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Igor Novikov served as head of the Department of Relativistic Astrophysics at the Space Research Institute and head of the Department of Theoretical Astrophysics at the Lebedev Physical Institute, both in Moscow, and then as director of the Theoretical Astrophysics Center of the University of Copenhagen. He is now at the Niels Bohr Institute.

The beginning of my scientific career in the middle of the 1960s coincides with the events that caused the astrophysical community to become aware of the real existence of the CMBR.

I outline here the development of the situation at this period in the Soviet Union. It so happened that I played a part at this stage of the story. This description relies on my recollections, published and unpublished material of my colleagues, and special recent discussions with participants of the events. I have used also some material from the books Zel'dovich and Novikov (1983) and Novikov (1990), and from the paper Novikov (2001).

I would like to remind you that at the beginning of the 1950s the theory of an expanding, indeed an evolving, universe with the beginning of time at some finite period ago was practically forbidden in the USSR. There was a postulate that only an eternal universe without directed evolution as a whole is compatible with the materialistic attitude.

Only at the beginning of the 1960s did the first serious discussions and publications on the physics of the expanding universe became possible. At that period some important works on the structure of the cosmological singularity and gravitational instability of the expanding universe were published by E. Lifshitz, I. Khalatnikov, and I. Novikov. But these works did not discuss the physical conditions in the early universe. In the early 1960s Yakov Zel'dovich began turning his attention to cosmology. He very quickly became one of the greatest cosmologists of the last century.

In 1962 Zel'dovich published a paper (Zel'dovich 1962) in which he modernized the cold universe scenario. According to this scenario at the initial stage of the evolution of the universe the matter consists of a mixture of protons, electrons, and neutrinos in equal amounts and the entropy is low. Then at high density (on the order of nuclear density) and at zero temperature neutrinos and electrons form a degenerate relativistic Fermi gas. The process of interaction of protons with electrons with the formation of neutrons and neutrinos is forbidden since the neutrino states that are energetically obtainable in this process are occupied. Upon expansion such a substance remains pure cold hydrogen. It was assumed that all other elements were generated much later, in stars. According to this model the CMBR radiation should not exist in our epoch.

Zel'dovich's hypothesis was widely discussed in the USSR. One of the main reasons for the hypothesis was some indication in the literature at that time that the helium mass fraction in the oldest stars is much less than 20%.

Probably this means that the primeval helium mass fraction is essentially smaller than predicted in the theory of the hot universe, 25%. Zel'dovich believed also that according to the hot universe theory the matter density of the CMBR should be of the order of the modern average nucleon density. In this paper his conclusion was: "These deductions are incompatible with the observations."

Curiously, Ya. Zel'dovich at that period, as well as the originators of the hot model, was mainly interested in the integral properties of the relic radiation (CMBR) - its density, pressure, and temperature - but not its spectrum.

Here my story begins. At that time I had just completed the postgraduate course at Moscow University; my science adviser was Professor Zel'manov. My adviser was mostly interested in the mechanics of motion of masses in cosmological models when no simplifying assumptions are made about their uniform distribution. He was less interested in specific physical processes in the expanding universe. At that time, I knew almost nothing about the hot universe model.

Not long before the end of my postgraduate term, I was attracted to the following problem. We know how different types of galaxies produce electromagnetic radiation in different ranges of wavelength. With certain assumptions about the evolution of galaxies in the past, and having taken into account the reddening of light from remote galaxies owing to the expansion of the universe, one can calculate the present distribution of the integrated galactic emission as a function of wavelength. In this calculation, one has to remember that stars are not the only sources of radiation, and that many galaxies are extremely powerful sources of radio waves in the meter and decimeter wavelength ranges.

I began the necessary calculations. Having completed the postgraduate term, I joined the group of Professor Ya. B. Zel'dovich; our interests focused mostly on the physics of processes in the universe.

All calculations were carried out jointly with A. Doroshkevich, who I met when I joined Zel'dovich's group. We obtained the calculated spectrum of galactic radiation, that is, of the radiation that must fill today's universe if one takes into account only the radiation produced since galaxies were born and stars began to shine. This spectrum, shown in Figure 4.5, predicted a high radiation intensity in the meter wavelength range (such wavelengths are strongly emitted by radio galaxies) and in visible light (stars are powerful emitters in the visible range), while the intensity in the centimeter, millimeter and some still shorter wavelength ranges of electromagnetic radiation must be considerably lower.

Fig. 4.5. From Doroshkevich and Novikov (1964). Spectrum of the metagalaxy. Curves (a)-(d): the integrated radiation from galaxies under several assumptions about the cosmology and the evolution of the galaxies. Curve (e): equilibrium Planck radiation with T = 1K. Crosses denote experimental points. ©1964 American Institute of Physics.

Fig. 4.5. From Doroshkevich and Novikov (1964). Spectrum of the metagalaxy. Curves (a)-(d): the integrated radiation from galaxies under several assumptions about the cosmology and the evolution of the galaxies. Curve (e): equilibrium Planck radiation with T = 1K. Crosses denote experimental points. ©1964 American Institute of Physics.

