Dark matter - Particle Candidates and Models

Dark Matter Candidates: A Comprehensive Overview Introduction One of the most pressing questions in modern physics is: what is dark matter? We know dark matter exists because of its gravitational effects on galaxies and the universe's large-scale structure, but we don't yet know what it's made of. Scientists have proposed various candidates that fall into two main categories: particle candidates and compact object candidates like primordial black holes. This guide surveys the leading proposals and what makes them viable dark matter. Why Weakly Interacting Particles Matter for Dark Matter A fundamental challenge for any dark matter candidate is this: dark matter particles cannot easily lose energy through collisions. If they could, they would cool and collapse into compact objects like stars and black holes, which we would observe. The fact that dark matter remains smoothly distributed throughout galaxies suggests it interacts very weakly with ordinary matter and even with itself. This is why most particle candidates interact only through gravity and possibly the weak nuclear force—the weakest of the fundamental forces. Understanding this constraint is essential for evaluating any dark matter proposal. Weakly Interacting Massive Particles (WIMPs) WIMPs are hypothetical elementary particles that represent one of the most well-studied dark matter candidates. They interact via gravity and the weak nuclear force, but not through the stronger electromagnetic or strong nuclear forces. Why WIMPs Were a Leading Candidate WIMPs have a special property that makes them naturally suited for dark matter: the thermal relic abundance mechanism. Here's the key idea: In the hot early universe, WIMPs were continuously created and destroyed in particle collisions. As the universe expanded and cooled, these reactions slowed down. Eventually, WIMPs became too rare to find each other and destroy, "freezing out" at a fixed abundance. Remarkably, a WIMP with a mass near 100 GeV/c² (roughly the mass of a heavy nucleus) with a self-annihilation cross-section of about $3 \times 10^{-26}\,\text{cm}^3\,\text{s}^{-1}$ naturally produces the exact abundance of dark matter we observe today. This "coincidence" is called the WIMP miracle and explains why physicists were excited about WIMPs. Supersymmetric WIMPs A particularly motivated class of WIMPs comes from supersymmetry, a theory proposing that every particle has a heavier "partner." The neutralino—the lightest supersymmetric particle—is protected from decay by a quantum number called R-parity and represents an ideal WIMP candidate. The neutralino's mass and interaction strength naturally arise from the theory's structure, making it theoretically compelling. Direct Detection Attempts Physicists have searched for WIMPs using direct-detection experiments that look for nuclear recoils—tiny kicks given to atomic nuclei when WIMPs collide with them. Sensitive detectors cooled to cryogenic temperatures can measure these rare interactions. However, no confirmed WIMP detection has yet been made, despite decades of increasingly sensitive searches. The Supersymmetry Problem A significant issue has emerged: the Large Hadron Collider (LHC), which can create particles at extremely high energies, has not discovered any supersymmetric particles. This absence argues against the simplest supersymmetry models and their natural WIMP candidates. While more exotic supersymmetry models remain viable, the lack of LHC discoveries has substantially dampened enthusiasm for WIMPs. Axions and Axion-Like Particles Axions represent a very different approach to the dark matter problem. Instead of requiring new particle physics primarily for dark matter, axions were proposed to solve an entirely different puzzle: the strong CP problem in quantum chromodynamics. The Strong CP Problem and the Peccei-Quinn Solution In the Standard Model of particle physics, certain fundamental symmetries lead to a prediction that the strong nuclear force should violate a symmetry called CP (charge-parity symmetry). This violation should produce measurable effects in the decays of neutrons and other particles. However, experiments show these effects are strikingly absent—they're smaller than predicted by roughly a trillion times. This discrepancy is the "strong CP problem." In 1977, Roberto Peccei and Helen Quinn proposed an elegant solution: introduce a new global U(1) symmetry that automatically prevents CP violation in the strong force. When this symmetry breaks spontaneously (like water freezing into ice, which breaks the symmetry of rotational freedom), it leaves behind a new particle: the axion. Why Axions Make Excellent Dark Matter An axion with mass below about 60 keV/c² is long-lived and interacts only extremely weakly with ordinary matter. Several properties make axions compelling dark matter candidates: They're extremely light, so vast numbers would exist without exceeding the total dark matter density They interact so weakly that they elude direct detection They have a specific theoretical origin in solving the strong CP problem, so their existence isn't purely hypothetical The Misalignment Mechanism Axions are produced in the early universe through the misalignment mechanism. Initially, the axion field has some value determined by random quantum fluctuations. As the universe cools after the Big Bang, this field oscillates around its minimum energy state, effectively creating a population of axion particles. The abundance depends on two parameters: the axion mass and the decay constant $fa$ (a measure of the symmetry scale). For decay constants around $10^{11}$–$10^{12}$ GeV, the predicted axion abundance matches observations. Experimental Detection of Axions Detecting axions is challenging because they interact so weakly. The leading experiment is the Axion Dark Matter eXperiment (ADMX), which uses a clever approach: In a strong magnetic field, axions can convert to photons (ordinary light particles). ADMX uses a resonant microwave cavity—similar in principle to a radio tuner—to search for this conversion. By varying the cavity's resonance frequency, experimenters can scan across different axion masses. ADMX has reached sensitivity to axion masses in the micro-electronvolt range and continues to push toward discovery. Other detection methods include helioscope experiments like IAXO, which search for solar axions produced in the sun's core. Primordial Black Holes as Dark Matter Primordial black holes (PBHs) represent a strikingly different dark matter hypothesis: instead of new particles, dark matter could consist of black holes that formed in the