Number and Entropy of Halo Black Holes

TL;DR

This paper estimates the number and entropy of halo black holes based on astrophysical constraints, suggesting dark matter could predominantly be composed of black holes with significant entropy contribution.

Contribution

It provides new estimates for the total number and entropy of intermediate mass black holes considering microlensing and binary survey constraints.

Findings

Dark matter may be mostly black holes of about 10^5 solar masses.

The universe's entropy could be dominated by black holes, reaching ten million googols.

Black holes could account for over 99% of the universe's entropy.

Abstract

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}$.