Primordial Black Holes from Passive Density Fluctuations
Chia-Min Lin, Kin-Wang Ng
TL;DR
This work shows that passive density fluctuations during slow-roll inflation, which can be blue-tilted and enhanced at small scales, naturally seed primordial black holes (PBHs) near the end of inflation. By analyzing three concrete mechanisms—axion inflation with gauge-field inverse decay, trapping-induced particle production, and quantum stress tensor fluctuations—the authors demonstrate that PBHs with masses up to $M \lesssim 10^{15}$ g can form and subsequently evaporate, thus not serving as dark matter but potentially influencing early-universe processes. The study highlights that even within a single-field slow-roll framework, passive fluctuations offer a robust path to PBH production, linked to a strongly blue small-scale spectrum, and it discusses observational implications such as non-Gaussianity constraints and potential links to antiproton fluxes in low-scale inflation scenarios. Overall, the paper identifies passive fluctuations as a generic and testable mechanism for PBH formation, motivating future CMB and large-scale structure probes to constrain their role in, and impact on, early-universe cosmology.
Abstract
In this paper, we show that if passive fluctuations are considered, primordial black holes (PBHs) can be easily produced in the framework of single-field, slow-roll inflation models. The formation of PBHs is due to the blue spectrum of passive fluctuations and an enhancement of the spectral range which exits horizon near the end of inflation. Therefore the PBHs are light with masses $\lesssim 10^{15}g$ depending on the number of e-folds when the scale of our observable universe leaves horizon. These PBHs are likely to have evaporated and cannot be a candidate for dark matter but they may still affect the early universe.