Theory of dark matter superfluidity
Lasha Berezhiani · Justin Khoury
Abstract and summary · read the original at the source
In one page
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.
Why it matters hereThis 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.
What it claims
01Dark 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
Published and peer-reviewed02Requiring 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)
Published and peer-reviewed03For 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)
Published and peer-reviewed04The 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)
Published and peer-reviewed05The 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
What to watch06Superfluidity 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
What to watch
Read it · abstract
Abstract
We propose a novel theory of dark matter (DM) superfluidity that matches the successes of the ΛCDM model on cosmological scales while simultaneously reproducing the MOdified Newtonian Dynamics (MOND) phenomenology on galactic scales. The DM and MOND components have a common origin, representing different phases of a single underlying substance. DM consists of axion-like particles with mass of order eV and strong self-interactions. The condensate has a polytropic equation of state P ∼ ρ³ giving rise to a superfluid core within galaxies. Instead of behaving as individual collisionless particles, the DM superfluid is more aptly described as collective excitations. Superfluid phonons, in particular, are assumed to be governed by a MOND-like effective action and mediate a MONDian acceleration between baryonic matter particles. Our framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not): due to the higher velocity dispersion in clusters, and correspondingly higher temperature, the DM in clusters is either in a mixture of superfluid and normal phase, or fully in the normal phase. The rich and well-studied physics of superfluidity leads to a number of observational signatures: array of low-density vortices in galaxies, merger dynamics that depend on the infall velocity vs phonon sound speed; distinct mass peaks in bullet-like cluster mergers, corresponding to superfluid and normal components; interference patterns in super-critical mergers. Remarkably, the superfluid phonon effective theory is strikingly similar to that of the unitary Fermi gas, which has attracted much excitement in the cold atom community in recent years. The critical temperature for DM superfluidity is of order mK, comparable to known cold atom Bose-Einstein condensates. Identifying a precise cold atom analogue would give important insights on the microphysical interactions underlying DM superfluidity. Tantalizingly, it might open the possibility of simulating the properties and dynamics of galaxies in laboratory experiments.
The way in
https://doi.org/10.1103/PhysRevD.92.103510Published as Physical Review D 92, 103510 (2015). The author copy, arXiv:1507.01019v2, is posted under the arXiv.org perpetual non-exclusive licence rather than a Creative Commons licence, and the APS deposit carries the APS default licence, so this page carries the summary, the claims and the authors’ own abstract and sends the reader to the source, which is free to read in full at arxiv.org/abs/1507.01019.
How to cite it
Lasha Berezhiani, Justin Khoury (2015) Theory of dark matter superfluidity. doi:10.1103/PhysRevD.92.103510
Where it sits in the curriculum