Short gravitational waves as a tool for searching for cosmic domain walls ✦
Gravitational-wave transients GW190521 and GW231123, interpreted as mergers of very massive black holes, were tested against the topological dark-matter model with domain walls. Fitting the wall template to LIGO data favored the black-hole hypothesis with log Bayes factors of 12.2 and 11.3, which is lower than values obtained when injecting black-hole signals into noise. A joint analysis was performed for the first time, where both events simultaneously constrain the parameters of a single scalar field; although black holes remain preferred, the events are compatible with a common field, and the wall parameters agree for independent noise realizations and sky locations. Injected wall transients were systematically recovered as black holes with high spins, indicating morphological degeneracy. Multi-event parameter-consistency tests offer a new tool for searching for domain walls in future observing runs of gravitational-wave observatories.
Context
The nature of dark matter remains one of the biggest unsolved problems in physics, and the pioneering work of Vera Rubin on galaxy rotation only deepened the mystery. Recent short gravitational-wave events like GW190521 and GW231123, detected by LIGO, have anomalous parameters (extreme masses and spins), challenging standard stellar evolution scenarios. An alternative hypothesis suggests they could be signals from macroscopic configurations of a quantum field — domain walls — formed during symmetry breaking in the early expanding universe.
Methods
The authors applied Bayesian analysis to LIGO Hanford and Livingston data, using an eight-parameter interferometer response model developed with crucial contributions from Rainer Weiss. The signal template accounts for variations in the effective fine-structure constant and a dispersive phase shift caused by the interaction of the scalar field with the electromagnetic sector. For comparison, they used a fifteen-parameter black hole merger model (NRSur7dq4). Signal injections into noise were performed to calibrate Bayesian factors.
Results
Hypothesis comparison yields a log10 Bayes factor of 12.2 for GW231123 and 11.3 for GW190521 in favor of black holes, but these values are significantly lower than the medians (23.1 and 40.9) obtained from injections of realistic BBH signals. A joint fit revealed consistency of dark sector parameters: log10(mϕ/eV) ≈ -12.09, log10(m0/eV) ≈ -11.19, wall speed vDW ≈ 0.026c, and discrete ratio Nratio = 3 for both events. The dark energy coupling parameter gDW is weakly constrained, with a median ~10^-27, corresponding to a variation of the fine-structure constant at the 10^-27 level.
Implications
The results demonstrate that short gravitational-wave transients can carry information about macroscopic field structures linked to dark matter. While the standard interpretation remains preferred, the anomalously low Bayes factors and morphological degeneracy between domain walls and highly spinning black holes call for caution. Multi-event analysis opens a new path to test the topological dark matter hypothesis.
Future development
Future LIGO observing runs will increase the sample of short transients, allowing us to build statistics for a joint analysis of scalar field parameters. If multiple events show the same field masses and coupling constants, that would be a strong case for domain walls. Developing unmodeled burst search methods and specialized templates will improve sensitivity to such signals. Moreover, refining the role of dark energy in symmetron models could link the observed wall speed to cosmological evolution.
Impact
This work touches on fundamental questions of dark matter physics, black hole astrophysics, and early universe cosmology. It also showcases the potential of gravitational-wave observatories as multi-purpose detectors beyond standard astrophysics. The concept of signals propagating at subluminal speed of light calls for a revision of coincidence algorithms to register non-gravitational transients.
Next steps
A specialized pipeline for domain wall searches is needed, one not limited by light travel time coincidence criteria. It is also important to explore additional interferometer response channels, such as the coupling of the scalar field to fermion masses. Joint analysis with atomic clock data would help cross-check variations in fundamental constants.
Key open problems
This research is directly linked to the unsolved problem of the nature of dark matter and to the question of whether anomalous gravitational-wave events are due to unknown astrophysical processes or a manifestation of new physics. It also touches on the hierarchy problem in dark energy (symmetron) models and in quantum field theory, where domain walls could be relics of phase transitions.
🎯 If the domain wall interpretation is correct, then one such wall, tens of meters thick, could pass through Earth every day — but you'd only notice it with giant interferometers.
🎬 The idea of macroscopic quantum objects passing through Earth is reminiscent of the 'Dark Forest' — the concept of hidden cosmic structures that can interact with our world unnoticed. In Liu Cixin's novel 'The Three-Body Problem', invisible cosmic 'strings' piercing through space are also explored.
Key numbers
- wall speed: 0.026c
- log Bayes factor for GW231123: 12.2
- log scalar field mass: -12.09 eV
- wall thickness: ~100 m
- event frequency: up to one per day