Astrophysicists have an idea that can help solve two puzzles: the reason for the strange abundance of super-high-energy radiation fired from the center of our galaxy and the identity of invisible things called dark matter which has puzzled 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 dark matter particles (whatever they are) are sucked into one of these portals, they destroy each other and spit them out shockingly strongly. gamma ray.
This line of thinking could possibly explain why the galactic center – where dense clusters of dark matter presumably hide – is full of gamma rays; and it can shed light on how dark matter acts and can sometimes interact with the normal matter of our universe.
Related: The 11 biggest unanswered questions about dark matter
What should be dark matter
More than 80% of 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 the existence of dark matter came in the 1970s, when astronomer Vera Rubin noticed that galaxies were spinning far too fast – without an additional, hidden source of gravity, they must have torn themselves apart centuries ago.
For decades, astronomers did not know whether to change their notion of gravity, add more frequent, but really dull matter to the universe, or include a brand new ingredient. But year after year, observation after observation limited the choices. No theory of modified gravity can explain all observations. And physicists have severely limited the amount of normal matter (bright, dull and everything in between) in the cosmos.
This leaves dark matter to explain the fast galaxies. This case would be a new kind of particle, with an unknown identity (or identity). It does not vary with light, otherwise we would have seen it by now. It does not work with the strong nuclear power – which binds particles matter – otherwise scientists would have noticed its influence in atomic experiments. It may speak to the weak nuclear force, but the force is so weak and short-range that the observation of deviations in the expected results is difficult.
Trillions of dark matter particles may be currently flowing through us, invisible and silent.
However, dark matter betrays its presence through gravity, because every form of mass and energy in the universe exerts a kind of gravitational influence. The only sure way to study dark matter, therefore, is through the gravitational interaction with normal matter, such as the motions of stars in galaxies.
But there may be another way.
The case of the excess electrons
In a study published January 28 in the preprint database arXiv, 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 in this hunt for dark matter. The clue comes in the form of a strange abundance of gamma rays observed from the center of our Milky Way galaxy.
Related: 11 Fascinating Facts About Our Milky Way
Gamma rays are the form of radiation with the highest energy possible, and they usually only emerge from serious energy events, such as stars becoming supernovae. But there are more gamma rays than you would expect in the galactic center, given how rare such catastrophic events are. It is therefore possible, according to this theory, that gamma rays may arise 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 gamma rays, come from some source and move through the galactic center. The electrons themselves are undetectable (they are very, very small), but as they flow through interstellar space, they can occasionally strike in a random photon (a light particle).
That photon, probably something harmless and low-energy, collides with the aggressive electron; the collision amplifies the photon’s energy so that it begins to emit gamma rays that we can see.
Those collisions may explain the excess gamma rays, but where do the high-energy electrons come from?
Jump through the portal
Let’s take what we know. One, dark matter works only by gravity. Two, high energy leptons the swing around the galactic center can explain the extra gamma rays we see there. Three, because the nucleus in our own galaxy has the highest density of matter, we think that there is also a large concentration of dark matter.
Coincidence? Or conspiracy?
The link between these two observations has an appropriate name: leptophilic gravity portals, as outlined by Sun Xu-Dong and Dai Ben-Zhong, of the China Key Laboratory of Astroparticle Physics, in their arXiv article. The study has yet to be peer-reviewed.
Let’s start with the section “gravity portals”. As far as we understand gravity, it just attracts things. Earth draws to the moon; the sun draws on the earth; stars in a galaxy attract each other, and so on. And gravity does a very good job of pulling.
At first glance, the only thing gravity can do to dark matter is … pull.
But our understanding of gravity is incomplete. Physics can state that gravity works on a large scale, but there is no so-called quantum theory of gravity that describes the strong gravity that works on a very small scale. And in this regime, gravity can expect some surprises.
The other forces of nature can destroy, transform and create particles. The weak nuclear force, for example, can turn a proton into a neutron, causing radioactive decay. A particle and its antiparticle can be connected via the electromagnetic force and destroy each other in an eruption.
Maybe gravity can, in extreme cases, bring together and destroy two dark matter particles and turn them into … anything, really.
And according to the theoretical model set out by the researchers, the particles of dark matter might turn into leptons. Hence the ‘leptophile’ part of the name, which means ‘lepton-loving’.
According to the new theory, dark matter particles can sometimes destroy each other through nothing but accidental gravitational interactions. These random interactions are known in physics jargon as “gravitational portals”, as they provide a way for particles to communicate through gravity alone. The product of the collision is a high-energy electron. These interactions occur much more in the galactic center, where the density of dark matter is probably the highest. Those electrons then move further and eventually strike a low-energy photon and turn into a gamma ray, causing the excess we observe.
Yes, this idea is a piece. But since physicists are in the dark as far as the identity of dark matter is concerned, new ideas are always welcome. And this theory is specifically designed to correspond to the observation of gamma rays. But once the door is opened so that dark matter particles can transform into ordinary matter (leptons, in this case), more theoretical work can be done to see if there are other ways to test the theory.
Originally published on Live Science.