Dark matter superfluidity and galactic dynamics

Abstract

We propose a unified framework that reconciles the stunning success of MOND on galactic scales with the triumph of the ΛCDM model on cosmological scales. This is achieved through the physics of superfluidity. Dark matter consists of self-interacting axion-like particles that thermalize and condense to form a superfluid in galaxies, with ∼mK critical temperature. The superfluid phonons mediate a MOND acceleration on baryonic matter. Our framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not): dark matter has a higher temperature in clusters, and hence is in a mixture of superfluid and normal phase. The rich and well-studied physics of superfluidity leads to a number of striking observational signatures.The standard Λ Cold Dark Matter (ΛCDM) model does very well at fitting large scale observables. On galactic scales, however, a number of challenges have emerged. Disc galaxies display a tight correlation between total baryonic mass and asymptotic velocity, M b ∼ v 4 c , known as the Baryonic Tully-Fisher Relation (BTFR) [1,2]. Hydrodynamical simulations can reproduce the BTFR by tuning baryonic feedback processes, but their stochastic nature naturally results in a much larger scatter [3]. Furthermore, the mass [4,5] and phase-space [6-9] distributions of dwarf satellites in the Local Group are puzzling.A radical alternative is MOdified Newtonian Dynamics (MOND) [10], which replaces dark matter (DM) with a modification of gravity at low acceleration: a a N (a N a 0 ); a √ a N a 0 (a N a 0 ), with best-fit value a 0 1.2 × 10 −8 cm/s 2 . This empirical force law has been remarkably successful at explaining a wide range of galactic phenomena [11]. In the MOND regime, a test particle orbits an isolated source according to v 2 /r = G N M b a 0 /r 2 . This gives a constant asymptotic velocity, v 2 c = √ G N M b a 0 , which in turn implies the BTFR. The empirical success of MOND, however, is limited to galaxies. The predicted temperature profile in galaxy clusters conflicts with observations [12]. The Tensor-Vector-Scalar (TeVeS) relativistic extension [13] fails to reproduce the CMB and matter spectra [14,15]. The lensing features of merging clusters [16,17] are problematic [18]. This has motivated various hybrid proposals that include both DM and MOND, e.g., [19][20][21][22][23].In this Letter, together with a longer companion paper [24], we propose a novel framework that unifies the DM and MOND phenomena through the physics of superfluidity. Our central idea is that DM forms a superfluid inside galaxies, with a coherence length of order the size of galaxies. The critical temperature is ∼ mK, which intriguingly is comparable to Bose-Einstein condensation (BEC) critical temperatures for cold atom gases. Indeed, in many ways our DM behaves like cold dark atoms.The superfluid nature of DM dramatically changes its macroscopic behavior in galaxies. Instead of evolving as independent particles, DM is more aptly described as collective excitations. Superfluid phonons,...

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“…Besides it being an interesting substance with interesting properties on its own, that can be manufactured in laboratories from known particles, superfluids have been theorised to be potentially relevant even in dark matter context. In particular, in some scenarios dark matter may undergo superfluid phase-transition cosmologically and/or in localised structure [6][7][8][9][10][11][12][13][14][15]. In some other cases, the superfluid He has been proposed as a target material for dark matter detection [16][17][18][19].…”