An Entropic Interpretation of Friedmann Vacuum Pressure

The cosmological vacuum pressure appearing in the Friedmann acceleration equation is normally treated either as a phenomenological property of a dark-energy fluid or as a direct consequence of a cosmological constant. In this revised note, we show that invariant negative value of the cosmological equation of state parameter, w, follows naturally from an entropic cosmological model (ECM) in which spacetime dynamics are governed by the acceleration of an informational horizon. By correctly identifying the Friedmann acceleration with the second time derivative of a horizon scale, we obtain a dimensionally consistent and sign-correct constitutive relation. This relation yields negative pressure of the observed magnitude without introducing a fundamental vacuum energy density or modifying Einstein’s equations. The Friedmann equation using parameters derived from ECM, yields a constant value of P/ρ = w = −13/3; however, due to the time-inverse scaling of the input parameters, this does not indicate a pathological runaway acceleration. Rather, it reveals the natural state of dynamical spacetime having expanding horizons.