Testing the nature of dark compact objects: a status report

TL;DR

<3-5 sentence high-level summary>The paper surveys exotic compact objects (ECOs) as potential alternatives to black holes, focusing on how to test the Kerr BH paradigm with gravitational waves and horizon-scale shadows. It develops a framework based on the closeness parameter $\epsilon$ and curvature scales to classify ECOs, and reviews a broad zoo of models from boson stars to fuzzballs, highlighting their formation, stability, and observational imprints. The dynamical and wave-phenomenology—quasinormal modes, photon spheres, and gravitational-wave echoes—provides concrete pathways to distinguish ECOs from BHs, while current observations (GW detections, shadows, TDEs) place increasingly stringent bounds on horizonless alternatives. The work underscores that advancing detector sensitivity and waveform modeling will enable tests of near-horizon physics and potential quantum-gravity effects, ultimately strengthening or challenging the BH paradigm.

Abstract

Very compact objects probe extreme gravitational fields and may be the key to understand outstanding puzzles in fundamental physics. These include the nature of dark matter, the fate of spacetime singularities, or the loss of unitarity in Hawking evaporation. The standard astrophysical description of collapsing objects tells us that massive, dark and compact objects are black holes. Any observation suggesting otherwise would be an indication of beyond-the-standard-model physics. Null results strengthen and quantify the Kerr black hole paradigm. The advent of gravitational-wave astronomy and precise measurements with very long baseline interferometry allow one to finally probe into such foundational issues. We overview the physics of exotic dark compact objects and their observational status, including the observational evidence for black holes with current and future experiments.

Figures (18)

• Figure 1: An equatorial slice of a very compact object, together with the most significant (from a geodesic perspective) locations. At large distances away from the central region, physics is nearly Newtonian: planets -- such as the small dot on the figure -- can orbit on stable orbits. The external gray area is the entire region where stable circular motion is possible. At the innermost stable circular orbit ($r=6M$), timelike circular motion is marginally stable, and unstable as one moves further within. High-frequency EM waves or GWs can be on circular orbit in one very special location: the light ring ($r=3M$). Such motion is unstable, and can also be associated with the "ringdown" excited during mergers. For horizonless objects, as one approaches the geometric center another significant region may appear: a second, stable light ring. Once rotation is turned on, regions of negative energy ("ergoregions") are possible. The astrophysical properties of a dark compact object depends on where in this diagram its surface is located.
• Figure 2: A source (for example, a star) emits photons in all directions in a region of spacetime where a compact object exists (black circle). Photons with high impact parameter are weakly bent (dashed, black curve), while those with small impact parameter (short-dashed blue) are absorbed and hit the object. The separatrix corresponds to photons that travel an infinite amount of time around the light ring (solid red curve) before being scattered or absorbed. Such critical photons have an impact parameter $b=3\sqrt{3}M$MTB. The gray shaded area is the photon sphere.
• Figure 3: Left: Critical escape trajectories of radiation in the Schwarzschild geometry. A locally static observer (located at $r=r_{\rm obs}$) emits photons isotropically, but those emitted within the colored conical sectors will not reach infinity. The gray shaded area is the photon sphere. Right: Coordinate roundtrip time of photons as a function of the emission angle $\psi>\psi_{\rm esc}$ and for $\epsilon \ll 1$.
• Figure 4: Buchdahl's theorem deconstructed.
• Figure 5: Left: Comparison between the total mass of a boson star ( complex scalar or vector fields) and an oscillaton ( real scalar or vector fields), as a function of their radius $R$. $R$ is defined as the radius containing 98% of the total mass. The procedure to find the diagram is outlined in the main text. From Ref. Brito:2015yfh. Right: Mass-radius diagram for nonspinning fluid stars in GR. The red dashed (blue dotted) lines are ordinary NSs (quark stars) for several representative equations of state Lattimer:2006xbOzel:2016oaf (data taken from OzelFreireWeb); the black continuous lines are strongly-anisotropic stars Raposo:2018rjn. Note that only the latter have photon spheres in their exterior and violate Buchdahl's bound.