Exotic compact objects: a recent numerical-relativity perspective

Abstract: Beyond black holes and neutron stars, new hypothetical compact objects have been proposed as potential astrophysical entities. In general, their properties have not yet been fully explored or understood, nor has it been proven whether or not they exist in nature. They are the so-called $\textit{exotic compact objects}$, theoretical equilibrium configurations in the strong regime of gravity that involve new exotic physical phenomena deeply related to fundamental questions of theoretical physics (e.g., the nature of dark matter, the formation of singularities, or the presence of horizons). Among these exotic objects, there are those that require the existence of new fields and particles beyond the Standard Model, such as boson stars and ultralight bosons; those that seek to describe the dense equation of state of neutron stars as an even more extreme state of matter made up of free quarks; or those that exhibit additional properties related to extensions of General Relativity or even quantum gravity. However, in order to move from theoretical objects to astrophysical objects, their dynamics and stability must first be assessed by performing numerical-relativity simulations under the premise that solutions that are unstable on dynamical timescales will never completely form, being, at most, a transient state that will not be able to play any astrophysical role. Furthermore, numerical simulations also make it possible to extract the gravitational radiation from relevant astrophysical scenarios, such as the collapse of exotic stars or binary mergers, which could then be compared with current and upcoming LIGO-Virgo-KAGRA gravitational-wave detections.