Dark matter is one of the biggest mysteries in modern science.
Astronomers believe it makes up most of the matter in the universe, yet nobody has ever directly seen it.
Unlike ordinary matter, dark matter does not interact with light, making it effectively invisible. Scientists can only detect its presence through gravity.
Now, researchers from Massachusetts Institute of Technology and several European institutions have developed a new method that could help reveal dark matter by studying gravitational waves produced when black holes collide.
Their findings were published in the journal Physical Review Letters.
Gravitational waves are ripples in space and time created by extremely violent cosmic events, such as two black holes spiraling into each other and merging. These waves travel across the universe and can eventually be detected on Earth by giant observatories such as the LIGO-Virgo-KAGRA Collaboration.
The scientists believe that if black holes collide inside a dense cloud of dark matter, the dark matter could slightly change the pattern of the gravitational waves produced during the merger. By carefully studying these patterns, researchers may be able to look for hidden signs of dark matter.
To test this idea, the research team created computer models that predicted what gravitational waves should look like if black holes merged inside dark matter rather than empty space.
They focused on one possible type of dark matter made from extremely lightweight particles called “light scalar” particles. Scientists think these particles may behave both like particles and waves near black holes.
According to theory, when these dark matter waves encounter a rapidly spinning black hole, the black hole’s energy can strengthen the waves through a process known as superradiance. This effect can dramatically increase the density of dark matter around the black hole.
The researchers wanted to know whether this dense dark matter environment could leave a detectable fingerprint on gravitational waves.
Using detailed simulations, the team studied many possible black hole mergers with different masses, sizes, and dark matter conditions. They then compared their predictions with real gravitational-wave data collected during the first three observing runs of the LVK detector network.
The researchers examined 28 of the clearest gravitational-wave signals detected so far.
Most of the events matched the standard expectation that the black holes merged in empty space. However, one signal, known as GW190728, behaved differently.
The pattern of this gravitational wave appeared to fit the researchers’ dark matter model better than the vacuum model. The signal was first detected on July 28, 2019, and scientists previously determined it came from two black holes with a combined mass about 20 times greater than the sun.
The researchers stress that this is not proof of dark matter. The signal is not statistically strong enough to claim a discovery. However, the result suggests that some black hole mergers may occur inside dense dark matter environments, and current methods could be overlooking them.
The new technique gives scientists a powerful way to search for dark matter using gravitational waves, opening a completely new approach to studying one of the universe’s greatest mysteries.
As gravitational-wave observatories continue collecting more data in the coming years, researchers hope they may eventually find stronger evidence that dark matter is leaving hidden fingerprints on black hole collisions across the cosmos.
