Soviet astrophysicist, originally a specialist in nuclear physics. His cosmological theories led to more accurate determinations of the abundance of helium in older stars.
Zel'dovich was born in Minsk, studied at Leningrad (now St Petersburg), and in 1931 began work at the Soviet Academy of Sciences. During World War II he contributed research towards the war effort. He later worked at the Institute of Cosmic Research at the Space Research Institute of the Soviet Academy of Sciences in Moscow.
During the 1930s, Zel'dovich was involved in a research programme aimed at discovering the mechanism of oxidation of nitrogen during an explosion. He also wrote about the chemical reactions of explosions, the subsequent generation of shockwaves, and the related subjects of gas dynamics and flame propagation.
Zel'dovich participated in early work on the mechanism of fission during the radioactive decay of uranium. In the 1950s he developed an interest in cosmology and in such subjects as quark annihilation and neutrino detection. In 1967 he proposed that in its initial stages the universe was uniform in all directions, but that as it has expanded, this isotropy has diminished.