Int'l team lends credence to "cold dark matter" model

Source: Xinhua| 2017-07-27 07:41:45|Editor: Zhang Dongmiao
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SAN FRANCISCO, June 26 (Xinhua) -- An international team of cosmologists has used data from the intergalactic medium, namely the vast, largely empty space between galaxies, to narrow down what dark matter could be and cast doubt on a relatively new theory called "fuzzy dark matter."

Instead, their findings published in the journal Physical Review Letters lend credence to a different model called "cold dark matter."

Dark matter is the aptly named unseen material that makes up the bulk of matter in our universe. Scientists have never directly detected dark matter. Over decades, they have proposed a variety of theories about what type of material, from new particles to primordial black holes, could comprise dark matter and explain its many effects on normal matter.

The new research could inform efforts to detect dark matter directly, especially if scientists have a clear idea of what sorts of properties they should be seeking.

"For decades, theoretical physicists have tried to understand the properties of the particles and forces that must make up dark matter," said lead author Vid Irsic, a postdoctoral researcher in the Department of Astronomy at the University of Washington (UW). "What we have done is place constraints on what dark matter could be; and 'fuzzy dark matter,' if it were to make up all of dark matter, is not consistent with our data."

The "fuzzy" and "cold" dark-matter theories are drawn up to explain the effects that dark matter appears to have on galaxies and the intergalactic medium between them.

Cold dark matter is the older of these two theories, dating back to the 1980s, and is currently the standard model for dark matter. It posits that dark matter is made up of a relatively massive, slow-moving type of particle with "weakly interacting" properties. It helps explain the unique, large-scale structure of the universe, such as why galaxies tend to cluster in larger groups.

The cold dark matter theory has some drawbacks and inconsistencies. For example, it predicts that our own Milky Way Galaxy should have hundreds of satellite galaxies nearby. Instead, we have only a few dozen small, close neighbors.

According to the newer "fuzzy" theory, which addresses the deficiencies of the "cold" model, dark matter consists of an ultralight particle, rather than a heavy one, and also has a unique feature related to quantum mechanics. For many of the fundamental particles in our universe, their large-scale movements, traveling distances of meters, miles and beyond, can be explained using the principles of "classic" Newtonian physics. Explaining small-scale movements, such as at the subatomic level, requires the complex and often contradictory principles of quantum mechanics. But for the ultralight particle predicted in the fuzzy dark matter theory, movements at incredibly large scales, such as from one end of a galaxy to the other, also require quantum mechanics.

With these two theories in mind, Irsic and his colleagues set out to model the hypothetical properties of dark matter based on relatively new observations of the intergalactic medium, or IGM. The IGM consists largely of dark matter along with hydrogen gas and a small amount of helium. The hydrogen within IGM absorbs light emitted from distant, bright objects, and astronomers have studied this absorption for decades using Earth-based instruments.

The team looked at how the IGM interacted with light emitted by 125 quasars, which are distant, massive, star-like objects.

Using a supercomputer at the University of Cambridge in England, the researchers simulated the IGM and calculated what type of dark matter particle would be consistent with the quasar data. They discovered that a typical particle predicted by the fuzzy dark matter theory is simply too light to account for the hydrogen absorption patterns in the IGM. A heavier particle, similar to predictions of the traditional cold dark matter theory, is more consistent with their simulations.

An ultralight "fuzzy" particle could still exist, according to the authors. But it cannot explain why galactic clusters form, or other questions like the paucity of satellite galaxies around the Milky Way, said Irsic. A heavier "cold" particle remains consistent with the astronomical observations and simulations of the IGM.

"The mass of this particle has to be larger than what people had originally expected, based on the fuzzy dark matter solutions for issues surrounding our galaxy and others," he was quoted as saying in a news release from UW.