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This study examines how light spectator axions affect primordial black hole formation during cosmic inflation within string theory frameworks, specifically using the Fibre Inflation model. The researchers found that axions with very small decay constants have negligible impact on curvature perturbations, demonstrating the model's robustness, while axions with larger sub-Planckian decay constants can enhance perturbation growth and facilitate primordial black hole seeding. The work provides a quantum gravity-consistent mechanism for producing primordial black holes that could account for observed dark matter.
Why it matters
This research advances our understanding of how primordial black holes, which are candidates for dark matter, could have formed in the early universe through physically consistent models grounded in string theory. The findings help constrain theoretical models of cosmic inflation and could inform future observational searches for primordial black holes and their signatures.
Understand the Science
Abstract: We study the impact of light spectator axions on the seeding of primordial black holes (PBHs) during inflation in string theory. Primordial black holes exhibit unique and novel phenomenology, and may constitute the observed dark matter. Cosmic inflation provides a mechanism for producing them, but such inflation models typically feature Planckian field excursions, necessitating an ultraviolet completion into quantum gravity. String theory provides a natural framework for doing so, and indeed Fibre Inflation has been shown to produce PBHs while satisfying constraints from cosmic microwave background data. In this work we study the dynamics of axions during Fibre Inflation, and find a diverse and rich set of possibilities, including turns in field space and enhancement of primordial perturbations. We find that across most of parameter space, notably an axion with a far sub-Planckian decay constant $fll M_{rm Pl}$, there is a negligible impact on the power spectrum of curvature perturbations, indicating an overall robustness of the model. On the other hand, an axion with a larger but still sub-Planckian decay constant, $fgtrsim {cal O}(0.1) M_{rm Pl}$, and an exponentially small prefactor of its non-perturbative potential, can enhance the growth of perturbations, making it easier to achieve the amplification needed to seed PBHs, effectively realizing axion-assisted PBHs in string theory.
Source: Spectator Axions in String Inflation and Primordial Black Holes