The Penrose process, superradiance and ergoregion instabilities

TL;DR

This paper clarifies the conditions under which superradiance and ergoregion instabilities occur, demonstrating their presence in flat spacetime with fermions and horizonless geometries, and analyzing their implications for astrophysical objects.

Contribution

It resolves the apparent tension between superradiance and the Penrose process by showing superradiance in flat spacetime with fermions and in horizonless geometries, and discusses their physical consequences.

Findings

Superradiance occurs for self-interacting fermions in flat spacetime.

Superradiance leads to ergoregion instabilities in horizonless geometries.

Superradiant instabilities can power structure formation outside rotating stars.

Abstract

Superradiant scattering is a radiation enhancement process that takes place in many contexts, and which has recently found exciting applications in astro and particle physics. In the framework of curved spacetime physics, it has been associated with the classical Penrose process for particles. Superradiance is usually also associated with bosonic fields around geometries with ergoregions and horizons. These notions are in clear tension however: the Penrose process occurs for horizonless geometries, and particles are composed of fermions. Here, we resolve the tension in its different aspects, by showing that (i) superradiance occurs for self-interacting fermions on flat spacetime; (ii) superradiance occurs also for horizonless geometries, where it leads to an ergoregion instability. Ultracompact, horizonless geometries will usually respond with echoes of growing amplitude, until rotational (or electrostatic) energy is extracted from the object; (iii) the Fourier-domain analysis leads to absence of superradiance when horizons are not present. We elucidate why this analysis fails to give meaningful results; (iv) finally, we show that superradiant, ergoregion instabilities have a particle analog of similar growth timescales and which can power the formation of a structure outside a compact, rotating star.