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Black holes, or not black holes?

International team led by AEI researcher establishes new method to test the nature of compact object mergers

The LIGO-Virgo-KAGRA Collaboration’s detectors observe gravitational-wave signals from coalescences of compact objects, most of which are expected to be binary black holes. However, there are several proposed alternatives to the classical vacuum solution of Einstein’s field equations. An international team led by AEI researchers has now proposed and demonstrated a new method to determine if the observed signals originate from regular black holes or exotic compact objects (ECOs). Unlike existing methods, their method tracks a source property directly related to the merger morphology: the objects’ compactness (the ratio of mass to size). Compactness is determined by measuring how close the binary components are at merger and by infering from gravitational-wave data when the signal departs from that of a binary black hole. Using 69 signals from the GWTC-3 catalog, the researchers obtain the first population-level measurement of compactness. They find that the observed population of compact binary mergers is consistent with that of binary black holes, with no significant evidence for ECOs. The team demonstrates that their method does not inherently favor the black hole hypothesis, and they establish an upper limit of 3% for the ECO merger rate relative to that of binary black holes.

Paper abstract

Classically, black holes (BHs) are the most compact objects predicted in nature with C=0.5 in the Schwarzschild limit; C is defined as the mass-to-radius ratio in geometric units. In this work we perform a novel measurement on the nature of putative BH mergers in the gravitational wave (GW) data by directly probing the binary’s closest approach through an effective compactness parameter. We confidently show all such high-significance signals in GWTC-3 are consistent with the BH hypothesis for the first time. Our hierarchical analysis yields Ceff = 0.5^+0.3_−0.1, and we further limit the merger rate of low-compactness exotic binaries to <0.7 Gpc⁻³ yr⁻¹. This work establishes compactness as a key observable in GW astronomy.