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September 4, 2014
University of Kansas
Astrophysicists believe that about 80 percent of the substance of ouruniverse is made up of mysterious "dark matter" that can't be perceivedby human senses or scientific instruments.
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A new model for a cosmological enigma -- dark matter: Solving long-standing and troublesome puzzles
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Credit: University of Kansas / KU News Service [Click to enlarge image]
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Orange waves and blue rays represent the effect of quantum evaporation.
strophysicists believe that about 80 percent of the substance of ouruniverse is made up of mysterious "dark matter" that can't beperceived by human senses or scientific instruments.
"Dark matter has not yet been detected in a lab. Weinfer about it from astronomical observations," saidMikhail Medvedev, professor of physics andastronomy at the University of Kansas, who has justpublished breakthrough research on dark matter thatmerited the cover of Physical Review Letters.
Medvedev proposes a novel model of dark matter,dubbed "flavor-mixed multicomponent dark matter."
"Dark matter is some unknown matter, most likely anew elementary particle or particles beyond theStandard Model," Medvedev said. "It has never beenobserved directly, but it reveals itself via gravity itproduces in the universe. There are numerousexperiments around the world aimed at finding itdirectly."
Medvedev's theory rests on the behavior of elementary particles that have been
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Medvedev's theory rests on the behavior of elementary particles that have beenobserved or hypothesized. According to today's prevalent Standard Model theory ofparticle physics, elementary particles -- categorized as varieties of quarks, leptons andgauge bosons -- are the building blocks of an atom. The properties, or "flavors," ofquarks and leptons are prone to change back and forth, because they can combinewith each other in a phenomenon called flavor-mixing.
"In everyday life we've become used to the fact that each and every particle or an atomhas a certain mass," Medvedev said. "A flavor-mixed particle is weird -- it has severalmasses simultaneously -- and this leads to fascinating and unusual effects."
Medvedev compared flavor-mixing to white light that contains several colors and cangenerate a rainbow.
"If white was a particular flavor, then red, green and blue would be different masses --masses one, two and three -- that mix up together to create white," he said. "Bychanging proportions of red, green and blue in the mix, one can make different colors,or flavors, other than white."
Medvedev said that dark matter candidates are also theorized to be flavor-mixed --such as neutralinos, axions and sterile neutrinos.
"These are, in fact, the most preferred candidates people speak about all the time,"Medvedev said.
"Previously we discovered that flavor-mixed particles can 'quantum evaporate' from agravitational well if they are 'shaken' -- meaning they collide with another particle," hesaid. "That's a remarkable result, as if a spacecraft made of flavor-mixed matter andhauled along a bumpy road puts itself into space without a rocket or any other meansor effort by us."
Medvedev included the physics process of quantum evaporation in a "cosmologicalnumerical code" and performed simulations using supercomputers.
"Each simulation utilized over a 1,000 cores and ran for a week or so," he said. "Thisyearlong project consumed about 2 million computer hours in total, which is equal to230 years."
Medvedev said that dark matter may interact with normal matter extremely weakly,which is why it hasn't been revealed already in numerous ongoing direct detectionexperiments around the world. So physicists have devised a working model ofcompletely collisionless (noninteracting), cold (that is, having very low thermalvelocities) dark matter with a cosmological constant (the perplexing energy densityfound in the void of outer space), which they term the "Lambda-CDM model."
But the model has hasn't always agreed with observational data, until Medvedev'spaper solved the theory's long-standing and troublesome puzzles.
"Our results demonstrated that the flavor-mixed, two-component dark matter modelresolved all the most pressing Lambda-CDM problems simultaneously," said the KUresearcher.
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University of Kansas. "A new model for a cosmological enigma -- dark matter:Solving long-standing and troublesome puzzles." ScienceDaily. ScienceDaily, 4September 2014. <www.sciencedaily.com/releases/2014/09/140904121241.htm>.
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Medvedev performed the simulations using XSEDE high-performance computationfacilities, primarily Trestles at the San Diego Supercomputer Center and Ranger at theTexas Advanced Computing Center.
Pictured above: The top photo, published in the Physics Review Letters, representsthe distribution of dark matter in the universe computed within the two componentflavor-mixed dark matter paradigm. The bottom photo, published in a previous paper,represents the effect of "quantum evaporation."
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The above story is based on materials provided by University of Kansas. Note:Materials may be edited for content and length.
Journal Reference:
1. Mikhail V. Medvedev. Cosmological Simulations of Multicomponent Cold DarkMatter. Physical Review Letters, 2014; 113 (7) DOI:10.1103/PhysRevLett.113.071303
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