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