Black Holes May Hold the Key to Solving the Mystery of Dark Matter
A Groundbreaking Discovery
A groundbreaking study published in Physical Review Letters suggests that black holes might hold the key to solving the long-standing mystery of dark matter. Researchers reveal that the dark regions captured in Event Horizon Telescope images could serve as powerful detectors for the universe's unseen mass.
Dark matter accounts for nearly 85% of all matter in the universe, yet its true nature remains a mystery. While scientists have long explored various detection techniques, a new study proposes an innovative approach—using black hole imagery as a tool for discovery. The Event Horizon Telescope's remarkable images of supermassive black holes not only unveil spacetime's geometry but also provide a new avenue for probing dark matter.
In an interview with Phys.org, co authors Jing Shu of Peking University and Yifan Chen of the Niels Bohr Institute shared their insights.
"I've always been captivated by instruments such as the Event Horizon Telescope, which let us explore the extreme conditions surrounding supermassive black holes and push the limits of known physics," Shu remarked.
Chen added, "The notion of using black holes as detectors for undiscovered particles fascinates me. Their immense gravity acts as a natural lens for matter, bridging particle physics, gravity and astrophysical study."
The researchers concentrated on a captivating detail seen in black hole imagery—the shadowed region visible in the Event Horizon Telescope (EHT) observations of M87* and Sagittarius A*.
A Cosmic Darkroom
The Event Horizon Telescope (EHT) is a worldwide collaboration of radio observatories that combine their power to form an Earth-sized virtual telescope using Very Long Baseline Interferometry. Operating at a frequency of 230 GHz, it captures synchrotron radiation—light emitted as electrons spiral through the intense magnetic fields surrounding supermassive black holes.
To interpret the data from their observations, astrophysicists rely on advanced computer simulations. The magnetically arrested disk (MAD) model has consistently aligned most closely with results from the Event Horizon Telescope. In this model, powerful magnetic fields thread through the accretion disk, controlling the inflow of matter and fuelling the twin jets shoot out perpendicularly. The MAD model also accounts for the darkness of black hole shadows, as most electrons are concentrated within the disk, leaving the jet regions relatively sparse and producing stark contrast in the images.
"Ordinary astrophysical plasma is often expelled through powerful jets, leaving the shadow region unusually dim," explained Chen. "Dark matter, however, might continuously inject new particles that emit radiation within this zone."
Because dark matter is predicted to cluster densely near a black hole's core, even faint annihilation signals could emerge clearly against the weak astrophysical background, making the shadow region a prime site for testing.
Modelling Dark Matter
The immense gravitational force of supermassive black holes draws dark matter tightly toward them, creating what physicists describe as a "dark matter spike." These regions reach densities far greater than anywhere else in the galaxy.
Because dark matter annihilation rates rise with the square of its density, these extreme concentrations could, in theory, yield observable signals—provided annihilation actually occurs.
To explore this, researchers built an advanced model expanding upon the MAD framework by incorporating dark matter physics into established astrophysical simulations. Using general relativistic magnetohydrodynamic (GRMHD) modelling and detailed particle propagation analyses, the team simulated how electrons and positrons from hypothetical dark matter annihilation would move through the magnetic fields predicted by the MAD model.
Unlike earlier studies that relied on oversimplified spherical assumptions, this new method employs the intricate, asymmetric magnetic field structures drawn from MAD simulations—the very fields responsible for shaping the observed astrophysical emissions.
"What appears in black hole images isn't the black hole itself, but light produced by ordinary electrons in the accretion disk, whose dynamics we can model using established physical laws," explained Shu.
If dark matter particles were to annihilate near the black hole, they would generate additional electrons and positrons, producing radiation subtly distinct from the standard emission.
Distinguishing the Dark Matter Signature
The key difference lies in spatial distribution. In the MAD model, electrons are densely packed within the accretion disk but remain sparse in the jet regions, producing the characteristic dark shadow.
By contrast, electrons and positrons generated through dark matter annihilation would spread more evenly throughout both the disk and jet zones, as annihilation continuously injects particles even in regions where astrophysical processes yield few.
The researchers explored two annihilation pathways—bottom quark—antiquark and electron-positron pairs—spanning dark matter masses from below a giga-electronvolt (GeV) up to roughly 10 tera-electronvolts (TeV).
For every scenario, the researchers computed the expected synchrotron radiation and produced simulated black hole images, incorporating both the astrophysical emission from the MAD model and potential contributions from dark matter.
Morphology as a Probe
The team's method, which focuses on the shape of black hole images rather than just their overall brightness, gives the study a unique edge. They ensured that any dark matter annihilation signals would remain below the level of astrophysical emission throughout the image, especially within the central shadow.
"By comparing our models with actual EHT observations of the dark region, we can hunt for faint signals that might betray the presence of dark matter," Shu explained.
Focusing on the shape of black hole images rather than just total brightness has proven far more powerful. This approach rules out large sections of previously unexplored parameter space, constraining annihilation cross sections down to around 10⁻²⁷ cm³/s for current EHT data.
"Even with current observations, our results probe regions of parameter space that were previously inaccessible, outperforming searches based on similar density profiles." Chen explained.
The limits hold firm despite uncertainties in astrophysical factors such as black hole spin and plasma temperatures, which often complicate indirect dark matter studies.
Future Prospects
The full potential of this method will be unlocked with upcoming EHT upgrades. Future enhancements are expected to boost dynamic range by nearly 100-fold and reach angular resolutions to a single gravitational radius, allowing astronomers to investigate the deepest parts of black hole shadows.
"The crucial improvement is enhancing the telescope's dynamic range, letting us detect very faint structures adjacent to intensely bright features," Chen said.
"It's similar to the HDR mode on smartphones, which reveals details in both shadows and highlights within the same shot," he added.
These upgrades could allow astronomers to detect dark matter with annihilation cross sections approaching the thermal relic benchmark, for masses up to around 10 TeV.
Expanding the Horizon
Looking forward, the team plans to explore multiple avenues to expand this research.
"The black hole shadow is far more than a static silhouette; it acts as a dynamic, multi-layered laboratory," Shu explained. "Polarization measurements from the EHT provide further insight, revealing how magnetic fields and plasma influence the emitted radiation."
Shu emphasizes that multi-frequency observations will be vital. Varying radiation mechanisms respond differently to frequency, enabling astronomers to identify the radiation source and use different "colours" to separate dark matter signals from astrophysical noise.
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