In popular science, dark matter is a hotly discussed topic. With various theories regarding its existence and interaction with regular matter, many scientists agree that these are some questions that remain unresolved. But thanks to work by scientists like Prof. Justin Khoury, physics and astronomy, University of Pennsylvania, significant steps to understanding this phenomenon are being made. As part of the Kieval lecture series, Khoury led a talk at Cornell on Monday, to discuss new approaches to solving these mysteries.
There is significant evidence for the existence of dark matter. A simple example is the existence of bullet clusters. These consist of two colliding galaxies and astronomers have observed that dark matter passes through hot gas without slowing down but the gas itself will slow down.
“In addition, most of the mass, which is dark matter, is segregated from where most of the ordinary matter lies,” Khoury said. “On a cosmic web scale, this does well in fitting with the observations. On a galaxy cluster scale, it also does pretty well. However, when on the scale of galaxies, it does not fit.”
Because of the way dark matter interacts in bullet clusters, Khoury believes that dark matter is collisionless.
Khoury uses the analogy of a political spectrum to describe his argument. On one end of the extreme are “right wing evangelical” thinkers, who maintain that dark matter is “all feedback” and consists of very simple particles. On the other end are the “left wing hippy” thinkers, who say that “there is no dark matter, it’s all modified gravity”. In the “moderate middle,” however, is Khoury’s position, which states that dark matter exists and is a superfluid. Superfluidity is a characteristic property of a fluid which allows it to have zero viscosity and thus, interact in frictionless collisions and maintain its kinetic energy.
Khoury’s theory is not one that is supported by most scientists, who take the “right wing evangelical” view. The “middle ground” hypothesis argues that dark matter behaves like a cold, collisionless fluid on large scales. This helps explain the theory of MOdified Newtonian Dynamics, which aims to account for the fact that the speed of stars in galaxies cannot be explained solely by newtonian mechanics. Whereas those arguing that dark matter is actually just modified gravity would claim that MOND is a law that originates from modifying the laws of gravity, Khoury argues that MOND is a law that originates from the fundamental nature of dark nature.
The superfluid dark matter argument may help to better identify and distinguish between galaxies and galaxy clusters. In galaxies, there is superfluid movement of dark matter and MOND applies. However, in galaxy clusters, there is no superfluid movement of dark matter and MOND does not apply.
Despite the numerous theories swirling around the existence of dark matter, Khoury hopes to prove the merits of the theory he propagates. Because understanding the existence and interactions of dark matter could provide other insights into our universe, solving this dark mystery is a priority.