Reclaiming the Ether Plenum: The Measured Mass-Energy Density of Space and What It Does and Does Not Establish

The companion paper The Architecture of the Void argues on conceptual grounds that twentieth-century physics rebuilt the space-filling plenum under other names. This paper supplies the arithmetic. Space is filled with cosmic microwave background radiation at a measured temperature of 2.72548 kelvin. The Stefan-Boltzmann relation gives an energy density of 4.175 times ten to the minus fourteen joules per cubic metre, and Einstein's mass-energy equivalence converts this to a mass-equivalent density of 4.645 times ten to the minus thirty-one kilograms per cubic metre. Every step is either a measurement or a standard relation; none is an assumption specific to the Theory of NeoGravity. Space therefore contains a measured, non-zero mass-equivalent density, and this is arithmetic rather than hypothesis. The paper is equally explicit about what the calculation does not establish, and two limitations are stated at length. First, the total vacuum energy density is not known: naive quantum-field-theoretic estimates exceed the observed value by a factor of order ten to the one hundred twenty-third, the cosmological constant problem, which no framework including this one has resolved. This paper therefore claims only the measured radiative contribution and does not claim to know the vacuum's total energy content. Second, and more consequentially for the series, the Theory of NeoGravity constrains a ratio rather than an absolute density. The refractive index of the medium depends on the fractional quantity rho(r) divided by rho-nought, and reproducing the observed solar light deflection requires a fractional density change of approximately 8.5 times ten to the minus six at the solar limb whatever the baseline density turns out to be. The measured cosmic microwave background density is therefore suggestive of a physically real medium but is not pinned down by the theory as its baseline. The paper concludes that the medium's reality is established by measurement, its magnitude is partially known, and its identification with the specific plenum the series requires remains an open question stated as such.