Noise and limitations on sensitivity

Abstract

There are two major sources of noise in space. One was discovered in 1965 when Arno Penzias and Robert Wilson at Bell Laboratories in New Jersey were calibrating a microwave communication receiver. They were unable to identify a noise that was present at the same signal strength irrespective of the direction in which the antenna was pointed. They were about to write it off as a footnote in their report when they received a telephone call from Robert Dicke at the University of Princeton, who had heard of their plight. Dicke had calculated that if the universe started in a Big Bang, there should be a background signal in the microwave region, and he suggested this was what the antenna had found. As it happened, the discovery had been made using a radiometer designed by Dicke which compensated system-gain and noise variation by alternating back and forth between the target and a calibrated source of noise. The excess antenna temperature of 3.5K was consistent with Dickers prediction for a Big Bang. As observations were obtained at other wavelengths, it was established that the power level as a function of wavelength matched thermal blackbody emission at a temperature of 2.75K with its peak at 160.2 gigahertz, equivalent to a wavelength of 1.9 millimeters. In fact, unknown to Dicke, such cosmic background radiation had been predicted in 1948 by George Gamow, Ralph Alpher and Robert Herman. For their discovery, Penzias and Wilson shared a Nobel Prize in 1978. The major source of what amounts to noise at optical wavelengths is the Sun and stars, as the emission of a blackbody with a surface temperature of several thousand degrees has its peak in the visible rather than the microwave region (Figure 9.1).