12.2 Dark matter evidence and candidates
5 min read•august 1, 2024
Dark matter remains one of the biggest mysteries in physics. We can't see it directly, but its gravitational effects are everywhere. From galaxy rotation curves to cosmic , the evidence for dark matter is compelling.
But what exactly is dark matter? Scientists have proposed various candidates, from to to primordial black holes. Each has its strengths and weaknesses, and the hunt is on to detect these elusive particles or objects.
Observational Evidence for Dark Matter
Galaxy Dynamics and Gravitational Effects
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- Galaxy rotation curves exhibit discrepancy between observed rotational velocities and those predicted by visible matter suggesting presence of unseen mass
- Outer regions of galaxies rotate faster than expected based on visible matter alone
- Flat rotation curves indicate presence of dark matter halo extending beyond visible disk
- observations reveal mass distributions in galaxy clusters exceeding visible matter
- Light from distant galaxies bends around massive objects more than accounted for by visible matter
- Examples include Bullet Cluster and Abell 1689 galaxy cluster
- Bullet Cluster observations show separation between distribution of visible matter and center of gravitational mass
- Hot gas (visible matter) lags behind dark matter during cluster collision
- Provides direct empirical evidence for dark matter's existence separate from normal matter
Cosmic Structure and Evolution
- (CMB) anisotropies require presence of to explain observed patterns
- Temperature fluctuations in CMB map reflect density variations in early universe
- Amplitude and distribution of fluctuations consistent with dark matter-dominated universe
- Large-scale structure formation necessitates dark matter to produce observed galaxy distributions
- Cosmic web of galaxies, clusters, and filaments requires non- to form in observed timeframe
- N-body simulations of galaxy formation and evolution reproduce observed structures only with dark matter
- Abundance of light elements from Big Bang nucleosynthesis constrains amount of baryonic matter
- Observed ratios of hydrogen, helium, and lithium set upper limit on baryonic matter density
- Necessitates non-baryonic dark matter to account for total matter density inferred from other observations
Dark Matter Candidate Particles
Weakly Interacting Massive Particles (WIMPs)
- Hypothetical particles interacting via weak force and gravity
- Masses ranging from 1 GeV to 1 TeV
- Naturally explain observed dark matter abundance through thermal freeze-out mechanism
- Motivated by and other extensions of Standard Model
- Lightest supersymmetric particle (LSP) often proposed as WIMP candidate
- Extensive experimental searches conducted using various detection methods
- Direct detection experiments (XENON, LUX, )
- Indirect detection searches (Fermi-LAT, IceCube)
- Collider experiments (LHC searches for missing energy signatures)
Alternative Particle Candidates
- Axions proposed to solve strong CP problem in quantum chromodynamics
- Light, neutral particles with potential dark matter properties
- Searched for using resonant cavity experiments (ADMX) and solar axion telescopes
- Sterile neutrinos hypothesized as particles not interacting via Standard Model forces except gravity
- Could explain dark matter and neutrino mass simultaneously
- Potential signatures in X-ray observations of galaxy clusters
- Self-interacting dark matter (SIDM) models propose dark matter particles interacting through unknown force
- Potentially explains small-scale structure observations (core-cusp problem)
- Requires introduction of new dark sector physics
Non-Particle Candidates
- Primordial black holes represent compact objects formed in early universe
- Non-particle dark matter candidate
- Various formation mechanisms proposed (density fluctuations, cosmic string collapse)
- Modified gravity theories attempt to explain dark matter phenomena without new particles
- Examples include (MOND) and Tensor-Vector-Scalar gravity (TeVeS)
- Face challenges explaining full range of dark matter observations
Strengths and Weaknesses of Dark Matter Candidates
WIMP Scenario Evaluation
- Strengths of WIMP hypothesis
- Naturally explains observed dark matter abundance (WIMP miracle)
- Connects dark matter to known particle physics (weak interaction)
- Testable through multiple experimental approaches
- Weaknesses and challenges
- Lack of detection despite extensive experimental searches
- Tension with some small-scale structure observations
- Requires extension of Standard Model not yet confirmed experimentally
Assessment of Alternative Candidates
- Axion strengths and limitations
- Solves strong CP problem and dark matter mystery simultaneously
- Well-defined experimental signatures
- Requires fine-tuning of parameters and faces detection challenges
- Sterile neutrino pros and cons
- Connects dark matter to neutrino physics
- Potential explanation for neutrino masses
