Vacuum Energy with Natural Bounds: A Spectral Approach without Fine-Tuning

We present an updated formulation of the Spectral Bounded Vacuum (SBV) model, in which the quantum vacuum is described as a dynamically bounded spec- trum with physically motivated infrared and ultraviolet limits, linked to the ther- mal and hadronic evolution of the Universe. In this framework, the present-epoch vacuum energy arises within a spectrally bounded description from a balance be- tween spectral contribution and time-dependent damping, reflecting the influence of gravitation, thermodynamic (entropic) processes, and hadronic interactions. The characteristic spectral scale is determined by the geometric mean of the infrared and ultraviolet boundaries, leading to a robust scaling law without fine-tuning. A nu- merical evaluation with standard parameters for the current epoch yields a vacuum energy density consistent with the observed cosmological constant. The model is extended with the Quantum Entropic Vacuum (QEV), in which the entropic con- tribution of the vacuum is explicitly included as a dynamic component. The QEV picture describes how fluctuations in quantum entropy contribute to the tem- poral evolution of the vacuum energy, offering an additional explanation for the stability of the cosmological constant at late times.