4.2 Observational Signatures
What is Superfluid?
- If a container holding superfluid helium is rotated slowly, the helium will stay completely still.
- Once the container rotates quickly enough, the helium will begin to rotate as well, but not homogeneously as one would expect with normal fluids.
- Instead small dynamic vortices are formed throughout the fluid; the faster the fluid spins, the more small vortices are formed.
- Galaxies are spinning, so the dark matter within galaxies may well have its own spin. If that dark matter is superfluidic then it should also form vortices rather than rotate homogeneously.
- These vortices, if detected, would provide observational evidence for the superfluidity of dark matter. However, these vortices are relatively small (about 100 of them could be contained within our solar system) and thin (about 1mm in thickness). These vortices would be difficult to detect.
Collision of Milky Way and Andromeda Galaxies
- Simulations of the collision between the Milky Way galaxy and Andromeda galaxy show that they will, after many years of slowly sloshing together, form a new combined galaxy.
- The fact that the two galaxies do not simply slingshot past each other at high velocities indicates the presence of some sort of friction or damping force that cause the to galaxies merge.
- This force is really just the gravitational interactions of the many dark matter particles within each galaxy. This interaction is referred to as dynamical friction.
- Dynamical friction occurs because, as one object approaches and passes through a cloud or group of other objects, the mass of the cloud behind it grows in relation to the remaining matter in front of it. This means that as the object passes through the cloud, the force of gravity increasingly works to oppose its motion.
Superfluid Dark Matter During Galactic Merges
- The simulated effect of dynamical friction due to dark matter is more dramatic than would be the case if the dark matter is in the superfluid state.
- Because this dark matter has almost zero viscosity, it will flow past itself easily and therefore decrease the effect of dynamical friction.
- There is some observational evidence of this from the Fornax dwarf galaxy. Globular clusters of stars in orbit around the galaxy should have long ago fallen into the galaxy itself if dark matter does not behave like a superfluid.
- These clumps of stars are suspended in the dark matter superfluid, so they do not “feel” the dynamical friction.
- Similarly, galaxies at the center of galaxy clusters are so close together that their dark matter halos should be overlapping, leading them to merge, yet they do not merge. This can also be explained by the very different merger properties of the superfluid dark matter that is theorized to exist in the cores of galaxy clusters.
Interference in Dark Matter Interactions
- In many simulations of superfluid dark matter, clumps of dark matter that get gravitationally attracted to each other and interact exhibit “fringes”, like the fringes of light and dark caused by light interference.
- This interference can be explained by conventional models of dark matter, but they could also be evidence for more quantum behavior by superfluid dark matter and may have as-yet undetermined influences on large-scale structure in the universe.