Images from the Event Horizon Telescope reveal a new method for detecting dark matter

According to a new Physical Examination Letters study, black holes could help solve the mystery of dark matter. The dark regions in images of black holes captured by the Event Horizon Telescope may act as ultra-sensitive detectors for the invisible matter that makes up most of the matter in the universe.

Dark matter makes up about 85% of the matter in the universe, 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 new detection method, which has distinct advantages.

Stunning images of supermassive black holes taken by the Event Horizon Telescope have revealed much more than just the geometry of space-time; they opened an unexpected window into the search for dark matter.

Phys.org spoke with co-authors Jing Shu of Peking University and Yifan Chen of 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 limits of known physical laws,” Shu said.

Chen added: “I was 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 together to achieve Earth-scale resolution using very long baseline interferometry. Operating at a frequency of 230 GHz, the telescope captures synchrotron radiation, the light produced when electrons spiral along intense magnetic field lines near supermassive black holes.

To understand what they see, astrophysicists perform complex computer simulations.

The magnetically arrested disk (MAD) model consistently provided the best agreement with the EHT observations. The MAD model depicts powerful magnetic fields penetrating the accretion disk, where they regulate both the flow of incoming material and the power jets erupting perpendicular to the disk.

The MAD model crucially 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 particularly weak,” Chen explained. “Dark matter, however, could continually inject new particles radiating into this region.”

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

Dark matter modeling

The gravitational pull of supermassive black holes causes a dramatic concentration of dark matter nearby, forming what physicists call a “dark matter peak.” These regions reach densities an order of magnitude higher than anywhere else in the galaxy.

Since dark matter annihilation rates depend on the square of the density, these increased densities could produce detectable signals, if annihilation occurs.

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

The team applied general relativistic magnetohydrodynamic (GRMHD) simulations as well as detailed modeling of particle propagation. With this framework, they could model the behavior of electrons and positrons resulting from a hypothetical annihilation of dark matter 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 configurations of magnetic fields extracted from MAD simulations, the same fields that shape the astrophysical emission we observe.

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

“If dark matter particles annihilate near the black hole, they would produce additional electrons and positrons whose radiation appears slightly different from normal emission.”

The crucial distinction appears in the 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 more evenly distributed in disk and jet regions, because dark matter annihilation continually supplies particles, even where astrophysical processes produce few electrons.

The team examined two annihilation channels – bottom quark-antiquark pairs and electron-positron pairs – in masses of dark matter ranging from less than GeV to around 10 TeV.

For each scenario, they calculated the resulting synchrotron radiation and generated synthetic images of black holes combining both astrophysical emissions (from MAD) and potential dark matter signals.

Morphology as a probe

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

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

“By comparing these predictions with real EHT images in the ‘dark room,’ we can look for subtle signals that might reveal dark matter,” Shu said.

This morphological approach turns out to be significantly more powerful than previous constraints based on total intensity alone. The analysis excludes substantial regions of previously unexplored parameter space, setting the limits of annihilation cross sections at 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, outperforming other research that assumes similar density profiles,” Chen said.

The constraints remain robust in the face of astrophysical uncertainties, notably variations in black hole rotation and plasma temperature parameters, factors which generally introduce significant uncertainties in indirect searches for dark matter.

Future outlook

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

“The main improvement is to improve the dynamic range of the telescope, that 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 found on many smartphones, which uses advanced processing to bring out details in dark shadows and highlights in the same image.”

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

Looking ahead, the researchers are considering several directions to expand this research.

“The black hole shadow is not just a static image; it is a dynamic laboratory on many levels,” Shu said. “Beyond intensity maps, EHT polarization data also open new windows, because polarization encodes how magnetic fields and plasma shape radiation.”

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

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