Kazimir S Stankevich Investigation of the background radiation in the early years of its discovery

Kazimir Stankevich is Honored Worker of Science and the head of the astrophysics section of the Radiophysical Research Institute, Nizhny Novgorod.

The second Russian research center for radio astronomy (the first was in Moscow, in the Physical Institute of the Academy of Sciences, FIAN) was formed in the State University in Gorky (now Nizhny Novgorod) in the 1950s. Experimental investigations, which started in 1947, developed in the radiophysics department of the Institute of Physics and Technology under the guidance of Professor G. S. Gorelik. The head of the radio astronomy section in the Laboratory of Oscillation in the Physical Institute of the Academy of Sciences, Professor S. E. Haykin, contributed a lot to the formation of experimental radio astronomy. He got the section of radiophysics to take part in a program of research into radio-wave propagation in the atmosphere using the Sun, the Moon and other cosmic objects as sources of radiation. At the same time measurements of radiation fluxes, angular sizes and coordinates of the sources of radiation were carried out. It was in that section where the radio astronomical research group (under the guidance of V. S. Troitsky) arose; several radio telescopes were created, and the research for the program was successfully conducted. The influence of Professor V. L. Ginzburg (the head of the subfaculty of radio-wave propagation at the university at that time, later a Nobel Prize winner) on the development of radio astronomy was tremendous. Students and graduate students fell under his influence and were carried away with radio astronomy. His lectures on radio astronomy and the origin of cosmic rays became the introduction to the most important problems of this new science and promoted formation of lines of investigation.

After my graduation in 1957 from the graduate school, subfaculty of radio-wave propagation, I was assigned to the section of microwave radio astronomy headed by V. S. Troitsky in the newly formed radiophysical research institute attached to the State University. The section of Vsevolod Sergeyevich always gave a lot of attention to the development of new methods of investigation. In the 1960s, on Troitsky's initiative, we began to elaborate the technique for precise measurements of weak radiation noise intensity from cosmic sources. The method developed by V. S. Troitsky, V. D. Krotikov, K.S. Stankevich, and N. M. Tsetlin uses temperature calibration of the antenna by means of blackbody radio-wave radiation in the Fraunhofer zone of an antenna.18 By means of this method flux densities

18 The calibrating disk, or "artificial Moon," typically had diameter close to 1 m, and was placed on a cliff far enough away that the angular size of the disk at the telescope was smaller than the were measured with an uncertainty of 2-3%. This method was called "artificial Moon," since Vsevolod Sergeyevich took an active part in the study of radio-wave radiation from the Moon and pioneered the use of this method applied to this object. Precision measurements of the Moon's disk temperature in a wide range of frequencies were necessary to him for remote probing of its surface layers. A model of the Moon's surface layers was created and the properties of the lunar material were predicted based on these results. I applied this technique for calibration of fluxes in the spectra of strong, discrete sources for the purpose of increasing the precision of primary standards for the absolute radio astronomical scale of flux densities from cosmic objects.

In 1960 there took place an important event which influenced my research interests a lot. Iosif Samuilovich Shklovsky (1960) published his theory of a long-term decrease of the flux density from a supernova remnant. He showed that the difference could be seen when the fluxes are measured a few years apart. Before that nobody could presume that cosmic sources were able to vary their power sufficiently fast. Evolution was expected to be possible but at insignificantly low rates. Hence, if this effect were seen, a new branch of radio astronomy would appear, the study of evolution of the radiation from supernova remnants and of the energy processes inside them. Precision measurements of fluxes and spectra of radio-wave sources were required to detect and study the evolution of young remnants of supernovae. Based on the results of observations during 1961 to 1964 we detected the decrease of flux density from the supernova Cas A at several wavelengths in the centimeter range (Lastochkin and Stankevich 1964). We were among the first groups to confirm Shklovsky's theory.

