Self-Interacting Dark Matter: Evidence and Open Questions

Self-interacting dark matter (SIDM) is a leading modification to the standard cold dark matter model, positing that dark matter particles can scatter off each other through a force beyond gravity. This article explains the astrophysical evidence that motivates SIDM, the challenges it faces, and the key open questions that researchers are actively investigating. You will learn what SIDM predicts, where it succeeds, where it struggles, and what observations could confirm or rule it out.

Formula

The core of SIDM is not a single formula but a set of predictions derived from particle scattering. The most relevant quantity is the momentum-transfer cross-section per unit mass, often written as σ/m (in cm²/g). This parameter controls how often dark matter particles collide and exchange momentum. In a halo, the average number of collisions per particle over a time t is approximately:

N_coll ≈ (σ/m) × � × v × t

where � is the local dark matter density (g/cm³), v is the relative velocity of the particles (cm/s), and t is the time elapsed (s). A higher σ/m means more collisions, which can thermalize the inner region of a halo and create a constant-density core instead of a steep central cusp.

Worked Example

Using the default values from the calculator above: finish time = 50 minutes, distance = 10 km. The average pace is:

1. Pace = 50 min / 10 km = 5.0 min/km.
2. Speed = 10 km / (50/60 h) = 12.0 km/h.

The calculator returns an average pace of 5.00 min/km and an average speed of 12.00 km/h. This is a simple example to illustrate the format; for SIDM, the relevant calculation is the collision rate, which requires density and velocity inputs from simulations or observations.

Background: From CDM to SIDM

The standard cosmological model, Lambda-CDM (ΛCDM), treats dark matter as collisionless—particles that interact only gravitationally. This model has been remarkably successful at explaining the large-scale structure of the universe, such as the distribution of galaxies and the cosmic microwave background. However, on smaller scales—within individual galaxies and dwarf galaxies—ΛCDM faces several discrepancies.

The most cited problems are the cusp-core problem, the missing satellites problem, and the too-big-to-fail problem. In ΛCDM, dark matter halos should have a steep density profile (a cusp) at their centers, but many dwarf galaxies appear to have a flat core. Also, simulations predict far more small satellite galaxies than we observe, and the most massive predicted satellites are not seen. SIDM was proposed as a way to solve these small-scale issues while preserving the successes of ΛCDM on large scales.

How Self-Interacting Dark Matter Works

In SIDM, dark matter particles have a non-gravitational interaction that allows them to scatter off each other. The strength of this interaction is characterized by the cross-section per unit mass (σ/m). When two dark matter particles collide, they exchange energy and momentum. In the dense central regions of a halo, this can lead to a process called gravothermal collapse or thermalization, which redistributes energy and flattens the central density profile.

The key effect is that collisions transfer heat from the outer, hotter parts of the halo to the inner, cooler parts, or vice versa, depending on the velocity distribution. This erases the cusp and creates a core with a size that depends on σ/m and the halo’s history. For example, a constant cross-section of σ/m ≈ 1 cm²/g can produce cores of a few hundred parsecs in dwarf galaxies, matching observations better than collisionless CDM.

Evidence Supporting SIDM

Several observations are often cited as evidence for SIDM:

• Dwarf galaxy cores: Many dwarf spheroidal galaxies in the Local Group have central density profiles that are shallower than the cusps predicted by CDM. SIDM naturally produces these cores.
• Galaxy cluster collisions: The Bullet Cluster and similar systems show a separation between dark matter and gas, but the dark matter halos are also slightly offset and less concentrated than expected for collisionless particles. Some studies suggest that a small self-interaction cross-section can explain these offsets.
• Diversity of rotation curves: The rotation curves of low-surface-brightness galaxies are very diverse, which is difficult to reproduce with collisionless CDM but can be explained by SIDM if the cross-section depends on velocity.
• Missing satellite problem: While SIDM does not directly reduce the number of subhalos, it can make them less dense, making them harder to observe and potentially explaining why some predicted satellites are not seen.

