Emergent Cosmic Acceleration from an Effective Viscoelastic Medium (EVM)

This work presents a cosmological framework in which the late-time accelerated expansion of the Universe emerges from an effective viscoelastic response of matter, rather than from a fundamental cosmological constant. In the Effective Viscoelastic Medium (EVM) framework, gravitational interactions propagate at the speed of light, while the large-scale redistribution of matter occurs over a finite dynamical timescale. This separation leads to an effective gravitational memory characterized by a relaxation timescale of the form τ = κ/H, where κ is a dimensionless constant and H is the Hubble parameter. The resulting dynamics generate an additional effective contribution to the expansion history, producing an emergent accelerated expansion without introducing dark energy. A closed-form analytical solution for the expansion rate is derived, explicitly separating the instantaneous matter contribution from the delayed-response component. The model is tested against observational data using cosmic chronometer H(z) measurements and Type Ia supernova datasets. A standard χ² likelihood analysis shows that the EVM framework provides a fit comparable to, and slightly improved over, the standard ΛCDM model, with reduced chi-square values χ²ν ≈ 0.542 (EVM) and χ²ν ≈ 0.697 (ΛCDM). The residual distribution shows no significant systematic bias. A key implication of the framework is the natural emergence of a characteristic acceleration scale a₀ ~ cH₀, providing a direct physical connection between cosmological expansion and galactic dynamics. This suggests that cosmic acceleration may be understood as an emergent phenomenon arising from the finite dynamical response of matter in an expanding Universe. This work establishes a minimal and predictive alternative to ΛCDM, unifying cosmological expansion and galactic dynamics within a single physical framework. Future work will extend the model to joint analyses including cosmic microwave background anisotropies, large-scale structure, and gravitational lensing.