'Gravity portals' could morph dark matter into ordinary matter, astrophysicists propose
Astrophysicists have an idea that could help to solve two mysteries: the reason for the bizarre abundance of super-high-energy radiation shooting from the center of our galaxy and the identity of invisible stuff called dark matter that has perplexed the world since its discovery some 50 years ago.
And the idea has a super-cool name: gravity portals. The idea goes, when two dark matter particles (whatever they are) get sucked into one of these portals, they obliterate each other and spit out shockingly strong gamma rays.
This line of thinking can potentially explain why the galactic center — where dense clusters of dark matter are thought to lurk — is full of gamma rays; and it could shed light on how the dark matter behaves and might occasionally interact with the normal matter of our universe.
What dark matter has to be
More than 80% of the matter in our universe is of a form unknown to the standard model of particle physics. Scientists call it "dark matter," because it does not interact with light. The first hint of dark matter's existence came in the 1970s, when astronomer Vera Rubin noticed that galaxies were rotating far too quickly — without an additional, hidden source of gravity, they should have torn themselves apart eons ago.
For decades, astronomers didn't know if they needed to change their understanding of gravity, add more regular-but-really-dim matter to the universe, or include a brand new ingredient. But year after year, observation after observation have limited the choices. No theory of modified gravity can explain all observations. And physicists have placed firm limits on the amount of normal matter (bright, dim and everything in between) in the cosmos.
That leaves dark matter to explain the speedy galaxies. This matter would be a new kind of particle, with some unknown identity (or identities). It doesn't interact with light, otherwise we would've seen it by now. It doesn't interact with the strong nuclear force — which binds together particles of matter — otherwise scientists would have detected its influence in atomic experiments. It might talk to the weak nuclear force, but that force is so feeble and short-range that observing any deviations in the expected results is challenging.
Trillions of dark matter particles may be streaming through you right now, invisible and silent.
However, dark matter betrays its presence through gravity, because every single form of mass and energy in the universe exerts some sort of gravitational influence. So the only surefire way to study dark matter is through its gravitational interactions with normal matter, such as the motions of stars inside galaxies.
But there may be another way.
The case of the excess electrons
In a study published Jan. 28 to the preprint database arXiv, physicists proposed a new theory to explain what dark matter is and how it behaves. But before diving into their idea, we have to introduce one more clue into this dark-matter hunt. The clue comes in the form of a strange abundance of gamma rays observed emanating from the center of our Milky Way galaxy.
Gamma rays are the highest-energy form of radiation possible, and they usually only come into existence from some seriously high-energy events, such as stars going supernova. But there are more gamma rays than you'd expect in the galactic center given how rare such cataclysmic events there are. So, it's possible, this theory proposed, that gamma rays may emerge as a byproduct of high-energy electrons.
These high-energy electrons, which are a kind of particle known as "leptons" and much easier to produce than gamma rays directly, emanate from some source and travel throughout the galactic center. The electrons themselves are undetectable (they are very, very tiny), but as they flood through interstellar space, they can occasionally slam into a random passing photon (a light particle).
That photon, most likely something innocuous and low-energy, collides with the aggressive electron; the collision boosts the photon's energy so much that it begins to give off gamma rays that we can see.
Those collisions potentially explain the excess gamma rays, but where do those high-energy electrons come from?
Jumping through the portal
Let's recap what we know. One, dark matter interacts only through gravity. Two, high-energy leptons floating around the galactic center could explain the extra gamma rays we see there. Three, because in our own galaxy the core has the highest density of matter, we think that there is also a large concentration of dark matter there.
Coincidence? Or conspiracy?
The link between these two observations has a suitably awesome name: leptophilic gravity portals, as detailed by Sun Xu-Dong and Dai Ben-Zhong, of the China Key Laboratory of Astroparticle Physics, in their arXiv paper. The study has yet to be peer-reviewed.
Let's start with the "gravity portals" part. As far as we understand gravity, it just pulls on stuff. Earth pulls on the moon; the sun pulls on Earth; stars in a galaxy pull on each other, and so on. And gravity does a really, really good job of pulling.
So on the face of it, the only thing gravity can do to dark matter is … pull.
But our understanding of gravity is incomplete. Physics can explain gravity operates on the large scale, but there's no so-called quantum theory of gravity, which would describe strong gravity operating at very tiny scales. And in this regime, gravity might have some surprises in store.
The other forces of nature are capable of annihilating, transforming and creating particles all the time. The weak nuclear force, for instance, can change a proton into a neutron, triggering radioactive decay. A particle and its antiparticle can connect via the electromagnetic force, annihilating each other in a burst of radiation.
So maybe gravity, in extreme cases, can bring two dark matter particles together and destroy them, turning them into … anything, really.
And according to the theoretical model outlined by the researchers, maybe those dark matter particles can transform into leptons. Hence the "leptophilic" part of the name, which means "lepton-loving."
According to the new theory, dark matter particles can occasionally annihilate each other through nothing more than chance gravitational interactions. These chance interactions are known in the physics jargon as "gravity portals," since they offer a way for particles to interact through gravity alone. The product of that collision is a high-energy electron. These interactions would be much more common in the galactic center, where the density of dark matter is likely highest. Those electrons would then travel on, eventually striking a low-energy photon and turning into a gamma ray, causing the excess that we observe.
Yes, this idea is a stretch. But given that physicists are in the dark when it comes to the identity of dark matter, new ideas are always welcome. And this theory was specifically designed to match the observation of the gamma rays. But once that doorway has been opened, allowing dark matter particles to transform into regular matter (leptons, in this case), more theoretical work can be done to see if there are other ways to test the theory.
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