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H Spectrum of the cosmic background radiation as taken from ground-based observations. Most are at centimeter wavelengths where the Earth's atmosphere is very transparent. Measurements are the black data points with error bars.The smooth blue curve through those points is the best fit to the data.The red curve represents the COBE satellite (see below).The horizontal axis is essentially frequency plotted as log(i/A), where A is in centimeters. [© Edward L.Wright, used with permission]

However, until it was shown unambiguously that the spectrum is that of a blackbody, there were still some ways for steady-staters to produce something like the background radiation. As Fig. 21.5 shows, the discovery and subsequent confirmations of the background radiation were on the long wavelength side of the peak of a 3 K black-body spectrum (which is at about 1 mm). The most convincing observations would be to show that the spectrum does, indeed, turn over at wavelengths shorter than 1 mm.

However, even at wavelengths shorter than 1 cm, the Earth's atmosphere becomes sufficiently opaque that ground-based radio observations of sufficient precision are virtually impossible. More recently, observations from space have helped clarify the situation. Before we discuss those, however, an interesting experiment performed in the late 1960s (and still being improved upon) is worth some mention. This experiment involves optical observations of interstellar cyanogen (CN), which was first discovered in 1939.

The basic idea behind the CN experiment is shown in Fig. 21.6. The optical absorption lines are observed when the CN makes an electronic transition from its ground state to the first excited state. In Chapter 14 we saw that, for any electronic state,

H Arno A. Penzias and Robert W.Wilson, who discovered the cosmic background radiation in 1967, in front of the telescope they used, at Bell Laboratories in New Jersey. [Reprinted with permission from Lucent Technologies' Bell Labs]

H Spectrum of the cosmic background radiation as taken from ground-based observations. Most are at centimeter wavelengths where the Earth's atmosphere is very transparent. Measurements are the black data points with error bars.The smooth blue curve through those points is the best fit to the data.The red curve represents the COBE satellite (see below).The horizontal axis is essentially frequency plotted as log(i/A), where A is in centimeters. [© Edward L.Wright, used with permission]

Low Temp

High Temp

Starlight Passing Through Cloud

Background Star

Interstellar Cloud with CN Molecules

_ Rotational Split

(Exaggerated)

Electronic Transitions

Rotational Split

(Exaggerated)

Wavelength Low Temp

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