Superfluid Dark Matter

Abstract

The superfluid dark matter model offers an elegant solution to reconcile discrepancies between the predictions of the cold dark matter paradigm and observations on galactic scales. In this scenario, dark matter is composed of ultralight bosons with self-interactions that can undergo a superfluid phase transition in galactic environments. In this review, we explore the theoretical foundations of dark matter superfluidity, detailing the conditions required for the formation and stability of superfluid cores of astrophysical size. We examine the phenomenological consequences for galactic dynamics, including the impact on galaxy mergers, the formation of vortices, the behavior near supermassive black holes, modifications to dynamical friction, and the emergence of long-range interactions. By synthesizing theoretical developments with observational constraints, we aim to provide a comprehensive overview of the current status and future prospects of dark matter superfluidity as a viable extension of the standard cosmological model.

Figures (12)

• Figure 1: Schematic illustration of ultralight DM models in the mass/self-interactions plane. This review focuses on the Superfluid DM model.
• Figure 2: Comparison between the thermal radius $R_\text{\tiny T}$ and Jeans scale $\lambda_{\rm J}$ for superfluids with two-body interactions. Blue/orange/red solid lines indicate Jeans scales of size $\lambda_{\rm J} = 0.1,2,6$ kpc in degenerate regions of the Milky Way DM halo. The green region corresponds to degeneracy pressure case ($\zeta\lesssim 1$). The black solid curve indicates the parameter space that generates a thermal radius $R_\text{\tiny T} = \lambda_{\rm J}$. On its right, the Milky Way DM halo is in global thermal equilibrium. We see that it is impossible to have an interaction pressure-dominated core with $\lambda_{\rm J} \gtrsim R_\text{\tiny T}$. Figure inspired from Ref. Berezhiani:2021rjs.
• Figure 3: Visual depiction of the central superfluid area, approximately of size $R_\text{\tiny T}$. This zone features a central superfluid soliton of size $\lambda_{\rm J}$, encircled by a series of lightly interacting streams of superfluid remnants revolving around it. These remnants stem from the tidal disintegration of superfluid droplets that originated in the periphery of the thermal core. Whether the halo is in a state of thermal equilibrium or not determines the presence of an outer region consisting of out-of-equilibrium degenerate particles, extending up to the virial radius of the halo. Figure taken from Ref. Berezhiani:2023vlo.
• Figure 4: Parameter space for the model of SDM with two-body interactions. The quartic coupling has been rescaled in terms of the scattering cross section per unit of DM mass. The colored regions are constrained by various observational bounds, while the white ones are allowed. The dotted, dashed and solid black lines denote solitons of size $\lambda_{\rm J}=0.1,2$ and $30$ kpc, respectively. See the main text for details.
• Figure 5: Pictorial representation of the cosmological evolution for SDM.