Cosmic Revelations: Satellite homes in on the infant universe

Like beaming parents showing off pictures of their newborn, astronomers this week proudly unveiled the sharpest snapshot of the baby universe ever taken. The scientists had a lot to smile about.

Their infant portrait, revealed by the remnant glow from the Big Bang, pegs the universe’s age to an unprecedented accuracy of 1 percent. Rather than using more approximate numbers, astronomers can now say the universe is 13.7 billion years old, the researchers report. The new data also confirm that the universe began with a brief but humongous growth spurt, dubbed inflation. Inflation stretched to cosmic scales random patches of the fabric of space-time that had minuscule fluctuations in density, creating the lumps from which galaxies arose.

The images of the Big Bang’s afterglow, known as the cosmic microwave background, also delineate the cosmos’ composition: 4 percent is ordinary matter; 23 percent is invisible stuff called cold dark matter, which prompted the galaxies to coalesce; and 73 percent is so-called dark energy, which has accelerated the rate at which the universe expands (SN: 5/25/02, p. 333: Available to subscribers at More evidence for a revved-up universe).

What’s more, the data recorded by the Wilkinson Microwave Anisotropy Probe (WMAP), a NASA satellite, reveal that the universe had already made an abundance of stars when it was only 200 million years old. That’s about one-fifth the age that many cosmologists had predicted.

At a NASA press briefing on Feb. 11, Charles L. Bennett of NASA’s Goddard Space Flight Center in Greenbelt, Md., and David N. Spergel of Princeton University presented the snapshot from WMAP, launched in June 2001.

“For cosmology, this announcement represents a rite of passage from philosophical uncertainty to precision science,” comments John N. Bahcall of the Institute for Advanced Study in Princeton, N.J. “Every astronomer will remember where he or she was when they first heard the WMAP results.”

The sky map generated by WMAP shows tiny hot and cold spots in the otherwise uniform cosmic microwave background averaging a frigid 2.73 kelvins. Just a few millionths of a degree above or below the average, the hot and cold spots reveal the earliest phases of clumping of photons and matter. Viewing the fluctuations with 45 times the sensitivity and 33 times the spatial resolution of its predecessor, the Cosmic Background Explorer satellite, WMAP has nailed down several key cosmological parameters.

WMAP measured the age of the universe by detecting the size of the hot and cold spots as seen from Earth. The older the universe, the smaller these spots would appear.

By measuring the peaks in temperature fluctuations on different spatial scales, WMAP has essentially measured the contents of the universe. Although the microwave background was generated during the Big Bang, telescopes see the radiation as it appeared when it first streamed freely into space a few hundred thousand years later.

Before that, the universe was so hot that there were no neutral atoms, only ions and electrons that trapped the cosmic microwave background radiation.

Imprinted on that radiation are acoustic oscillations, generated by the primal tug-of-war between the gravitational pull of matter and the outward pressure exerted by photons while they were still trapped. These oscillations created regions of slightly higher or lower pressure, generating places that were slightly hotter or colder than average.

The satellite has also measured the polarization of the cosmic microwave background. This measurement of the tendency for light waves to vibrate in a particular direction is a major technical feat–the polarization signal is one-hundredth the strength of the tiny temperature fluctuations.

Photons become polarized when they scatter off free electrons. That’s why polarization dates two important epochs of the universe. The first is the time–380,000 years after the Big Bang–when the last free electrons became bound to atomic nuclei, permitting radiation to stream into space.

The second epoch marks the time when stars first lit up the universe and reionized atoms into nuclei and free electrons. From the polarization measurements, Spergel’s team deduced that the cosmos became a star-making factory only 200 million years after its birth.

Where there were hordes of stars, there were probably also quasars and galaxies. Yet the most distant known galaxies and quasars date to when the universe was 800 million years old. “We’re saying there’s a lot of objects farther out that we haven’t seen,” says Spergel.

Still, he notes, the most profound result is that “everything fits” with the current cosmological model. “For the first time, we are making measurements with such precision that we have a standard model for the evolution of the universe, in the same way that particle physicists have a standard model” of the subatomic world, says Spergel.

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