Physicists have been striving to understand dark matter—the invisible material that makes up most of the universe—for decades. In the standard cosmological model, this mysterious substance is a cold, thinly dispersed medium composed of weakly interacting particles, which are considered “collisionless” in that they either hardly interact with each other, or don’t interact at all. Additionally in this model, dark matter keeps the outermost galaxies within galaxy clusters from eluding the grasp of gravity and drifting into distant space, or from being torn apart.
However, a new theory proposed by physicist Justin Khoury, a professor in the School of Arts and Sciences, and his former postdoctoral researcher Lasha Berezhiani, now at the Max Planck Institute for Physics, stands to shake up how scientists consider dark matter. Their hypothesis that dark matter is a liquid has the potential to explain cosmological mysteries that have previously eluded researchers. The findings have been published in Journal of Cosmology and Astroparticle Physics.
As a theoretical principle, the standard rendering of dark matter functions well to explain large-scale cosmological observations, such as the gravitational retention of outer galaxies within a galaxy cluster, but issues arise when applied to smaller-scale mysteries in the universe, such as the rotation curves of individual galaxies.
A rotation curve plots the orbital speeds of objects in relation to their distance from a central mass. In a solar system, objects revolve around the concentrated mass at the center of the system, and, based on Kepler’s second law of planetary motion, the speed of rotating objects decreases as they move farther away from the center.
Galaxy rotation curves are unusual. Objects far from the galactic center orbit at a speed independent from their distance from the center, instead of the decreasing speed expected from Kepler’s second law. Because the velocity becomes constant instead of decreasing, this behavior is usually interpreted as evidence for dark matter. Alternatively, physicists sometimes resort to modifying Newton’s law of gravitational force to explain these galactic observations.
Khoury and Berezhiani approach this differently. Their theory suggests that dark matter in galaxies behaves as a special type of fluid, known as a superfluid, which lacks viscosity and flows indefinitely when stirred. In this approach, the excitations of the superfluid result in a new long-range force, on top of the usual gravitational force, in such a way that explains the unexpected behavior seen in galaxies. Meanwhile, on the large scale of galaxy clusters, according to their theory, dark matter behaves as in the conventional model.
Even at the absolute zero temperatures found in space, a superfluid remains a smoothly flowing liquid, and its individual particles behave as a collective entity. For something to reach a superfluid state, along with being exposed to extremely low temperatures, the fluid needs to have densely-packed atoms.
“A superfluid is a very interesting kind of fluid,” says Khoury. “Most substances we find in the laboratory form a solid when cooled to a low enough temperature. But a certain set of substances, such as liquid helium, effectively remain a liquid all the way to absolute zero temperature. The uncertainty principle of quantum mechanics forbids these atoms from arranging into crystalline structure, and instead they remain fluid, but it’s a very peculiar kind of fluid. It flows without viscosity, doesn’t carry any entropy, and has many other strange behaviors.”
For earlier work in this area, Khoury and Berezhiani received the 2017 Buchalter Cosmology Prize, and the researchers have continued exploring the topic. Receiving the prize, Khoury says, was an honor, rewarding innovative new ideas in the field of cosmology that will hopefully fill the gaps in the current understanding of the universe.
While researchers can calculate the density of the total amount of dark matter in the universe, no one has determined the mass of an individual particle. But it is known that to create a superfluid on Earth, scientists squeeze particles together—such as liquid helium particles—to create extremely dense matter, and then rapidly cool the matter to near absolute zero. This bodes well for the superfluid theory, since the gravitational pull of a galaxy creates the squeeze. The leading paradigm of dark matter already suggests it is extremely cold, which indicates dark matter could be a viable superfluid in certain states.
“What we discovered, essentially, was an analogy,” Khoury says. “If you look at the types of theories people utilized to explain these galactic observations using modified gravity, those equations take the same form as those of a superfluid. We realized that if the dark matter was this particular kind of superfluid, then you could explain both sets of observations, the large scale and the small scale.”
The work was supported by the U.S. Department of Energy, the Université de Strasbourg, the National Science Foundation, NASA, and the Charles E. Kaufman Foundation of the Pittsburgh Foundation.
Justin Khoury is a professor in the Department of Physics and Astronomy at the University of Pennsylvania.
Homepage photo: An artistic rendering displays how superfluid vortices might appear within the dark matter halo around individual galaxies. (Image courtesy of Markos Kay for Quanta Magazine).