Tidal deformations of compact objects from the membrane paradigm

The tidal deformability is a key observable to test the nature of compact objects in a binary coalescence. Within vacuum general relativity, the tidal Love numbers of a four-dimensional black hole are strictly zero, while they are nonzero and model-dependent for material objects, matter distributions around black holes, or in alternative theories of gravity. Here, we develop a model-agnostic framework based on the membrane paradigm, where the tidal properties of a spherically symmetric object are encoded in the response of a fictitious membrane in terms of viscosity coefficients. We show that both neutron stars and exotic compact objects (in particular, we provide an explicit example for thin-shell gravastars) are included in this framework, provided that the bulk and shear viscosity coefficients of the membrane have a nontrivial frequency dependence. We derive a general result for the electric and magnetic tidal Love numbers for nonrotating and spherically symmetric compact objects in terms of the viscosity coefficients, discussing their behavior with the compactness of the object, and identifying the conditions for the logarithmic behavior found in previous literature for various ultracompact objects. Finally, we show that the membrane shear viscosity coefficients associated with neutron star models feature novel quasiuniversal relations, which depend only mildly on the neutron-star equation of state.