early universe before any stars existed. Formation in the Early Universe In the extremely dense early universe, regions with slightly higher density than average would experience stronger gravitational collapse. Normally, such collapse is halted when the collapsing material heats up enough to resist further compression (as happens in star formation). However, in the very early universe—during or shortly after cosmic inflation—conditions were so extreme that gravitational collapse could overcome this resistance, creating black holes without any supernova or stellar explosion. These primordial black holes would be non-baryonic (not made of ordinary matter) and could have masses ranging from the Planck scale (extremely tiny) to supermassive scales. Recent Motivation: LIGO Detections Interest in primordial black holes received a major boost when the LIGO gravitational-wave detector observed merging black holes with masses around 30 solar masses in 2015. These were unexpected because stellar-process black hole formation typically produces either much lighter or much heavier black holes. This gap made primordial black holes an attractive explanation. Observational Constraints However, various observations constrain how much of dark matter can be PBHs: Gravitational microlensing surveys (EROS-2, OGLE) look for the magnification of background starlight caused by massive objects passing in front of them. These surveys limit MACHOs (Massive Compact Halo Objects like brown dwarfs and stellar remnants) to less than a few percent of the galactic halo, and similar constraints apply to PBHs in certain mass ranges. Hawking radiation would cause small primordial black holes to evaporate. The radiation from this evaporation would have observable effects, ruling out PBHs below certain masses. Gravitational-wave observations from LIGO and Virgo probe the abundance of PBHs in the $10$–$100\ M{\odot}$ range by detecting mergers. These constraints rule out PBHs as 100% of dark matter in specific narrow mass ranges—if all PBHs have the same mass (a monochromatic mass spectrum). However, this assumption may not hold. Extended Mass Distributions Relax Constraints Modern inflation models predict that primordial black holes would form with a range of masses rather than all having identical mass. An extended mass distribution means the observational constraints on narrow mass ranges don't immediately rule out PBHs as all or part of the dark matter. This possibility keeps primordial black holes viable as a dark matter candidate. Fine-Tuning Challenges for Primordial Black Hole Formation Creating primordial black holes requires a significant challenge: generating sufficiently large density fluctuations in the early universe. The Standard Inflation Problem Standard inflation theory, with its slow-roll evolution of the inflaton field, predicts very small density fluctuations. These fluctuations are far too small to collapse gravitationally and form black holes. To form primordial black holes, density fluctuations must be amplified by roughly a trillion-fold or more. Exotic Inflation Models To overcome this problem, theorists have proposed exotic inflation models featuring special structures in the inflaton potential: Inflection points: places where the potential flattens temporarily, slowing inflation and amplifying fluctuations Bumps or plateaus: features that create similar amplitude-amplification effects Other non-standard features: various mechanisms designed to enhance fluctuations during specific cosmological epochs These models can produce the required large fluctuations, but they introduce a complication. The Tuning Problem The abundance of primordial black holes is exponentially sensitive to the amplitude of density fluctuations. Small changes in parameters produce enormous changes in black hole abundance. This means that successful primordial black hole models must adjust their parameters with extreme precision to match observations. This fine-tuning is unaesthetic and suggests to many theorists that primordial black holes may not be the full answer to the dark matter problem, though they could constitute part of it. <extrainfo> Dynamical Dark Matter Framework Keith R. Dienes and Brooks Thomas introduced the theoretical framework of Dynamical Dark Matter in a 2012 survey paper, proposing that dark matter could consist of a diverse ensemble of particles with varying properties and cosmological abundances that evolve over cosmic time. This framework suggests dark matter might not be a single simple component but rather a complex system. </extrainfo> Other Exotic Candidates Beyond WIMPs, axions, and primordial black holes, physicists have proposed additional dark matter candidates: Sterile neutrinos are hypothetical right-handed neutrinos that don't participate in the weak nuclear force interactions that affect ordinary neutrinos. With keV-scale masses, sterile neutrinos can constitute warm dark matter—a form of dark matter whose particles move fast enough to smooth out small-scale structure in the universe. This differs from "cold" dark matter (like axions and WIMPs) where particles move slowly. Dark photons are massive vector bosons that can interact with ordinary photons through kinetic mixing—a subtle quantum effect that creates a bridge between the Standard Model and a hidden sector of new particles. This mixing provides a potential detection pathway for dark photons. Hidden-sector particles more broadly refer to new particles existing in a parallel sector of physics with minimal direct interactions with ordinary matter, detectable primarily through their gravitational effects. These exotic candidates each address specific theoretical motivations and offer different experimental signatures, providing complementary search strategies to the more conventional candidates. Summary: The Dark Matter Landscape The search for dark matter remains genuinely open. Each candidate offers distinct advantages and faces specific challenges: WIMPs provide natural thermal relic abundances but lack experimental confirmation and face challenges from LHC non-discoveries Axions solve the strong CP problem while naturally producing dark matter, with experimental searches actively underway Primordial black holes require no new particle physics but face fine-tuning issues and observational constraints Other candidates (sterile neutrinos, dark photons, etc.) provide alternative theoretical frameworks and detection signatures The answer likely involves one or more of these candidates—or perhaps an entirely unexpected discovery. This uncertainty is what makes dark matter research so compelling and actively pursued worldwide.