Wednesday, April 15, 2015

Repulsive keV Dark Matter

The case for 2-10 keV mass dark matter has gotten a lot stronger in 2015.
First, as mentioned in the previous post, the Planck 2015 results significantly lowered the value of the optical depth for photon-electron scattering during reionization and significantly lowered the z-value at which reionization occurred. Effectively, this pushes back the time at which the first stars and galaxies formed, and therefore indirectly suggests that dark matter took longer to clump together than predicted by GeV cold dark matter theories. As can be seen in the last figure in that previous post, a lower value of optical depth is possible for thermal relics with rest masses of ~2-3 keV and is incompatible (at 1-2 sigma) with CDM theories.

Second, just today, it was announced that there is a good chance that dark matter is actually self-repulsive. (Of course, it's been know indirectly for awhile that dark matter is self-repulsive because there is a missing core of dark matter in the center of galaxies...which can be explained by fermion repulsion between identical particles.) The news today is that there appears to be repulsion between the dark matter halos in a 'slow collision.'  This should be contrasted with the lack of repulsion when two dark matter halos (such as in the Bullet Cluster) collide in a 'fast collision.'

So how do we reconcile all of this information?  Actual, the answer is quite simple.
Dark matter halos are made of Fermi particles of keV rest mass that are quantum degenerate when their density is high and non-degenerate when their density is low.

When two fermi degenerate halos of uncharged-particles collide with velocities much greater than their Fermi velocity, the clusters of particle pass right through each other. Pfenniger & Muccione calculated what would happen in collisions between Fermi particles (or we could imagine two degenerate halos...doesn't matter provided that we are talking about a degenerate halo of particles or a single particle.)
To quote Pfenniger & Muccione:  "An interesting aspect developed for example by Huang (1964, chap. 10), is that the first quantum correction to a classical perfect gas... caused purely by the bosonic or fermionic nature of the particles is mathematically equivalent to an particle-particle interaction potential:

φ(r) = −kT ∙ ln [1 ± exp (−2π r2 /  λdB2 )] ."

When going from a two-particle collision to a collision between 2 halos, the main difference is that the deBroglie wavelenth of the particle would be replaced by the effective degeneracy radius of the halo.
When the directed velocity is large compared with the thermal velocity of the cluster, then the Fermi clusters pass right through each other. The center of mass is nearly same as if these were classical particles. In other words, there would be no separation between the center of the dark matter mass and the center of the solar matter mass.

In the next case below, the directed velocity of the two particles (or clusters) is decreased 3-fold. In this case, there is some slight repulsion between clusters. In this case, there would be a slight separate between the center of the dark matter mass and the center of the non-dark matter mass because the solar matter mass will pass through unaffected by the DM collision (unless there was actually a solar-solar collision...however unlikely.)

Finally, in the last case, the directed velocity of the two particles (or clusters) is decreased a further 3-fold. In this case, the particles have the time to interact, and can actually gravitationally coalesce and entangle.

This means that we should expect the "cross section of interaction" to depend greatly on how quickly the dark matter clusters are colliding.