First 3D Wave Simulations of 'Fuzzy' Dark Matter Match a Real Lensed Quasar
If dark matter is made of ultralight particles that ripple like waves, those ripples should shift the multiple images of a distant quasar. The simulations matched a real lensed quasar better than standard models.
Dark matter makes up about 85% of the matter in the universe, yet no one knows what it is. A team of astronomers in Beijing and Hong Kong has built a new way to test one of the leading ideas: that dark matter is made of particles so light they behave less like tiny billiard balls and more like waves.
These hypothetical ultralight particles are often called "fuzzy" dark matter. Because they weigh so little, their quantum wavelengths can stretch across thousands of light-years. Inside a galaxy, the waves would overlap and interfere, piling up in some places and cancelling in others, much like ripples crossing on a beach. The result would be a galaxy-sized pattern of dense lumps and thin gaps in the dark matter, something ordinary "cold" dark matter would not produce.
The researchers, led by doctoral student Jiajun Zhou in Zong-Hong Zhu's group at Beijing Normal University, working with Jeremy Lim and Amruth Alfred of the University of Hong Kong, asked how those ripples would change what telescopes see. Their tool was gravitational lensing. When a massive galaxy sits between Earth and a distant quasar, its gravity bends the quasar's light, as Einstein predicted, and can split it into several images. The exact positions of those images depend on how the lensing galaxy's mass is spread out, lumps and all.
Earlier studies had approximated the effect with simplified models. This team did something new. It ran full three-dimensional computer simulations of fuzzy dark matter's wave behavior and calculated the lensing directly from the simulated mass. The paper, published in The Astrophysical Journal Letters, is the first to make lensing predictions straight from such wave simulations. When the team compared the results with a well-studied lensed quasar, the simulated galaxies reproduced the observed image positions more closely than the commonly used models the study compared against.
"We were very excited to see what the simulations predicted as we did not know beforehand what observable signatures to expect," the researchers said. "Jiajun worked very hard to rapidly get out these remarkable results and we believe this has opened entirely new avenues for future research on the nature of dark matter using lensing."
The result is not proof that dark matter is fuzzy. One lens system can be fitted in several ways, and small dark matter clumps from other causes can also nudge image positions. What the study does show is that sharp, high-resolution observations of lensed quasars could in principle tell the wave picture apart from the standard one. The team is now working on other signatures, including how the ripples would change the relative brightness of the multiple images.
The timing is good for that kind of test. New telescopes such as the Vera C. Rubin Observatory and the European Space Agency's Euclid mission are expected to find many thousands of new lensed systems, and radio arrays and the James Webb Space Telescope can measure image positions with great precision. If dark matter really ripples, those images could be where the ripples first show.
