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

Hybrid 5D Gravity (H5DG) is a twelve-paper theory of gravity that addresses whether a deliberately constrained five-dimensional braneworld can connect relativistic gravity, galactic dynamics, gravitational lensing, a collisionless dark sector, precision-gravity tests, and late-time cosmology. 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 resulting 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 radion problem, obtain a controlled leading-order ultra-light radion estimate, invert the exact curvature-position relation, and show that the Randall-Sundrum dimensional-reduction limit is exponentially accurate. They do not, however, dynamically select the observed cosmological curvature. The galactic sector derives an effective AQUAL equation whose physically normalised warped projection preserves the deep-Milgromian power while suppressing canonical contamination. The projection does not derive the observable acceleration scale: a0 (z) = cH (z) / (2π) is retained as an independent, falsifiable normalisation conjecture and redshift test. Bullet-Cluster and galaxy-galaxy weak-lensing analyses select a fully-lensing infrared response and show that a scale-uniform CDM-like compact-object population is not viable across galaxies and clusters. The preferred microscopic infrared carrier is therefore developed as 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 the tree-level tensor propagation speed to cT = c. The spin-0 and coupled metric-aether papers derive a finite scalar kinetic normalisation and establish the local eikonal stability condition 0 < (xμ) ' < 1, selecting n ≤ 1 within the standard interpolation family. A gravitational-slip companion derives ηIR = 1 at leading weak-field order and an isolated post-linear correction of relative order v²/c². A preferred-frame analysis derives the locally screened result α1 approximately equal to -1. 4 × 10^-7 at 1 AU and a finite longitudinal regulator relevant to α2, while leaving the complete global moving-source response conditional. 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 construction 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 cosmological propagation is implemented as a two-fluid Boltzmann system. With no additional parameter fitted to cosmological data, the adopted benchmark 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 quantified tensions: a roughly 4 percent to 6 percent deficit in the CMB lensing potential and extensive conversion during the Lyman-alpha forest epoch. H5DG is intentionally presented as an incomplete and falsifiable research framework rather than a closed theory. The exact radion spectrum and one-loop modulus potential, microscopic derivation of the acceleration normalisation, complete coupled gradient matrix, full post-linear and bulk-matched slip, global preferred-frame solution, numerical alpha-2 response, strong-field pulsar parameters, non-eikonal and uniformly deep-Milgromian behaviour, vector cosmological perturbations, radiative matching, and a full cosmological likelihood analysis remain open. These limitations and current observational tensions are stated explicitly rather than absorbed through additional adjustable parameters.