By Richard M. Todaro

With special thanks to Dr. Paul L. Richards, Professor of Physics at the University of California, Berkeley for his kind and patient telephone tutorial on Big Bang cosmology.

A trio of findings about cosmic background radiation may help explain why matter is irregularly distributed throughout the universe with the observed “large-scale structure” of galactic super-clusters.

A sample of four false-color maps of intensity variations in the cosmic microwave background (CMB) from different parts of the sky created from data collected by the University of Chicago's Degree Angular Scale Interferometer (DASI). The maps depict tiny deviations, on the order of one hundred thousandth of one degree, in the otherwise uniform 2.73 K background. The maps are a snapshot of the universe as it looked 14 billion years ago. DASI is funded by the National Science Foundation. (Graphic courtesy of the DASI Collaboration)

While the existence of cosmic background radiation has been known since the 1960s, the variations in the radiation have only been known since 1991. In that year, NASA's Cosmic Background Explorer satellite first measured the slight variations in the cosmic background radiation, which can be measured as either variations in frequency/wavelength or in absolute temperature. (Absolute temperature is measured on something called the Kelvin scale. A temperature of 0 Kelvin defines the situation in which thermal motion ceases - it is never actually reached - and it corresponds to -273C.)

This past April, three teams of scientists presented their findings on variations in the cosmic background radiation at an American Physical Society conference in Washington, D.C. Each of the teams measured the cosmic background radiation in different parts of the sky and at somewhat different wavelengths. Two of the three teams had already presented preliminary findings on the largest variations in the background radiation last year.

The experiments are each known by their acronyms: DASI, BOOMERANG, and MAXIMA. The first stands for Degree Angular Scale Interferometer, and it used 13 distinct ground-based radio telescopes at the South Pole to measure cosmic background radiation frequencies in the low energy part of the electromagnetic spectrum (weak microwaves). The other two were both balloon-borne experiments that measured the cosmic background radiation at slightly higher frequencies in the millimeter region.

BOOMERANG, short for the Balloon Observations Of Millimetric Extragalactic Radiation ANd Geophysics, consisted of a very sensitive millimeter telescope that was carried aloft by a large balloon 120,000 feet or about 37 kilometers into the upper atmosphere over Antarctica. The balloon and its attached instruments circumnavigated the Antarctic continent over almost an 11 day period from late December 1998 to early January 1999, measuring the cosmic background radiation.

MAXIMA, the Millimeter Anistrophy eXperiment IMaging Array, also measured the background radiation in the millimeter region, but it flew for just a seven-hour period on the night of August 2, 1998 high above Texas.

The minute variations in the cosmic background radiation recorded by all three experiments are a very significant finding. Today's cosmic background radiation is a faint "echo" of the much more intense radiation that filled the universe several hundred thousand years after the Big Bang/inflation event when it had cooled just enough to permit ordinary (electrically neutral) matter to form. This radiation last interacted with matter when the matter in the universe was still a hot plasma of electrically charged particles. This occurred about 300,000 years after the Big Bang, or about 14 billion years ago. Thereafter, the universe cooled enough to allow ordinary matter (mostly hydrogen atoms) to form. That last interaction is called the “time of last scattering.”

Since then, this radiation has been moving in all directions across the universe (only very tiny bits of it intercepted by matter). The radiation has “cooled” over the age of the universe - roughly 14 billion years - from about 4,000 Kelvin at the time of last scattering to about 2.74 Kelvin, which falls in the microwave region of the electromagnetic spectrum. (By contrast, the Sun radiates most strongly in the visible light wavelength with a corresponding temperature of roughly 5,000 Kelvin.)

In these earlier findings, each had found that this cosmic background radiation alternates between “hot”and “cold” patches on the order of 50 millionths to 100 millionths of a Kelvin (i.e. 50 to 100 micro-Kelvins) and at a spatial separation of about a half a degree (which is about the size of the full moon as seen in the sky).

The refined findings by two of the teams combined with the new findings by a third team showed more detailed structure within the hot and cold patches. Much like a musical note as a fundamental “harmonic”, the new findings showed additional harmonics of hot and cold patches within larger hot and cold patches.

The analogy to sound waves is actually rather apt. In the early universe, before it cooled enough to allow ordinary matter to form and instead was a hot plasma (with negatively charged electrons moving too fast to be captured by positively charged protons), minor fluctuations in the plasma behaved like acoustic (sound) waves. Areas of compression in the plasma density corresponded to hotter patches and areas of expansion in the plasma density corresponded to colder patches. In this way, the cosmic background radiation that fills space today is a slowly fading echo of those sound waves that permeated the plasma.

These density fluctuations also allowed matter, once it had formed, to clump together. Without such fluctuations, matter could not have collected together and gravity could not have worked to further cluster more matter together. Ultimately, it allowed stars to form, which in turn allowed galaxies and clusters of galaxies and clusters of the clusters (called super-clusters) to form. What is referred to as "large-scale structure" in the cosmos, with matter showing a “lumpiness” in its distribution across the universe, arose from such fluctuations. The tiny hot and cold patches observed in the cosmic background radiation are not only the fading echoes of that awesome music of creation, they also suggest why stars and galaxies are arrayed as they are across the cosmos.

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