Dark Matter Candidates

Why This Matters

Dark matter constitutes roughly 27% of the universe's total energy density, yet we've never directly detected a single dark matter particle. In Astrophysics II, you're being tested on your ability to evaluate competing theoretical frameworks—understanding not just what candidates exist, but why each addresses specific observational puzzles like galactic rotation curves, the bullet cluster, and large-scale structure formation. The dark matter problem sits at the intersection of particle physics, cosmology, and gravitational dynamics, making it one of the most integrative topics you'll encounter.

Don't fall into the trap of memorizing a list of exotic particle names. Instead, focus on the underlying physics: What interaction mechanisms does each candidate propose? How would we detect it? What cosmological problems does it solve—or create? When you encounter an FRQ asking you to evaluate evidence for dark matter, you need to connect observational signatures to theoretical predictions. Know the detection strategies, mass scales, and structure formation implications for each candidate category.

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Thermal Relic Candidates

These particles were produced in thermal equilibrium in the early universe and "froze out" as the universe expanded and cooled. Their present-day abundance depends on their annihilation cross-section—the famous "WIMP miracle" connects weak-scale physics to the observed dark matter density.

Weakly Interacting Massive Particles (WIMPs)

• Mass range of $10 \text{ GeV}$ to several $\text{TeV}$—this weak-scale mass naturally produces the correct relic abundance through thermal freeze-out
• Interact via weak nuclear force, enabling three detection strategies: direct detection (nuclear recoils), indirect detection (annihilation products), and collider production
• Cold dark matter behavior supports hierarchical structure formation, matching observations of galaxy clustering and cosmic web filaments

Gravitinos

• Superpartner of the graviton in supersymmetric extensions of the Standard Model—their existence would confirm SUSY
• Extremely weak interactions (gravitational strength only) make direct detection essentially impossible with current technology
• Thermal history implications—gravitino abundance constrains reheating temperature after inflation, connecting dark matter to early universe cosmology

Kaluza-Klein Particles

• Arise from extra-dimensional theories where Standard Model particles have heavier "copies" propagating in compactified dimensions
• Lightest Kaluza-Klein particle (LKP) is stable in many models, analogous to how the lightest supersymmetric particle is stable
• Collider signatures would appear as missing energy plus Standard Model particles, similar to WIMP searches at the LHC

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Ultra-Light Boson Candidates

These candidates have extremely small masses, causing their de Broglie wavelengths to extend to astrophysical scales. Quantum mechanical wave behavior becomes relevant for structure formation, producing distinctive signatures in galactic cores.

Axions

• Originally proposed to solve the strong CP problem—the puzzle of why QCD doesn't violate CP symmetry despite having no reason not to
• Mass range $10^{-6}$ to $10^{-3} \text{ eV}$ makes them ultra-light, with enormous occupation numbers behaving as a classical field
• Detectable via axion-photon coupling in strong magnetic fields—experiments like ADMX search for resonant conversion in microwave cavities

Fuzzy Dark Matter

• Ultra-light bosons with masses $\sim 10^{-22} \text{ eV}$—de Broglie wavelengths reach kiloparsec scales, comparable to galactic cores
• Wave interference creates solitonic cores that naturally avoid the cusp-core problem plaguing standard cold dark matter simulations
• Suppresses small-scale structure below the de Broglie wavelength, potentially explaining the "missing satellites" problem

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Warm Dark Matter Candidates

Warm dark matter particles have intermediate velocities at decoupling—fast enough to erase small-scale density perturbations but slow enough to preserve large-scale structure. This addresses tensions between cold dark matter simulations and observed dwarf galaxy properties.

Sterile Neutrinos

• Do not interact via standard weak force—"sterile" means they only mix with active neutrinos through mass terms, not gauge interactions
• Mass range $\sim 1-10 \text{ keV}$ places them in the warm dark matter regime, suppressing structure below $\sim 100 \text{ kpc}$
• Radiative decay produces X-ray line at $E = m_s/2$—the contested 3.5 keV line in galaxy cluster spectra remains a possible detection signature

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Self-Interacting and Modified Candidates

These approaches address small-scale structure problems by modifying dark matter dynamics rather than just changing particle mass. Self-interactions or modified gravity can redistribute matter in galactic cores, potentially resolving discrepancies between simulations and observations.

Self-Interacting Dark Matter (SIDM)

• Dark matter particles scatter off each other with cross-section $\sigma/m \sim 1 \text{ cm}^2/\text{g}$—strong enough to affect cores, weak enough to preserve cluster dynamics
• Thermalizes galactic cores through heat conduction, converting cusps to cores and explaining observed rotation curves of dwarf galaxies
• Velocity-dependent cross-sections can satisfy constraints across mass scales, from dwarfs to clusters

Modified Gravity Theories

• Alter gravitational dynamics instead of adding matter—MOND modifies Newtonian acceleration below $a_0 \sim 10^{-10} \text{ m/s}^2$
• Successfully predicts rotation curves using only baryonic matter distribution, following the Tully-Fisher relation naturally
• Struggles with galaxy clusters and cosmology—the Bullet Cluster's offset between mass (lensing) and baryons strongly disfavors pure modified gravity

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Compact Object Candidates

These candidates are macroscopic objects rather than elementary particles. Their gravitational effects are identical to particle dark matter on large scales, but detection relies on gravitational signatures rather than particle interactions.

Primordial Black Holes

• Formed from density fluctuations in the early universe—not from stellar collapse, so they can have masses from $10^{-18} M_\odot$ to thousands of $M_\odot$
• Constrained across most mass ranges by microlensing surveys, CMB distortions, and gravitational wave observations from LIGO/Virgo
• Asteroid-mass window ($10^{-16}$ to $10^{-11} M_\odot$) remains viable—too light for microlensing, too heavy for Hawking evaporation

Massive Compact Halo Objects (MACHOs)

• Baryonic objects like brown dwarfs, neutron stars, or stellar-mass black holes residing in galactic halos
• Detected via gravitational microlensing—temporary brightening of background stars as MACHOs pass through the line of sight
• Cannot comprise all dark matter—microlensing surveys (EROS, MACHO project) rule out dominant contribution in the $10^{-7}$ to $10 M_\odot$ range

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Quick Reference Table

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Self-Check Questions

1. Both axions and fuzzy dark matter are ultra-light bosons—what distinguishes their mass scales, and how does this affect their observational signatures?

2. Which two candidates specifically address the cusp-core problem in dwarf galaxies, and through what different mechanisms?

3. Compare and contrast the detection strategies for WIMPs versus sterile neutrinos. Why is one searched for in underground laboratories while the other is searched for with X-ray telescopes?

4. If an FRQ presents the Bullet Cluster as evidence, which dark matter candidate category does it most strongly disfavor, and why does the spatial offset between lensing mass and X-ray emission matter?

5. Primordial black holes and MACHOs are both compact objects—explain why one is consistent with Big Bang nucleosynthesis constraints while the other is not, and identify which mass ranges remain viable for primordial black holes.