Universal truths: distant quasars reveal content, age of universe.

Using 3,000 recently discovered quasars as searchlights on the distant universe, astronomers have mapped with unprecedented precision the distribution of the diffuse gas between galaxies. By combining these measurements with observations of the faint microwave glow of radiation left over from the Big Bang and other cosmological data, the researchers report that they have pinned down the age of the universe to an accuracy 5 times greater than ever before. By their reckoning, the cosmos is 13.6 billion years old, give or take 200 million years.

The findings also uphold a leading model of cosmic evolution known as inflation, says study collaborator Uros Seljak of Princeton University. Inflation posits that the infant universe underwent a brief but enormous growth spurt that locked in and magnified subatomic fluctuations to astronomical-sized wrinkles. Those wrinkles provided the seeds for all the clusters of galaxies and voids seen in the cosmos today.

Inflation predicts exactly how matter in the universe should clump on a variety of length scales. The new quasar study, in combination with data from a NASA satellite studying the Big Bang's glow, provides "the most rigorous test to date" of inflation and shows that the theory "passes with flying colors," Seljak says. He and his colleagues have posted their findings online at http://xxx.lanl.gov/abs/astro-ph/0407378, http://xxx.lanl.gov/abs/ astro-ph/0407377, and http://xxx.lanl.gov/abs/astro-ph/0407372.

The quasars were found with the Sloan Digital Sky. Survey, an ongoing investigation covering one-quarter of the heavens (SN: 11/01/03, p. 275). All the quasars lie between 8 billion and 10 billion light-years from Earth and represent a population 100 times that used in any previous analysis of intergalactic gas.

The study "is impressive, and the Sloan data ... look great," says Dick Bond of the University of Toronto. Those data, which enabled the scientists to map the density of intergalactic gas on a smaller scale than ever before, "considerably extend the reach" of current models of the universe, Bond adds.

As a quasar's light traverses the billions of light-years that lie between its home galaxy and Earth, some of the radiation is absorbed by intervening clumps of hydrogen gas. Each parcel of gas shows up as a different dip in the quasar's spectrum. The spacing and depth of these dips indicate several key properties of the universe.

For instance, the quasars' spectra place the tightest limits to date on the mass of elementary particles called neutrinos. The analysis eliminates the possibility of an additional family of massive neutrinos, which some particle physics experiments have suggested. The quasar study also lends support to the idea that the density of dark energy, the mysterious force that is causing the universe to expand at an ever-faster rate, is constant over time (SN: 5/22/04, p. 330).

Studying the spectra of quasars to reveal the distribution of either gas or galaxies in the universe isn't a new endeavor. But until now, no study has used enough quasars to generate a reliable mapping of the intergalactic gas. This map serves as the best available signpost for dark matter--the invisible material that makes up most of the mass in the universe.

The gas clumps are "an independent tracer of the dark matter distribution and are more sensitive than galaxies to small-scale variations" in cosmic structure, comments David N. Spergel of Princeton University.