4.5 Dark matter candidates

Dark matter candidates are hypothetical forms of matter that could explain the missing mass in the universe. These range from baryonic objects like brown dwarfs to exotic particles like WIMPs and axions. Understanding these candidates is crucial for unraveling the nature of dark matter.

The search for dark matter involves various detection methods, including direct detection experiments, indirect searches for annihilation products, and collider searches. Each approach offers unique insights, contributing to our understanding of this elusive component that makes up a significant portion of the universe's matter content.

Types of dark matter

• Dark matter is a hypothetical form of matter that does not interact with electromagnetic radiation, making it invisible to direct observation
• Dark matter is thought to make up a significant portion of the universe's total matter content, accounting for the missing mass needed to explain various astrophysical phenomena
• The nature of dark matter remains one of the greatest mysteries in modern physics, with several candidates proposed based on their properties and interactions

Baryonic vs non-baryonic

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• Baryonic dark matter consists of ordinary matter made up of baryons (protons and neutrons) that do not emit or absorb light, such as cold gas, dust, or compact objects like black holes and neutron stars
• Non-baryonic dark matter is composed of particles that are not part of the Standard Model of particle physics, such as hypothetical particles like weakly interacting massive particles (WIMPs) or axions
• Observations suggest that the majority of dark matter in the universe is non-baryonic, as the amount of baryonic matter is constrained by Big Bang nucleosynthesis and the cosmic microwave background radiation

Cold vs hot dark matter

• Cold dark matter (CDM) consists of particles that move at non-relativistic speeds, allowing for the formation of small-scale structures like galaxies and galaxy clusters
• Hot dark matter (HDM) is made up of particles that move at relativistic speeds, such as neutrinos, which would suppress the formation of small-scale structures and lead to a top-down formation scenario
• The current cosmological model favors cold dark matter, as it better explains the observed large-scale structure of the universe

Particle vs non-particle candidates

• Particle dark matter candidates are hypothetical particles that do not interact strongly with ordinary matter, such as WIMPs, axions, or sterile neutrinos
• Non-particle dark matter candidates include objects like primordial black holes, or alternative theories such as modified gravity or scalar fields
• The search for the nature of dark matter involves both particle physics experiments and astrophysical observations, aiming to identify the properties and interactions of the dark matter candidate

Baryonic dark matter

• Baryonic dark matter refers to dark matter composed of ordinary matter (baryons) that does not emit or absorb significant amounts of electromagnetic radiation
• Although baryonic dark matter contributes to the total dark matter content of the universe, observations suggest that it cannot account for all of the dark matter

Massive compact halo objects (MACHOs)

• MACHOs are astronomical objects that emit little or no radiation, such as black holes, neutron stars, brown dwarfs, or rogue planets
• These objects could potentially contribute to the dark matter content of galactic halos
• Gravitational microlensing surveys have searched for MACHOs in the Milky Way's halo, but results suggest that they cannot account for a significant portion of the dark matter

Brown dwarfs, neutron stars, and black holes

• Brown dwarfs are substellar objects that are not massive enough to sustain hydrogen fusion in their cores, making them faint and difficult to detect
• Neutron stars are extremely dense remnants of massive stars that have undergone supernova explosions
• Black holes are regions of spacetime with such strong gravitational fields that not even light can escape from within the event horizon
• While these objects contribute to the baryonic dark matter content, their abundances are constrained by various observations and theoretical arguments

Constraints on baryonic dark matter

• Big Bang nucleosynthesis sets limits on the total amount of baryonic matter in the universe, as the abundance of light elements depends on the baryon-to-photon ratio
• The cosmic microwave background radiation also constrains the baryonic matter content through its effect on the power spectrum of temperature fluctuations
• Observations of the large-scale structure of the universe, such as galaxy clusters and the Lyman-alpha forest, provide further evidence that baryonic dark matter cannot account for all of the dark matter in the universe

Non-baryonic dark matter

• Non-baryonic dark matter is composed of particles that are not part of the Standard Model of particle physics
• These hypothetical particles are thought to have very weak interactions with ordinary matter, making them difficult to detect directly
• Non-baryonic dark matter is favored by current observations and cosmological models, as it can explain the missing mass in the universe without conflicting with constraints on baryonic matter

