Do we need fine-tuning to create primordial black holes?

Abstract

Recently, the formation of primordial black holes (PBHs) from the collapse of primordial fluctuations has received much attention. The abundance of PBHs formed during radiation domination is sensitive to the tail of the probability distribution of primordial fluctuations. We quantify the level of fine-tuning due to this sensitivity. For example, if the main source of dark matter is PBHs with mass $10^{-12}{M}_{\odot }$, then anthropic reasoning suggests that the dark matter to baryon ratio should range between 1 and 300. For this to happen, the root-mean-square amplitude of the curvature perturbation has to be fine-tuned within a $7.1\%$ range. As another example, if the recently detected gravitational-wave events are to be explained by PBHs, the corresponding degree of fine-tuning is $3.8\%$. We also find, however, that these fine-tunings can be relaxed if the primordial fluctuations are highly non-Gaussian, or if the PBHs are formed during an early-matter-dominated phase. We also note that no fine-tuning is needed for the scenario of a reheating of the universe by evaporated PBHs with Planck-mass relics left to serve as dark matter.

6 pages

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• Primordial black holes from single-field inflation: a fine-tuning audit
• PBH remnants as dark matter produced in thermal, matter and runaway-quintessence post-inflationary scenarios
• Primordial black holes from null energy condition violation during inflation
• Constraints on a mixed model of dark matter particles and primordial black holes from the Galactic 511 keV line
• Ultra-low mass PBHs in the early universe can explain the PTA signal
• Detectability of Gravitational Waves from the Coalescence of Massive Primordial Black Holes with Initial Clustering
• Primordial Black Hole Formation in Non-Standard Post-Inflationary Epochs
• Solar Mass Primordial Black Holes in Moduli Dominated Universe
• Development of generic no-scale inflation