Astrophysicists have very little idea of what dark matter is and they have yet to detect a dark matter particle. But they do know that the gravity of clumps of dark matter can distort light from distant objects. An international team of astrophysicists is using this distortion — called strong gravitational lensing — to learn more about the properties of the mysterious substance.
No one knows what dark matter is, but this invisible form of matter makes up roughly a quarter of the Universe. Image credit: NASA’s Goddard Space Flight Center.
“The standard model for dark matter holds that this substance is ‘cold,’ meaning that the particles move slowly compared to the speed of light,” said Professor Chris Fassnacht, a researcher in the Department of Physics at the University of California, Davis.
“This is also 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.”
“The model of cold (more massive) dark matter holds at very large scales, 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. ‘Hot’ dark matter with particles moving close to the speed of light has been ruled out by observations.”
Professor Fassnacht, graduate student Jen-Wei Hsueh and their 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,” Professor 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.”
“We hope 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.”
The findings are published in the Monthly Notices of the Royal Astronomical Society.
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J.-W. Hsueh et al. SHARP – VII. New constraints on the dark matter free-streaming properties and substructure abundance from gravitationally lensed quasars. MNRAS, published online November 15, 2019; doi: 10.1093/mnras/stz3177
