A Bound on Vacuum Dissolution Heating from the Temperature of the Low-Redshift Lyman-Alpha Forest

If the vacuum energy density observed today is the residue of a much larger density that largely dissolved, a natural question is whether the dissolution, or the surviving density itself, still exchanges energy with ordinary gas. This paper parametrises any such exchange as a density-independent volumetric heating rate Q = f u_L / t_H, where u_L = 5.25e-09 erg per cubic centimetre is the surviving vacuum energy density, t_H is the Hubble time, and f is the fraction of u_L delivered to gas per Hubble time. Intergalactic gas in the low-redshift Lyman-alpha forest is an exceptionally sensitive calorimeter for such a term, because its density is so low that radiative cooling per unit volume is feeble. Requiring the equilibrium temperature of gas at the cosmic mean density (n_H = 2.89e-07 per cubic centimetre at z = 0.15) to stay below 20,000 K bounds f at 5.60e-11; a stricter cap of 12,000 K bounds f at 8.09e-12. At f = 1e-09 the mean-density gas has no radiative equilibrium at all: the heating exceeds the peak of the cooling curve and the gas warms without limit in this model. The bound is the tightest yet obtained from diffuse-gas calorimetry, and it closes the option of explaining halo or cluster heating with a uniform dissolution term. All figures are computed from stated assumptions; nothing here is a measurement.