What is Raj Koothrappali's scientific profession?

Raj Koothrappali's scientific profession is astrophysicist.

Raj works in the physics department at Caltech, where his research focuses on astrophysics and related space science. His career places him alongside Sheldon, Leonard, and Howard, but his specialty is the study of astronomical objects and phenomena rather than engineering or laboratory biology.

The series gives Raj several notable research storylines. He studies trans-Neptunian objects, becomes associated with a planetary discovery nicknamed “Planet Bollywood,” and later identifies a comet with Penny's help. He also develops an interest in astroparticle physics, linking astronomical observations with particle physics.

Viewers sometimes confuse Raj with a general physicist because he works in Caltech's physics department. “Astrophysicist” is the more precise answer: it describes a physicist whose work centers on stars, planets, galaxies, cosmic objects, and the larger universe.