In early 1960s, Zeldovich started working in astrophysics and physical cosmology. In 1964, he and independently Edwin Salpeter were the first to suggest that accretion discs around massive black holes are responsible for the huge amounts of energy radiated by quasars. From 1965, he was a professor at the Department of Physics of the Moscow State University and a head of the division of Relativistic Astrophysics at the Sternberg Astronomical Institute. In 1966, he and Igor Novikov were the first to propose searching for black hole candidates among binary systems in which one star is optically bright and X-ray dark and the other optically dark but X-ray bright (the black hole candidate).