Number and Entropy of Halo Black Holes

Based on constraints from microlensing and disk stability, both with and without limitations from wide binary surveys, we estimate the total number and entropy of intermediate mass black holes. Given the visible universe comprises $10^{11}$ halos each of mass $\sim 10^{12} M_{\odot}$, typical core black holes of mean mass $\sim 10^7 M_{\odot}$ set the dimensionless entropy ($S/k$) of the universe at a thousand googols. Identification of all dark matter as black holes sets the dimensionless entropy of the universe at ten million googols, implying that dark matter can contribute over 99% of entropy, which favors all dark matter as black holes in the mass regime of $\sim 10^{5} M_{\odot}$.

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