Historical research guide · 0409 · 2016
Dark matter superfluidity and galactic dynamics
Lasha Berezhiani · Justin Khoury
In one minute
Lasha Berezhiani and Justin Khoury, at the University of Pennsylvania, propose that dark matter is a superfluid. Their particles are light and axion-like, they interact with each other, and inside a galaxy they cool below a critical temperature of about a thousandth of a degree above absolute zero and condense into a single coherent quantum fluid the size of the galaxy. Two things follow. The fluid’s own sound waves — phonons — pull on ordinary matter with exactly the extra force that Milgrom’s modified dynamics puts in by hand, which is why galaxy rotation curves come out right. And because clusters of galaxies are hotter, their dark matter is only partly condensed, which is precisely where that modified force is known to fail. The authors argue this reconciles two rival accounts that each work in their own domain, and they list what to look for: vortices threading the disc, unusually slow galaxy mergers, and interference patterns in colliding halos.
Connection
Chapter 5 treats the vacuum as a quantum fluid rather than empty space, and this is that idea carried into cosmology by mainstream theorists: a coherent condensate the size of a galaxy, whose sound waves are a force. The authors even suggest a cold-atom system with the same equation of state would let galactic dynamics be simulated in a laboratory.
From the source
Dark matter consists of self-interacting axion-like particles that thermalize and condense to form a superfluid inside galaxies, with a coherence length of order the size of the galaxy and a critical temperature of about a milliKelvin — intriguingly comparable to the Bose–Einstein condensation temperatures of cold atom gases. Instead of evolving as independent particles, the dark matter is more aptly described as collective excitations.Abstract; section DM condensation, Equations 1 to 3
The superfluid’s phonons mediate a MOND-like force on baryonic matter. Because superfluidity only occurs at low temperature, the framework distinguishes by itself between galaxies, where the modified force law is successful, and galaxy clusters, where it is not: the larger velocity dispersion in clusters gives a higher temperature, so their dark matter sits in a mixture of superfluid and normal phases.Abstract; section Superfluid phase, Equation 4
The condensate’s equation of state is pressure proportional to density cubed, which suggests the superfluid arises through three-body interactions, and which distinguishes it from earlier Bose–Einstein-condensate dark matter, where pressure goes as density squared and the resulting low sound speed is in tension with observations. The non-analytic kinetic term is strikingly reminiscent of the Unitary Fermi Gas of cold atom physics. What to watch: finding a cold atom system with the same equation of state, which the authors say might allow laboratory simulations of galactic dynamics.Introduction; Equation 7
With a particle mass of 0.6 eV and a scale Λ of 0.2 meV, the hydrostatic solution gives dark matter halos of realistic size — about 125 kpc for a halo of 10¹² solar masses. The profile is cored: the density varies slowly through most of the volume, unlike the more steeply varying Navarro–Frenk–White profile, and the superfluid core is expected to be surrounded by a cloud of dark matter particles in the normal phase.Equations 9 and 10; Fig. 2
A superfluid spun faster than a critical velocity develops vortices, and the typical angular velocity of halos is well above that critical value, so an array of dark matter vortices should permeate the disc. What to watch: detecting those vortices through substructure lensing, for example with ALMA.Observational implications, Vortices
Landau’s criterion gives two merger outcomes. If the infall velocity is below the phonon sound speed, halos pass through each other with negligible dissipation, giving multiple encounters and a longer merger time — consistent with the Bullet Cluster. Above it, the encounter excites particles out of the condensate, producing dynamical friction and a rapid merger. The authors note that this picture matches the lensing map of the Abell 520 train wreck, which shows peaks on the galaxies and separate peaks on the X-ray gas.Observational implications, Galaxy mergers and Bullet cluster
The way in
https://doi.org/10.1016/j.physletb.2015.12.054How to cite it
Lasha Berezhiani, Justin Khoury (2016) Dark matter superfluidity and galactic dynamics doi:10.1016/j.physletb.2015.12.054