Cosmic Elastic Theory: Cosmic Acceleration and General Relativity as Local Phase-Transition Phenomena in a Causal Thermodynamic Spacetime

Abstract This work develops a thermodynamic interpretation of spacetime in which energy propagation is governed by elastic modes regulated by local density. By integrating heterogeneous cosmological datasets—spanning Pantheon+ supernovae, DES weak-lensing, and JWST early-universe observations as illustrative examples—we demonstrate that the residual and variance fields follow a common elastic hierarchy. In this picture, spacetime behaves as a continuous medium characterized by modes of tension that determine its capacity to store, propagate, and dissipate energy. At low density, the effective gravitational coupling is suppressed, and energy is freely converted into local tension, producing measurable redshift elongation. At high density, the elastic tension saturates, and the system enters a stable regime in which dissipation is confined to microscopic channels and General Relativity is recovered. This equilibrium regime corresponds to a state of isotension—a condition in which local tension gradients vanish and the elastic medium redistributes energy through a process analogous to osmotic balance. In this state, curvature does not disappear but becomes stationary: the system stores energy elastically while maintaining constant mean tension, defining the thermodynamic ground state of spacetime. Between these limits lies a transition zone, internally structured into (i) a weak-coupling subregime, where the coupling remains residual and variance increases with density gradients, and (ii) a critical threshold at Kcrit = 1.39, where the coexistence of dissipation and coupling generates a peak of instability analogous to a second-order phase transition. The empirical consistency of these behaviors across independent probes suggests that cosmic acceleration and weak-gravity anomalies share a common thermodynamic origin: a dissipative relaxation of the elastic spacetime medium toward equilibrium. The coexistence of two dynamical regimes—one dominated by dissipation and another by elastic coupling—has analogues in previous frameworks such as Timescape cosmology. However, while Timescape emphasized temporal variance between regions, the present analysis anchors the duality in the elastic constitution of spacetime itself. When tension modes are coupled to density, the observed cosmological anomalies arise as natural thermodynamic responses of a medium whose capacity to store and release energy decreases as density declines. In this view, redshift acquires a dual character: a kinematic term associated with expansion, and a stretching term generated by cumulative dissipation along photon trajectories. What is commonly interpreted as cosmic acceleration is thus redefined as the thermodynamic trace of dissipative redshift, not a change in the dynamical scale factor. At high density, the elastic medium reaches saturation and stabilizes through geometric buoyancy, recovering the predictions of General Relativity as the equilibrium limit of the system. Gravity therefore emerges as a manifestation of curvature elasticity: an effective restoring force acting within a medium that resists unbounded stretching. Although phenomenological in formulation, this framework produces testable predictions that show quantitative consistency across independent observations—including supernovae (Pantheon+), weak-lensing variance (DES), high-z galaxies (JWST), and wide stellar binaries. In this sense, spacetime is treated not as a passive background but as an active thermodynamic field that dissipates, stores, and equilibrates energy through geometric friction. The term geometric friction denotes the regulatory channel that governs the exchange between elastic dissipation and curvature coupling. It is not the source of entropy production but its moderator: when the coupling is absent (K = 0), dissipation proceeds freely through spontaneous relaxation; as the coupling strengthens (0sat)geometric friction rises, competing with the dissipative flow and producing the instability characteristic of the transition regime; once saturation is reached (K →Ksat), friction balances dissipation, restoring isotension equilibrium and reproducing the idealized limit of General Relativity. This competition defines the entropic rhythm of spacetime—the dynamic rate at which tension, curvature, and entropy coevolve toward equilibrium.