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2003 Gruber Cosmology The Science

Professor Rashid A. Sunyaev has a fine record of influential work in physical cosmology and high-energy astrophysics, which covers a broad range and now extends over thirty-five years.

Modern "physical cosmology" began in 1965, with the discovery by Penzias and Wilson of the cosmic background radiation, and its interpretation as the afterglow of the universe's hot, dense beginning. Sunyaev was then a young protégé of the great Soviet cosmologist Y.B. Zeldovich.

In the late 1960s and early 1970s, a series of papers by Sunyaev and Zeldovich laid out, in the context of the "hot big bang" theory, many of the basic ideas regarding structure formation and the history of the intergalactic medium. Specially important were papers showing how the distortion of the blackbody spectrum sets a constraint on early heat input into the universe and on how incipient structures in the universe would manifest themselves by small fluctuations over the sky in the microwave background temperature.

The latter research paralleled what was being done concurrently in the West, by Peebles, Silk and others (and also by Japanese researchers). Its importance has come to full fruition only in the last decade, when temperature fluctuations have been detected with amplitudes consistent with what is expected if these fluctuations are the embryos for later structures.

A separate and very distinctive type of angular fluctuations in the microwave background, due to hot gas in clusters of galaxies at modest redshift, has become known as the Sunyaev-Zeldovich Effect and is of wide current interest, both as a probe of clusters and because it offers a way of measuring the scale of the universe.

In the early 1970s, Sunyaev's other main contributions were in high-energy astrophysics. It was at that time that the first evidence emerged for the actual existence of black holes, which would manifest themselves by the intense radiation emitted by gas swirling into these "accretion discs." His paper with Shakura on the structure of steady accretion discs became a classic, establishing many of the key phenomena that are still believed relevant to galactic nuclei and compact X-ray sources. He was particularly interested in the implications of X-ray astronomy, and in work with Basko and others he explored the distinctive spectroscopic signatures of reflection and scattering in accretion discs and binary stars. At the time, there were no X-ray telescopes with the combination of spectral resolution and sensitivity to utilize these results, but they are of great current interest, in the era of the Chandra and Newton-XMM satellites.

In the 1980s, Sunyaev shifted his interests towards observation and experimental projects in X-ray astronomy, and took up a senior position in the Space Research Institute. A particular success was the "Granot" International Orbiting Observatory. This project, for which Sunyaev deserved much organizational and scientific credit, led to the discovery of three black hole candidates (X-ray novae,) and the detection of quasiperiodic oscillations, and important work on the "superluminal" galactic source GRS 1915+105. Sunyaev has been particularly associated with work which helped to distinguish between compact objects involving black holes and those involving neutron stars. His work has also led to important and ingenious conclusions about the black hole in our Galactic Center and the changes in its activity over the last few thousand years.

In the last decade, Sunyaev has spent most of his time as a Scientific Member of the Max Planck Institut für Astrophysik in Munich. There he has resumed a more active theoretical program, but at the same time has maintained strong enough links with Moscow to have, despite great difficulty, salvaged some of the scientific excellence from the former Soviet space program.

Sunyaev is universally admired and respected for his wide-ranging achievements, his deep insights, and his excellent judgment. He is a dynamic personality, with a high profile on the international scientific scene. He has received substantial recognition, both in Russia and in the West - in the last few months, his list of awards was recently lengthened by the Dannie Heineman Prize, from the American Astronomical Society, and the Friedman Prize for Cosmology, from the Russian Academy of Sciences.

By Sir Martin Rees
Astronomer Royal and Royal Society Research Professor, Cambridge University