There were plenty of discoveries in radio astronomy in the 1960s. Quasars, the strongest sources of extragalactic radio-wave radiation, were detected in 1960, and by 1963 they were shown to be quasi-stellar objects. At that time the first catalogs of discrete radio sources and their spectra were published. Other notable discoveries include the emission line of hydroxyl, OH, at A = 18 cm, astrophysical OH masers, the radio recombination lines of atomic hydrogen, and the variability of radio-wave radiation from quasars and radio galaxies. The discovery of the extragalactic microwave background radiation in 1965 was an outstanding event in radio astronomy. It was detected by means of techniques and equipment used in radio astronomy and hence angular resolution of the telescope, but large enough to be an appreciable source of radiation. The disk was close to a perfect absorber at the wavelength of observation, making it a source of blackbody radiation at the disk temperature.

there appeared a new object in the realm of radio astronomy - the whole universe!

Before that discovery none of my colleagues in FIAN, Kharkiv, or NIRFI (the Radiophysical Research Institute in Nizhny Novgorod) or other observatories was engaged in cosmology; everybody was carried away with the achievements of radio astronomy. Sometimes we heard about cosmological problems during the All-USSR conferences. I recall a dispute over a singularity and the search for solutions which exclude it. Certainly there was a search for objects with large redshift within the scope of radio and optical astronomy, but it was nothing but accumulation of data. We still were far from cosmological generalizations. Deep observations of the sky and counting of radio-wave sources started some later, after 1965. From our point of view the radiation at 3 K was the first invasion of cosmology into the realm of radio astronomy (or vice versa). After the discovery of radiation at 3K cosmology became an experimental science: the model of the hot universe was supported, and besides it was the second fact in favor of evolution of the universe, now with a hot commencement. By the way, the singularity which scared everybody so much has moved elegantly to the realm of physics of extreme states of matter at high pressure and energy density. Cosmology became an interesting and attractive science, and we, radio astronomers, could be of use in the study of it. Several years later as a professor in the chair of radio astronomy and radio-wave propagation I was giving lectures on cosmology. And I am deeply convinced that cosmology should be taught to students specializing in any branch of physics.

I learned about the discovery of the excess isotropic cosmic radiation at wavelength 7.35 cm (Penzias and Wilson 1965a) and its interpretation as relic radiation of the hot universe (Dicke et al. 1965) published in the summer of 1965 from my Moscow colleagues, since foreign journals were available in Moscow several months earlier than at the periphery. I recall that I read the papers in October or November in the library of the Physical Institute of the Academy of Sciences.

At the time the radiation at 3 K was discovered I was already experienced in precision measurements of the absolute values of flux densities from discrete sources by the method of the "artificial Moon," which calibrated the antenna temperature by thermal radiation of a discoid blackbody in the beam (far-field) region of the radiation pattern. We used this concept to measure the background radiation temperature. Calibration of the antenna temperature during reception of the background radiation was performed by means of thermal radiation from two reference absorbers. They were kept at different temperatures: that of liquid nitrogen and of the surroundings.

The antenna radiation pattern was shielded from the ground. Our research group (the engineers, V. P. Lastochkin and V. A. Torkhov, and me, the senior researcher) possessed all the necessary tools and materials to measure the temperature of background radiation at the wavelength of 3.2 cm. The whole year of 1966 was devoted to construction of the reference radiators and improvement of the measurement technique. So it was not until the winter of 1966-1967 that we were able to perform the measurements on the roof of the radiophysical institute. The air temperature was —25 °C. That was believed to be the best most stable weather conditions.

The background radiation temperature we measured was 2.2 ± 0.3K at 3.2 cm. On March 4, 1967, we submitted our paper to the journal Radiofizika (Stankevich, Lastochkin and Torkhov 1967). By that time a measurement at this wavelength was already published by Roll and Wilkinson (1966). We were not pioneers, but we achieved our goal - we developed an alternative technique of absolute measurements of background radiation. It seemed to be of great value since this method could be applied in a wide range of wavelengths, from millimeters to decimeters, which was important for detailed study of the background radiation spectrum. The main task at that time was to prove that the spectrum of this radiation at 3 K is consistent with the spectrum of a blackbody of the same temperature. From this point of view the millimeter range of wavelengths was of first-rate interest.

