Historical research guide · 0619 · 2015
Theory of dark matter superfluidity
Lasha Berezhiani · Justin Khoury
In one minute
Lasha Berezhiani and Justin Khoury, at the University of Pennsylvania, lay out the full theory behind a single idea: dark matter is a superfluid. Their particles are light — around one electronvolt, roughly a billionth of a proton mass — and they push on each other hard enough to share heat inside a galaxy. There, the galaxy’s own gravity does the job a magnetic trap does in a cold-atom laboratory: the particles cool below a critical temperature of a few thousandths of a degree above absolute zero and condense into one coherent quantum fluid the size of the halo. The consequences are the payoff. Sound waves in that fluid — phonons — carry an extra pull on ordinary matter that reproduces Milgrom’s modified dynamics, so rotation curves come out right without tuning each galaxy by hand. Clusters of galaxies run hotter, so their dark matter is only partly condensed, which is exactly where that modified force is known to fail. Galaxies, in this picture, obey the same equations as helium and trapped cold atoms.
Connection
This is the long-form case that a superfluid — a single quantum fluid with a coherence length the size of a galaxy — is a working description of the universe at the largest scales, which is the model chapter five uses for the vacuum itself. The shorter companion paper, Berezhiani and Khoury’s ‘Dark matter superfluidity and galactic dynamics’, is read in full elsewhere in this library; this one supplies the derivations behind it.
From the source
Dark matter and the MOND phenomenon are two phases of one substance: axion-like particles of about an electronvolt with strong contact self-interactions, which condense into a superfluid core inside galaxies and stay in the normal phase in hotter galaxy clusters.Abstract; Section 1.3
Requiring the particles’ de Broglie wavelengths to overlap at galactic densities caps the particle mass at about 2 electronvolts, sharpened by equation (11) to 2.3 electronvolts for a halo of 10^12 solar masses formed at redshift two.Section 2, equations (8) and (11)
For the condensate to be coherent across the halo the particles must thermalise faster than the galaxy’s dynamical time, which sets a lower bound on the self-interaction cross-section per unit mass of about 0.1 square centimetres per gram for a 0.6 electronvolt particle — just inside the 0.5 limit from cluster mergers.Section 2, equations (14) and (15)
The critical temperature works out at 6.5 millikelvin for an electronvolt particle in a halo formed at redshift two, the same range as laboratory Bose-Einstein condensates — lithium-7 condenses at about 0.2 millikelvin.Section 2, equation (16)
The phonon effective theory of this dark matter is strikingly similar to that of the unitary Fermi gas, so a cold-atom system could serve as a laboratory analogue of galactic dynamics.Abstract; Section 1.3
Superfluidity predicts what to look for: an array of low-density vortices threading the galactic disc, mergers that pass through each other with almost no friction when the infall speed is below the phonon sound speed, and two distinct lensing mass peaks in bullet-like cluster collisions — a pattern consistent with the Abell 520 map.Section 1.3 bullet list; Sections 9 to 11
The way in
https://doi.org/10.1103/PhysRevD.92.103510How to cite it
Lasha Berezhiani, Justin Khoury (2015) Theory of dark matter superfluidity doi:10.1103/PhysRevD.92.103510