A baryonic universe might also be consistent with adiabatic fluctuations, but in this case we require the proto-clouds to have sufficiently high density that they decouple from the CMB well ahead of recombination. The answer to this question depends on quite what mass needs to be explained. Moreover the arguments given in the preceding section suggest that the dark matter is not formed from baryons which were smoothly distributed at recombination, or even at nucleosynthesis. Although this is clearly speculative, it is the most economical hypothesis available. For equilibrium, the power radiated from each cloud must be balanced by some heat generated within.
Thermal stability is another matter.
Because the heating of each cloud is largely independent of its temperature, the radiative cooling must actually decrease, with increasing T , if it is to be thermally stable. If it were otherwise, the following scenario would take place. Suppose the cloud contracts slightly from an initial equilibrium condition, then its temperature increases, consequently it radiates more efficiently, but the heating rate remains the same, so cooling then outstrips heating and this causes further contraction. As this is much less than the Hubble time, this is not an acceptable model. The difficulty we then face is that, in order to construct a thermally stable model, we need to identify a radiative cooling process which becomes less effective at higher temperatures.