Periodic Orbits of Van der Waals Black Holes

Abstract

Black holes in asymptotically anti-de Sitter (AdS) spacetime whose extended-phase-space thermodynamics reproduces that of a van der Waals fluid exactly form a distinctive class, the van der Waals black holes (VBHs). The timelike geodesics of VBHs are solved, their periodic orbits are catalogued, and the gravitational waves emitted along them are computed, so as to trace how the thermodynamic pressure P and the molecular-volume parameter b shape the orbital dynamics. Integrating the geodesic equations numerically yields the energy, angular momentum and radius of the innermost stable circular orbit (ISCO) and maps out the family of bound orbits. To delimit the parameter window in which the effective potential stays well behaved, we introduce a critical angular momentum Ls. Within the orbit-classification scheme, we compute the energies corresponding to rational q for several parameter sets and display the resulting orbits. The waveforms produced along these orbits are generated within the "Kludge" formalism, which makes it possible to quantify the imprint of b on the wave amplitude and on the period. Both the orbital architecture and the gravitational-wave signature turn out to be strongly sensitive to b. The dynamical behavior of VBHs and Schwarzschild-AdS (SAdS) black holes differs markedly, providing a new perspective for probing black hole thermodynamics via gravitational wave observations.