A Review of f(R) Gravity: From Variational Principles to the Starobinsky Inflationary Model
Author: Abolfazl Mohammadi Yaychi
Department: Department of Physics, K. N. Toosi University of Technology, Tehran, Iran
Date: August 2026
This paper provides a systematic review of f(R) gravity, covering the variational principle, Einstein-Hilbert action, modified field equations, and the Starobinsky inflationary model. The paper is intended as a self-contained pedagogical guide for readers interested in the transition from General Relativity to modified gravity theories.