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Dark matter is notoriously antisocial, refusing to interact with light and “normal” matter, making it effectively invisible. But what scientists aren’t sure about is whether dark matter interacts with itself.
If any particles form dark matter Doing self-interacting by colliding and possibly even destroying each other, new research suggests that galaxy clusters could be used as natural dark matter accelerators. This dark matter detection method would rely on two of these massive galaxy clusters meeting and colliding.
Because these galaxy clusters are packed with dark matter, astronomers could examine these cosmic crash sites for possible signs of dark matter interacting with itself.
While such a clue doesn’t tell us exactly which particles dark matter is made of, it would help eliminate dark matter models that don’t allow for self-interaction, bringing researchers closer to the real dark matter candidate.
“Galaxy clusters are also dominated by dark matter,” Jacqueline McCleary, an assistant professor of physics at Northeastern University, said in a statement. “Eighty to ninety percent of their mass is dark matter, and the more massive an object is, the faster its constituent dark matter particles will be moving. You’re essentially studying very high-energy collisions.”
Related: Dark matter could be ‘coupler’ for supermassive black holes
Are Dark Matter Particles Antisocial ‘Cosmic Ghosts’?
The lack of interactions is one of the things that makes dark matter so hard to understand. Despite making up more than 80% of the material universe and providing the gravitational pull that literally keeps galaxies from flying apart, dark matter is effectively invisible because it doesn’t interact with light. In the same way, dark matter particles slip through matter particles like cosmic ghosts.
These antisocial properties have led scientists to realize that dark matter cannot be made of atoms composed of electrons, protons and neutrons, which Doing interact with light and with each other. This has fueled the search for a viable candidate particle for dark matter.
“Everything in the universe is a particle, a wave, and a field, so the bottom line [assumption] “Dark matter has to be a particle,” McCleary said. “The question is, what kind of particle, because a ‘particle’ is very vague.”
The candidate particles range from hypothetical “Weakly Interacting Massive Particles” or “WIMPS” to “Massive Compact Halo Objects” or “MACHOs” (we see you, scientists), to tiny, nearly massless particles called axions or sterile neutrinos. Even tiny black holes left over from the Big Bang, called primordial black holes, have been tipped as candidates for dark matter.
The problem is that all of these suspects have not been discovered and therefore remain frustratingly hypothetical.
The phantom nature of dark matter means we can’t simply load dark matter particles into a particle accelerator on Earth, such as the Large Hadron Collider
(LHC), to smash them together and determine their composition.
While the LHC has unlocked some secrets of the so-called ‘standard model of particle physics’ by smashing protons into pieces and observing the resulting particle showers, dark matter, which falls outside that model, seems out of reach for the world’s most powerful particle accelerator.
The only way we can actually infer the presence of dark matter is through the one interaction it appears to have. Dark matter has mass and therefore distorts space-time (a four-dimensional union of time and space), giving it a gravitational influence.
That influence can tell everyday matter and light how to move, and we can detect that. From this indirect approach, we know that galaxies are surrounded by huge halos of dark matter, and that dark matter also exists in large quantities near the cores of galaxies.
Scientists also theorize that when galaxies group together into huge clusters, it is the result of enormous tendrils of invisible dark matter spreading out through the universe like the invisible web spun by a cosmic spider. This cosmic web also helps individual galaxies accumulate mass and grow, as gravity pulls in ordinary matter.
McCleary and colleagues proposed abandoning a man-made particle accelerator in favor of a natural one. They reasoned that if galaxy clusters and the dark matter they carry collided with sufficient force, detectable dark matter interactions could occur — if dark matter is indeed interacting with itself.
These interactions could be as simple as dark matter particles colliding and bouncing off each other, or they could result in mutual annihilation and a flash of energy. When particles and antiparticles—for example, an electron and its antiparticle, called a positron—meet, they annihilate each other and release the energy they contain. Some dark matter models suggest that it is its own antiparticle, meaning that one form of interaction could be self-annihilation caused when two dark matter particles meet.
To test this idea, the team created simulations of galaxy collisions on computers powerful enough to probe the collisions down to the interactions between particles. This required programming the simulation with physical models of interactions between stars, gas, and dark matter.
Using this simulation and others like it, scientists could create a roadmap for observing real dark matter collisions. From there, researchers could gather clues that would help them determine some of the basic properties that a hypothetical dark matter particle should possess. This could narrow down the suspects of dark matter.
“If you can measure some self-interaction, or if you can put an upper limit, you either rule out classes of particles that could be dark matter, or you rule them in,” McCleary said. “If you measure some self-interaction, then you bring in a different class of particles. You allow for a different class of theories.
“What this paper also says is that if you study, say, 100 clusters of galaxies that are merging, you have a chance of measuring the self-interaction of dark matter up to a certain threshold.”
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“It seems counterintuitive at first, because we always want a positive answer: What is it?” McCleary concluded. “But in this case we’re forced to say: What isn’t it? It’s a process of elimination.”
The team’s research was published in April in the journal Monthly Notices of the Royal Astronomical Society.