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Dark matter may spawn more of itself from ordinary matter, like a cosmic ice-9
Physicists propose an new exponential growth mechanism for dark matter creation.
Dark matter, one of weirdest substances in the universe, just got weirder.
Turns out, this mysterious (and invisible) material could multiply by converting garden-variety matter into more dark matter, like some kind of cosmic ice-9, researchers reported Nov. 3 in the journal Physical Review Letters.
This type of multiplication is called exponential growth, and it shows up all the time in models of nature, from bacterial colonies to parasitic wasp populations, according to previous research published in Nature.
"We were just wondering, could you make dark matter exponentially grow based on its abundance?" Joshua Ruderman, a particle physicist at New York University and co-author of the Physical Review Letters paper, told Live Science. "And we realized a simple way to do it."
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Dark matter has been a cosmological enigma since the legendary Swiss astronomer Fritz Zwicky first described it in a 1933 paper. It's the invisible, untouchable twin of regular, or "baryonic," matter, which makes up all of the stuff we can see in the universe: Earth, the stars, trees and even people. Scientists aren't sure what dark matter is made of or how it came to be — but they know that it makes up roughly 85% of all matter in the universe, according to CERN.
But if dark matter doesn’t interact with light, and so scientists can't see it, how can they tell that dark matter is so ubiquitous? Dark matter divulges its existence through its gravitational effects on other celestial objects, like stars and even whole galaxies. For example, NASA researchers have caught certain star clusters exerting far more gravity than should be possible based on the amount of visible matter they contain. "Those observations wouldn’t make sense without the presence of dark matter," Kevork Abazajian, a cosmologist at the University of California, Irvine, told Live Science.
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The newly proposed mechanism suggests new dark matter comes from regular matter. One prominent older mechanism, called "freeze-out," posits that dark matter formed in the early universe, when all particles were mashed together in a hot, dense soup called the "thermal bath," Adam Green, a physicist at the University of St. Thomas in Minnesota, wrote on the blog ParticleBites. According to this theory, the vast majority of these particles started off as dark matter, which were destroyed as soon as they came into contact with regular matter (a known phenomenon called annihilation). The reverse of this theoretical reaction, in which dark matter replaced abundant regular matter in the early universe, is known as "freeze-in."
Ruderman and his co-authors took the freeze-in model to the extreme. They suggest that at first, regular matter outweighed dark matter by a lot — but that began to change very rapidly. When a particle of dark matter came into contact with another particle, they posited, it annihilated the other particle and spit out two particles of dark matter. This is similar to the way other fundamental particles, like positrons and electrons, annihilate one another to produce two photons. Unlike the freeze-in mechanism, this model produces dark matter at an exponential, rather than logarithmic (or gradually slowing), rate. "It's a totally different mechanism. And it’s interesting to explore," said Abazajian, who was not involved in the study. If this is indeed the true mechanism, it offers a solid explanation for the current six-to-one ratio of dark matter to regular matter.
But if dark matter converts everything else into more dark matter, does that mean the entire universe is doomed to become one big dark matter blob?
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Not necessarily. "If it continued too long, you would end up with a universe with way more dark matter than our universe," Ruderman said. But the new mechanism only works at an extremely high temperature. As the universe cooled, Ruderman explained, the reaction ceased, and the amount of dark matter became fixed. "It's a universal property-of-equilibrium thing."
However, the exponential growth model remains just one of several possible mechanisms to explain dark matter. "Right now," Ruderman said, "We have more questions than answers."
Originally published on Live Science.
Joanna Thompson is a science journalist and runner based in New York. She holds a B.S. in Zoology and a B.A. in Creative Writing from North Carolina State University, as well as a Master's in Science Journalism from NYU's Science, Health and Environmental Reporting Program. Find more of her work in Scientific American, The Daily Beast, Atlas Obscura or Audubon Magazine.