Dark matter is thought to make up most of the matter in the universe, but it only interacts with its surroundings through gravity. If two colliding black holes spiral through a dense region of dark matter and merge, gravitational waves could carry an imprint of that dark matter. Researchers at MIT and in Europe have developed a method to predict how gravitational waves would appear if produced by black holes moving through dark matter, rather than empty space. They applied this technique to publicly available gravitational-wave data recorded by LIGO-Virgo-KAGRA (LVK) during the first three observing runs. From 28 of the clearest signals, the team found that 27 originated from black holes merging in a vacuum, as expected. However, one signal, GW190728, showed possible signs of a dark matter imprint. The scientists emphasize that they have not detected dark matter directly, but this method allows for screening gravitational-wave data for hints of dark matter that can be followed up with other techniques.
"We know 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 are reported in a study appearing today in Physical Review Letters.
Dark matter is an invisible, hypothetical form of matter that does not interact with electromagnetic forces. It can pass through light and magnetic fields without leaving a trace. The only evidence for dark matter is its apparent interaction with gravity. Astronomers have inferred the existence of dark matter by observing how gravity bends around distant galaxies, suggesting an extra force beyond the galaxies' own gravitational pull. This extra force is suspected to be dark matter, accounting for over 85 percent of the universe's matter. However, the exact nature of dark matter is still a topic of great debate.
To predict what a gravitational wave from a black hole binary would look like if it carried a dark matter imprint, the research team developed a model that involved detailed numerical simulations. They considered various properties of colliding black holes and the density of dark matter. They then applied their model to LVK's publicly available data, focusing on the clearest signals. Among the 28 analyzed signals, GW190728 showed a pattern that may match their dark matter model, although the statistical significance is not high enough to claim a detection of dark matter.
"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. "It is an exciting time to search for new physics using gravitational waves."
Blogger's Review: This research opens up a novel approach to detecting dark matter, leveraging gravitational waves as a probing tool. The implications for understanding the composition of the universe are profound, and this method could lead to significant discoveries about the elusive nature of dark matter. The use of black holes in this context provides a fresh perspective on fundamental physics and could reshape our understanding of cosmic phenomena.