Brendan M. Lynch
KU News Service

Researcher advances a new model for dark matter

Thu, 09/04/2014

LAWRENCE — Astrophysicists believe that about 80 percent of the substance of our universe is made up of mysterious “dark matter” that can’t be perceived by human senses or scientific instruments.

“Dark matter has not yet been detected in a lab. We infer about it from astronomical observations,” said Mikhail Medvedev, professor of physics and astronomy at the University of Kansas, who has just published breakthrough research on dark matter that merited the cover of Physical Review Letters, the world’s most prestigious journal of physics research.

Medvedev proposes a novel model of dark matter, dubbed “flavor-mixed multicomponent dark matter.”

“Dark matter is some unknown matter, most likely a new elementary particle or particles beyond the Standard Model,” Medvedev said. “It has never been observed directly, but it reveals itself via gravity it produces in the universe. There are numerous experiments around the world aimed at finding it directly.”

Medvedev’s theory rests on the behavior of elementary particles that have been observed or hypothesized. According to today’s prevalent Standard Model theory of particle physics, elementary particles — categorized as varieties of quarks, leptons and gauge bosons — are the building blocks of an atom. The properties, or “flavors,” of quarks and leptons are prone to change back and forth, because they can combine with 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 atom has a certain mass,” Medvedev said. “A flavor-mixed particle is weird — it has several masses simultaneously — and this leads to fascinating and unusual effects.”

Medvedev compared flavor-mixing to white light that contains several colors and can generate 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. “By changing 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 a gravitational well if they are ‘shaken’ — meaning they collide with another particle,” he said. “That's a remarkable result, as if a spacecraft made of flavor-mixed matter and hauled along a bumpy road puts itself into space without a rocket or any other means or effort by us.”

Medvedev included the physics process of quantum evaporation in a “cosmological numerical code” and performed simulations using supercomputers.

“Each simulation utilized over a 1,000 cores and ran for a week or so,” he said. “This yearlong project consumed about 2 million computer hours in total, which is equal to 230 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 detection experiments around the world. So physicists have devised a working model of completely collisionless (noninteracting), cold (that is, having very low thermal velocities) dark matter with a cosmological constant (the perplexing energy density found 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’s paper solved the theory’s long-standing and troublesome puzzles.

“Our results demonstrated that the flavor-mixed, two-component dark matter model resolved all the most pressing Lambda-CDM problems simultaneously,” said the KU researcher.

Medvedev performed the simulations using XSEDE high-performance computation facilities, primarily Trestles at the San Diego Supercomputer Center and Ranger at the Texas Advanced Computing Center.

Pictured above: The top photo, published in the Physics Review Letters, represents the distribution of dark matter in the universe computed within the two component flavor-mixed dark matter paradigm. The bottom photo, published in a previous paper, represents the effect of "quantum evaporation."

With graduation just a few months away, James Robert Wilson, senior in sport management, took this photo of the Memorial Campanile while looking forward to KU commencement traditions. After walking through the campanile and down the Hill in May, Wilson plans to take a summer road trip, then pursue a master’s degree and help coach track and field. Wilson, who is from Abilene, Kansas, says, "Coming to KU has put me in contact with people from all over the world and opened my eyes to many new cultures.” His advice to all Jayhawks: "Make the most of your time here by trying new things.” Our advice to graduating Jayhawks: Enjoy your last semester. Where will your time at KU take you? Tags: #exploreKU #Graduation University of Kansas School of Education

#KUresearch targets pathogens that kill children, plague ranchers & leave U.S. open to attack.
KU welcomes President Obama Jan. 22, 2015, was a historic day on the Hill: President Barack Obama visited the University of Kansas campus (, the first sitting president to do so in a century. More than 7,000 people — including many students and faculty who had spent hours in line to get tickets for the event — packed inside KU’s Anschutz Sports Pavilion to hear the president speak. Welcomed by Chancellor Bernadette Gray-Little with “Barack Chalk, Jayhawk!” Obama told the gathering “I’m a Kansas guy,” because his mother was from Wichita and grandparents were from Augusta and El Dorado. In his 35-minute talk, the president discussed themes (see official White House transcript from his 2015 State of the Union address, including his goal to lower the cost of attending college.

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