Primordial Black Holes and Gravitational Waves in Multi-Axion-Chern-Simons Inflation

TL;DR

This paper explores how a string-inspired two-axion inflation model can enhance primordial black hole and gravitational wave production, potentially explaining dark matter and producing observable GW patterns distinguishable by future detectors.

Contribution

It introduces a novel multi-axion Chern-Simons inflation model with modulated fluctuations, leading to enhanced PBH and GW production, and compares it with conventional axion monodromy models.

Findings

Enhanced PBH and GW production due to axion field modulations.

Distinct GW spectra signatures between the models.

PBHs could account for a significant fraction of Dark Matter.

Abstract

We study aspects of inflation and the possibility of enhanced production of primordial black holes (PBHs) and gravitational waves (GWs) in a string-inspired model of two axion fields coupled to Chern-Simons gravity, which results in a running-vacuum-model inflation. Fluctuations of the scale invariant spectrum, consistent with the cosmological data, are provided in this model by world-sheet (non-perturbative) instanton terms of the axion field arising from string compactification. As a result of such modulations, there is an enhanced production of PBHs and GWs in such cosmologies, which may lead to observable in principle patterns in the profile of GWs during the radiation era. Moreover, we demonstrate that the PBHs may provide a significant amount of Dark Matter in this Universe. For comparison, we also discuss a two-stage inflation cosmological model of conventional string-inspired axion monodromy, involving again two axion fields. The resulting modifications imprinted on the GWs spectra between these two classes of models are distinct, and can, in principle, be distinguished by future interferometers. We consider models with more or less instantaneous reheating. We also make some remarks on the effects of a prolonged reheating period in leading to further enhancement of the power spectrum and thus fractions of PBHs that play the role of Dark matter.