GR-Razor Stress Tests, Pillar 2: Inhomogeneous Backreaction as a GR-Internal Resolution to the JWST Early Galaxy Excess

Pillar 2 of the GR-Razor Stress Tests series applies the series' structured five-axis guillotine framework to inhomogeneous backreaction mechanisms as candidate GR-internal resolutions to the JWST early galaxy excess: the observed overabundance of massive, mature galaxies at redshifts z > 10-12 relative to standard ΛCDM predictions by factors of 10-100. Three sub-mechanisms are evaluated: Buchert scalar averaging as primary, and LTB void models and Szekeres spacetimes as supporting candidates. All three operate within pure general relativity without new fields, modified gravity, or free parameters, satisfying both intake metrics of the series.

All three fail on amplitude grounds. Buchert's kinematic backreaction term Q is constrained by observation and simulation to at most a few percent correction to the effective Friedmann equation at z > 10, two to three orders of magnitude below the required growth enhancement. LTB and Szekeres exact models avoid the averaging scheme problem but remain amplitude-limited by CMB-consistent initial conditions. The averaging scheme non-covariance of Buchert, unresolved for two decades, adds a structural vulnerability absent from exact models. CMB acoustic peak constraints rule out the initial conditions that would drive sufficient backreaction at high redshift.

This is the second consecutive Pillar to fail decisively on amplitude grounds, establishing a pattern: GR-internal mechanisms operating at the linear perturbation or averaging level cannot bridge the JWST gap within observationally constrained parameter space. The methodology, intake criteria, and series context are documented in the Foundation Document (Pillar 1, same Zenodo series).