Dark matter occurs almost everywhere in the cosmos. But what it consists of is a mystery. Now astronomers have a new idea for how to get to the bottom of the mysterious dark matter particles: gravitational waves from merging black holes could carry the signature of these particles. If dark matter consists of lighter particles such as axions, they would have to build up to a higher density in the vicinity of black holes – and this leaves traces in the signal, as the team explains.
The invisible dark matter, which probably only interacts via gravity, makes up more than 85 percent of all matter in the universe. Yet we know hardly anything about it, not even what particles it is made of. For a long time, astrophysicists favored massive, weakly interacting particles, so-called WIMPs (Weakly Interacting Massive Particles).
But after various detectors and experiments failed to find any evidence of such heavy dark matter particles, light particles such as axions are now considered the most promising candidates. These hypothetical particles, which belong to the bosons, are said to be several orders of magnitude lighter than an electron, have no spin and hardly interact with other particles. According to some theories, such light scalar particles should behave like coordinated waves under certain circumstances.
Black holes can accumulate dark bosons
This is where the new proof idea from Soumen Roy from the Catholic University of Louvain and his colleagues comes into play. “Gravitational interactions near black holes can trigger the formation of dense scalar configurations,” explains the team. The enormous gravity of the black holes would therefore have to cause dark matter particles in their immediate surroundings to accumulate to high densities through various mechanisms such as superradiance and mutual interactions.
“Such superradiant clouds could reach up to ten percent of the mass of the black hole and densities of more than a billion grams per cubic centimeter – that is 30 orders of magnitude more than in the galactic dark matter background,” write the astrophysicists. “If we were to detect these effects, it would be a completely new approach to exploring fundamental physics and dark matter.” Then one could check whether the light dark matter particles actually exist.
Telltale signatures in gravitational waves
The highlight: There are already instruments with which we can detect such condensations of dark matter on black holes. Because they would have to leave detectable traces in the gravitational waves of colliding black holes – the vibrations in space-time that are released when two black holes merge. “The predicted densities are in the detectable range of LIGO, Virgo and KAGRA,” explain the astrophysicists.
(Video: GRChombo)
To find out what this signal of condensed dark matter would look like in the gravitational waves, the researchers carried out detailed numerical simulations in which they reconstructed the dark matter gravitational wave signature for different sizes and masses of the colliding black holes, as well as different dark matter densities. They then compared the result with the catalog of gravitational wave events that the detectors LIGO in the USA, Virgo in Italy and KAGRA in Japan have recorded so far. The focus was on 28 events captured particularly clearly and in high resolution.
An event with promising features
It turned out that of the 28 gravitational wave events recorded in particularly great detail, 26 showed no evidence of dark matter. “The parameters correspond to those of a vacuum and therefore provide strong evidence for a density close to zero,” the team writes. But two events stood out. “The notable anomalies are events GW190814 and GW190728, which show some evidence of increased density,” said Roy and his colleagues. They therefore examined in more detail whether these events could come from an enrichment in the density of dark matter.
These analyzes revealed only weak evidence of such a connection for the gravitational wave event GW190814. However, this was different for event GW190728, which was detected on July 28, 2019. These gravitational waves came from the merger of two black holes of approximately the same weight with a total mass of around 20 solar masses. The wave parameters of their signal showed some features that indicate the presence of condensed dark matter, as the researchers report.
First indications of a very light particle
“But the statistical significance is not yet high enough to be considered proof,” emphasizes co-author Josu Aurrekoetxea from the Massachusetts Institute of Technology (MIT). However, if the results are confirmed, the gravitational wave signal could help to further narrow down the mass of the long-sought dark matter particles: “Our interpretation suggests the existence of a new scalar particle with a mass of 10<-sup>-12 electron volts,” the team writes.
According to astrophysicists, it is definitely worth continuing to specifically search gravitational wave events for dark matter signals in the future. “We now have the potential to detect dark matter around black holes because the LVK detectors will collect more data in the coming years,” says Roy. Next-generation gravitational wave detectors such as the Einstein Telescope or the Cosmic Explorer could provide even more insights. Because these planned observatories will be able to capture gravitational waves in even better quality and with higher signal strength.
Source: Soumen Roy (Université Catholique de Louvain, Louvain-La-Neuve, Belgium) et al., Physical Review Letters, 2026; doi: 10.1103/fv9z-zkxx