And D Sheds On Bldg



Fig. 3.3. Illustration of the ability of a Dicke radiometer to detect thermal radiation. Below is a sketch of the skyline of Cambridge in 1945, seen from the roof of Temporary Building 20 on the MIT campus. Above is a strip-chart recording of the response of the radiometer to objects at different temperatures. Warmer objects are recorded as deflections down on the chart, to higher temperature. The metallic dome at F is cool because it reflects the beam from the cooler sky.

can extrapolate to what would be detected in the limit where the distance through the atmosphere vanishes - where there is no atmospheric emission. This would be the observed temperature of space beyond the Earth's atmosphere.33

Dicke et al. (1946) used this "tipping experiment" method to establish that "there is very little (<20°K) radiation from cosmic matter" at the

33 We should add that some of the variation in measured temperature shown in Figure 3.3 results from what radio astronomers term "side-lobe pickup." An antenna does not produce a sharply defined beam on the sky. Rather, there are subsidiary diffraction maxima, known as "side lobes," that allow radiation to leak into the receiver from substantial angles away from the direction of the main beam. As the zenith angle increases, these side lobes pick up more and more radiation from ground that is at a temperature of about 300 K. Side-lobe pickup bedeviled early attempts to detect the small variations in the CMBR temperature across the sky produced by the lumpy mass distribution.

microwave wavelength (near 1 cm) they measured. Ironically, this paper appears in the same volume of the journal Physical Review as Gamow's 1946 letter on element formation in the early stages of expansion of a big bang cosmology. Gamow was not yet discussing a hot big bang, however -the first publication on that subject was in 1948 - so it is not surprising that neither paper made reference to the possible significance of the other. The bound Dicke and colleagues placed on how hot space might be is well above what would be expected in the cool big bang situation Hayashi and Nishida (1956) later analyzed. It is not far from the situation Gamow (1948a) proposed and Alpher and Herman (1948) calculated, however. And it is not far from what was later found to be temperature of the fossil radiation from the big bang, T0 = 2.725 K. But the connection between what Dicke's radiometer can measure and what might be expected from a hot big bang was not noticed for another two decades.

In the course of research on other subjects in the 1950s and early 1960s measurements equivalent to the Dicke et al. (1946) tipping experiment were repeated, and the CMBR eventually detected and recognized. Detection happened first as a byproduct of research at Bell Telephone Laboratories on the development of low-noise maser amplifiers for communication systems (De Grasse et al. 1959; Ohm 1961; Jakes 1963).

Figure 3.4 shows a particularly detailed tipping measurement from this communications program. These data are from Project Echo, which demonstrated communication by microwave signals sent from the ground and reflected back to the ground by a satellite (a large balloon with a conducting surface). The paper on this measurement (Ohm 1961) presents the following numbers. When the Echo receiver was pointed to the zenith it detected microwave radiation equivalent to blackbody radiation at temperature Tsystem = 22.2 ± 2.2K at 2390 MHz (12.6-cm wavelength). When the instrument was tipped away from the zenith the system temperature increased because it was looking through more atmosphere. From that variation Ohm could estimate that the atmosphere contributed the equivalent of Tatm = 2.3 ± 0.2 K in the direction of the zenith. This plus estimates of the radiation originating in the instrument and that entering the horn antenna from the ground was, in this experiment, estimated to total Tlocal = 18.9 ± 3.0 K. The difference,

is a measure of what might be entering the atmosphere from cosmic sources. This is a considerable improvement over the Dicke et al. (1946) measurement, Texcess < 20 K. The measured value in equation (3.15) is consistent

Fig. 3.4. A tipping measurement (Ohm 1961). The top curve shows the sum of the microwave radiation flux from the detector, ground, atmosphere, and whatever comes in from above the atmosphere. The lower curve is the result of subtracting estimates of what came from the instrument and ground. Reprinted with permission of Lucent Technologies/Bell Labs.

Fig. 3.4. A tipping measurement (Ohm 1961). The top curve shows the sum of the microwave radiation flux from the detector, ground, atmosphere, and whatever comes in from above the atmosphere. The lower curve is the result of subtracting estimates of what came from the instrument and ground. Reprinted with permission of Lucent Technologies/Bell Labs.

with the Alpher and Herman (1948) estimate of the Gamow condition (in equation 3.7), within the uncertainties. It is also close to the CMBR temperature (equation 2.6). But it is also formally consistent with zero. Hogg (p. 72) describes the situation in more detail.

The measurement was repeated in the Telstar Project that demonstrated transmission of a television signal from the ground to a satellite that rera-diated the signal back down to the ground. Jakes (1963) reported that (at 7.2-cm wavelength) "The over-all system noise temperature was measured to be somewhat less than 17°K pointing at the zenith, which included about 4.5°K for waveguide losses, 2.5°K sky noise, 2.5°K for antenna side lobes and heat losses and 5°K for the maser." The sum and difference - which Jakes does not state - amounts to

This is close to the central value of the range of estimates of Texcess from Project Echo, and again close to the CMBR temperature.

The consistency of central values of Texcess from the Echo and Telstar systems did not force attention to the idea that there might be a detectable sea of extraterrestrial microwave radiation: a reader of these papers could imagine that local sources of radiation had been slightly underestimated. That might be what Ohm (1961) had in mind in writing that "the '+' temperature possibilities of Table II" listing local noise contributions "must predominate." We know in hindsight that there was no need to assume the system temperatures in the Echo and Telstar systems were systematically underestimated, and that the contribution of radiation entering the horn antenna from the ground likely is an overestimate. All this became clear later in the 1960s when Penzias and Wilson added a low-temperature calibrator to the Telstar system, for the purpose we mentioned on page 46 and they explain in Chapter 4. That made the difference Texcess between what was detected and what was expected from the instrument, ground and atmosphere a clear and pressing issue for them. It then became a pressing issue for the cosmology community.

The next chapter presents recollections of Dicke's reaction to this issue that are consistent with the idea that when he asked Roll and Wilkinson to look for the CMBR he did not know the Bell experiments suggested excess noise. We have no evidence whether he was even aware of these communications experiments. But memories are complicated: Dicke's younger colleagues recall having to remind him that in 1946 he had published a measurement that placed a limit of 20 K on the CMBR temperature.

The broader reaction in the science community to this issue of excess noise was conditioned by the state of research in cosmology in the early 1960s. We consider this next.

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