Entropy Production and the Maximum Entropy of the Universe

The entropy of the observable universe has been calculated as S_uni ~ 10¹⁰⁴ k and is dominated by the entropy of super massive black holes. Irreversible processes in the universe can only happen if there is an entropy gap between the entropy of the observable universe S_uni and its maximum entropy S_max: = S_max - S_uni. Thus, the entropy gap is a measure of the remaining potentially available free energy in the observable universe. To compute one needs to know the value of S_max. There is no consensus on whether S_max is a constant or is time-dependent. A time-dependent S_max(t) has been used to represent instantaneous upper limits on entropy growth. However, if we define S_max as a constant equal to the final entropy of the observable universe at its heat death: S_max S_max,HD, we can interpret T as a measure of the remaining potentially available (but not instantaneously available) free energy of the observable universe. The time-dependent slope dS_uni/dt (t) then becomes the best estimate of current entropy-production and T dS_uni/dt(t) is the upper limit to free energy extraction.