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The Cosmic Symphony

Wayne Hu & Martin White

Scientific American 290N2 44 (2004)

New observations of the cosmic microwave background radiation show that the early universe resounded with harmonious oscillations

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In the beginning, there was light. Under the intense conditions of the early universe, ionized matter gave off radiation that was trapped within it like light in a dense fog. But as the universe expanded and cooled, electrons and protons came together to form neutral atoms, and matter lost its ability to ensnare light. Today, some 14 billion years later, the photons from that great release of radiation form the cosmic microwave background (CMB).

Tune a television set between channels, and about 1 percent of the static you see on the screen is from the CMB. When astronomers scan the sky for these microwaves, they find that the signal looks almost identical in every direction. The ubiquity and constancy of the CMB is a sign that it comes from a simpler past, long before structures such as planets, stars and galaxies formed. Because of this simplicity, we can predict the properties of the CMB to exquisite accuracy. And in the past few years, cosmologists have been able to compare these predictions with increasingly precise observations from microwave telescopes carried by balloons and spacecraft. This research has brought us closer to answering some age-old questions: What is the universe made of? How old is it? And where did objects in the universe, including our planetary home, come from?

Arno Penzias and Robert Wilson of AT&T Bell Laboratories detected the CMB radiation in 1965 while trying to find the source of a mysterious background noise in their radio antenna. The discovery firmly established the big bang theory, which states that the early universe was a hot, dense plasma of charged particles and photons. Since that time, the CMB has been cooled by the expansion of the universe, and it is extremely cold today &mdash comparable to the radiation released by a body at a temperature of 2.7 kelvins (that is, 2.7 degrees Celsius above absolute zero). But when the CMB was released, its temperature was nearly 3,000 kelvins (or about 2,727 degrees C).

In 1990 a satellite called COBE (for Cosmic Background Explorer) measured the spectrum of the CMB radiation, showing it to have exactly the expected form. Overshadowing this impressive achievement, however, was COBE's detection of slight variations &mdash at the level of one part in 100,000 &mdash in the temperature of the CMB from place to place in the sky. Observers had been diligently searching for these variations for more than two decades because they hold the key to understanding the origin of structure in the universe: how the primordial plasma evolved into galaxies, stars and planets.

Since then, scientists have employed ever more sophisticated instruments to map the temperature variations of the CMB. The culmination of these efforts was the launch in 2001 of the Wilkinson Microwave Anisotropy Probe (WMAP), which travels around the sun in an orbit 1.5 million kilometers beyond Earth's. The results from WMAP reveal that the CMB temperature variations follow a distinctive pattern predicted by cosmological theory: the hot and cold spots in the radiation fall into characteristic sizes. What is more, researchers have been able to use these data to precisely estimate the age, composition and geometry of the universe. The process is analogous to determining the construction of a musical instrument by carefully listening to its notes. But the cosmic symphony is produced by some very strange players and is accompanied by even stranger coincidences that cry out for explanation.

Our basic understanding of the physics behind these observations dates back to the late 1960s, when P. James E. Peebles of Princeton University and graduate student Jer Yu realized that the early universe would have contained sound waves. (At almost the same time, Yakov B. Zel'dovich and Rashid A. Sunyaev of the Moscow Institute of Applied Mathematics were coming to very similar conclusions.) When radiation was still trapped by matter, the tightly coupled system of photons, electrons and protons behaved as a single gas, with photons scattering off electrons like ricocheting bullets. As in the air, a small disturbance in gas density would have propagated as a sound wave, a train of slight compressions and rarefactions. The compressions heated the gas and the rarefactions cooled it, so any disturbance in the early universe resulted in a shifting pattern of temperature fluctuations.

Timeline illustration: Bryan Christie Design