Weakly interacting massive particles (WIMPs)

• WIMPs are hypothetical particles that have masses in the GeV to TeV range and interact with ordinary matter through the weak nuclear force and gravity
• These particles are predicted by various extensions of the Standard Model, such as supersymmetry or extra dimensions
• WIMPs are a popular dark matter candidate because their predicted abundance from thermal production in the early universe is consistent with the observed dark matter density

Axions and other light particles

• Axions are extremely light, weakly interacting particles that were originally proposed to solve the strong CP problem in quantum chromodynamics (QCD)
• These particles have very small masses (in the $\mu$eV to meV range) and are expected to have extremely weak interactions with ordinary matter
• Other light particle candidates include sterile neutrinos and hidden sector particles, which could also contribute to the dark matter content of the universe

Sterile neutrinos

• Sterile neutrinos are hypothetical neutrinos that do not interact through the weak nuclear force, only gravitationally
• These particles are motivated by various extensions of the Standard Model and could have masses in the keV range
• Sterile neutrinos could be produced in the early universe through oscillations with active neutrinos or through other mechanisms, and their abundance could be consistent with the observed dark matter density

Particle dark matter candidates

• Particle dark matter candidates are hypothetical particles that are predicted by various extensions of the Standard Model of particle physics
• These particles are expected to have very weak interactions with ordinary matter, making them difficult to detect directly
• The search for particle dark matter involves a combination of theoretical work, astrophysical observations, and experimental efforts, such as direct and indirect detection experiments and collider searches

Supersymmetric particles

• Supersymmetry (SUSY) is a theoretical extension of the Standard Model that predicts the existence of a partner particle for each known particle, differing by half a unit of spin
• The lightest supersymmetric particle (LSP), such as the neutralino, is often stable and could serve as a dark matter candidate
• SUSY particles are expected to have masses in the GeV to TeV range and could be produced in the early universe through thermal processes

Kaluza-Klein particles

• Kaluza-Klein (KK) particles arise in theories with extra spatial dimensions, such as string theory or extra-dimensional models
• In these models, Standard Model particles can propagate in the extra dimensions, giving rise to a tower of massive KK states
• The lightest KK particle (LKP) could be stable and serve as a dark matter candidate, with properties similar to WIMPs

Other exotic particles

• Various other exotic particles have been proposed as dark matter candidates, such as gravitinos (the supersymmetric partner of the graviton), or particles arising from other extensions of the Standard Model
• These particles could have a wide range of masses and interactions, and their viability as dark matter candidates depends on their specific properties and production mechanisms in the early universe
• The search for exotic particle dark matter candidates is an active area of research, driven by both theoretical work and experimental efforts

Non-particle dark matter candidates

• Non-particle dark matter candidates include objects or phenomena that could explain the missing mass in the universe without invoking new particles
• These candidates range from compact astrophysical objects to alternative theories of gravity or scalar fields
• While non-particle candidates are less favored than particle dark matter, they are still actively investigated and could provide insights into the nature of dark matter

Primordial black holes

• Primordial black holes (PBHs) are hypothetical black holes that formed in the early universe through the collapse of dense regions in the primordial plasma
• These black holes could have a wide range of masses, from the Planck mass to supermassive black holes, depending on their formation time and the conditions in the early universe
• PBHs could contribute to the dark matter content of the universe, but their abundance is constrained by various astrophysical observations, such as the effects on the cosmic microwave background and the gamma-ray background

Modified gravity theories

• Modified gravity theories, such as $f(R)$ gravity or tensor-vector-scalar (TeVeS) theory, attempt to explain the missing mass in the universe by modifying the laws of gravity on large scales
• These theories aim to reproduce the observed gravitational effects attributed to dark matter, such as galactic rotation curves and gravitational lensing, without invoking new particles
• While modified gravity theories can provide alternative explanations for some astrophysical phenomena, they often face challenges in reconciling with other observations and theoretical constraints

Scalar fields and dark energy

• Scalar fields, such as quintessence or k-essence, are hypothetical fields that could permeate the universe and contribute to its energy density
• These fields are often invoked to explain the observed accelerated expansion of the universe, attributed to dark energy
• Some scalar field models, such as fuzzy dark matter or wave dark matter, propose that the dark matter content of the universe could be explained by the presence of ultra-light scalar fields with masses around $10^{-22}$ eV
• These models aim to provide a unified description of dark matter and dark energy, but they face challenges in reproducing all the observed properties of dark matter and the large-scale structure of the universe

