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Fig. 4.8. Illustrations of the origin of the quasiperiodic oscillations of the baryon and radiation distributions (from Sunyaev and Zel'dovich 1970c).

that time, at his "dacha" near Leningrad in the summer of 1967. Zel'dovich just loved to explain the results obtained within the framework of the general theory of relativity in a customary Newton language. At the same time all of us knew that the behavior of acoustic waves during the radiationdominated era of the universe was analyzed by Evgeny Lifshitz (1946) in his classic work. We knew that the universe was a unique object at the early stages of its expansion. The velocity of sound was close to the velocity of light, and therefore a Jeans wavelength was just a bit smaller than the horizon scale. In the very early universe observationally interesting wavelengths were larger than the Jeans wavelength. They became smaller than the Jeans length as the universe expanded. And at recombination the baryons decoupled from the radiation and the Jeans wavelength for the baryons sharply dropped.

In the very early universe only the growing mode of density perturbations survived, meaning all acoustic waves were launched with the same phase. The sharp decoupling cut off fluctuations of different wavelengths at different phases. This led to a wonderful prediction of the quasiperi-odic dependence of the perturbation amplitude after recombination on the length scale. This effect now is called the "baryonic acoustic oscillations." (The presence of dark matter complicates this picture, but not the physical essence.) This quasiperiodic behavior in the distribution of baryons and electrons and their velocities, naturally, had to be reflected in the quasiperiodic behavior of the angular distribution of CMBR temperature. (See Figure 2 in Zel'dovich, Rakhmatulina and Sunyaev 1972, where an attempt was made to show how to use a spherical harmonic expansion for the demonstration of acoustic peaks. This effect was discovered by the Boomerang, Maxima-1, and WMAP experiments.) In Doroshkevich, Zel'dovich and Sunyaev (1978) the Silk damping was taken into account.

YaB liked to get cozy and vet article manuscripts. Unfortunately, my typewritten drafts were very long and he was never able to read the whole text. With "a touch of a master," he would go through the first several pages and briefly look through the figures. However, he used to edit abstracts of the articles many times. I remember how upset I was when, in the abstract of our article (Sunyaev and Zel'dovich 1970c), he crossed out my words on the importance of observing the quasiperiodic scale-dependence of the amplitude of the CMBR angular fluctuations. He wrote that the effect was very small and could hardly be observed. To calm me down he said that the physics of the phenomena described in the paper was beautiful, and the article needed to be published. Another time, after a long conversation with A. D. Sakharov, who used to visit Zel'dovich quite often at the Institute of

Applied Mathematics, Zel'dovich told me that A. D. Sakharov was nervous and changing every day his opinion on an important, however far from scientific, issue, and that we needed to call the quasiperiodic dependence in our article "Sakharov oscillations." That would be a great joke! I tried to protest that I had never heard anything similar to our results from Sakharov, and that we would have to have some reference. YaB responded: "Let's refer to Sakharov's (1965) article on a cold universe, where every second paragraph starts with the words 'As Zel'dovich told me.'"

Sergei Shandarin told me once that if Zel'dovich ever thought that these baryonic oscillations and acoustic peaks would be observed and become of importance then he would never have made such jokes.

The situation around the name of Andrei Sakharov in the second half of 1969 and early in the 1970s continued to deteriorate. It was obvious that any joke about him was inappropriate. Zel'dovich did everything possible to keep the name of Sakharov alive in Soviet astrophysical literature, mentioning in particular again and again "Sakharov oscillations" in his papers and books. This completely changed the initial meaning of "Sakharov oscillations" as Zel'dovich had introduced them. Now after practically 40 years it is possible to write how and why this term originated.

Terms are functions of time, but acoustic peaks and baryonic oscillations will be present on the sky for the next billions of years.

Epilogue

In the four decades since the events I have described cosmology has made unbelievable progress: much of what seemed impossible then has been studied in detail and measured with a high degree of accuracy. WMAP spacecraft (after Boomerang and Maxima-1 balloon flights) made a most significant contribution to the revolution in observational CMB cosmology. I remember my last conversation with David Wilkinson at the Kennedy Space Flight Center on Cape Canaveral after the WMAP launch. David recalled an old question of mine: why MAP was going to be placed in a low orbit, while for the Relikt-2 project Igor Strukov and Dmitry Skulachev chose the great opportunity of launching to the second Lagrangian libration point (Strukov and Skulachev 1991). This possibility was reviewed in detail at the Space Research Institute in Moscow by Pavel Eliasberg and Ravil Nazirov's group. At that time David asked me to tell Alan Bunner at NASA's Headquarters about this plan. It was amazing that David managed to find time to thank me and (indirectly Strukov and Skulachev) for this advice, when WMAP was already on the way to the Lagrange point.

I regret that limits of time and space do not allow me to say much about Relikt-1, the first spacecraft to scan the sky to look for CMBR angular fluctuations. Everything including the detectors was designed and produced in the USSR. The first map of the microwave sky brightness was obtained by this experiment. Emission from the plane of the Galaxy was mapped in detail. The amplitude and direction of the CMBR dipole component were measured, and strict limits placed on CMBR angular fluctuations, before the launch of COBE (Klypin et al. 1987; Strukov et al. 1987; Skulachev and Skulachev 1988; and Klypin, Strukov and Skulachev 1992).

The Strukov and Skulachev group was very close to the great result later obtained by COBE/DMR (Smoot et al. 1992). All that was missing was the presence of at least one additional frequency channel and a modest factor in detector sensitivity - or mission lifetime. But this is the point of view of a person who was not knowledgeable in the nuances of the experiments.

It would be a pleasure to write about Semen Gershtein and Zel'dovich's (1966) idea on the upper limit for the muonic neutrino mass based on cosmological data; about the upper limit on the amount of neutrino species, obtained by Viktoriy Shvartsman (1969); on Andrei Sakharov's seminal articles and ideas including his thoughts about the nature of baryon asymmetry in the universe (Sakharov 1967); on Georgiy Zatsepin Vadim Kuz'min (1966) with their prediction of the high energy cutoff in the ultrahigh cosmic ray spectrum due to the presence of CMBR (Greisen-Zatsepin-Kuz'min limit); and about an influential article by Vitaly Ginzburg and Leonid Ozernoi (1966), where they for the first time raised the issue of mechanisms of heating and ionization of the intergalactic gas. It would be possible to write about many other things, but this is enough. It suffices to say that I remember that as a happy time when everything turned out well and every day gave us something new and unexpected.

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