An Energy Budget for the Warm-Hot Intergalactic Medium: How Much Dissolved Vacuum Energy It Could Hold

The warm-hot intergalactic medium (WHIM) holds roughly forty percent of the universe's baryons at temperatures of 1e5 to 1e7 K, and its total thermal energy content is small, finite, and measured in aggregate by the baryon census. This paper uses that census as a calorimeter for the vacuum dissolution picture, in which today's vacuum energy density u_L = 5.25e-09 erg per cubic centimetre is the residue of far larger densities that dissolved. The WHIM's thermal energy density, computed at an effective temperature of 3e6 K for forty percent of the mean baryon density, is 1.39e-16 erg per cubic centimetre — a fraction 2.65e-08 of u_L. Spread over its baryons, the surviving vacuum density represents 1.31e10 eV per baryon, against the 869 eV per baryon the WHIM actually holds as heat. If any fraction phi of a vacuum reservoir's energy density had thermalised into WHIM baryons, the census therefore bounds phi at 2.65e-08 for the surviving density itself, at 6.61e-50 for a QCD-scale reservoir of 1.01e-05 GeV^4, and at 2.88e-62 for an electroweak-scale reservoir of 2.32e7 GeV^4. Full thermalisation of even the surviving density would have heated WHIM baryons to 1.13e14 K. The dissolution of the vacuum, whatever carried its energy away, did not deposit it in the diffuse baryons: the WHIM's modest warmth is a nearly perfect exclusion of that channel. Computed from stated assumptions; not a measurement.