May 31, 2007

Radio Waves from the Big Bang

Arno A. Penzias and Robert W. Wilson, A Measurement of Excess Antenna Temperature at 4080 Mc/s, 1965

Robert H. Dicke, P. James E. Peebles, Peter G. Roll, and David T. Wilkinson, Cosmic Black-body Radiation, 1965

Science teachers often present established facts and theories about the world around us without giving much information about the observations and evidence that require these explanations. Perhaps this is necessary simplification for the sake of educating without overwhelming, but paring things down to the bare conclusions, however true they may be, always leaves doubt about their accuracy. To really believe in some of the more astonishing things we've discovered about the universe, it helps to know what real people saw with their own eyes that forced them to these conclusions. The Big Bang is one of these discoveries.

In 1964 Bell Laboratories turned over a sensitive radio receiver, originally designed to detect signals bounced off of Echo, NASA's first communications satellite, to Arno Penzias and Bob Wilson, who planned to use it to study radio emissions from our galaxy. No ordinary radio receiver, the Horn Antenna is a freakish looking contraption, 15m long and 10m high, its form entirely determined by its function. (Go look at the pictures!) It is an exquisitely sensitive instrument, able to detect ridiculously tiny signals, amplify them, and filter out background noise. The horn itself acts much like the more familiar dish antennas in concentrating faint signals. Background noise is quantified and filtered out by repeatedly switching the detector between the signal from the horn and a null radio source (a bath of liquid helium kept at -269°C). Background noise remains constant while the signal varies with the same timing as the switch. The signal is amplified by a maser, also cooled by liquid helium to reduce internal static.

From July 1964 through April 1965, Penzias and Wilson struggled to quantify all the sources of background noise in their antenna at a radio frequency of 4,080 megacycles per second. They calculated the amount of radio emission being received from the atmosphere. They calculated the amount of radio emission received from the ground. They calculated the amount of radio emission generated by the telescope itself. (Practically anything that has a temperature emits small amounts of radio, along with much larger amounts of infrared radiation.) They set up a radio transmitter nearby to measure the effects of man-made radios, and pointed the antenna at major cities such as New York. They measured the static generated in every component of the detector. They rebuilt the maser, finding it blameless. They carefully cleaned and realigned the joints in the telescope, and put aluminum tape over the rivets and seams to help eliminate any possible noise from these imperfections in the telescope. They even evicted some pigeons which had taken to roosting in the horn and cleaned up their mess.

Throughout this process, a residual signal equivalent to a radio temperature of 3.5°±1.0°K remained unexplained. It did not vary with direction. It did not vary with time, even over the course of several months. Penzias and Wilson were confident that they had identified every source of noise generated by their equipment, and the invariance of the signal ruled out any possibility that it could come from a single source in the sky, or even a large source like the Milky Way. They concluded that the whole universe must be permeated by this tiny glow of background radiation.

A chance discussion put them in contact with Robert Dicke, a theoretician with an explanation. Since Hubble's work on redshifted galaxies we've known that the universe is expanding. Extrapolating backwards and taking things to an extreme suggests that at some point in the past everything might have been crammed into a very small amount of space. At sufficiently high densities, the average temperature of all matter in the universe would be high enough to prevent atoms from forming. Unlike the universe of today, where a photon can travel billions of light-years without hitting anything, all the light in a high density universe regularly hits free-roaming electrons, meaning that all the light in such a universe quickly reaches black-body equilibrium. When the temperature drops below ~3,000°K, atoms can form, at which point collisions between photons and electrons decrease significantly, an event that physicists refer to as "the decoupling of light and matter". From that point on, the 3,000°K black-body spectrum of most of the universe's light is largely unaffected by anything except the expansion of the universe, which steadily redshifts it down the spectrum.

So, to sum up: Penzias and Wilson made the first measurement of what is now called the Cosmic Microwave Background (CMB), which turns out to look like a black-body spectrum from light interacting with matter at a temperature of 2.726°K. The fact that it's there at all tells us that the universe must at some point have had a density high enough to give everything in it a temperature of at least 3,000°K. The fact that it has been redshifted down to 2.726°K tells us that the universe has expanded by a factor of ~1100 since then.


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