Strange ripples in space may point to dark matter near merging black holes

A faint ripple from deep space may be offering physicists a new way to look for one of the universe’s biggest missing pieces.

Dark matter is thought to make up most of the matter in the cosmos, yet it has never been directly observed. It does not emit, absorb, or reflect light. So far, the clearest signs of it come from gravity. For example, galaxies bend light and move as though they contain far more mass than astronomers can see.

Now a team led by physicists at MIT and several European institutions has developed a way to search for possible traces of dark matter in gravitational waves. These are the distortions in space-time produced when massive objects like black holes collide.

After testing their method on publicly available data from the LIGO-Virgo-KAGRA network, they found that one known event, called GW190728, may fit the pattern expected if two black holes merged inside a dense cloud of dark matter. This would be rather than in empty space.

That is not a dark matter detection.

But it is a new kind of lead.

“We know that dark matter is around us. It just has to be dense enough for us to see its effects,” said Josu Aurrekoetxea, a postdoctoral researcher in MIT’s Department of Physics and one of the study’s authors. “Black holes provide a mechanism to enhance this density, which we can now search for by analyzing the gravitational waves emitted when they merge.”

The findings appear in Physical Review Letters. Aurrekoetxea worked with Soumen Roy of Université Catholique de Louvain in Belgium, Rodrigo Vicente of the University of Amsterdam, Katy Clough of Queen Mary University of London, and Pedro Ferreira of Oxford University.

The work focuses on one proposed class of dark matter candidates, extremely light scalar particles. Theorists have suggested that near black holes, these particles may behave less like isolated specks and more like coordinated waves.

In some cases, a fast-spinning black hole could transfer some of its rotational energy to that surrounding dark matter. The process, known as superradiance, could amplify the material into a very dense cloud. The researchers note that such clouds could reach densities above 10^9 grams per cubic centimeter around stellar-mass black holes. This is far beyond the thin background dark matter spread across galaxies.

That matters because a dense environment can change how a pair of black holes spirals together.

If two black holes lose or gain angular momentum through interactions with a scalar cloud during their inspiral, the gravitational wave they emit should drift slightly away from the pattern expected in a vacuum. The signal would still look like a black hole merger, but its timing and phase evolution could carry a subtle distortion. This distortion is a fingerprint of the surrounding environment.

To test that idea, the team built a semianalytic waveform model that predicts what a merger signal should look like if the binary is moving through a scalar-field environment. They then checked that model against more detailed numerical relativity simulations. These simulations solve the coupled equations of gravity and the scalar field directly.

Those simulations showed something important. A standard vacuum model can misread the system, biasing key parameters such as the chirp mass because it tries to explain the altered inspiral without accounting for the extra environment. The new model did a better job recovering the injected parameters and matching the altered frequency evolution seen in the simulations.

The authors are careful here. Their model makes strong assumptions and is Newtonian in a regime where the late inspiral becomes highly relativistic. They also say the density parameter inferred from the model should not be treated as a precise measurement, only as the right order of magnitude. That limitation matters, especially when claims hinge on small differences in waveforms.

Still, the method was good enough to attempt something bigger: a search through real gravitational-wave events.

The group analyzed 28 compact binary merger signals from the GWTC-3 catalog, selecting the clearest events from the first three LIGO-Virgo-KAGRA observing runs. For each one, they compared two possibilities. The first is that the merger happened in vacuum. The second is that it unfolded inside a scalar environment.

Twenty-seven events lined up with vacuum expectations.

One did not.

GW190728 was detected on July 28, 2019, and earlier work had identified it as a black hole merger with a total mass of about 20 Suns. In the new analysis, that event showed a statistical preference for the scalar-environment model.

Under broad assumptions, the researchers found that GW190728 could be consistent with a merger moving through a dense scalar cloud. When they imposed tighter priors based on superradiance, which restrict the allowed particle mass and cloud properties to physically motivated values, the result still leaned toward the dark matter scenario. In that case, the preferred particle mass landed around 10^-12 electron volts.

Notably, the evidence is still modest. The reported Bayes factor is around ln B ≈ 3.5, which is not high enough to claim a discovery.

“The statistical significance of this is not high enough to claim a detection of dark matter, and further checks should be performed by independent groups,” Aurrekoetxea said. “What we think is important to highlight is that without waveform models like ours, we could be detecting black hole mergers in dark matter environments, but systematically classifying them as having occurred in vacuum.”

Soumen Roy, who led the data analysis, said the approach opens a fresh route for future searches as detectors improve and the event catalog grows.

“We now have the potential to discover dark matter around black holes as the LVK detectors keep collecting data in the coming years,” Roy said. “It is an exciting time to search for new physics using gravitational waves.”

Rodrigo Vicente, who developed the analytical signal model, put the appeal more simply: “Using black holes to look for dark matter would be fantastic. We would be able to probe dark matter at scales much smaller than ever before.”

The research also lays out several reasons for caution. The authors say other effects could mimic part of the same waveform distortion. They cannot fully rule out degeneracies with astrophysical environments or with missing features in vacuum models. Even so, they report that GW190728 showed no substantial evidence for eccentricity or line-of-sight acceleration. In addition, routine parameterized tests of the inspiral remained consistent with vacuum in other respects.

There is also tension with some earlier spin-based constraints on ultralight scalars around black holes, though the team argues those measurements can be model dependent and do not exclude the full mass range favored here.

So the new result lands in an awkward but interesting place. It does not overturn anything. It does not prove dark matter is made of light scalars. What it does is introduce a tool that could keep unusual events from being filed away too quickly as ordinary vacuum mergers.

That alone could matter.

If the method holds up, gravitational-wave observatories could become dark matter experiments as well as black hole detectors. Instead of only telling physicists that two compact objects merged, future signals might also reveal what kind of environment those objects moved through on the way to collision.

That would broaden the role of instruments like LIGO, Virgo, and KAGRA, especially as louder signals arrive in future observing runs. Next-generation detectors such as the Einstein Telescope and Cosmic Explorer could push these tests much further. They could give researchers a better chance to separate genuine dark matter effects from look-alike explanations.

Even if GW190728 turns out to be a false alarm, the study provides a framework for searching future events for the same kind of imprint.

Research findings are available online in the journal Physical Review Letters.

The original story "Strange ripples in space may point to dark matter near merging black holes" is published in The Brighter Side of News.

Like these kind of feel good stories? Get The Brighter Side of News' newsletter.