March/April 1999

Scope Floats continued

Roughly translated, anisotropy (pronounced an-i-SAH-tro-pee) means "lumpiness" or "unevenness." As scientists look into the universe, they see lumps in every direction: stars, galaxies, galaxy clusters, and galaxy superclusters — chains of galaxies spanning hundreds of millions of light-years. But the lumps are distributed unevenly. In other words, the universe is anisotropic.

But scientists find it difficult to explain how matter in the early universe began clumping together to make those lumps. That is especially true on the largest scale — the scale of superclusters. It's clear that galaxies congregated in clusters and superclusters from the beginning. Yet there wasn't enough time for the gravity of all those newborn galaxies to grab onto each other and cause them to form many of the superclusters. Instead, like a child inheriting blue eyes or curly hair from a parent, the universe must have inherited its large-scale structure — its lumpiness — from the Big Bang.

Technicians attach the parachute and explosive device to the balloon.

BAM looks for the "lumpiness gene" in the very early universe by measuring tiny fluctuations in the cosmic background radiation — a faint glow from the expanding fireball created by the Big Bang.

"The thing that I find particularly exciting is we're studying how clumpy [the universe] started. We are really measuring, directly, the conditions of the Big Bang," says Halpern.

Big Bang theory says that all matter, energy, and even space and time were born in a single instant of cosmic creation, probably about 12 billion to 15 billion years ago.

At first, the rapidly expanding universe was incredibly hot and dense — too hot, in fact, for matter to form. It was also opaque, so you wouldn't have been able to see from one "side" of the universe to the other.

About 300,000 years after the Big Bang, though, the fireball had cooled to 4,000 degrees Kelvin — a little cooler than the surface of the Sun. (One degree Kelvin is equal to one degree Celsius, but the Kelvin scale starts at absolute zero, which is -273 degrees Celsius and -459 degrees Fahrenheit.) Hydrogen atoms formed, and the universe became transparent.

But the glow of the opaque fireball is still visible, because as astronomers look deeper into the universe, they're looking farther back in time. The expansion of the universe has stretched the light from the fireball and lowered its equivalent temperature, so it now appears at a temperature of about 2.7 degrees Kelvin. It forms a faint glow across the entire sky, which is detectable only by microwave instruments. Studying the background radiation from Earth's surface is difficult because the atmosphere distorts the view.

Mark Halpern makes last-minute adjustments to BAM's instruments.

The cosmic background was discovered in 1965, and at first glance it appeared isotropic, with the same intensity (or temperature) in every direction. In 1992, though, the Cosmic Background Explorer (COBE) satellite mapped subtle variations corresponding to temperature differences of just 0.0002 degrees Kelvin. These variations indicate that the universe was already showing some clumpiness at an age of just 300,000 years — long before the formation of individual galaxies.

The size of the clumps can also help scientists determine the mass of the universe; more big clumps means more mass. That's important because the fate of the universe may depend on its mass. If the universe contains enough stars, planets, and "dark matter," their combined gravity could halt the expansion of the universe and drag everything together again. Most observations suggest there isn't enough mass in the universe to cause such a collapse. (A recent discovery also suggests that the universe contains a "repulsive" energy that is causing the expansion of the universe to accelerate. See page 20 for details.)

COBE mapped the background radiation across the entire sky, but each "point" on its map was as wide as 14 full Moons. To fully understand the structure of the early universe, astronomers must see finer details.

"A lot of groups are pursuing this in different ways," says Halpern. "Even right here at the balloon base, in the last month there have been three different experiments. There are ground-based efforts...and there's a satellite effort, called MAP [Microwave Anisotropy Probe], that I'm a part of. All these different efforts are trying to do different parts of this puzzle."

BAM itself consists of a telescope that reflects microwave energy into a sensitive detector cooled to 0.3 degrees Kelvin. "The cosmic microwave background has a temperature of three degrees above absolute zero, and...your detector must be colder than the object you're trying to measure," explains BAM scientist Greg Tucker, a physicist at Brown University in Rhode Island.

BAM will see the sky much more clearly than COBE did, but it will only view a small sliver of sky for a short time. On the other hand, MAP, the satellite scheduled for launch next year, will view the entire sky, and will observe for several months.

"If there will be a satellite, why not just wait?" Halpern asks with a chuckle. "I guess that's not the scientific spirit. I think we can do a reasonable job now of measuring some important things — nowhere near as crisp and precise as one can do from a satellite, but we can do a careful enough job here, much more cheaply, much more quickly."

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