- Existence not confirmed and faces X-ray observation constraints
- SIDM model evaluation
- Addresses small-scale structure issues in cosmology
- Introduces new complexity with dark sector interactions
- Requires careful tuning to satisfy observational constraints
Considerations for Non-Particle Solutions
- Primordial black hole scenario assessment
- Offers non-particle solution to dark matter
- Utilizes known physics (general relativity)
- Faces tight observational constraints on allowed mass ranges
- Modified gravity theories evaluation
- Attempt to explain dark matter phenomena without new particles
- Struggle to account for full range of observations (Bullet Cluster)
- Often require additional dark components to match all data
Experimental Techniques for Dark Matter Detection
Direct Detection Methods
- Nuclear recoil experiments aim to observe collisions between dark matter particles and atomic nuclei
- Cryogenic bolometers measure temperature rise from recoil energy (CDMS, CRESST)
- Noble liquid detectors use scintillation and ionization signals (XENON, LUX, DarkSide)
- Semiconductor detectors employ charge collection in crystalline materials (SuperCDMS)
- Low-threshold detectors developed to probe lower mass dark matter candidates
- Examples include DAMIC (silicon CCDs) and SENSEI (skipper CCDs)
- Underground laboratories provide shielding from cosmic rays and other background sources
- Deep underground locations (SNOLAB, Gran Sasso) reduce muon flux
- Careful material selection and handling minimize radioactive backgrounds
Indirect Detection Approaches
- Search for products of dark matter annihilation or decay
- Gamma rays detected by space-based (Fermi-LAT) and ground-based (H.E.S.S., MAGIC) telescopes
- Neutrinos observed by large-volume detectors (IceCube, ANTARES)
- Antimatter signatures sought in cosmic rays (AMS-02 on ISS)
- Target regions with high dark matter density
- Galactic center, dwarf spheroidal galaxies, galaxy clusters
- Challenges include distinguishing dark matter signals from astrophysical backgrounds
- Multi-wavelength observations and spectral analysis techniques employed
Collider and Specialized Searches
- Collider experiments attempt to produce dark matter particles in high-energy collisions
- Large Hadron Collider (LHC) searches for missing energy signatures
- Future colliders (ILC, CLIC) may offer improved sensitivity
- Axion searches employ specialized techniques
- Resonant cavity experiments (ADMX) probe axion-photon coupling
- Solar axion telescopes (CAST) search for axions produced in the Sun
- Novel experimental approaches explore new parameter spaces
- Atomic interferometry for ultra-light dark matter (MAGIS)
- Topological defect detectors for macroscopic dark matter structures
Key Terms to Review (18)
Axions: Axions are hypothetical elementary particles proposed to solve the strong CP problem in quantum chromodynamics and are also considered as candidates for dark matter. They are lightweight, neutral, and predicted to have very weak interactions with ordinary matter, making them elusive and difficult to detect. Axions play a critical role in addressing fundamental questions in particle physics, including limitations of existing models and the nature of dark matter.
Baryonic matter: Baryonic matter refers to the type of matter that is composed of baryons, which are particles such as protons and neutrons that make up atomic nuclei. This form of matter constitutes the ordinary matter found in stars, planets, and living organisms, distinguishing it from dark matter, which does not interact with electromagnetic forces and is not visible. Understanding baryonic matter is crucial for piecing together the universe's composition and the evidence surrounding dark matter.
Cosmic microwave background: The cosmic microwave background (CMB) is the faint radiation left over from the hot, dense state of the early universe, providing a snapshot of the cosmos approximately 380,000 years after the Big Bang. This relic radiation not only supports the Big Bang theory but also serves as crucial evidence for various unsolved problems in particle physics, such as the nature of dark matter and baryogenesis.
Dark halo: A dark halo is a theoretical region surrounding galaxies that contains dark matter, which does not emit or interact with electromagnetic radiation, making it invisible. This halo is crucial for explaining the gravitational effects observed in galaxies and galaxy clusters, providing strong evidence for the existence of dark matter as a major component of the universe's mass-energy content.
Galactic rotation curves: Galactic rotation curves are graphs that depict the rotational velocity of stars and gas in a galaxy as a function of their distance from the galactic center. These curves are crucial in understanding the dynamics of galaxies and provide significant evidence for the existence of dark matter, as they reveal discrepancies between observed velocities and those predicted by visible matter alone.
