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Self-Interacting Dark Matter (SIDM) vs Superfluid Dark Matter

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Dark Matter Candidates· within family

	Self-Interacting Dark Matter (SIDM) Frontier	Superfluid Dark Matter Frontier
Proposed	2000 / 2018	2015
Key figures	David Spergel, Paul Steinhardt, Sean Tulin, Hai-Bo Yu	Lasha Berezhiani, Justin Khoury
In one sentence	Dark matter that interacts with itself via some non-gravitational force, with cross sections tuned so the interactions thermalize the inner regions of dwarf galaxies (creating cores instead of cusps) but barely affect large-scale structure.	Lasha Berezhiani and Justin Khoury proposed in 2015 a hybrid dark matter framework: a particle that condenses into a superfluid phase in the central regions of galaxies, producing MOND-like phenomenology on galactic scales via phonon excitations, while remaining ordinary cold dark matter on cosmological scales. Berezhiani and Khoury's 2015 and 2016 papers (Phys. Rev. D 92, 103510 and Phys. Lett. B 753, 639) attempt to bridge particle-DM ontology with MOND-style modified-gravity phenomenology in a single framework.
Predictions	Cored density profiles in dwarf and low-surface-brightness galaxies, with core sizes correlated with halo mass and velocity dispersion Reduced central densities and rounder inner halos in galaxy clusters compared to pure CDM, but a smaller effect than in dwarfs (because clusters have higher velocity dispersion and shorter halo crossing times relative to the SIDM mean free path) Possible offsets between dark matter and galaxies in merging cluster systems, depending on the velocity dependence of the cross-section	Galactic rotation curves are explained by phonon excitations in the superfluid phase of dark matter; the framework reproduces the MOND empirical relations on galactic scales Galaxy cluster dynamics are governed by standard CDM (the superfluid phase is destroyed at cluster densities), reproducing the empirical successes of dark matter on large scales Cosmological observables (CMB, large-scale structure) are essentially CDM, since the superfluid phase exists only in dense galaxy interiors Specific predictions for the transition radius between superfluid and normal CDM phases in each galaxy, depending on the central density and the dark-matter particle properties
Where it breaks	Baryonic physics (star formation feedback, AGN) can also produce cores within CDM, reducing the necessity of SIDM Cluster shapes and ellipticity from gravitational lensing constrain σ/m below the value needed to affect dwarf cores, requiring velocity-dependent cross-sections that some models can produce but not all Missing-satellites and too-big-to-fail problems aren't fully addressed by SIDM alone; they require additional fixes Critics view SIDM as introducing a free parameter (the cross-section) rather than proposing a specific candidate particle	Galaxy cluster constraints (specifically, the Bullet Cluster and related merging-cluster observations) place upper bounds on the dark-matter particle's self-interaction cross-section; the superfluid framework's parameter space is constrained by these observations The framework requires fine-tuning the dark-matter particle's mass and self-interaction parameters to specific ranges that enable superfluid condensation in galaxies but not in clusters; the deep physical motivation for these values is not provided Citation counts for the framework remain below what some researchers consider significant; its current impact is more modest than the bridge-framework ambition suggests Alternative hybrid frameworks (Verlinde Entropic Gravity, modified-inertia variants of MOND) cover similar territory; the choice among bridge frameworks is not currently observationally constrained
Key unresolved problem	The mimicry problem: ordinary effects like exploding stars and black-hole winds can carve out the same smooth galaxy centers that self-interacting dark matter (SIDM) would, so nothing observed yet clearly requires the particles to collide with each other.	The fine-tuning problem: the particle's mass and interactions must sit in a very narrow range so the dark matter turns into a frictionless superfluid inside galaxies but not in clusters, and there is no deeper reason those exact values should hold.
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Self-Interacting Dark Matter (SIDM)

2000 / 2018 · Frontier

Superfluid Dark Matter

2015 · Frontier

Proposed

2000 / 2018

2015

Key figures

David Spergel, Paul Steinhardt, Sean Tulin, Hai-Bo Yu

Lasha Berezhiani, Justin Khoury

In one sentence

Dark matter that interacts with itself via some non-gravitational force, with cross sections tuned so the interactions thermalize the inner regions of dwarf galaxies (creating cores instead of cusps) but barely affect large-scale structure.

Lasha Berezhiani and Justin Khoury proposed in 2015 a hybrid dark matter framework: a particle that condenses into a superfluid phase in the central regions of galaxies, producing MOND-like phenomenology on galactic scales via phonon excitations, while remaining ordinary cold dark matter on cosmological scales. Berezhiani and Khoury's 2015 and 2016 papers (Phys. Rev. D 92, 103510 and Phys. Lett. B 753, 639) attempt to bridge particle-DM ontology with MOND-style modified-gravity phenomenology in a single framework.

Predictions

Cored density profiles in dwarf and low-surface-brightness galaxies, with core sizes correlated with halo mass and velocity dispersion
Reduced central densities and rounder inner halos in galaxy clusters compared to pure CDM, but a smaller effect than in dwarfs (because clusters have higher velocity dispersion and shorter halo crossing times relative to the SIDM mean free path)
Possible offsets between dark matter and galaxies in merging cluster systems, depending on the velocity dependence of the cross-section

Galactic rotation curves are explained by phonon excitations in the superfluid phase of dark matter; the framework reproduces the MOND empirical relations on galactic scales
Galaxy cluster dynamics are governed by standard CDM (the superfluid phase is destroyed at cluster densities), reproducing the empirical successes of dark matter on large scales
Cosmological observables (CMB, large-scale structure) are essentially CDM, since the superfluid phase exists only in dense galaxy interiors
Specific predictions for the transition radius between superfluid and normal CDM phases in each galaxy, depending on the central density and the dark-matter particle properties

Where it breaks

Baryonic physics (star formation feedback, AGN) can also produce cores within CDM, reducing the necessity of SIDM
Cluster shapes and ellipticity from gravitational lensing constrain σ/m below the value needed to affect dwarf cores, requiring velocity-dependent cross-sections that some models can produce but not all
Missing-satellites and too-big-to-fail problems aren't fully addressed by SIDM alone; they require additional fixes
Critics view SIDM as introducing a free parameter (the cross-section) rather than proposing a specific candidate particle

Galaxy cluster constraints (specifically, the Bullet Cluster and related merging-cluster observations) place upper bounds on the dark-matter particle's self-interaction cross-section; the superfluid framework's parameter space is constrained by these observations
The framework requires fine-tuning the dark-matter particle's mass and self-interaction parameters to specific ranges that enable superfluid condensation in galaxies but not in clusters; the deep physical motivation for these values is not provided
Citation counts for the framework remain below what some researchers consider significant; its current impact is more modest than the bridge-framework ambition suggests
Alternative hybrid frameworks (Verlinde Entropic Gravity, modified-inertia variants of MOND) cover similar territory; the choice among bridge frameworks is not currently observationally constrained

Key unresolved problem

The mimicry problem: ordinary effects like exploding stars and black-hole winds can carve out the same smooth galaxy centers that self-interacting dark matter (SIDM) would, so nothing observed yet clearly requires the particles to collide with each other.

The fine-tuning problem: the particle's mass and interactions must sit in a very narrow range so the dark matter turns into a frictionless superfluid inside galaxies but not in clusters, and there is no deeper reason those exact values should hold.

Reader vote

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