Inflation and Primordial Black Holes

TL;DR

This paper reviews mechanisms in inflationary models that can amplify curvature fluctuations to produce primordial black holes, discussing single and multiple field scenarios with illustrative examples.

Contribution

It provides a comprehensive overview of how inflationary dynamics can lead to primordial black hole formation, highlighting new mechanisms and models including axion inflation and rapid-turn two-field models.

Findings

Identification of background gradient effects in single field inflation

Role of tachyonic instabilities in multi-field inflation

Pedagogical presentation of key calculations and models

Abstract

We review conceptual aspects of inflationary scenarios able to produce primordial black holes, by amplifying the size of curvature fluctuations to the level required for triggering black hole formation. We identify general mechanisms to do so, both for single and multiple field inflation. In single field inflation, the spectrum of curvature fluctuations is enhanced by pronounced gradients of background quantities controlling the cosmological dynamics, which can induce brief phases of non--slow-roll inflationary evolution. In multiple field inflation, the amplification occurs through appropriate couplings with additional sectors, characterized by tachyonic instabilities that enhance the size of their fluctuations. As representative examples, we consider axion inflation, and two-field models of inflation with rapid turns in field space. We develop our discussion in a pedagogical manner, by including some of the most relevant calculations, and by guiding the reader through the existing theoretical literature, emphasizing general themes common to several models.