Why the Vacuum Pressure Does Not Enter Interstellar Phase Balance: A Gradient and Curvature Accounting

The surviving vacuum energy density u_L = 5.25e-09 erg per cubic centimetre, expressed as a pressure over Boltzmann's constant, is 3.80e07 K per cubic centimetre — four orders of magnitude above the P/k of roughly 3,000 K per cubic centimetre at which the interstellar medium's cold and warm phases balance. This coincidence of units invites a claim one meets occasionally: that vacuum pressure must somehow enter, shift, or even set interstellar phase equilibrium. This paper shows by explicit arithmetic that it cannot, and quantifies the two channels through which a uniform w = -1 component could in principle touch a cloud. The pressure-gradient channel is identically zero, because the component is uniform. The general-relativistic channel, through the active gravitational density rho + 3P/c^2 = -2 rho_L, produces an outward acceleration of 2.02e-16 cm per square second at the edge of a 20 pc cloud — a fraction 5.79e-08 of that cloud's self-gravity (3.48e-09 in the same units), 1.4e-03 of the thermal pressure-gradient acceleration, and 5.7e-07 of the Galactic tidal acceleration at the same radius. Over a full free-fall time of 4.68 million years the vacuum acceleration displaces the cloud's edge by 7.14e-07 pc. The vacuum's own gravitational timescale is 50.7 billion years. Phase equilibrium is set by heating and cooling microphysics, as the standard theory holds; the vacuum sector's enormous pressure is a property of the spacetime average, not a force available to local gas. Computed from stated assumptions; not a measurement.