The Structural Natal Index (SNI) and Evidence for Galactic Demographic Decline: Implications for Cosmic Dynamics

Standard cosmology explains redshift primarily through metric expansion but neglects the demographic evolution of galaxies. Here we introduce the Structural Natal Index (SNI), a dimensionless order parameter that quantifies the balance between galaxy formation and quenching across cosmic time. Using JWST deep fields, SDSS DR17, and IllustrisTNG simulations, we measure SNI(z = 0) = 0.42 ± 0.18 and SNI(z = 2) = 1.6 ± 0.3, revealing a critical transition from a fertile universe to a quiescent regime. Within the Dead Universe Theory (DUT), this behavior arises from the asymmetric thermodynamic retraction of the spacetime substrate, reinterpreting expansion as a dynamical response to structural senescence. Spacetime is treated as a non-radiative, entropy-saturated substrate that exhausts the energy available for structure formation, rendering dark energy unnecessary. Authorship notes. The Structural Natal Index (SNI) is introduced in this manuscript by Joel Almeida (computational cosmologist and software architect, Ph.D.). Code for SNI estimation and galaxy-demographic tables is made available to the scientific community (see "Code and Data"). Joel Almeida also created the FOSSIL method for galaxy death analysis, also available in the ExtractoDAO Labs repository on GitHub.