Constraints on exotic compact objects and scalar fields from gravitational-wave observations

In the gravitational-wave emission from a binary merger, the late part of the signal is produced by the single remnant object as it settles down from its perturbed state.

This signal allows to probe directly the nature of the remnant object.

In black hole spectroscopy, we measure the characteristic spectrum of modes expected for black holes in general relativity, facilitating tests of its predictions.

Additional post-merger signals may reveal the final object to be a more exotic alternative or deviations from general relativity.

If scalar fields are present around a black hole, they may couple to and drive gravitational wave modes with their characteristic frequency.

We search for such signals in observed data to constrain the presence of these scalar fields and their coupling, and predict future prospects using third-generation instruments.

If instead of a black hole the final object is one of several proposed alternatives, characteristic signatures may be present in the post-merger emission.

A horizonless compact object would at late times emit a modified spectrum of weak but long-lived modes following the initial unmodified signal.

With methods adapted to this type of signal, we analyse data from the most promising detections, placing strict bounds on the location of possible deviations from the Kerr geometry.

For a compact object consisting of fluid matter, r-mode oscillations could lead to a similar long-lived signal, and we apply the developed methods to constrain the corresponding fluid's properties.