Nobody but A. E. Salomonovich, the head of the laboratory for millimeter radio astronomy at the Physical Institute, could possess a sufficiently sensitive and stable radiometer for the 8-mm wavelength range. He was the first in the Soviet Union to construct radiometers of that kind. Alexander Efimovich agreed to execute a joint measurement of the 3-K radiation temperature using the technique we developed for a similar measurement at wavelength 3.2 cm. He expected that the radiometer for a measurement at 8.2 mm, which was constructed by his order in the machine shop of the Institute of Radio-Engineering and Electronics of the Academy of Sciences, should be finished by June of that year (1967). The radiometer was developed by engineer V. I. Puzanov. I had to elaborate the technique and produce the reference blackbodies, including the one cooled down to the temperature of liquid nitrogen. So, by spring (April) of 1967 the list of authors was formed and the time of joint measurements, June, was clarified.

Radio radiation from the cloudless atmosphere is quite changeable and in summer is 5 to 6 times greater than the background radiation at 3 K. This is why to separate the contribution from the atmosphere one had to determine the brightness temperature of the atmosphere during the experiment. This goal was achieved by measuring the absolute value of the radiation intensity received at two angles to the horizon, 90° and 30°. For this purpose the reference radiators were constructed in such a way that one could tilt them to use the same cooled radiator at the two angles. This is why the volume of the cavity had noticeably increased and we needed about 50 l of liquid nitrogen per filling. For the whole experiment we would need not less than two cubic meters of liquid nitrogen. It was impossible to get such an amount of liquid nitrogen in Gorky. A. E. Salomonovich proposed that the experiment be performed at the radio astronomy station of FIAN in Pushchino. Alexander Efimovich organized a meeting of the radio astronomers at FIAN in Pushchino before the experiment. I presented the measurement technique and detailed the plan of the experiment. Our colleagues L. I. Matveenko and R. L. Sorochenko supported us and took part in the organization of observations later on. Our proposals were approved. In the upshot the experimental setup was installed at the radio astronomy station and liquid nitrogen was delivered from Moscow in a vacuum flask container once a week, supplied by the cryogenic laboratory of FIAN.

The experiment to measure the background radiation temperature at the wavelength of 8.2 mm started at the end of June and was successfully completed by early August. The temperature was found to be equal to 2.9 ±0.7K, and the Planckian character of the background radiation was confirmed as far as 8-mm wavelength. The paper Measurements of the Temperature of the Primordial Background Radiation at 8.2-mm Wavelength was submitted on August 17, 1967 (Puzanov, Salomonovich and Stankevich 1967). At that time there were no published papers on measurements of the CMBR in the millimeter range. There was a publication of a measurement at wavelength 1.5 cm (Welch et al. 1967), where the temperature of the background radiation was reported to be 2.0 ± 0.8 K, so there was some confusion about the spectrum of the radiation. Our paper improved the situation and favored the Planckian character of the CBR.

