Into the Lair: Gravitational-Wave Signatures of Dark Matter

The nature and properties of dark matter (DM) are both outstanding issues in physics. Besides clustering in halos, the universal character of gravity implies that self-gravitating compact DM configurations might be spread throughout the universe. The astrophysical signature of these objects may be used to probe fundamental particle physics, or even to provide an alternative description of compact objects in active galactic nuclei. Here we discuss the most promising dissection tool of these configurations: the inspiral of a compact stellar-size object and consequent gravitational-wave emission. The inward motion of this "test probe" encodes unique information about the nature of the central, supermassive DM configuration. When the probe travels through some compact DM profile we show that, within a Newtonian approximation, the quasi-adiabatic evolution of the inspiral is mainly driven by DM accretion into the small compact object and by dynamical friction, rather than by gravitational-wave radiation-reaction. These effects circularize the orbits and leave a peculiar imprint on the gravitational waves emitted at late time. When accretion dominates, the frequency and the amplitude of the gravitational-wave signal produced during the latest stages of the inspiral are nearly constant. In the exterior region we study a relativistic model in which the inspiral is driven by the emission of gravitational and scalar waves. Resonances in the energy flux appear whenever the orbital frequency matches the mass of the DM particle and they correspond to the excitation of the central object's quasinormal frequencies. Unexpectedly, these resonances can lead to large dephasing with respect to standard inspiral templates, to such an extent as to prevent detection with matched filtering techniques. We discuss some observational consequences of these effects for gravitational-wave detection.