How could we finally observe dark matter in an original way, this invisible substance that makes up the greatest part of the mass of the Universe? A promising lead may have just been discovered by listening to the very subtle distortions of spacetime produced by black holes.
The key to this approach lies in the study of gravitational waves (explanations at the end of the article), those tiny ripples in the fabric of space that propagate at the speed of light. When a small black hole orbits a much more massive monster located at the heart of a galaxy, it emits a continuous signal of these waves for thousands of years before finally merging. This slow evolution constitutes a unique signature that future instruments could capture with unprecedented precision.
Diagram of gravitational waves generated by two black holes orbiting closely around each other, shortly before their collision (more precisely, coalescence). The team from the Institute of Physics at the University of Amsterdam has developed a complete mathematical model based on Einstein's theory of general relativity. This model allows for a highly accurate description of how a small compact object interacts with its immediate environment as it falls towards a supermassive black hole. This is a notable advance, as previous work often used approximations to simulate these interactions.
This theoretical framework is particularly useful for studying the dense regions of dark matter (see below) that could form around central black holes, often called "spikes." By integrating this new relativistic description into wave prediction models, the physicists show how these structures would leave a measurable imprint in the signals. The European Space Agency's LISA space observatory, whose launch is scheduled for 2035, is designed to record these signals for months or years, thus following hundreds of thousands of orbital cycles.
The ability to model these gravitational waves precisely paves the way for an indirect mapping of dark matter. By analyzing the tiny modifications to the signal caused by the presence of this invisible matter, scientists could determine how it is distributed around black holes. This method thus offers a new observation window for understanding the fundamental nature of this enigmatic component of the Universe, without needing to see it directly.
When two black holes orbit each other and merge, they emit gravitational waves detectable on Earth. By studying the precise shape of these waves, scientists could probe the environment of black holes and better understand dark matter.
Credit: ESA
The results of this research, published in
Physical Review Letters, represent an important step towards using gravitational waves as a cosmic probe. They set the stage for the era of large space observatories, where listening to the whispers of spacetime could teach us much about the invisible composition of our cosmos.
Gravitational waves, messengers of spacetime
Gravitational waves are distortions in the very fabric of space and time that propagate through the Universe at the speed of light. They are produced by cataclysmic events involving enormous masses in accelerated motion, such as the merger of two black holes or the explosion of stars. These ripples, predicted by Albert Einstein in 1916, were first directly detected in 2015 by the LIGO and Virgo observatories, thus confirming a fundamental aspect of modern physics.
These waves are extremely weak because spacetime is a very rigid fabric. To measure them, scientists use laser interferometers several miles (kilometers) long, capable of detecting distance variations smaller than a billionth the size of an atom. Each gravitational wave carries a unique signature that informs about the nature of the objects that emitted it, such as their mass, distance, and how they orbited each other before merging.
The study of these signals constitutes a new astronomy, completely different from observing light. It allows exploration of phenomena that remain invisible to classical telescopes, such as isolated black holes or certain events occurring in regions obscured by dust. By listening to these cosmic vibrations, researchers are opening an unprecedented window onto the most energetic and intriguing aspects of our Universe.
Dark matter, the invisible force that structures the cosmos
Dark matter is a form of matter that neither emits nor absorbs light, making it completely invisible to traditional telescopes. Its existence is inferred indirectly from its gravitational effects on visible matter, such as the rotation of galaxies or the bending of light from distant objects. Current observations indicate that it constitutes about 85% of all matter present in the Universe, forming a vast cosmic web upon which galaxies are built.
Despite its abundance, the fundamental nature of dark matter remains one of the great unanswered questions in astrophysics and particle physics. Leading theories propose that it might be composed of exotic particles that interact very weakly with ordinary matter. Its presence is essential to explain how large-scale structures formed after the Big Bang.
Research often focuses on regions where dark matter could be more concentrated, such as around supermassive black holes at the centers of galaxies. These accumulations, sometimes called "spikes," could influence the motion of stars and other nearby objects. By using gravitational phenomena like gravitational waves to probe these environments, scientists hope to finally crack the enigma of this hidden component that gives shape to our Universe.