Astrophysicists have an idea that could help solve two mysteries: the reason for the strange abundance of ultra-high energy launches from the center of our galaxy, and the identity of invisible things called dark matter which has confused the world since its discovery about 50 years ago.
And the idea has a super cool name: gravity portals. The idea is that when two particles of dark matter (whatever they are) are sucked into one of these portals, they erase each other and spit out shockingly gamma rays.
This line of thought could potentially explain why the galactic center ̵
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What dark matter it must be
More than 80% of the matter in our universe is in 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 the existence of dark matter came in the 1970s, when astronomer Vera Rubin noticed that galaxies rotated too fast – without an additional, hidden source of gravity, they had to be torn apart eons ago.
For decades, astronomers did not know whether to change their understanding of gravity, add more regular but really dark matter to the universe, or add a whole new ingredient. But year after year, observation after observation limits choice. No theory of modified gravity can explain all observations. And physicists have set strict limits on the amount of normal matter (bright, obscure, and everything in between) in space.
This leaves dark matter to explain fast galaxies. This question would be a new kind of particle, with some unknown identity (or identities). It does not interact with light, otherwise we would have seen it by now. It does not interact with strong nuclear force – which binds particles of matter – otherwise scientists would detect its influence in atomic experiments. It can speak with a weak nuclear power, but this power is so weak and short-range that observing any deviations in the expected results is a challenge.
Trillions of particles of dark matter can radiate through you in the moment, invisible and silent.
However, dark matter betrays its presence through gravity, because every form of mass and energy in the universe exerts some kind of gravitational influence. So the only sure way to study dark matter is through its gravitational interactions with normal matter, such as the motions of stars in galaxies.
But there may be another way.
The case of excess electrons
In a study published Jan. 28 in the arXiv preprint database, physicists proposed a new theory to explain what dark matter is and how it behaves. But before we dive into their idea, we need to introduce another clue to this hunt for dark matter. The indication comes in the form of a strange abundance of gamma rays observed from the center of our Milky Way galaxy.
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Gamma rays are the most energetic form of radiation possible, and they usually only arise from certain serious high-energy events, such as supernova-going stars. But there is more gamma-ray radiation than you would expect in the galactic center, given how rare such cataclysms are. So, it is possible, this theory suggests, that gamma rays appear as a by-product of high-energy electrons.
These high-energy electrons, which are a type of particle known as leptons and much easier to produce than direct gamma rays, come from a source and travel throughout the galactic center. The electrons themselves are undetectable (they are very, very small), but as they flood through interstellar space, they can sometimes collide with a randomly passing photon (light particle).
This photon, most likely something harmless and low-energy, collides with the aggressive electron; the collision amplifies the energy of the photon so much that it begins 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?
Jumping through the portal
Let’s summarize what we know. One, dark matter interacts only through gravity. Two, high energy leptons Flying around the galactic center may explain the additional gamma rays we see there. Three, because in our own galaxy the nucleus has the highest density of matter, we believe that there is also a high concentration of dark matter.
A coincidence? Or a conspiracy?
The connection between these two observations bears an aptly great name: leptophilic gravitational portals, as described in detail by Sun Xu-Dong and Dai Ben-Zhong, of China’s key laboratory for astroparticle physics, in their paper arXiv. The study has not yet been reviewed.
Let’s start with the “gravity portals” section. As far as we understand gravity, it just pulls things. The earth pulls on the moon; the sun pulls on the Earth; stars in the galaxy are drawn to each other, and so on. And gravity does a really good job.
So at first glance, the only thing gravity can do with dark matter is … pull.
But our understanding of gravity is incomplete. Physics can explain that gravity acts on a large scale, but there is no so-called quantum theory of gravity to describe strong gravity operating on a very small scale. And in this mode, gravity can have some surprises.
Other natural forces are capable of destroying, transforming and creating particles all the time. A weak nuclear force, for example, can change a proton into a neutron, causing radioactive decay. A particle and its antiparticle can be connected by electromagnetic force, destroying each other in a burst of radiation.
So maybe gravity, in the extreme, can unite two particles of dark matter and destroy them, turning them into … everything, really.
And according to the theoretical model outlined by the researchers, perhaps these particles of dark matter can be transformed into leptons. Hence the “leptophilic” part of the name, which means “leptophile”.
According to the new theory, particles of dark matter can sometimes destroy each other through nothing more than random gravitational interactions. These random interactions are known in physical jargon as “gravitational portals” because they offer a way for particles to interact only by gravity. The product of this collision is a high-energy electron. These interactions would be much more common in the galactic center, where the density of dark matter is probably highest. These electrons will then continue, eventually hitting a low-energy photon and turning into a gamma ray, causing the excess 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 has been specially developed to correspond to the observation of gamma rays. But once this door is open, allowing dark matter particles to transform into ordinary matter (in this case leptons), more theoretical work can be done to see if there are other ways to test the theory.
Originally published in Live Science.