“At first, we thought it was absurd. How else could you respond to the idea that black holes generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together? We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy,” said theoretical physicist Asimina Arvanitaki, at the Perimeter Institute for Theoretical Physics referring to the heated debate about the standard model for dark matter that proposes that it is ‘cold,’ meaning that the particles move slowly compared to the speed of light which is tied to the mass of dark matter particles. The lower the mass of the particle, the ‘warmer’ it is and the faster it will move.
On January 9, NASA physicists using the Hubble Space Telescope reported that although the type of particle that makes up dark matter is still a mystery, a compelling observational test for the cold dark matter passed “with flying colors,” The NASA team used a new “cosmic magnifying glasses” technique that found that dark matter forms much smaller clumps than previously known, confirming one of the fundamental predictions of the widely accepted “cold dark matter” theory.
Physicists at the University of California, Davis, taking the temperature of dark matter, the mysterious substance that makes up about a quarter of our universe now report that the model of cold (more massive) dark matter holds at very large scales” said Chris Fassnacht, a physics professor at UC Davis, “but doesn’t work so well on the scale of individual galaxies.” That’s led to other models including ‘warm’ dark matter with lighter, faster-moving particles and ‘hot’ dark matter with particles moving close to the speed of light that have been ruled out by observations.
Fassnacht and colleagues are using the distortion caused the gravity of clumps of dark matter that distorts light from distant objects, called gravitational lensing, to learn more about the properties of dark matter.
Former UC Davis graduate student Jen-Wei Hsueh, Fassnacht and colleagues used gravitational lensing to put a limit on the warmth and therefore the mass of dark matter. They measured the brightness of seven distant gravitationally lensed quasars to look for changes caused by additional intervening blobs of dark matter and used these results to measure the size of these dark matter lenses.
If dark matter particles are lighter, warmer and more rapidly-moving, then they will not form structures below a certain size, Fassnacht said. “Below a certain size, they would just get smeared out.”
The results put a lower limit on the mass of a potential dark matter particle while not ruling out cold dark matter, he said. The team’s results represent a major improvement over a previous analysis, from 2002, and are comparable to recent results from a team at UCLA.
Fassnacht hopes to continue adding lensed objects to the survey to improve the statistical accuracy. “We need to look at about 50 objects to get a good constraint on how warm dark matter can be,” he said.
The Daily Galaxy, Max Goldberg, via UC Davis
Image credit: The Hubble image at the top of the page shows galaxies using gravitational lensing. that revealed cold dark matter concentrations 1/10,000th to 1/100,000th times the mass of the Milky Way’s dark matter halo.(NASA, ESA, A. Nierenberg, T. Treu)