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According to a new study, black holes could help solve the dark matter mystery. The shadowy regions in black hole images captured by the Event Horizon Telescope can act as ultra-sensitive detectors for the invisible material that makes up most of the universe's matter.

Dark matter makes up roughly 85% of the universe's matter, but scientists still don't know what it actually is. While researchers have proposed countless ways to detect it, this study introduces black hole imaging as a fresh detection method—one that comes with some distinct benefits.

The Event Horizon Telescope's stunning images of supermassive black holes have revealed more than just the geometry of spacetime; they've opened an unexpected window into the search for dark matter.

Â鶹ÒùÔº spoke to co-authors Jing Shu from Peking University and Yifan Chen from the Niels Bohr Institute.

"I have always been fascinated by instruments like the Event Horizon Telescope (EHT), which allow us to probe the extreme environments around supermassive black holes and challenge the boundaries of known physical laws," Shu said.

Chen added, "I've been fascinated by the idea of using black holes as detectors for new particles. Their extreme gravity makes them natural concentrators of matter, creating a unique meeting point for particle physics, gravity, and astrophysical observation."

The research team focused on a striking feature of black hole images: the shadow region that appears dark in EHT observations of M87* and Sagittarius A*.

A cosmic darkroom

The Event Horizon Telescope is a global network of radio observatories working in concert to achieve Earth-sized resolution through Very Long Baseline Interferometry. Working at a frequency of 230 GHz, the telescope captures synchrotron radiation—light produced when electrons spiral along the intense magnetic field lines near supermassive black holes.

To understand what they're seeing, astrophysicists run complex computer simulations.

The magnetically arrested disk (MAD) model has consistently delivered the best agreement with EHT observations. The MAD model depicts strong magnetic fields penetrating the accretion disk, where they both regulate the flow of infalling matter and power jets that erupt perpendicular to the disk.

Crucially, the MAD model explains why black hole shadows appear dark: most electrons reside in the accretion disk, while the jet regions above and below are relatively particle-poor, creating a sharp contrast in the images.

"Ordinary astrophysical plasma is often expelled by powerful jets, leaving the shadow region especially faint," Chen explained. "Dark matter, however, could continuously inject new particles that radiate in this region."

Because dark matter is expected to concentrate densely near the black hole's center, even faint annihilation signals could stand out against this low astrophysical background, making the shadow an ideal testing ground.

Modeling dark matter

The gravitational pull of supermassive black holes causes dark matter to concentrate dramatically in their vicinity, forming what physicists call a "dark matter spike." These regions achieve densities orders of magnitude higher than anywhere else in the galaxy.

Since dark matter annihilation rates depend on density squared, these enhanced densities could produce detectable signals—if the annihilation occurs at all.

The research team developed a sophisticated framework that builds directly on the MAD model by adding dark matter physics to the astrophysical baseline.

The team applied general relativistic magnetohydrodynamic (GRMHD) simulations along with detailed particle propagation modeling. With this framework, they could model how electrons and positrons from hypothetical dark matter annihilation would behave in the magnetic field structures extracted from the MAD model.

Unlike previous studies that relied on simplified spherical models, this approach uses the realistic, asymmetric magnetic field configurations extracted from the MAD simulations—the same fields that shape the astrophysical emission we observe.

"What we see in black hole images is not the black hole itself, but light emitted by ordinary electrons in the surrounding accretion disk, whose behavior we can model using well-known physics," Shu said.

"If dark matter particles were annihilating near the black hole, they would produce extra electrons and positrons whose radiation looks slightly different from the normal emission."

The critical distinction emerges in spatial distribution. In the MAD model, electrons concentrate in the accretion disk with sparse populations in the jet regions—creating the dark shadow.

But electrons and positrons from dark matter annihilation would be distributed more uniformly throughout both disk and jet regions, because dark matter annihilation continuously supplies particles even where astrophysical processes produce few electrons.

The team examined two annihilation channels—bottom quark-antiquark pairs and electron-positron pairs—across dark matter masses ranging from sub-GeV to approximately 10 TeV.

For each scenario, they calculated the resulting synchrotron radiation and generated synthetic black hole images that combined both astrophysical emission (from MAD) and potential dark matter signals.

Morphology as a probe

The researchers' approach to exploiting the morphology of the black hole images rather than just the total brightness makes the work stand out.

They required that dark matter annihilation signals remain below astrophysical emission at every point in the image, particularly within the inner shadow region.

"By comparing these predictions with real EHT images at the 'darkroom,' we can search for subtle signals that may reveal dark matter," Shu said.

This morphological approach proves significantly more powerful than previous constraints based on total intensity alone. The analysis excludes substantial regions of previously unexplored parameter space, setting limits on annihilation cross sections down to approximately 10^-27 cm³/s for current EHT observations.

"Our exclusions based on current EHT observations already probe large regions of previously unexplored parameter space, surpassing other searches that assume similar density profiles," Chen said.

The constraints remain robust against astrophysical uncertainties, including variations in black hole spin and plasma temperature parameters—factors that typically introduce significant uncertainties in indirect dark matter searches.

Future prospects

The true power of this approach will be realized with anticipated EHT upgrades. Future improvements promise to increase dynamic range by nearly 100 times and achieve angular resolution equivalent to approximately one gravitational radius, enabling them to probe deeper into the darkest regions of the shadow.

"The key upgrade is improving the telescope's dynamic range, which is its ability to reveal very faint details right next to extremely bright features," Chen explained.

"A common example is the 'high dynamic range' (HDR) mode on many smartphones, which uses advanced processing to bring out details in both dark shadows and bright highlights in the same image."

These enhancements could enable detection of dark matter with annihilation cross sections near the thermal relic value, a theoretically well-motivated target, for masses up to approximately 10 TeV.

Looking ahead, the researchers envision several directions for expanding this research.

"The black hole shadow is not just a static image; it is a dynamic, multi-layered laboratory," Shu said. "Beyond the intensity maps, polarization data from the EHT also open new windows, because polarization encodes how magnetic fields and plasma shape the radiation."

Multi-frequency observations will also prove crucial, according to Shu. Different radiation mechanisms scale differently with frequency, allowing researchers to determine the source of radiation—essentially using multiple colors to distinguish dark matter signals from astrophysical backgrounds.

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