Question

How are theoretical physics, astrophysics, cosmology and string theory related to one another?

Answer 1

They all try to explain the Universe using current and developing theories in physics.

Explanation:

String theory is a quest to provide a Unified theory for physics that sees particles not as particles but vibrating strings. It is a developing field. Theoretical physics, astrophysics and cosmology use relativity and quantum mechanics to explain how the Universe was formed and it's current behavior.