What branch of physics is Sheldon Cooper's specialty?

Sheldon Cooper’s specialty is theoretical physics.

In The Big Bang Theory, Sheldon works as a theoretical physicist at Caltech. His research is concerned with mathematical models and fundamental questions about how the universe works, rather than primarily with building laboratory apparatus or conducting hands-on experiments. The show uses this specialization as a major part of his identity and his often-competitive relationship with Leonard and other scientists.

Sheldon’s interests extend beyond his formal specialty. He frequently discusses cosmology, string theory, quantum mechanics and related areas, and he displays broad knowledge of many sciences. That range can make “physics” seem like the only necessary answer, but the more precise canonical description is theoretical physics.

The distractors describe different fields or approaches. Optical physics focuses on light and its interactions, geophysics applies physics to Earth, and experimental physics emphasizes observation and laboratory testing. Sheldon’s defining professional label is therefore theoretical physicist, not experimental physicist.