Volume as an index of a subalgebra

We propose a new way to understand the volume of certain subregions in the bulk of AdS spacetime by relating it to an algebraic quantity known as the index of inclusion. This index heuristically measures the relative size of a subalgebra $\mathcal{N}$ embedded within a larger algebra $\mathcal{M}$. According to subregion-subalgebra duality, bulk subregions are described by von Neumann algebras on the boundary. When a causally complete bulk subregion corresponds to the relative commutant $\mathcal{N}' \cap \mathcal{M}$ -- the set of operators in $\mathcal{M}$ that commute with $\mathcal{N}$ -- of boundary subalgebras, we propose that the exponential of the volume of the maximal volume slice of the subregion equals the index of inclusion. This ``volume-index'' relation provides a new boundary explanation for the growth of interior volume in black holes, reframing it as a change in the relative size of operator algebras. It offers a complementary perspective on complexity growth from the Heisenberg picture, and has a variety of other applications, including quantifying the relative size of algebras dual to the entanglement wedge and the causal wedge of a boundary region, as well as quantifying the violation of additivity of operator algebras in the large $N$ limit. Finally, it may offer insights into the volume growth of de Sitter space through the changes in North and South pole observer algebras in time.