A revolution in cosmology: a closer look at the cosmic microwave background

Posted 05/05/01

By Robert Caldwell
Assistant Professor of Physics and Astronomy

In the past few years, and especially in the past few weeks, we have witnessed a series of fascinating cosmic discoveries. We have seen the blueprints for the structure of the universe. We have measured the geometry of the universe. We have recorded the earliest sounds in the universe. All from the cosmic microwave background, or CMB for short.

What is it? The CMB is a ubiquitous bath of very cold radiation (a temperature of 2.726 K, with a peak frequency in the microwave range), comprised of photons surviving from the depths of the universe, when it was only 400,000 years old. Just prior to that time, the universe was filled with a hot gas of ever-colliding atomic nuclei, electrons and photons. As the universe cooled and expanded, the nuclei combined with the electrons to form neutral atoms and released the photons to travel freely. This process took place everywhere. (I remember where I was when I first learned this: my advisor and I were eating lunch at a local Greek restaurant, and drawing diagrams on napkins.) And those photons have been traveling, largely undeflected ever since, for the last 15 or so billion years. Looking out in the sky, looking back in time, the CMB appears to us to have originated on the surface of a sphere with a radius of nearly 15 billion lightyears. We can see nothing further - beyond it the universe becomes opaque to electromagnetic radiation. The CMB is a wall of light.

The CMB was discovered in 1965 by Arno Penzias and Bob Wilson as a faint buzz in their radio antenna, located at Bell Labs in New Jersey. Physicists had theorized that such a bath of ancient radiation might exist, and just down the road at Princeton, an experiment to search for precisely this was under way when Penzias and Wilson announced their results. The discovery of the CMB, and its elucidation as a relic from the early universe, set the seal of approval on the Big Bang and transformed cosmology into a scientific discipline.

Later observations found the CMB to be remarkably uniform, with the same temperature (or photon flux) in all directions on the sky.

Everything changed in 1991 with the first results from the Cosmic Background Explorer, COBE, satellite, launched by NASA. Physicists finally had an instrument sensitive enough to detect the slight variations in the otherwise pristine glow of the CMB radiation. COBE revealed a pattern of hot and cold spots that had been long awaited by cosmologists - the fluctuations correspond to the slight irregularities in the density of the matter in the universe when it was just 400,000 years old. These irregularities are thought to have grown and amplified, due to gravity, into the galaxies seen today. This was our first glimpse of the blueprints for the structure of the universe.

In 2000, a sublime discovery was made by three independent teams, revealing the geometry of the universe. One team, TOCO (which I was a part of), perched an antenna atop a mountain in northern Chile, while the two others, BOOMERANG and MAXIMA, launched high altitude balloons carrying their CMB detectors. All measured the angular size of the hottest and coldest spots on the sky. What does this mean? Each spot defines a triangle, where the base is the width of the spot, and the arms of the triangle are the light travel distance of the photons since the birth of the CMB. It turns out that we know the length of the arms very well, and we have excellent control of the theory of the origin of the hot and cold patches, and know how big they are. We didn't know the geometry. In plane geometry, as we learned in high school, the sum of the interior angles of a triangle add up to 180 degrees. For curved space the geometry is non-Euclidean, like a triangle drawn on a sphere for which the sum is greater than 180 degrees. It turns out that the geometry affects the apparent angular size of the patches. The prediction of 1 degree for flat space (but dramatically larger or smaller for curved space) was confirmed by the three teams.

The hot and cold spots are caused by an oscillating competition between gravity, which squeezes the matter during the emission of the CMB photons, and radiation pressure, which resists the compression. These oscillations are literally rarefactions and compressions in the matter, or sound waves. The hottest and coldest spots are due to the cycling of the fundamental mode. At an extraordinary frequency of 1 cycle per 400,000 years, this is the basso-superprofundo boom of the birth of the CMB.

Now, the breathtaking results from DASI, an array of small radio dishes observing the sky from the South Pole, along with BOOMERANG AND MAXIMA, are revealing the deeper structure of the universe. These experiments have now registered the higher harmonics of the cosmic boom, at frequencies approximately 3 and 4.5 times the fundamental mode. I don't know if anyone has transcribed these notes up to the audible range, but this music of the celestial sphere is a dramatic confirmation of our theories. It tells us the proportionate composition of the matter in the universe. The results indicate that only 5 percent of the matter in the universe is "normal," or made of nuclei and electrons and the like - the stuff of stars and planets and people. Another 30 percent is an invisible "dark matter" which likely resides in galaxies and clusters of galaxies today. And the remaining 65 percent is a complete and total mystery, a "dark energy" which is causing the universe to accelerate, but that's another story. It's also my field of research. My Web page has more on this.

I'm looking forward to this summer, when NASA's Microwave Anisotropy Probe, MAP, satellite is launched. I'm planning to go down to Florida to watch the launch, as it carries the hopes of physicists to squeeze ever more information out of the CMB. If we're lucky, a few years later we may discover that the CMB photons are polarized. And if they are (and if our predictions are correct) we may be able to glimpse the universe in the earliest instant of the Big Bang, a raw 10^(-35) seconds after the origin of the cosmos.

Dark Energy and its implications