Since the hot and cold universe scenarios were eagerly discussed in our group (consisting of Zel'dovich, Doroshkevich, and myself), the paper that Doroshkevich and I prepared for publication added to the total the putative radiation surviving from the early universe if it indeed had been hot. This hot universe radiation was expected to lie in the centimeter and millimeter ranges and thus fell into the very interval of wavelengths in which the radiation from galaxies is weak! Hence, the relic radiation (provided the early universe had been hot!) was predicted to be more intense, by a factor of many thousands or even millions, than the radiation of known sources in the universe in this range of wavelengths.

This background could, therefore, be observed! Even though the total amount of energy in the microwave background is comparable with the visible light energy emitted by galaxies, the relic radiation would be in a very different range of wavelengths and thus could be observed. Here is what Penzias (1979a) said about our work with Doroshkevich (Doroshkevich and Novikov 1964) in his Nobel lecture:

The first published recognition of the relic radiation as a detectable microwave phenomenon appeared in a brief paper entitled "Mean Density of Radiation in the Metagalaxy and Certain Problems in Relativistic Cosmology" by A. G. Doroshkevich and I. D. Novikov in the spring of 1964. Although the English translation appeared later the same year in the widely circulated "Soviet Physics -Doklady", it appears to have escaped the notice of other workers in this field. This remarkable paper not only points out the spectrum of the relic radiation as a blackbody microwave phenomenon, but also explicitly focuses upon the Bell Laboratories 20-ft horn-reflector at Crawford Hill as the best available instrument for its detection!

Our paper was not noticed by observers. Neither Penzias and Wilson, nor Dicke and his coworkers, were aware of it before their papers were published in 1965; Penzias told me several times that this was very unfortunate.

I want to mention a strange mistake related with an interpretation of one of the conclusions of the Doroshkevich and Novikov (1964) paper. Penzias (1979a) wrote: "Having found the appropriate reference (Ohm 1961), they [Doroshkevich and Novikov] misread its result and concluded that the radiation predicted by the 'Gamow theory' was contradicted by the reported measurements." Also in the paper Thaddeus (1972) one can read: "They [Doroshkevich and Novikov] mistakenly concluded that studies of atmospheric radiation with this telescope (Ohm 1961) already ruled out isotropic background radiation of much more than 0.1 K."

Actually in our paper there is not any conclusion that the observational data exclude the CMBR with the temperature predicted by the hot universe. We wrote in our paper: "Measurements reported in [14] [Ohm 1961] at a frequency v = 2.4 • 109 cps give a temperature 2.3 ±0.2 K, which coincides with theoretically computed atmospheric noise (2.4 K). Additional measurements in this region (preferably on an artificial Earth satellite) will assist in final solution of the problem of the correctness of the Gamow theory."

Thus we encouraged observers to perform the corresponding measurements! We did not discuss in our paper the interpretation of the value 2.4K obtained by Ohm with help of a technology developed specially for measuring the atmospheric temperature (see discussion in Penzias 1979a).

Below I will tell more about our discussions with some radio astronomers in the USSR.

At that time the possibilities for communications with our foreign colleagues were very restricted. I learned about the discovery of the CMBR radiation at a conference in London in the summer of 1965. When I was back in Moscow I informed Professor Ya. Zel'dovich.

At the first moment when I told Ya. Zel'dovich about the discovery he obviously did not remember the details of my paper with Doroshkevich and started to scold us that we had not included in our paper the figure with the predicted spectrum of the CMBR. When I immediately showed him the corresponding figure in the reprint of our paper he started to scold us for the absence of the effective propaganda of our paper.

It was clear that this discovery means the strict proof of the hot universe. This discovery was widely discussed among Soviet physicists and astronomers. Zel'dovich abandoned his hypothesis of the cold universe and became an ardent proponent of the theory of the hot universe. In his letter to Professor Dicke he wrote on September 15, 1965 (unpublished and kindly provided to me by J. Peebles): "I am not more so cock-sure in my cold universe hypothesis: It was based on the assumption that the initial helium content is much smaller than 35% by weight. Now I understand better the difficulty of helium determination."

Zel'dovich began active work on the hot model even before the discovery of the CMBR. V. M. Yakubov, a collaborator of Ya. Zel'dovich, repeated the earlier calculations of Hayashi (1950) of the process of the nucleosynthesis in the hot model. These calculations were much simpler and more transparent and based on new knowledge of the weak processes. These calculations were set forth in the paper Zel'dovich (1965). Thus the process of the big bang nucleosynthesis (BBNS) became known and understandable for us.

