Dear Colleagues,

Gravitational-wave astronomy is opening a new window into the strong field regime of gravity. To this day, the LIGO-Virgo-KAGRA (LVK) collaboration has detected over 50 gravitational-wave events, all of them consistent with black holes and neutron stars. Such outstanding achievement relies heavily on two fundamental pillars: numerical relativity and data analysis. The true (classical and quantum) nature of the observed compact objects remains, however, elusive due to several reasons. First, although the gravitational-wave data so far are well fitted by the Kerr hypothesis, no sufficiently developed alternative gravitational waveforms exist to challenge the prevailing black-hole binary merger interpretation. Second, current observations do not yet allow for a detailed observation of the characteristic ringdown emission from the remnant objects of these mergers, therefore providing strong evidence in favour or against the Kerr hypothesis. Alternative models of compact objects with unique features (ecos, UCOs) have been proposed, but numerical studies can be undertaken to study their properties and stability, and obtain the gravitational radiation emitted by such objects. These shall lead to their discovery of these exotic objects or to constraints on their populations. On the other hand, new data analysis techniques shall be needed to correctly identify the source of the gravitational waves detected by the LVK. A meeting point of these different fields would greatly improve our understanding of current and future gravitational-wave events.

This Special Issue aims to review current and future prospects of numerical relativity, gravitational waves, the physics of compact objects, and data analysis. In particular, of great interest are investigations involving the merger and ringdown stages of compact mergers, where strong gravity effects shall allow to resolve the nature of the involved objects. Therefore, novel ideas beyond the current binary black hole framework are especially welcome, both theoretical and experimental.

Dr. Nicolas Sanchis-Gual
Dr. Juan Calderón-Bustillo
Dr. Alejandro Torres-Forne
Guest Editors

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• gravitational waves
• numerical relativity
• data analysis
• black holes
• compact objects