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Astrophysicists have an idea that could help solve two mysteries: the reason for the bizarre abundance of very high energy radiation coming from the center of our galaxy and the identity of an invisible substance called black matter that has left the world perplexed since its discovery 50 years ago.
And the idea has a super cool name: gravity portals. The idea goes, when two dark matter particles (whatever they are) are sucked into one of these portals, they obliterate each other and spit out with shocking force. gamma rays.
This line of thought can potentially explain why the galactic center – where dense clusters of dark matter are believed to be lurking – is full of gamma rays; and it could shed light on how dark matter behaves and could sometimes interact with normal matter in our universe.
Related: The 11 biggest unanswered questions about dark matter
What dark matter must 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 clue to the existence of dark matter came in the 1970s, when astronomer Vera Rubin noticed that galaxies were spinning far too quickly – with no additional, hidden source of gravity, they should have been torn apart centuries ago.
For decades, astronomers weren’t sure whether to change their understanding of gravity, add more regular but truly dark matter to the universe, or include a whole new ingredient. But year after year, observation after observation has limited the choices. No theory of modified gravity can explain all the observations. And physicists have placed firm limits on the amount of normal matter (bright, dark, and everything in between) in the cosmos.
This leaves dark matter to explain fast galaxies. This matter would be a new type of particle, with an unknown identity (or identities). It does not interact with light, otherwise we would have seen it already. It does not interact with the strong nuclear force – which binds particles of matter – otherwise scientists would have detected its influence in atomic experiments. It could speak of the weak nuclear force, but this force is so weak and at close range that it is difficult to observe any deviation in the expected results.
Billions of dark matter particles can flow through you right now, invisible and silent.
However, dark matter betrays its presence by gravity, as every form of mass and energy in the universe exerts some sort of gravitational influence. The only surefire way to study dark matter is therefore its gravitational interactions with normal matter, such as the movements of stars inside galaxies.
But there may be another way.
The case of excess electrons
In a study published Jan. 28 in the arXiv preprint database, physicists came up with a new theory to explain what dark matter is and how it behaves. But before we dive into their idea, we need to introduce one more clue into this dark matter hunt. The clue comes in the form of a strange abundance of observed gamma rays emanating from the center of our Milky Way galaxy.
Related: 11 fascinating facts about our Milky Way galaxy
Gamma rays are the most energetic form of radiation possible, and they usually only appear from certain high-energy events, such as stars becoming supernovae. But there are more gamma rays than you would expect in the galactic center given the rarity of these cataclysmic events. It is therefore possible, according to this theory, that gamma rays 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 a source and travel through the galactic center. The electrons themselves are undetectable (they’re very, very small), but when they flood interstellar space, they can sometimes crash into a random passing photon (a luminous particle).
This photon, probably something harmless and low energy, collides with the aggressive electron; the collision increases the energy of the photon so much that it starts to emit gamma rays that we can see.
These collisions potentially explain the excess gamma rays, but where do these high energy electrons come from?
Jump through the portal
Let’s recap what we know. First, dark matter only interacts by gravity. Two, high energy leptons floating around the galactic center could explain the extra gamma rays we see there. Third, because in our own galaxy the core has the highest density of matter, we believe that there is also a large concentration of dark matter.
Coincidence? Or conspiracy?
The link between these two observations has a very brilliant name: leptophilic gravity portals, as detailed by Sun Xu-Dong and Dai Ben-Zhong, of the China Key Laboratory of Astroparticle Physics, in their article arXiv. The study has not yet been peer reviewed.
Let’s start with the “gravity portals” part. As far as we understand gravity, it just shoots things. Earth pulls on Moon; the sun shoots on the Earth; stars in a galaxy shoot at each other, and so on. And gravity does a very, very good job of pulling.
So at first glance, the only thing gravity can do on dark matter is… shoot.
But our understanding of gravity is incomplete. Physics can explain that gravity works on a large scale, but there is no quantum theory of gravity, which would describe strong gravity operating at very small scales. And in this regime, gravity could hold some surprises.
The other forces of nature are able to annihilate, transform and create particles at any time. Weak nuclear force, for example, can turn a proton into a neutron, triggering radioactive decay. A particle and its antiparticle can connect via electromagnetic force, annihilating each other in a burst of radiation.
So maybe gravity, in extreme cases, can pull two dark matter particles together and destroy them, turning them into… anything, really.
And according to the theoretical model sketched by the researchers, maybe these dark matter particles can turn into leptons. Hence the “leptophile” part of the name, which means “lepton loving”.
According to the new theory, dark matter particles can sometimes annihilate each other by nothing more than chance gravitational interactions. These chance interactions are known in physics jargon as “gravitational portals” because they provide a way for particles to interact only through gravity. The product of this collision is a high energy electron. These interactions would be much more frequent in the galactic center, where the density of dark matter is probably the highest. These electrons would then move around, eventually hitting a low-energy photon and turning into a gamma ray, causing the excess that we observe.
Yes, this idea is exaggerated. But since 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 correspond to the observation of gamma rays. But once that gate 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 for the theory.
Originally posted on Live Science.
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