Anisotropic stars as ultracompact objects in General Relativity

Anisotropic stresses are ubiquitous in nature, but their modeling in General Relativity is poorly understood and frame dependent. We introduce the first study on the dynamical properties of anisotropic self-gravitating fluids in a covariant framework. Our description is particularly useful in the context of tests of the black hole paradigm, wherein ultracompact objects are used as black hole mimickers but otherwise lack a proper theoretical framework. We show that: (i) anisotropic stars can be as compact and as massive as black holes, even for very small anisotropy parameters; (ii) the nonlinear dynamics of the 1+1 system is in good agreement with linearized calculations, and shows that configurations below the maximum mass are nonlinearly stable; (iii) strongly anisotropic stars have vanishing tidal Love numbers in the black-hole limit; (iv) their formation will usually be accompanied by gravitational-wave echoes at late times.Comment: 7+2 pages, 6 figures; v2: include extra material (general covariant framework for anisotropic fluids in General Relativity without symmetries and code validation); to appear in PR

Gravitational Wave Signatures of Highly Compact Boson Star Binaries

Solitonic boson stars are stable objects made of a complex scalar field with a compactness that can reach values comparable to that of neutron stars. A recent study of the collision of identical boson stars produced only non-rotating boson stars or black holes, suggesting that rotating boson stars may not form from binary mergers. Here we extend this study to include an analysis of the gravitational waves radiated during the coalescence of such a binary, which is crucial to distinguish these events from other binaries with LIGO and Virgo observations. Our studies reveal that the remnant's gravitational wave signature is mainly governed by its fundamental frequency as it settles down to a non-rotating boson star, emitting significant gravitational radiation during this post-merger state. We calculate how the waveforms and their post-merger frequencies depend on the compactness of the initial boson stars and estimate analytically the amount of energy radiated after the merger.Comment: 16 pages, 10 figure

Dynamical Chameleon Neutron Stars: stability, radial oscillations and scalar radiation in spherical symmetry

Scalar-tensor theories whose phenomenology differs significantly from general relativity on large (e.g. cosmological) scales do not typically pass local experimental tests (e.g. in the solar system) unless they present a suitable "screening mechanism". An example is provided by chameleon screening, whereby the local general relativistic behavior is recovered in high density environments, at least in weak-field and quasi-static configurations. Here, we test the validity of chameleon screening in strong-field and highly relativistic/dynamical conditions, by performing fully non-linear simulations of neutron stars subjected to initial perturbations that cause them to oscillate or even collapse to a black hole. We confirm that screened chameleon stars are stable to sufficiently small radial oscillations, but that the frequency spectrum of the latter shows deviations from the general relativistic predictions. We also calculate the scalar fluxes produced during collapse to a black hole, and comment on their detectability with future gravitational-wave interferometers.Comment: 21 pages, 17 figure

Well-posed evolution of field theories with anisotropic scaling: the Lifshitz scalar field in a black hole space-time

Partial differential equations exhibiting an anisotropic scaling between space and time -- such as those of Horava-Lifshitz gravity -- have a dispersive nature. They contain higher-order spatial derivatives, but remain second order in time. This is inconvenient for performing long-time numerical evolutions, as standard explicit schemes fail to maintain convergence unless the time step is chosen to be very small. In this work, we develop an implicit evolution scheme that does not suffer from this drawback, and which is stable and second-order accurate. As a proof of concept, we study the numerical evolution of a Lifshitz scalar field on top of a spherically symmetric black hole space-time. We explore the evolution of a static pulse and an (approximately) ingoing wave-packet for different strengths of the Lorentz-breaking terms, accounting also for the effect of the angular momentum eigenvalue and the resulting effective centrifugal barrier. Our results indicate that the dispersive terms produce a cascade of modes that accumulate in the region in between the Killing and universal horizons, indicating a possible instability of the latter.Comment: 22 pages, 8 figures, 1 table, comments are welcome

Spherical collapse in scalar-Gauss-Bonnet gravity: Taming ill-posedness with a Ricci coupling

We study spherical collapse of a scalar cloud in scalar-Gauss-Bonnet gravity—a theory in which black holes can develop scalar hair if they are in a certain mass range. We show that an additional quadratic coupling of the scalar field to the Ricci scalar can mitigate loss of hyperbolicity problems that have plagued previous numerical collapse studies and instead lead to well-posed evolution. This suggests that including specific additional interactions can be a successful strategy for tackling well-posedness problems in effective field theories of gravity with nonminimally coupled scalars. Our simulations also show that spherical collapse leads to black holes with scalar hair when their mass is below a mass threshold and above a minimum mass bound and that above the mass threshold, the collapse leads to black holes without hair, in line with results in the static case and perturbative analyses. For masses below the minimum mass bound, we find that the scalar cloud smoothly dissipates, leaving behind flat space