Temperature, like mass, suffers a lack of general definition in theoretical terms. Both are treated operationally. Yet thermodynamics must become generalized with both Relativistic and Quantum mechanics to progress beyond the current assemblage that is the Concordance of the cosmological standard model. In short, quantization needs Canons of general terms.
Consider Maxwell's convergence and divergence functions, curl and div. These commonly apply to coefficiencies of permeability (mu) and permittivity (epsilon) of a field associated with a mass. Mass : energy equivalence provides no law of proportionality to prescribe an upper density bound for any quantity of energy and so attempts to determine a finite field fall into a mathematic singularity.
In analysis, permeability and permittivity are seperate operations, yet in reality both would physically coexist. Both limit to a light velocity proportion in free space, but operate under opposite signs. The product would therefore be (-c^2), and to be consistent with mass : energy equivalence a postulate of proportionate density should also be quadratic. Thus a hypothetical upper density bound could be prescribed as;
lim = mass (mu * epsilon)^2 = mc^4
this does not in itself define what portion of the total mass must exist at that upper density, but that proportionate density would prescribe a finite quantity if a finite volume were derived as existant at a constant density. A differentiated quantity of energy would not need to become any more dense to become inertially bound as a finite field. And within that constant density volume temperature could equate to relativistic rate of response.