Primordial Black Holes as Dark Matter

TL;DR

This paper explores the possibility that a population of primordial black holes could account for dark matter and influence the universe's evolution, consistent with holographic inflation models and quantum fluctuation scenarios.

Contribution

It demonstrates that dense populations of primordial black holes can produce a matter-dominated era and are compatible with holographic inflation constraints.

Findings

Black holes can survive long enough to influence cosmic evolution.

Black hole densities are consistent with holographic inflation models.

Constraints on the slow roll parameter are mild and compatible with observations.

Abstract

We investigate models in which a spectrum of black holes with Hawking temperature of order the radiation temperature at the beginning of the radiation dominated era can survive long enough to produce a matter dominated era at the observed crossover between matter and radiation in our universe. We find that a sufficiently dense population of such black holes can indeed do so. The stronger observational constraint, that the black holes have lifetimes at least as long as the current age of the universe is harder to assess, because of black hole mergers during the matter dominated era. We then investigate whether the required densities and masses are consistent with the Holographic Space-time (HST) model of inflation. We find that they are, but put mild constraints on the slow roll parameter $\epsilon = - \frac{\dot{H}}{H^2}$ in that model to be small. The bound is no stronger than the observational bound on the model's prediction for tensor fluctuations. The required black hole density, at the reheat temperature, in a model with a single species of black hole, must be viewed as a quantum mechanical accident. In such a model, our universe exists because of a low probability quantum fluctuation.