When I started to work on these notes (autumn, 2003) I asked Professor V. Slysh (Lebedev Physics Institute, Moscow) for his recollections of the events of that period in the group of Professor I. Shklovsky at the P. K. Shternberg State Astronomical Institute in Moscow. He told me the following. In 1965 just after learning about the discovery of the CMBR I. Shklovsky asked V. Slysh to find in the Institute Library papers, published around 1940, concerning the interstellar absorption lines in the spectrum of the light coming from the star ( Ophiuchi; the absorption was caused by the molecule CN. I. Shklovsky himself was a specialist in the physics of the interstellar medium and remembered the papers. V. Slysh had found the papers by Andrew McKellar (1940, 1941). In these papers McKellar concluded that these lines (in the visible part of the spectrum) could have arisen only if the light was absorbed by rotationally excited CN molecules. The rotation must be excited by radiation at a temperature about 2-3 K. In the paper McKellar (1940) wrote: "Effective temperature of the interstellar space ...< or = 2.7K." In another paper McKellar (1941) wrote that the "Rotational temperature of interstellar space is about 2 K."

On the basis of this information I. Shklovsky wrote and published a paper Shklovsky (1966) where he declared that this temperature was the temperature of the whole universe rather than the temperature of only the interstellar medium as it had been declared by McKellar.

We are not yet through the chain of missed opportunities that plagued the discovery of the relic radiation.

Let us return to the question about the technical feasibility of detecting the cosmic microwave background radiation (CMBR). At what time did this become possible? Weinberg (1977) writes: "It is difficult to be precise about this, but my experimental colleagues tell me that the observation could have been made long before 1965, probably in mid-1950s and perhaps even in mid-1940s." Is this correct?

In the autumn of 1983, Dr. T. Shmaonov of the Institute of General Physics, Moscow, with whom I was not previously acquainted, telephoned me and said that he would like to talk to me about things relevant to the discovery of the CMBR. We met the same day and Shmaonov described how, in the middle of the 1950s, he had been doing postgraduate research in the group of the well-known Soviet radio astronomers S. Khaikin and N. Kaidanovsky: he was measuring radio waves coming from space at a wavelength of 3.2 cm. Measurements were done with a horn antenna similar to that used many years later by Penzias and Wilson. Shmaonov carefully studied possible sources of noise. Of course, his instrument could not have been as sensitive as those with which the American astronomers worked in the 1960s. Results obtained by Shmaonov were reported in 1957 in his PhD Thesis and published in a paper (Shmaonov 1957) in the Soviet journal Pribory i Tekhnika Eksperimenta (Instruments and Experimental Methods). The conclusion of the measurements was: "The absolute effective temperature of radiation background ... appears to be 4 ± 3K." Shmaonov emphasized the independence of the intensity of radiation on direction and time. Errors in Shmaonov's measurements were high and his 4-K estimate was absolutely unreliable, but nevertheless we now realize that what he recorded was nothing other than the CMBR. Unfortunately, neither Shmaonov himself, nor his science advisors, nor other astronomers who saw the results of his measurements knew anything about the possibility of the existence of the relic radiation and so failed to pay the results the attention that they deserved. They were soon forgotten. When Doroshkevich and I, having completed our calculations, were calling in 1963 and 1964 on several Soviet radio astronomers with the question "Do you know any measurements of the cosmic background in the centimeter and shorter wavelength ranges?" not one of them remembered Shmaonov's work.

It is rather amusing that even the person who made these measurements failed to appreciate their significance, not only in the 1950s - this is easy to explain - but even after the discovery of the microwave background by Penzias and Wilson in 1965. True, at that time Shmaonov was working in very different field. His attention turned to his old results only in 1983, in response to semiaccidental remarks, and Shmaonov gave a talk on the subject at the Bureau of the Section of General Physics and Astronomy of the USSR Academy of Sciences. This event took place 27 years after the measurements and 18 years after the publication of the results of Penzias and Wilson (1965a).

Shmaonov's observations were reanalyzed by N. Kaidanovsky and Yu. Pariiski (1987). Their conclusions were: "Thus, one can conclude that the contribution of the relic radiation [CMBR] was Trei = 2 ± 1K. Of course the result is rough but unambiguous." In autumn 2003 I discussed the situation with both N. Kaidanovsky and Yu. Pariiski. They confirmed this conclusion. Kaidanovsky emphasized that at the time of the measurements (1955-1956) the reality of Shmaonov's results was out of any doubt, but unfortunately there was not any theorist who could tell them the possible interpretation.

Fate takes unexpected and tortuous turns. Nevertheless, the entire story is very instructive. To hit upon a phenomenon is not yet equivalent to discovering it. One has to realize the significance of the find and give the correct explanation. A combination of circumstances and sheer luck does play a role here - no doubt about it. Nevertheless, success does not come by accident. Success requires lots and lots of work, vast knowledge, and persistence in the work itself and in bringing the results to the attention and recognition of others.

In conclusion I want to say that just after the discovery of the CMBR we in the group of Professor Zel'dovich started to work on the theory of the origin of galaxies and large-scale structure of the universe (see for example Doroshkevich, Zel'dovich and Novikov 1967) and on other physical processes in the hot model. Analogous works started in the groups of Professor V. Ginzburg, Professor I. Shklovsky, and other groups in the USSR.

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