Description
Three questions have troubled theoretical physics in the last century. What is the nature of the black hole singularity? What happens at the end of the evaporation process of a black hole? And what is dark matter? Remarkably, all three questions can be connected by a surprisingly simple hypothesis: dark matter consists of the remnants of a very special kind of black hole – a black hole without a singularity – which formed in the very early universe. These nonsingular black holes evaporate via Hawking radiation, as expected, but then late in their lifetime, instead of heating up, they begin to cool off, forming a kind of black hole remnant, which does not contain a singularity. Due to their thermodynamic properties, these nonsingular black hole remnants are a novel dark matter candidate. It is commonly assumed that low-mass primordial black holes cannot constitute a significant fraction of the dark matter in our universe due to their predicted short lifetimes from the conventional Hawking radiation and evaporation process. Assuming physical black holes are nonsingular -- likely due to a theory of modified gravity which includes quantum gravitational and high-energy physics -- we demonstrate that a large class of nonsingular black holes have finite evaporation temperatures. This can lead to slowly evaporating low-mass black holes or to remnant mass states that circumvent traditional evaporation constraints. As a proof of concept, we explore the limiting curvature hypothesis and the evaporation process of a nonsingular black hole solution in two-dimensional dilaton gravity. We identify generic features of the radiation profile and compare them with known regular black holes, such as the Bardeen solution in four dimensions. Remnant masses are proportional to the fundamental length scale, and we argue that slowly evaporating low-mass nonsingular black holes, or remnants, are viable dark matter candidates. Indeed, we show that nonsingular black holes formed during the early universe can comprise all of the dark matter in the observable universe today.
