Recent research challenges the long-held assumption that the supermassive object at the heart of the Milky Way, Sagittarius A (Sgr A ), is a black hole. Instead, scientists propose it could be an extraordinarily dense clump of dark matter – a concept with far-reaching implications for our understanding of galaxies and the nature of dark matter itself.
The Case for Dark Matter Over Black Holes
For years, astronomers have observed stars orbiting Sgr A* at incredible speeds – up to 10% the speed of light – confirming the presence of a powerful gravitational force. The orbits of dust-shrouded “G-sources” near the Galactic Center also align with this intense gravity. Traditionally, this has been attributed to a black hole with 4.6 million times the mass of our Sun. However, a new model suggests that an equally massive clump of dark matter could produce the exact same effects.
This isn’t just about swapping one invisible object for another. The key lies in the type of dark matter. The team’s model requires “fermionic” dark matter, composed of ultra-light particles, to form a compact core mimicking a black hole while simultaneously creating a vast, diffuse halo extending beyond the visible galaxy. Other dark matter compositions can’t replicate this structure.
Gaia’s Role in Challenging Existing Models
The evidence supporting this theory comes partly from data gathered by the European Space Agency’s Gaia mission. Gaia precisely mapped the rotation of stars in the Milky Way’s outer halo, revealing a slowdown in the galactic rotation curve – a phenomenon known as Keplerian decline.
The standard cosmological model, which assumes “cold” (slow-moving) dark matter, struggles to explain this decline. Fermionic dark matter, however, predicts a tighter, more compact halo that could account for the observed slowdown. This is crucial because it bridges the gap between observations at the galactic center and those in the outer halo.
The Shadow of Doubt: Explaining the Event Horizon Telescope Image
One major challenge to the dark matter hypothesis is the image of Sgr A* captured by the Event Horizon Telescope (EHT). The glowing ring around a dark central region appears consistent with the shadow cast by a black hole. However, researchers have demonstrated that a dense core of fermionic dark matter can also bend light strongly enough to create a similar shadow, despite being invisible to direct observation.
“Our model not only explains the orbits of stars and the galaxy’s rotation but is also consistent with the famous ‘black hole shadow’ image,” says Valentina Crespi, team leader at the Institute of Astrophysics La Plata.
Next Steps: Testing the Theory
While the dark matter model has successfully replicated observed behaviors, including stellar orbits, galactic structure, and the black hole shadow, it remains early days. The team proposes future observations using the Very Large Telescope (VLT) to search for “photon rings” around Sgr A*. Their presence would confirm a black hole; their absence could strengthen the case for a dark matter core.
The debate is far from settled. The Milky Way’s central mystery remains – a dark matter clump or a supermassive black hole? Only further observations will reveal the truth.
