The Entropic Genesis of Cosmic Acceleration: Quantum-Gravitational Entanglement Entropy as the Dynamical Origin of Dark Energy

We present a theoretical framework in which the observed late-time cosmic acceleration emerges not from a cosmological constant or a dynamical scalar field, but from the quantum-gravitational entanglement entropy between the causal patches of the observable universe. Formulating gravity within a quantum information-theoretic paradigm, we derive the fundamental entanglement entropy of the spacetime geometry from first principles, linking it to the Bekenstein-Hawking entropy via the holographic principle applied to the cosmic horizon. This entropy is not static but evolves with the causal structure of the universe. Its associated entanglement energy, obtained via the Clausius relation dE=T~dS applied to the future cosmic event horizon, manifests as a negative pressure component in the cosmological stress-energy tensor. We demonstrate that the equation of state parameter w of this entropic component is inherently dynamic, evolving from w \approx -1 at high redshifts to a slightly less negative value at present (z \approx 0), providing a precise fit to current observational data (Pantheon+SHOES, DESI, Planck) while naturally resolving the Hubble tension. The model predicts a unique, scale-dependent signature in the clustering of matter on very large scales (k < 0.01~hMpc^{-1}) and a specific pattern of deviations in the cosmic microwave background (CMB) temperature-polarization correlation spectrum at low multipoles (l < 30), which are testable with upcoming surveys (Euclid, Roman, CMB-S4). This work establishes a direct, testable link between quantum information, gravitation, and cosmology without invoking new fundamental fields or modifying general relativity on small scales.