‘Gravity portals’ could morph dark matter into ordinary matter, astrophysicists propose
Astrophysicists have proposed a solution to two mysteries: the origins of our galaxy’s extraordinary abundance of super-high-energy radiation and the nature of dark matter, an invisible substance that has perplexed researchers since its discovery 50 years ago.
And the concept has a catchy name: gravity portals. When two dark matter particles (whatever they are) are sucked through one of these portals, they obliterate one another and spit out shockingly powerful gamma rays, according to the hypothesis.
This line of thought may explain why the galactic center, where dense clusters of dark matter are thought to lurk, is full of gamma rays, as well as shed light on how dark matter behaves and may occasionally interact with regular matter in our universe.
What dark matter has to be
More than 80% of the matter in our universe is in a form that the conventional model of particle physics can not comprehend. Scientists call it “dark matter,” because it does not interact with light. The first hint of the presence of dark matter came in the 1970s, when astronomer Vera Rubin found that galaxies were rotating much too fast – without an additional, hidden source of gravity, they should have torn 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. However, year after year, observation after observation has narrowed the possibilities. No modified gravity theory can account for 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 account for the speedy galaxies. This matter would be a new kind of particle, with some unknown identity (or identities). It doesn’t interact with light, or we’d saw it by now. Which has no contact with the strong nuclear force, which binds matter particles together; otherwise, scientists would have detected its influence in atomic experiments. It might interact with the weak nuclear force, but that force is so weak and short-ranged that identifying any deviations from expected results is hard.
Trillions of invisible and quiet dark matter particles may be streaming through you right now.
Dark matter, on the other hand, exposes its presence through gravity, because every type of mass and energy in the cosmos has some gravitational influence. As a result, the only sure approach to study dark matter is through its gravitational interactions with normal matter, such as star motions inside galaxies.
But there may be another way.
The case of the excess electrons
Physicists proposed a new hypothesis to explain what dark matter is and how it behaves in a study published to the preprint database arXiv. But, before we go into their plan, we need to add one more clue to their dark-matter hunt. A strange abundance of gamma rays found emanating from the center of our Milky Way galaxy provides the clue.
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. However, considering how rare such cataclysmic events are, there are more gamma rays than you’d expect near the galactic center. So, it’s possible, this theory proposes, 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 are much easier to make directly than gamma rays, emanate from a source and move around the galactic center. The electrons themselves are undetectable (they are extremely small), but when they flood through interstellar space, they may collide with a passing photon (a light particle).
That photon, most likely something innocuous and low-energy, collides with the aggressive electron; the impact increases the photon’s energy so much that it begins to produce visible gamma rays.
Those collisions potentially explain the excess gamma rays, but where do those high-energy electrons come from?
Jumping through the portal
Let’s go over what we know. One, dark matter only interacts through gravity. Two high-energy leptons floating around the galactic center could explain additional gamma rays. Three, because the core of our galaxy has the highest density of matter, we believe there is also a significant concentration of dark matter there.
Coincidence? Or conspiracy?
Sun Xu-Dong and Dai Ben-Zhong of the China Key Laboratory of Astroparticle Physics described the relationship between these two observations in their arXiv paper: leptophilic gravity portals. The study has yet to be peer-reviewed.
Let’s begin with the “gravity portals” part. Gravity, as we understand it, just pulls on objects. The moon pulls on the Earth; the sun pulls on the Earth; stars in a galaxy pull on each other, and so on. And gravity does an excellent job of pulling.
On the ground, gravity’s only effect on dark matter is to… pull.
However, our understanding of gravity is inadequate. Although physics can explain gravity on a large scale, there is no so-called quantum theory of gravity that can predict strong gravity on extremely small scales. And gravity may have some surprises in store for everyone in this regime.
Nature’s other forces are constantly capable of annihilating, transforming, and creating particles. For example, the weak nuclear force can convert a proton into a neutron, resulting in radioactive decay. The electromagnetic force can link a particle and its antiparticle, annihilating each other in a burst of radiation.
So, in extreme cases, gravity may pull two dark matter particles together and destroy them, turning them into… anything.
And, according to the researchers’ theoretical model, such dark matter particles may be able to change into leptons. Hence the “leptophilic” part of the name, which means “lepton-loving.”
Dark matter particles, according to the new hypothesis, can occasionally annihilate one other through nothing more than random 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. A high-energy electron is the result of such collision. These interactions would be far more common at the galactic core, where dark matter density is most usually the best option. Those electrons would then travel on, eventually striking a low-energy photon and turning into a gamma ray, causing the excess that we observe.