A Near-Coasting Cosmology from 5D Brane Dynamics

his paper presents a consistent theoretical framework for a late-time "near-coasting" cosmic expansion, where the product of the Hubble parameter and cosmic age is approximately one (Ht≈1). While certain models have previously proposed such an expansion, they have often lacked a clear dynamical origin, instead imposing the coasting law as an ansatz. This work places the concept on a firm physical foundation using 5D brane-world gravity. We work within a 5D, Z2-symmetric, codimension-1 setup featuring a single Friedmann-Lemaître-Robertson-Walker (FRW) brane. The effective Friedmann equation on the brane is derived from the standard Israel junction conditions. The resulting equation contains the expected high-energy correction proportional to the square of the energy density (∝ρ2), a "dark radiation" term from the bulk Weyl tensor (∝a−4), and a spatial curvature term (−K/a2). We demonstrate that an approximate coasting regime arises naturally whenever the a−2 term is non-negligible at late times. This occurs most naturally in an open universe with mild negative spatial curvature (K<0), which drives the effective equation of state towards weff≈−1/3. Crucially, this mechanism avoids the null-brane pathology that affected earlier proposals of "light-speed expansion". The paper concludes by laying out a detailed and realistic roadmap for confronting the model with modern cosmological data, outlining the observational requirements and constraints from Big Bang Nucleosynthesis (BBN), Baryon Acoustic Oscillations (BAO), and the Cosmic Microwave Background (CMB)