Observational evidence for dark matter

• The existence of dark matter is inferred from various astrophysical and cosmological observations that cannot be explained by the presence of ordinary matter alone
• These observations span a wide range of scales, from individual galaxies to the large-scale structure of the universe
• The consistent picture emerging from these observations provides compelling evidence for the existence of dark matter, even though its nature remains unknown

Galactic rotation curves

• Galactic rotation curves describe the orbital velocities of stars and gas as a function of their distance from the galactic center
• In most galaxies, the observed rotation curves remain flat or slightly increasing at large distances, contrary to the expectation from the visible matter distribution
• This discrepancy can be explained by the presence of a dark matter halo surrounding the galaxy, providing the additional gravitational pull needed to maintain the high orbital velocities

Gravitational lensing

• Gravitational lensing occurs when the path of light from a distant source is bent by the gravitational field of an intervening massive object, such as a galaxy or galaxy cluster
• Strong gravitational lensing can produce multiple images, arcs, or rings of the background source, while weak lensing leads to subtle distortions in the shapes of background galaxies
• The strength of the gravitational lensing effect depends on the total mass distribution of the lens, including both visible and dark matter
• Observations of gravitational lensing provide a direct probe of the dark matter distribution in galaxies and clusters, confirming the presence of dark matter halos

Cosmic microwave background anisotropies

• The cosmic microwave background (CMB) is the remnant radiation from the early universe, observed as a nearly uniform background with small temperature fluctuations
• The power spectrum of these temperature anisotropies depends on the contents and evolution of the universe, including the dark matter density
• Precise measurements of the CMB power spectrum, such as those from the Planck satellite, provide constraints on the dark matter density and its properties, favoring cold, non-baryonic dark matter
• The CMB observations also support the overall cosmological model, which includes dark matter as a key component

Dark matter detection methods

• The search for dark matter involves a multi-pronged approach, combining theoretical work, astrophysical observations, and experimental efforts
• Dark matter detection methods can be broadly classified into direct detection, indirect detection, and collider searches
• Each method has its own strengths and limitations, and the combination of results from different approaches is crucial for identifying the nature of dark matter

Direct detection experiments

• Direct detection experiments aim to observe the interactions of dark matter particles with ordinary matter in low-background detectors
• These experiments typically use large, sensitive detectors made of materials such as germanium, silicon, or xenon, operated in deep underground laboratories to minimize background noise
• The expected signal is a rare, low-energy nuclear recoil caused by the elastic scattering of a dark matter particle off a target nucleus
• Examples of direct detection experiments include XENON, LUX, PandaX, and SuperCDMS, which have set stringent limits on the interaction cross-section of dark matter particles with nuclei

Indirect detection through annihilation products

• Indirect detection methods search for the products of dark matter annihilation or decay in astrophysical sources, such as galaxies, galaxy clusters, or the Sun
• If dark matter particles can annihilate or decay into Standard Model particles, these products could be detected as an excess of gamma rays, neutrinos, or cosmic rays above the expected astrophysical backgrounds
• Experiments such as Fermi-LAT, IceCube, and AMS-02 have searched for these signals in various astrophysical sources, setting constraints on the annihilation cross-section or decay lifetime of dark matter particles
• Indirect detection methods are complementary to direct detection efforts and can probe different regions of the dark matter parameter space

Collider searches for dark matter particles

• Collider experiments, such as those at the Large Hadron Collider (LHC), can search for dark matter particles produced in high-energy particle collisions
• If dark matter particles couple to Standard Model particles, they could be produced in colliders and detected through their missing energy signature, as they would escape the detector without interacting
• Collider searches can probe a wide range of dark matter masses and interaction types, complementing the sensitivity of direct and indirect detection methods
• Examples of collider searches include monojet, monophoton, and mono-Higgs analyses, which look for events with a single high-energy object recoiling against missing transverse energy
• Collider searches have set constraints on the interaction cross-sections of various dark matter candidates, such as WIMPs, axions, and dark sector particles