Dark matter is thought to make up most of the matter in the universe, but scientists have yet to observe it directly. Unlike normal matter, dark matter does not interact with light or electromagnetic forces, making gravity the only known way to detect its presence. Now, researchers believe that black hole collisions could provide a new way to search for clues about this invisible matter.
Physicists at MIT and several European institutions have developed a method to identify possible signatures of dark matter hidden within gravitational waves. These space-time ripples occur when massive objects, such as black holes, spiral and merge. If these black holes travel through dense clouds of dark matter before colliding, the resulting gravitational waves can carry subtle signatures of their interactions.
The researchers tested their approach using publicly available data collected by LIGO-Virgo-KAGRA (LVK), an international network of gravitational wave observatories that monitor black hole mergers and other distant cosmic events.
Searching for gravitational waves for clues to dark matter
The researchers analyzed signals collected during the first three observations of LVK. They focused on 28 of the most distinct gravitational wave events detected to date.
In 27 of these events, the signals matched what scientists would expect from black holes merging in empty space. But one signal, known as GW190728, looked different. The researchers’ analysis suggests that the gravitational wave pattern may contain evidence of interaction with dark matter.
The researchers stress that this does not amount to a confirmed discovery of dark matter. Instead, new techniques offer a way to scan gravitational wave data to find promising signals that can be further investigated later.
“We know that dark matter is all around us. To see its effects, it has to be dense enough,” says Josu Aurrekoetxea, a postdoctoral researcher in MIT’s Department of Physics. “Black holes provide a mechanism for increasing this density, and we can now study their density by analyzing the gravitational waves emitted when black holes merge.”
The findings will be published in Physical Review Letters. Aurrekoetxea co-authored the study with LVK member Soumen Roy of the Catholic University of Louvain in Belgium, Rodrigo Vicente of the University of Amsterdam, Katy Clough of Queen Mary University of London, and Pedro Ferreira of the University of Oxford.
How do black holes amplify dark matter?
Dark matter remains one of the biggest mysteries in physics. Scientists speculate about its existence because the gravity around galaxies appears to be stronger than can be explained by visible matter alone. Observations of gravitational lenses, which bend light around galaxies, suggest that even more invisible sources of mass are influencing the universe.
Current estimates suggest that dark matter may make up more than 85 percent of the matter in the universe. However, researchers still do not know what kind of material dark matter actually consists of.
One proposed format involves very lightweight particles called “light scalar” particles. The theory is that these particles could behave like cooperative waves near a black hole.
Scientists believe that when these waves encounter a rapidly rotating black hole, the black hole’s rotational energy is transferred to the dark matter waves, dramatically increasing their density. This process, known as superradiance, can be compared to whipping cream into butter.
When dense enough, dark matter can alter the gravitational waves produced when black holes collide.
Predicting the traces of dark matter in space and time
To investigate this possibility, the researchers constructed detailed simulations of black hole mergers under various conditions. They varied factors such as the mass and size of the black hole, the amount of dark matter surrounding it, and the density of that matter.
Using these simulations, the team predicted how gravitational waves would appear if black holes merged in a dense dark matter environment rather than in a vacuum.
The model also took into account how these waves change as they travel millions of light years before reaching detectors on Earth.
The researchers then compared their predictions to actual LVK observations. Of the 28 strongest signals investigated, GW190728 was the only event that showed agreement with the dark matter scenario.
GW190728 was first detected on July 28, 2019. Previous research found that the signal came from two black holes with a combined mass about 20 times that of the Sun. These black holes may have merged into dense clouds of dark matter, according to a new analysis.
A promising new tool for dark matter research
“This statistical significance is not high enough to claim the detection of dark matter, and further tests need to be carried out by an independent group,” said Aurekoetsea. “What we think is important to emphasize is that without a waveform model like ours, it is possible to detect a black hole merger in a dark matter environment and yet systematically classify it as having occurred in the vacuum.”
The researchers say this approach could become increasingly useful in the coming years as the number of gravitational wave observations increases.
“As the LVK detector continues to collect data for years to come, we may be able to discover dark matter around black holes,” said co-author Soumen Roy, who led the data analysis portion of the study. “It’s an exciting time to explore new physics using gravitational waves.”
“It would be great to use black holes to look for dark matter,” added co-author Rodrigo Vicente, who developed a model to analyze the signal. “We will be able to explore dark matter on a much smaller scale than ever before.”
This research was supported in part by the National Science Foundation and the Massachusetts Institute of Technology Center for Theoretical Physics (Reinweber Institute).
