In the brane-bulk framework, H−sector excitations (colour charge, glu- ons) propagate at an eective speed slightly dierent from the H+ photon speed, with fractional dierence δv/c ∼ (ℓP/Rbulk) 2 ∼10−122 at low energies undetectable. However, gravitational waves (metric perturbations) can take shortcuts through the bulk, travelling through the 5D AdS geometry rather than the 4D brane surface. This gives gravitational waves an eective speed slightly faster than c in vacuum. The delay between GW and electromagnetic signals from the same event (e. g. neutron star merger GW170817: delay < 1. 7 s over 40 Mpc) places an upper bound on the GW shortcut amplitude. The pre-QCD sump-free epoch imprints a spectral break in the primordial GW background at f ∼1/tQCD ∼100 mHz, within LISA's sensitivity band. Part of the One-Octonion Brane-Bulk Framework series. Anchor DOI: 10. 5281/zenodo. 19120873. Community: one-octonion-brane-bulk. Author: Bharathi Dasan Jagadeesan, M. D. , University of Minnesota. ORCID: 0000-0002-1143-941X.
Bharathi Jagadeesan (Fri,) studied this question.