Hybrid 5D Gravity (H5DG): A Step Towards Unification

Abstract

Hybrid 5D Gravity (H5DG) is a fourteen-paper theory of gravity and gravitational unification that investigates whether a deliberately constrained five-dimensional braneworld can connect relativistic gravity, galactic dynamics, gravitational lensing, a collisionless dark sector, precision-gravity tests, late-time cosmology, and a conservative embedding of the Standard Model.The theory begins with minimal five-dimensional Einstein gravity on a capped, de Sitter-sliced AdS5 cavity containing a single four-dimensional observer brane. The boundary-value problem supports a normalisable exactly massless graviton together with a gapped continuum of massive tensor modes, recovering four-dimensional gravity with calculable higher-dimensional corrections. The minimal geometry leaves one datum undetermined: the Einstein Boundary Modulus, equivalently the brane location, boundary tension, or induced de Sitter curvature. Companion fluctuation and modulus analyses derive the gauge-invariant scalar problem and exact curvature-position relation. A later nonperturbative solution finds a unique ultra-light sub-threshold scalar root, while the absolute canonical normalisation required to establish its final physical radion coupling remains open. The galactic sector derives an effective AQUAL response whose warped projection preserves the deep-Milgromian power. The observable normalisation a0(z) = cH(z)/(2π) remains an independent falsifiable conjecture. Bullet-Cluster and galaxy-galaxy weak-lensing analyses select a fully-lensing infrared response and exclude a scale-uniform CDM-like compact-object population across galaxies and clusters. The preferred microscopic infrared carrier is a five-dimensional Maxwell gauge field with a brane-localised unit-timelike constraint. Its zero mode reduces to a constrained Einstein-aether sector and structurally fixes cT = c at tree level. Spin-0 and coupled metric-aether analyses establish the local eikonal stability condition 0 < (xμ)' < 1, selecting n ≤ 1 within the standard interpolation family. The gravitational-slip calculation gives ηIR = 1 at leading weak-field order, while the preferred-frame analysis derives α1 approximately equal to -1.4 × 10^-7 at 1 AU and a finite longitudinal regulator relevant to α2. A dedicated Solar-Galactic external-field calculation subsequently shows that the original local universal AQUAL truncation predicts Q2 approximately equal to 4.20 × 10^-26 s^-2 and is excluded by the updated Cassini/DE440 constraint. The resulting no-go theorem motivates a minimal Helmholtz-filtered vector completion. At the geometric benchmark the filtered theory gives Q2 approximately equal to 7.7 × 10^-30 s^-2, comfortably below the Cassini bound, while preserving the galactic AQUAL phenomenology and leading weak-field lensing response. The dark sector consists of initially cold exotic compact objects whose merger recoil preferentially depletes galactic halos while preserving a dominant collisionless population in clusters. The benchmark requires approximately 88 percent galactic depletion, substantial conversion by redshift z approximately equal to 1, and conditional assumptions concerning ECO formation, primordial pairing, merger recoil, and energy transfer. Its two-fluid cosmological implementation shifts BAO distances by approximately +0.29 percent to +0.60 percent in the direction of the DESI DR2 low-Omega-m preference, while exposing a roughly 4 percent to 6 percent CMB-lensing-potential deficit and significant conversion during the Lyman-alpha forest epoch. The fourteenth paper extends H5DG conservatively beyond the purely gravitational sector by placing the empirically established Standard Model on the observer boundary without introducing new Standard-Model-charged particles, gauge groups, or phenomenological BSM matter fields. The resulting construction is a boundary effective field theory in which the observed Standard Model is coupled to the induced H5DG geometry, while the genuinely beyond-Standard-Model content remains gravitational and higher-dimensional.