Gravitational waves from binary coalescences provide one of the cleanest signatures of the nature of compact objects. It has been recently argued that the post-merger ringdown waveform of exotic ultracompact objects is initially identical to that of a black-hole, and that putative corrections at the horizon scale will appear as secondary pulses after the main burst of radiation. Here we extend this analysis in three important directions: (i) we show that this result applies to a large class of exotic compact objects with a photon sphere for generic orbits in the test-particle limit; (ii) we investigate the late-time ringdown in more detail, showing that it is universally characterized by a modulated and distorted train of "echoes"of the modes of vibration associated with the photon sphere; (iii) we study for the first time equal-mass, head-on collisions of two ultracompact boson stars and compare their gravitational-wave signal to that produced by a pair of black-holes. If the initial objects are compact enough as to mimic a binary black-hole collision up to the merger, the final object exceeds the maximum mass for boson stars and collapses to a black-hole. This suggests that -in some configurations -the coalescence of compact boson stars might be almost indistinguishable from that of black-holes. On the other hand, generic configurations display peculiar signatures that can be searched for in gravitational-wave data as smoking guns of exotic compact objects. arXiv:1608.08637v2 [gr-qc]

The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on ‘Black holes, Gravitational waves and Fundamental Physics’.

We present a method for solving the first-order field equations in a post-Newtonian (PN) expansion. Our calculations generalize work of Bini and Damour and subsequently Kavanagh et al., to consider eccentric orbits on a Schwarzschild background. We derive expressions for the retarded metric perturbation at the location of the particle for all -modes. We find that, despite first appearances, the Regge-Wheeler gauge metric perturbation is C 0 at the particle for all . As a first use of our solutions, we compute the gauge-invariant quantity U through 4PN while simultaneously expanding in eccentricity through e 10 . By anticipating the e → 1 singular behavior at each PN order, we greatly improve the accuracy of our results for large e. We use U to find 4PN contributions to the effective one body potentialQ through e 10 and at linear order in the mass-ratio. PACS numbers: 04.30.-w, 04.25.Nx, 04.30.Db I. INTRODUCTIONRecent years have seen a large amount of research in the regime of overlap between two complementary approaches to the general relativistic two body problem: gravitational self-force (GSF) and post-Newtonian (PN) theory. GSF calculations are made within the context of black hole perturbation theory, an expansion in the mass-ratio q ≡ µ/M of the two bodies, which is valid for all speeds. PN theory, on the other hand, is an expansion in small velocities (or equivalently, large separations), but is valid for any q. In the area of parameter space where the two theories overlap, they can check one another and also be used to compute previously unknown parameters.Much of the recent GSF/PN work has been performed with the eventual goal of detecting gravitational waves. With Advanced LIGO [1] now performing science runs, the need for accurate waveforms is immediate. One very effective framework for producing such waveforms is the effective one body (EOB) model [2]. Careful GSF calculations, when performed in the PN regime, can then be used to provide information on the PN behavior of EOB potentials.This calibration is possible only because gauge-invariant quantities can be computed separately using the GSF and PN theory. The first such invariant was the "redshift invariant" u t , originally suggested by Detweiler [3], who then computed u t numerically with a GSF code and compared it to its PN value through third PN order [4]. Subsequently, a number of papers used numerical techniques to compute u t to ever higher precision, and fit out previously unknown PN parameters at ever higher order [5][6][7][8][9]. Work by Shah et al. [7], in particular, opened a new avenue to obtaining high-order PN parameters in the linear-in-q limit. Rather than solve the first-order field equations through numerical integration, they employed the function expansion method of Mano, Suzuki and Takasugi (MST) [10,11]. Combining MST with computer algebra software like Mathematica, one can solve the field equations to hundreds or even thousands of digits when considering large radii orbits.In addition to u t , other local, circular orbit gaug...

We probe the gravitational interaction of two black holes in the strong-field regime by computing the scattering angle χ of hyperbolic-like, close binary-black-hole encounters as a function of the impact parameter. The fully general-relativistic result from numerical relativity is compared to two analytic approximations: post-Newtonian theory and the effective-one-body formalism. As the impact parameter decreases, so that black holes pass within a few times their Schwarzschild radii, we find that the post-Newtonian prediction becomes quite inaccurate, while the effective-one-body one keeps showing a good agreement with numerical results. Because we have explored a regime which is very different from the one considered so far with binaries in quasi-circular orbits, our results open a new avenue to improve analytic representations of the general-relativistic two-body Hamiltonian.