Something is emanating from the Milky Way’s center. It is a signal rather than a message or a disaster, at least not as far as anyone is aware. Actually, there were three of them. Astronomers have been observing these weak, continuous emissions from deep within our galaxy for years with the unsettling knowledge that none of the conventional explanations fully account for them. Cosmic rays, stellar remnants, supernovae—everything the textbooks suggest has been tried and failed.
Researchers at King’s College London now think they might be getting close to a solution. It won’t be merely a footnote in an astrophysics journal if they are correct. Scientists’ understanding of the invisible material that makes up the majority of matter in the known universe may be drastically altered.
| Detail | Information |
|---|---|
| Topic | Mysterious signals from the center of the Milky Way Galaxy |
| Lead Institution | King’s College London, Department of Physics |
| Lead Researcher | Dr. Shyam Balaji, Postdoctoral Research Fellow |
| Secondary Research | SETI Institute / Columbia University (Karen Perez) |
| Key Signal | 511-keV gamma-ray emission line + 2 MeV gamma-ray continuum |
| Location of Signal Origin | Galactic Centre / Central Molecular Zone (CMZ) |
| Proposed Cause | Excited Dark Matter (particle collision model) |
| Secondary Signal Source | Suspected millisecond pulsar near Sagittarius A* |
| Black Hole Reference | Sagittarius A* — 4 million solar masses |
| Detection Tool | ESA’s INTEGRAL space telescope; Breakthrough Listen Survey |
| Published In | The Astrophysical Journal |
| Reference Website | King’s College London – Physics |
Roughly 27% of all matter in the universe is dark matter. It neither emits nor absorbs light, and no human-made instrument has ever been able to directly detect it. Apparently, it keeps things together. The visible matter, such as stars, gas, and dust, is insufficient to produce the gravitational force required to keep large spiral galaxies intact when astronomers observe them rotating at the speeds they do. They ought to be tearing themselves to pieces.
They’re not. The holding is being done by something invisible. That something is dark matter, according to scientific consensus, and determining its nature has emerged as one of the most important issues in contemporary physics.
The lead author of the new study, Dr. Shyam Balaji, a Postdoctoral Research Fellow in the Department of Physics at King’s College London, has spent a lot of time considering a specific type of dark matter called “excited dark matter.” According to the theory, dark matter particles collide, momentarily absorb energy into what scientists refer to as an excited state, and then release that energy.
According to the theory, these collisions release positrons, which are positively charged electrons that travel throughout the galaxy and produce observable signals. It’s a beautiful mechanism. It was unclear if it had anything to do with what telescopes were really seeing.
The King’s College team now contends that the answer is yes, possibly for three signals at once. The first is the 511-keV emission line, a sharp peak in gamma rays at a very particular energy level. A tenuous connection between this signal and excited dark matter had already been suggested by earlier studies, but it remained isolated and speculative. The 2 MeV gamma-ray continuum, a wider, higher-energy emission, is the second signal.
The third, and possibly most fascinating, concerns abnormally high ionization levels in the Central Molecular Zone, a dense gas region close to the galactic center. Ionization has been attempted and failed to be explained by known sources. Positrons from dark matter collisions may be the cause, according to the King’s College model.
When you put those three pieces of information together, you can see why the research community is interested. A single unexplained signal might be an anomaly. Two begin to resemble a pattern. Three, all explained by one model? That begins to seem authentic.
“When we look at well-known astrophysical events, like star explosions,” Balaji said, “they haven’t been able to provide a full explanation for mysteries like the specific energy and shape we’ve observed coming from the centre of the Milky Way.” He and his colleagues contend that the excited dark matter model accomplishes what those events were unable to.
Meanwhile, another signal from the galactic center is attracting its own group of interested scientists from a different line of inquiry. A group headed by Karen Perez, a postdoctoral researcher at the SETI Institute who carried out the research while pursuing her doctorate at Columbia University, thinks they may have found something completely different hiding close to Sagittarius A*, the supermassive black hole that anchors the Milky Way’s core and has the mass of four million suns.
According to their data, which was taken from Breakthrough Listen’s Galactic Center Survey and published in The Astrophysical Journal, a rapidly rotating neutron star—the dense remnant core left behind after a massive star collapses—appears to be a pulsar. Every 8.19 milliseconds, the suspected object rotates once. That is about 122 times per second.
If confirmed, this pulsar would be located in one of the most extreme gravitational environments in the known galaxy. Pulsars are sometimes referred to as cosmic lighthouses because of the way their spinning beams periodically sweep past Earth.
Beyond its sheer weirdness, Einstein is the reason it matters. A pulsar in such close proximity to Sagittarius A* would be extremely sensitive to the warping of space-time brought on by the massive gravity of the black hole. Signals passing through such areas are predicted by general relativity to experience very specific effects, such as deflections and time delays.
In essence, a pulsar there would serve as a natural laboratory for testing those hypotheses in previously unattainable ways. One of the most exotic objects in physics sitting next to one of the most extreme environments in the universe, potentially offering a test of the rules of the universe itself, has an almost poetic quality that is difficult to ignore.
Researchers in this field feel that the galactic center is beginning to give up. For many years, it was too far away, too cluttered, and too covered in radiation and dust to be properly studied. The circumstances have changed as a result of improved instruments, more sensitive telescopes, and the public release of Breakthrough Listen’s data, which enables independent teams from all over the world to examine it.
Slowly and imperfectly, what was once a frustrating tangle of unexplained emissions is starting to resolve into something that could be understood.
As more data becomes available, it’s still unclear if the excited dark matter model will withstand scrutiny. The team recognizes that in order to properly test the predictions, next-generation space missions built especially to detect low-energy gamma rays will be needed. Confirmation would be important because it would be the first time an indirect mechanism for dark matter has been demonstrated to simultaneously explain several independent observational mysteries, not in some nebulous, all-important sense. That would provide researchers with a clear path to follow.
No one knows yet whether that thread leads somewhere or completely unravels. In the short term, that kind of uncertainty is unsettling, but in the long run, it can be beneficial. The deep galactic center is still sending out signals. For the first time in a long time, scientists believe they may finally know where to look.
