Understanding exotic black hole orbits using effective potentials

Abstract

Gravitational wave astronomy is an emerging observational discipline that expands the astrophysical messengers astronomers can use to probe cosmic phenomena. The gravitational waveform from a source encodes the astrophysical properties and the dynamical motion of mass within the system; this is particularly evident in the case of binaries, where the overall amplitude of the system scales with physical parameters like mass and distance, but the phase structure of the waveform encodes the orbital evolution of the system. Strongly gravitating systems can show interesting and unusual orbital trajectories, as is the case for ``extreme mass ratio inspirals,'' observable in the millihertz gravitational wave band by space-based gravitational wave detectors. These sources can exhibit ``zoom-whirl'' orbits, which make complicated waveforms that are useful for mapping out the gravitational structure of the system. Zoom-whirl behavior can be intuitively understood in the context of effective potentials, which should be familiar to students from classical orbital theory in mechanics. Here we demonstrate and explain zoom-whirl orbits using effective potential theory around Schwarzschild black holes, and present an interactive tool that can be used in classroom and other pedagogical settings.