Gravitational Lensing: Gravitational lensing is the bending of light from a distant object due to the gravitational field of a massive object located between the observer and the source. This phenomenon allows astronomers to study both the mass of the lensing object and the distribution of dark matter in the universe, as it provides evidence for the existence of mass that cannot be seen directly.
Jim Peebles: Jim Peebles is a Canadian theoretical physicist known for his groundbreaking work in cosmology, particularly in understanding the universe's structure and evolution. His research has provided significant insights into dark matter, dark energy, and the cosmic microwave background, helping to shape our modern understanding of the universe.
Lux-Zeplin: Lux-Zeplin is a proposed dark matter detection experiment that aims to identify weakly interacting massive particles (WIMPs) using liquid xenon as its detection medium. By employing advanced technology and large-scale detectors, the project seeks to uncover evidence of dark matter, which constitutes a significant portion of the universe's mass but remains elusive to direct observation.
Mass-to-light ratio: The mass-to-light ratio is a measurement used in astrophysics to compare the total mass of an astronomical object, such as a galaxy, to its total light output or luminosity. This ratio is critical for understanding the distribution of mass in the universe, especially in relation to dark matter, as it helps reveal the presence of unseen mass that does not emit light.
Modified Newtonian Dynamics: Modified Newtonian Dynamics (MOND) is a theoretical framework that adjusts Newton's laws of motion and gravity to account for the observed discrepancies in the motion of galaxies and galactic clusters without invoking dark matter. This approach proposes that at very low accelerations, the force of gravity behaves differently than predicted by traditional Newtonian physics. MOND aims to explain phenomena such as the flat rotation curves of galaxies, which cannot be fully accounted for by the presence of visible matter alone.
Non-baryonic matter: Non-baryonic matter refers to a type of matter that does not consist of baryons, which are particles like protons and neutrons. This concept is crucial in understanding dark matter, as non-baryonic matter is believed to make up a significant portion of the universe's mass-energy content, contributing to gravitational effects that cannot be explained by visible matter alone. It is essential for explaining various astrophysical phenomena and the structure of the universe.
PandaX: PandaX is a direct detection experiment designed to search for dark matter, specifically targeting Weakly Interacting Massive Particles (WIMPs). It aims to provide evidence for the existence of dark matter by looking for rare interactions between these elusive particles and ordinary matter in a highly sensitive environment, often situated deep underground to minimize cosmic radiation interference.
Structure formation: Structure formation refers to the process through which matter in the universe coalesces and evolves to form galaxies, clusters of galaxies, and larger cosmic structures. This process is influenced significantly by the presence of dark matter, which provides the necessary gravitational pull to facilitate the clustering of ordinary matter, leading to the development of the universe's large-scale structure over billions of years.
Supersymmetry: Supersymmetry is a theoretical framework in particle physics that posits a symmetry between bosons and fermions, suggesting that every known particle has a corresponding 'superpartner' with different spin characteristics. This concept aims to resolve several issues within the Standard Model and to provide a candidate for dark matter, while also offering insights into the fundamental nature of particles and forces.
Vera Rubin: Vera Rubin was an American astronomer whose pioneering work provided critical evidence for the existence of dark matter through her observations of galaxy rotation curves. She discovered that galaxies rotate at such speeds that, according to Newtonian physics, they should be flying apart, yet they remain intact. This discrepancy between the visible matter and the expected gravitational forces led to the conclusion that a significant amount of unseen mass, or dark matter, must be present.
Warm dark matter: Warm dark matter refers to a type of dark matter that has a mass and speed between that of cold dark matter and hot dark matter, typically associated with particles that have masses in the range of a few keV. This concept helps explain various cosmic structures and phenomena, providing insights into the formation and evolution of the universe on scales from galaxies to larger cosmic structures.
WIMPs: WIMPs, or Weakly Interacting Massive Particles, are hypothetical particles that are a leading candidate for dark matter. They are predicted to interact with normal matter through the weak nuclear force and gravity, making them difficult to detect directly. Understanding WIMPs is crucial for addressing the limitations of existing models and the ongoing search for explanations of dark matter and its role in the universe.
λCDM model: The λCDM model, or Lambda Cold Dark Matter model, is the standard cosmological model that describes the evolution of the universe. It incorporates dark energy (represented by Lambda, λ) and cold dark matter (CDM) to explain various astronomical observations, including the cosmic microwave background radiation, large-scale structure formation, and the accelerated expansion of the universe. This model is essential for understanding the composition, structure, and dynamics of the universe.