In 1963, he returned to academia to embark on pioneering contributions on the fundamental understanding of the thermodynamics of black holes and expanding the scope of physical cosmology.

They resided there until August 1941, when the family was evacuated together with the faculty of the Institute of Chemical Physics to Kazan to avoid the Axis Invasion of the Soviet Union.

[6]: 301  The centrality of his thesis focused towards the research on the Freundlich (or classical) adsorption isotherm, and Zeldovich discovered the theoretical foundation of this empirical observation.

[11]: 198–199  Soon after the discovery of nuclear fission (by German chemist Otto Hahn in 1939) Russian physicists had begun investigating the scope of nuclear-fission physics, and undertook seminars on that topic; Igor Kurchatov and Yulii Khariton were engaged in 1940.

[6]: 81  The work of Khariton and Zeldovich was extended into theories of ignition, combustion and detonation; these accounted for features which had not previously been correctly predicted, observed, nor explained.

[6]: 87–88  Zeldovich joined Igor Kurchatov's small team at this secretive laboratory in Moscow to launch the work on the nuclear combustion theory, and became a head of the theoretical department at the Arzamas-16 in 1946.

[6]: 89–90 In 1949, Zeldovich led a team of physicists that conducted the first nuclear test, the RDS-1, based roughly on the American design obtained through the atomic spies in the United States, though he continued his fundamental work on explosive theory.

[6]: 89–90  Zeldovich then began working on modernizing the successive designs of the nuclear weapon and initially conceived the idea of hydrogen bomb to Andrei Sakharov and others.

[citation needed] Between 1950 and 1953, Zeldovich performed calculations necessary for the feasibility of the hydrogen bomb that were verified by Andrei Sakharov, although the two groups worked in parallel on the development of the thermonuclear fusion.

Together with Semyon Gershtein he noticed the analogy between the weak and electromagnetic interactions, and in 1960, he predicted the muon catalysis (more precisely, the muon-catalysed dt-fusion) phenomenon.

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.

[citation needed] Zeldovich had another son with Nina Nikolaevna Agapova in 1958, whose name was Leonid Yakovlevich Agapov; he died in 2016 at the age of 58.