It now seems clear that dark matter is interacting more than just gravitationally. Previous studies have indicated this, and a new study further supports the idea. The interesting thing about this latest work is that it examines interactions of dark matter through entropy.
Entropy is a subtle and powerful concept in physics. It was first introduced as a property in thermodynamics, but plays a role in everything from black holes to the flow of time. It's also quite difficult to define without relying on math.
This is really entropy. Photo credit: Brian Koberlein
Entropy is often described as a measure of the disturbance in a system. Ice, for example, with its water molecules arranged in symmetrical crystals, has a lower entropy than water with its chaotic dance of water molecules. Since warmer things tend to be disordered, entropy is also related to the temperature of an object. The second law of thermodynamics therefore says that the entropy of a system cannot decrease, which also means that heat flows from warm to cold objects.
In this latest work, entropy is better described in terms of how likely it is that an object is in a certain state. Imagine lighting a scented candle in the corner of a room. While statistically the scent from the candle floats around the candle, the movement of air in the room will most likely spread the scent around the room. An evenly distributed fragrance is the most likely result, since it is the state with the maximum entropy. It is a state also known as thermodynamic equilibrium.
There are more ways to stir an egg than to decipher one. Photo credit: Brian Koberlein
In this new study, the team used computer models to calculate the state of maximum entropy for dark matter in dwarf galaxies. The distribution of dark matter determines how much the light affects the gravitational lens. When the team examined the theoretical lens through galactic dark matter at maximum entropy, it found that it was consistent with the observed lens around dwarf galaxies. Dark matter seems to be in a state of maximum entropy.
This means that dark matter must interact in some way. The scent in a room achieves maximum entropy because the aroma molecules interact strongly with air molecules. All of these interactions increase the entropy of space. Dark matter does not interact strongly with regular matter, so this study suggests that it interacts strongly with itself.
Without knowing exactly what dark matter is, we don't know how it can interact. But these and other studies further confirm the reality of dark matter.
Reference: Brinckmann, Thejs, et al. "The structure and assembly history of cluster-sized halos in self-interacting dark matter." Royal Astronomical Society monthly bulletin 474.1 (2018): 746-759.
Reference: Almeida, Jorge Sánchez, Ignacio Trujillo and Angel Ricardo Plastino. "The principle of maximum entropy explains the nuclei that are observed in the mass distribution of dwarf galaxies." Astronomy & Astrophysics 642 (2020): L14.