Findings from the cosmic microwave background. (News of the Early Universe).

The most detailed snapshots so far of the infant universe are confirming that the cosmos consists mostly of mystery material, called dark energy, that accelerates the universe's expansion.

The new evidence comes from the Arcminute Cosmology Bolometer Array Receiver (ACBAR), a South Pole network of 16 detectors that probes the temperature of the Big Bang's remnant radiation, known as the cosmic microwave background. That radiation provides an image of what the universe looked like about 400,000 years. after the Big Bang, when photons first streamed into space.

Although the radiation has cooled to an average temperature of 2.73 kelvins, the remnant light emanating from some patches of sky is slightly cooler or hotter. These tiny hot and cold spots reveal the earliest phases of gravitational clumping of matter and radiation, the seeds of galaxy formation.

The clumping also caused the early universe to ring like a bell. As gravity forced photons to bunch together, the photons resisted by exerting an outward pressure. That push and pull set up acoustic oscillations that remain imprinted in the cosmic microwave background today.

It's the peaks and valleys in those oscillations that cosmologists analyze to measure such cosmic traits as the overall curvature of the universe and the density of matter. ACBAR's sensitivity to temperature variations over a wide range of spatial scales enabled the array to measure both these traits. The results confirm that "we live in a bizarre universe" where dark energy reigns, says ACBAR researcher Jeffrey B. Peterson of the Carnegie Mellon University in Pittsburgh.

The study also provides a new hint of how photons of the microwave background interacted with hot gas in galaxy clusters in the 14 billion years that followed the Big Bang. Peterson and his colleagues recently posted their initial results online (http://xxx.lanl.gov/abs/astro-ph/0212289).

If further analyses uphold these findings, it could mean that galaxy clusters "are much more abundant than they appear from other observations," says Wayne Hu of the University of Chicago. That would indicate that cosmological models are incomplete.

Cosmologists are eagerly awaiting the findings from another device, the Microwave Anisotropy Probe (MAP) satellite. Unlike the ground-based ACBAR, the satellite has examined the microwave background over the entire sky. But MAP's resolution is less than that of ACBAR, so it can't discern as much detail.

"MAP will see the big picture, while ACBAR's high-resolution data enable it to zoom in on a small patch of sky," says MAP theorist David N. Spergel of Princeton University. ACBAR probes the detailed physics during the time that photons from the microwave background were beginning to be set free from matter and that acoustic oscillations were starting to be damped out, he notes.

Together, ACBAR and another high-resolution, ground-based device, the Cosmic Background Imager in Chile, "have started a new chapter in microwave-background research," says Max Tegmark of the University of Pennsylvania in Philadelphia. "The best is yet to come."