Black hole thermodynamics from Iyer–Wald formalism with variable Newton’s constant

Recent advances in black hole thermodynamics have intensified efforts to investigate its thermodynamic framework and analogues to that of classical thermal systems. Departing from the extended phase space approach, which involves a variable cosmological constant, the restricted phase space formalism focusing on the variations of Newton’s gravitational constant in black hole thermodynamics is proposed. This approach enables the construction of a self-consistent thermodynamic structure that features both the first law and the Euler relation. In this paper, we demonstrate that for any diffeomorphism-invariant gravitational theory, the Iyer–Wald formalism is applicable to restricted phase space thermodynamics and construct a self-consistent extensive black hole thermodynamic system. Our work thereby establishes the Iyer–Wald approach as the geometric foundation for black hole thermodynamics with variable Newton’s constant.