, astronomers discovered that there are hundreds of dark lines--dubbed the Lyman-alpha forest--in the infrared region of quasar spectra. Researchers had guessed that clouds of hydrogen gas between quasar and Earth might create the lines by absorbing some of the quasar's light. If the clouds were moving at different speeds and in different directions, the Doppler effect would shift each absorption band slightly. Hundreds of hydrogen clouds, therefore, would explain the forest.

But the hydrogen-cloud theory may be just pie in the sky. HongGuang Bi and Arthur Davidsen of Johns Hopkins University in Baltimore propose that the forest is actually created by a ubiquitous, undulating intergalactic medium composed of hydrogen and helium. By assuming that the medium was distributed in the early universe in a density described by simple calculations, the duo was able to explain the mysterious lines without invoking a more improbable distribution of gas in clouds.

Experts are impressed with the elegant theory. "That observation and theory agree so well means that this is a success--a big success," says Mike Norman, a computational astrophysicist at the University of Illinois.

A better understanding of gas distribution in the universe will help astrophysicists figure out how much extra matter the universe contains beyond its visible stars and galaxies. According to the Hopkins model, more than half of the baryonic matter--ordinary matter, made of protons and neutrons--in the early universe must have resided in the gas responsible for the Lyman-alpha forest. But some experts are unconvinced. Lennox Cowie, an astrophysicist at the University of Hawaii, believes that the theory overestimates the amount of baryonic matter. "It's a nice piece of work," he says. "Still, the question's not settled yet." Others, however, are ready to lay the matter to rest. "We finally feel we understand the Lyman-alpha forest," says Neal Katz, an astrophysicist at the University of Massachusetts. "Now we can use it as a tool."