Emergent Expansion Cosmology (EEC)

A Thermodynamic Closure for the Backreaction Problem and its Background-Level Observational ConfrontationIsmail Khan Independent Researcher,ismailkhan.mcs@gmail.comDOI: 10.5281/zenodo.19830980ORCID: 0009-0005-7485-8880.Preprint - Version 6 April 2026Thermodynamically motivated closure model for the Buchert backreaction equations of general relativity, where late-time cosmic acceleration emerges from the non-equilibrium free energy of the matter distribution at a coarse-grained nonlinear scale.Backreaction is linked to the variance of density fluctuations generated during gravitational collapse and structure formation, producing an effective contribution proportional to σ⁴(Rs). A density-dependent stiffness function inspired by halo-collapse statistics and non-equilibrium thermodynamics naturally yields an effective equation of state w = −1 at a characteristic density scale, without requiring a cosmological constant or additional propagating fields.The non-equilibrium free-energy density is written as:fneq = (κ₀/2)(ρm/ρc) exp(−ρm/ρc) ξ²where ξ = σ²(Rs,t).The corresponding stiffness function is:κ(ρm) = κ₀(ρm/ρc) exp(−ρm/ρc)The framework preserves the background Bianchi identity and satisfies the second law of thermodynamics during active structure formation. At linear perturbative order, no extra dynamical degrees of freedom arise; instead, the model predicts a modified Poisson coupling μ(z) with vanishing gravitational slip (Φ = Ψ).Fixing the characteristic density scale through the observed peak halo-formation epoch reduces the theory to three free cosmological parameters (ωm, H0, σ8), matching flat ΛCDM in parameter count.Joint fits to Pantheon+ supernovae, BOSS DR12 BAO and growth-rate data, DESI DR1 BAO, and Planck 2018 compressed matter-density likelihood give:H0 = 69.40 ± 0.37 km s⁻¹ Mpc⁻¹ S8 = 0.753 ± 0.036 Ωm = 0.2992 ± 0.0035 Ωneq = 0.7007 ± 0.0035The predicted S8 is consistent with weak-lensing surveys and significantly reduces the standard ΛCDM growth tension. The inferred Hubble constant lies between the Planck and SH0ES values.A remaining challenge is the CMB acoustic scale θ* = 0.01024, which differs from the Planck measurement 0.010409 ± 0.000031 by approximately 3.2σ, motivating a full Boltzmann-code CMB analysis.The full numerical codebase is publicly available at:https://github.com/ismailkhanmcs/EEC_NotebookNote: This preprint by Ismail Khan is canonically archived on Zenodo. This deposit exists solely for visibility. Please direct all citations to the original Zenodo record: https://doi.org/10.5281/zenodo.19830980Keywords: Ismail Khan, EEC cosmology, transient acceleration, Buchert backreaction, non-equilibrium thermodynamics, modified Friedmann equations, structure formation, modified gravity, gravitational slip, Press-Schechter, dark energy