Rubin studied for her Ph.D. at Georgetown University, the only university in Washington, DC that offered a graduate degree in astronomy. She was 23 years old and pregnant when she began her doctoral studies, and the Rubins had one young child at home. She began to take classes with Francis Heyden, who recommended her to George Gamow, her eventual doctoral advisor. Her dissertation, completed in 1954, concluded that galaxies clumped together, rather than being randomly distributed through the universe, a controversial idea not pursued by others for two decades. Throughout her graduate studies, she encountered discouraging sexism. In one incident she was not allowed to meet with her advisor in his office, because women were not allowed in that area of the Catholic university.