Compare · The Dark Universe

Self-Interacting Dark Matter (SIDM) vs Axions and axion-like particles

← Back to Self-Interacting Dark Matter (SIDM)

Dark Matter Candidates· within family

	Self-Interacting Dark Matter (SIDM) Frontier	Axions and axion-like particles Frontier
Proposed	2000 / 2018	1977 / 1983
Key figures	David Spergel, Paul Steinhardt, Sean Tulin, Hai-Bo Yu	Roberto Peccei, Helen Quinn, Steven Weinberg, Frank Wilczek, John Preskill, Pierre Sikivie
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.	Very light particles originally proposed to solve a fine-tuning problem in QCD (the strong-CP problem), with a tiny coupling to photons that makes them invisible to most experiments but also makes them a natural cold-dark-matter candidate.
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	A narrow radio signal from microwave cavity haloscopes (resonant chambers that amplify axion-to-photon conversion in a strong magnetic field), tuned to a frequency set by the unknown axion mass (f = m_a c² / h), with peak power fixed by the axion-photon coupling Stellar cooling anomalies: helium-burning stars and SN1987A would lose energy faster than observed if axion-photon couplings were too strong, bounding the coupling Time-varying signals in precision atomic clocks, NMR experiments, and interferometers from coherent oscillation of an axion dark matter field Spectral features from axion-photon conversion in galactic, stellar, and laboratory magnetic fields
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	Theoretically well-motivated but the allowed parameter space is vast: several orders of magnitude in mass and in coupling Some axion-production scenarios would leave a patchy imprint in the ancient light of the CMB (an isocurvature signal) that we do not see, so they survive only if the conditions during cosmic inflation are tuned No direct collider or laboratory hint; all motivation is theoretical and cosmological Critics argue axions are easy to 'rescue' with parameter tuning whenever an experiment finds nothing
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 unknown-mass problem: the axion's mass could fall anywhere across a vast range, and no experiment can yet sweep the whole plausible window in any reasonable amount of time.
Reader vote	No votes yet	No votes yet

Self-Interacting Dark Matter (SIDM)

2000 / 2018 · Frontier

Axions and axion-like particles

1977 / 1983 · Frontier

Proposed

2000 / 2018

1977 / 1983

Key figures

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

Roberto Peccei, Helen Quinn, Steven Weinberg, Frank Wilczek, John Preskill, Pierre Sikivie

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.

Very light particles originally proposed to solve a fine-tuning problem in QCD (the strong-CP problem), with a tiny coupling to photons that makes them invisible to most experiments but also makes them a natural cold-dark-matter candidate.

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

A narrow radio signal from microwave cavity haloscopes (resonant chambers that amplify axion-to-photon conversion in a strong magnetic field), tuned to a frequency set by the unknown axion mass (f = m_a c² / h), with peak power fixed by the axion-photon coupling
Stellar cooling anomalies: helium-burning stars and SN1987A would lose energy faster than observed if axion-photon couplings were too strong, bounding the coupling
Time-varying signals in precision atomic clocks, NMR experiments, and interferometers from coherent oscillation of an axion dark matter field
Spectral features from axion-photon conversion in galactic, stellar, and laboratory magnetic fields

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

Theoretically well-motivated but the allowed parameter space is vast: several orders of magnitude in mass and in coupling
Some axion-production scenarios would leave a patchy imprint in the ancient light of the CMB (an isocurvature signal) that we do not see, so they survive only if the conditions during cosmic inflation are tuned
No direct collider or laboratory hint; all motivation is theoretical and cosmological
Critics argue axions are easy to 'rescue' with parameter tuning whenever an experiment finds nothing

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 unknown-mass problem: the axion's mass could fall anywhere across a vast range, and no experiment can yet sweep the whole plausible window in any reasonable amount of time.

Reader vote

No votes yet