Inflation and Primordial Black Holes

We review conceptual aspects of inflationary scenarios able to produce primordial black holes by amplifying the size of curvature fluctuations to the level required to trigger 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.

Automated summary

We review the conceptual aspects of inflationary scenarios that can lead to the formation of primordial black holes by amplifying the size of curvature fluctuations. Various mechanisms are identified for both single-field and multiple-field inflation. In single-field inflation, the enhancement of curvature fluctuations occurs through non-slow-roll inflationary evolution induced by pronounced gradients of background quantities. In multiple-field inflation, amplification is achieved through appropriate couplings with additional sectors characterized by tachyonic instabilities.