Elastic Bounce Cosmology (EBC): A Phenomenological Framework for Quasi-Cyclic Cosmic Expansion with a Dynamic Spatial Restoring Force

We present Elastic Bounce Cosmology (EBC), a phenomenological framework in which the cosmic scale factor is governed by modified Friedmann equations incorporating a dynamic, scale-factor-dependent effective cosmological term. The central hypothesis is that space possesses an intrinsic elastic property — a geometric restoring force that drives oscillatory expansion and contraction without singularity. The cosmological constant is replaced by a single dynamical term Λ_EBC(a) = Λ_0 (a*/a − a/a*), where a* is the equilibrium scale factor. A non-singular bounce at a_min > 0 follows directly from energy conservation of the resulting effective potential, with no separate bounce term required. A key analytic result follows directly from this approach: the effective dark-energy equation of state in the current observational regime (a ≪ a*) is w_EBC(a) = −1 + (1+x²) / [3(1−x²)], with x = a/a*, yielding w_0 ≈ −2/3 in the limit a_0 ≪ a* without tuning of model parameters. This matches the central value of the DESI 2024 BAO measurement w_0 = −0.70 ± 0.10 within current uncertainties; the DESI result is preliminary and has not yet been confirmed by independent datasets. The framework offers conceptual reinterpretations of several open problems in ΛCDM: dark energy as a geometric property of space, the lithium-7 problem via selective nuclear photodisintegration at the bounce, the horizon and flatness problems through bounce geometry without an inflaton field, and baryogenesis distributed across multiple cycles. A joint analysis of DESI Y1 BAO, Pantheon+ supernovae and the CMB acoustic scale θ* disfavors EBC against ΛCDM at Δχ² ≈ 67, with the deficit dominated by the supernova Hubble-diagram shape; the Hubble tension is not alleviated at the background level. The framework therefore presents a viable analytic alternative whose strongest near-term test is the jerk parameter j_0: EBC predicts j_0 ≈ 0.32 versus j_0 = 1 in ΛCDM, a Δj_0 ≈ 0.7 separation accessible to Rubin LSST and DESI Year 5. A covariant derivation from a modified Einstein–Hilbert action and a full CMB acoustic-peak treatment remain open for future work.Version 1.1 (April 2026): Added email address and ORCID iD to title page. No changes to scientific content.Version 1.2 (April 2026): Removed duplicate author line. No changes to scientific content.April 2026: A German translation (EBC_paper_v2_DE.pdf) has been added to this record.Version v4 (April 2026): Major revision. Paper version jumps from v2 to v4.0; intermediate versions v3 and v3.x were developed internally but not published. Substantial conceptual and analytical changes since v2: the phenomenological bounce term has been removed from the Friedmann equations (the bounce now follows directly from energy conservation of the elastic potential); the first physical bounce is now explicitly characterized as radiation-dominated, with the elastic mode as a background modification; corrected predictions for the jerk parameter (j_0 ≈ 0.32, was incorrect in v2), the CPL parameter w_a, and the deceleration parameter q_0; new Appendix B with the explicit step-by-step derivation of w_EBC; new joint-fit analysis combining DESI Y1 BAO, Pantheon+ supernovae and CMB θ* (Section 8.1, Fit D), with three new figures (jerk diagnostic, Pantheon+ Hubble residuals, DESI BAO residuals); reorganized Section 8 into "Well-Defined Tasks" (8.1) and "Open Theoretical Problems" (8.2); references unified to NASA/ADS-style format; Hubble tension explicitly noted as not resolved by EBC.