Dark matter is believed to make up most of the matter in the universe, yet scientists still cannot observe it directly. Unlike ordinary matter, dark matter does not interact with light or electromagnetic forces, making gravity the only known way to detect its presence. Now, researchers think colliding black holes could provide a new way to search for clues about this invisible substance.
Physicists at MIT and several European institutions have developed a method to identify possible signs of dark matter hidden within gravitational waves. These ripples in space and time are created when massive objects such as black holes spiral together and merge. If those black holes travel through dense clouds of dark matter before colliding, the resulting gravitational waves could carry subtle traces of that interaction.
The team tested their approach using publicly available data collected by LIGO-Virgo-KAGRA (LVK), the international network of gravitational wave observatories that monitors black hole mergers and other distant cosmic events.
Searching Gravitational Waves for Dark Matter Clues
The researchers analyzed signals gathered during LVK’s first three observing runs. They focused on 28 of the clearest gravitational wave events detected so far.
For 27 of those events, the signals matched what scientists would expect from black holes merging in empty space. But one signal, known as GW190728, appeared different. According to the team’s analysis, the pattern of that gravitational wave may contain evidence of an interaction with dark matter.
The researchers stress that this does not amount to a confirmed discovery of dark matter. Instead, the new technique provides a way to scan gravitational wave data for promising signals that could later be investigated further.
“We know that dark matter is around us. It just has to be dense enough for us to see its effects,” says Josu Aurrekoetxea, a postdoc in the MIT Department of Physics. “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 co-authored the study with LVK member Soumen Roy of Université Catholique de Louvain (UCLouvain) in Belgium, Rodrigo Vicente of the University of Amsterdam, Katy Clough of Queen Mary University of London, and Pedro Ferreira of Oxford University.
How Black Holes Could Amplify Dark Matter
Dark matter remains one of the biggest mysteries in physics. Scientists infer its existence because gravity around galaxies appears stronger than visible matter alone can explain. Observations of gravitational lensing, where light bends around galaxies, suggest an additional unseen source of mass is influencing space.
Current estimates suggest dark matter could account for more than 85 percent of the matter in the universe. However, researchers still do not know what dark matter actually consists of.
One proposed form involves extremely lightweight particles called “light scalar” particles. Theories suggest these particles can behave like coordinated waves near black holes.
Scientists believe that when these waves encounter a rapidly spinning black hole, the black hole’s rotational energy can transfer into the dark matter waves, dramatically increasing their density. This process, known as superradiance, has been compared to whipping cream into butter.
If the density becomes high enough, the dark matter could alter the gravitational waves produced when black holes collide.
Predicting Dark Matter Imprints in Space-Time
To investigate this possibility, the researchers built detailed simulations of black hole mergers under many different conditions. They varied factors including the masses and sizes of the black holes, the amount of surrounding dark matter, and the density of that matter.
Using those simulations, the team predicted how gravitational waves would appear if black holes merged inside a dense dark matter environment rather than in a vacuum.
The model also accounted for how those waves would change as they traveled across millions of light years before reaching detectors on Earth.
The researchers then compared their predictions with actual LVK observations. Out of the 28 strongest signals examined, GW190728 was the only event that showed agreement with the dark matter scenario.
GW190728 was first detected on July 28, 2019. Earlier studies determined that the signal came from two black holes with a combined mass about 20 times that of the sun. According to the new analysis, those black holes may have merged within a dense cloud of dark matter.
A Promising New Tool for Dark Matter Research
“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 says. “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.”
Researchers say the growing number of gravitational wave observations could make this approach increasingly useful in the coming years.
“We now have the potential to discover dark matter around black holes as the LVK detectors keep collecting data in the coming years,” says co-author Soumen Roy, who led the data analysis part of the work. “It is an exciting time to search for new physics using gravitational waves.”
“Using black holes to look for dark matter would be fantastic,” adds co-author Rodrigo Vicente, who developed the analytical model of the signal. “We would be able to probe dark matter at scales much smaller than ever before.”
The research was supported in part by the U.S. National Science Foundation and MIT’s Center for Theoretical Physics — a Leinweber Institute.