Challenges and Tensions

Despite its successes, SIDM faces several challenges:

• Core collapse: In the densest halos, such as galaxy clusters, SIDM can cause the core to collapse further, leading to a very steep density profile. This is not observed in most clusters, which constrains the cross-section to be relatively small at high velocities.
• Velocity dependence: To solve both dwarf and cluster problems, the cross-section must be velocity-dependent: large at low velocities (to affect dwarfs) and small at high velocities (to avoid core collapse in clusters). This requires a specific particle physics model, such as a light mediator, which is not yet confirmed.
• Shape of halos: SIDM can make halos more spherical, but some observations of elliptical galaxies suggest that dark matter halos are triaxial. This tension depends on the exact cross-section and the halo’s assembly history.
• Substructure: SIDM can reduce the number of subhalos by making them less dense, but it can also destroy them through collisions. The observed number of satellite galaxies in the Milky Way is still a matter of debate.

How to Choose Between CDM and SIDM

Choosing between CDM and SIDM requires comparing their predictions to a wide range of observations. Key discriminators include:

• Central density profiles: Measure the inner slopes of dwarf galaxy density profiles. A cusp (slope ≈ -1) favors CDM; a core (slope ≈ 0) favors SIDM.
• Halo shapes: Measure the ellipticity of dark matter halos using gravitational lensing or stellar kinematics. SIDM predicts rounder halos in the inner regions.
• Subhalo abundance: Count satellite galaxies and compare to simulations. SIDM predicts fewer observable satellites because they are less dense.
• Cluster observations: Use strong lensing to measure the concentration of dark matter in clusters. SIDM with a constant cross-section predicts lower concentrations than CDM.

Common Misconceptions

A common misconception is that SIDM is a completely different type of dark matter. In reality, SIDM is a modification of the collisionless assumption; the particle could still be a weakly interacting massive particle (WIMP) or an axion, but with an additional self-interaction. Another misconception is that SIDM solves all small-scale problems. It does not solve the missing satellites problem directly; it only makes them harder to detect, which is a partial solution. Also, SIDM does not change the large-scale structure because the interaction is only significant in high-density regions.

Frequently Asked Questions

What is the cross-section of self-interacting dark matter?

The cross-section per unit mass (σ/m) is the key parameter. Observations of dwarf galaxies suggest σ/m ≈ 1 cm²/g at velocities of ~30 km/s, while cluster observations constrain it to be less than ~0.1 cm²/g at velocities of ~1000 km/s. This implies a velocity-dependent interaction.

Does self-interacting dark matter explain the missing satellites problem?

Partially. SIDM does not reduce the number of subhalos, but it makes them less dense, which can suppress star formation and make them harder to detect. This could bring simulations into better agreement with the observed number of satellite galaxies, but the problem is not fully solved.

How does SIDM affect the cosmic microwave background?

SIDM has a negligible effect on the CMB because the interaction is only significant in collapsed structures, which form after recombination. The CMB is primarily sensitive to the total dark matter density and its gravitational effects, which are unchanged.

What experiments can detect self-interactions?

Astrophysical observations are the primary probes, but particle physics experiments could also detect the mediator particle if it is light and couples to dark matter. Direct detection experiments, such as those looking for dark matter scattering off nuclei, could see an excess at low recoil energies if the mediator is very light.

Is SIDM ruled out by the Bullet Cluster?

No, the Bullet Cluster actually provides a constraint. The observed offset between dark matter and gas implies that the self-interaction cross-section is less than about 1 cm²/g for velocities of ~1000 km/s. This is consistent with velocity-dependent SIDM models, but rules out a constant cross-section of that size.

Future Directions and Open Questions

The most pressing open question is the exact velocity dependence of the cross-section. Current data favor a cross-section that decreases with velocity, but the functional form is unknown. Future observations of dwarf galaxies with the James Webb Space Telescope and large surveys like the Vera Rubin Observatory will provide better measurements of density profiles and satellite populations. On the theory side, more realistic simulations that include baryonic feedback (gas, stars, supernovae) are needed to disentangle the effects of SIDM from astrophysical processes. Also, particle physics models that naturally produce the desired cross-section are being explored, such as those with a light dark photon or a dark Higgs boson.

Another open question is whether SIDM can explain the diversity of rotation curves without over-suppressing the central densities of massive galaxies. Some models predict that SIDM should also affect the inner regions of galaxy clusters, but observations show that clusters have cusps, which is a challenge. Ultimately, SIDM is a viable extension of CDM, but it is not yet proven. It remains an active area of research, with the next decade of observations likely to provide decisive tests.

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in t | Finish time | min | 50 | 0
in d | Distance | km | 10 | 0
out pace | Average pace | min/km | t / d | 2
out speed | Average speed | km/h | d / (t / 60) | 2