Our published papers attracted the attention of the well-known physicists Ya. B. Zel'dovich and R. A. Sunyaev to our research. At that time they were engaged in the study of the evolution of matter and radiation in the hot model of the universe. They inferred that the spectrum of the background radiation might possess some peculiarities. Heating of the primordial plasma in the universe in a stage of expansion before recombination of hydrogen could result in deviations in the Rayleigh-Jeans part of the background radiation spectrum. Another issue in cosmology was the density of intergalactic plasma. It could produce detectable radio radiation in the spectrum of the CMBR. One had to search for signs of it at long wavelengths, A ~ 50 cm. By the end of 1967 the only known measurements of the background radiation temperature in the decimeter range were for the wavelengths of 20.7cm, with the result TCMBR = 2.8 ± 0.6K (Howell and Shakeshaft 1966), and 21.2cm, where Penzias and Wilson (1967) reported TCmbr = 3.2 ± 1.0K. Of course it was not enough to determine the peculiarities of the spectrum. The study of the background radiation implied intimate knowledge of the characteristics (temperature and spectral index) of nonthermal emission: continuous radiation from the Galaxy and unresolved extragalactic sources of radiation. Those quantities can be determined by simultaneous absolute measurements at three different wavelengths. My young colleague S. A. Pelyushenko and I performed such measurements according to the technique described above for wavelengths of 15 cm, 20.9 cm and 30 cm at the testing area of the institute in Zhimenki in the summer 1968. The paper (Pelyushenko and Stankevich 1969) was submitted on July 12, 1968. We determined that the temperature of the extragalactic background radiation does not appreciably vary with wavelength in the range 15-30cm, and that the temperature equals 2.5 ± 0.6K. This showed that radiation from intergalactic gas does not manifest itself in this wavelength range.

Weymann (1966) pointed out that radiation by intergalactic plasma could produce a 10-20% increase over the thermal spectrum at A ~ 30 cm, with a larger increase at longer wavelengths. Howell and Shakeshaft (1967b) found TCmbr = 3.7 ± 1.2 K for wavelengths of 49.3 cm and 73.5 cm and reported a high upper limit, 2 K, for possible deviation from the background radiation spectrum at 73.5 cm. This is why additional measurements were necessary to find at least a tighter upper limit on possible deviations.

Limitations due to mechanical problems excluded the use of horn antennas and reference blackbodies cooled down with liquid nitrogen for measurements at wavelengths above 30-40 cm. That led me to propose using the Moon as a screen from cosmic radio emission. The studies by V. S. Troitsky showed that its disk temperature is constant and well known in the decimeter wavelength range, and polarization effects are negligible for wavelengths longer than 3 cm. In other words, the Moon could be used as an intensity standard. The experiment had to be performed for two wavelengths by means of antennas with high angular resolution. At that time there was a lack of large parabolic antennas in the Soviet Union. I. S. Shklovsky solved this problem. He asked J. G. Bolton to invite me to perform measurements with the Parkes 210-ft radio telescope. My visit to Sydney, Australia, at the Division of Radiophysics CSIRO came about with financial support from the School of General Studies, Australian National University, in September 1968. With support of a grant from the School of Electrical Engineering,

University of Sydney, Richard Wielebinski and his postgraduate student W. E. Wilson took part in the project to accomplish work on absolute calibrations. Richard was engaged in the study of the spectrum and polarization of galactic radio emission. He was also interested in absolute calibrations of nonthermal radiation. Preparation for observations with the radio telescope took quite a while. The radio telescope was equipped with a radiometer system for 635 MHz. It was necessary to create a supernumerary channel for 408 MHz using a radiometer constructed by Richard Wielebinski, and construct and install an integrated feed assembly for simultaneous reception at two wavelengths. We performed rather complicated absolute measurements of the cosmic background in February and May 1969. We made use of the Parkes 210-ft radio telescope at wavelengths of 47.3 cm and 73.5 cm, and a large horn antenna was used at 73.5 cm to determine the absolute temperature of the sky in the calibration point at high galactic latitude. The temperature of the CMBR for these wavelengths was found to be 3.0 ±0.5K (Stankevich, Wielebinski and Wilson 1970), which is in good agreement with the average value 2.7 K from observations in centimeter and millimeter ranges. It followed that radiation from intergalactic plasma did not appreciably affect the background radiation spectrum in the decimeter wavelength range.

From the combined analysis of the Parkes results and the data for absolute measurements at wavelengths of 15 cm, 20.9 cm, and 30 cm it was found that the CMBR spectrum in the Rayleigh-Jeans range agrees with the radiation spectrum of a blackbody of 2.7K temperature, and no deviations from this spectrum as a result of emission by cosmic plasma at any stage of evolution of the universe were revealed. I consider this conclusion to be the main contribution from the work we accomplished.

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