Theory of dark matter superfluidity

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 ∼ ρ 3 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 patters 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.

Stellar streams as gravitational experiments

“…GCs with a weak external field and extended velocity dispersion profiles, such as NGC 2419, display similar tensions (Ibata et al 2011), but see also Gentile et al (2010) and Sanders (2012). Note however that, in theories like those of Berezhiani & Khoury (2015), such tensions would be expected since MOND would only be an effective description of gravity within a given distance from the host galaxy (∼100 kpc in the case of the MW). However, clusters which are closer to the Galactic center and dominated by the external gravitational field of the MW are also interesting as long as the external field is below a 0 .…”

“…This could mean that it is either an effective way to predict how dark matter (DM) is arranged in galaxies, or that it is an effective modification of gravity resulting from a broader theory of the dark sector (e.g. Blanchet & Heisenberg 2015;Berezhiani & Khoury 2015). To distinguish between these possibilities, it is absolutely mandatory to check for distinctive predictions of this paradigm in its a priori domain of validity, i.e.…”

Covariant version of Verlinde’s emergent gravity

“…For example it is interesting to note that the strange-seeming power of 2/3 in the Lagrangian was also used in [13]. In [13], the authors showed that a superfluid with this type of Lagrangian can reproduce MOND-like behavior in the superfluid phase.…”

“…For example it is interesting to note that the strange-seeming power of 2/3 in the Lagrangian was also used in [13]. In [13], the authors showed that a superfluid with this type of Lagrangian can reproduce MOND-like behavior in the superfluid phase. But -much like Verlinde's emergent gravity -has the benefit of displaying a different behavior on short distances, where a large gradient near sources of high energydensity (like, eg, stars) renders the superfluid approximation invalid.…”

Static structure of chameleon dark matter as an explanation of dwarf spheroidal galaxy cores

“…But this scenario is also not free from observational challenges as the recently measured abundance of ultrafaint lensed galaxies at z ≃ 6 in the Hubble Frontier Fields might provide stringent constraints on the success of ULA dark matter in explaining dSph cores [23]. Another dark matter candidate, which was proposed very recently [24], claims to solve the corecusp issue at small scales due to the superfluidic effect of the scalar-dark matter condensate at the small scale while replicating the success of CDM at the large scale.…”