Dark Matter Candidates

Dark matter candidates are crucial for understanding the universe's structure and evolution. This overview covers various potential particles and theories, from WIMPs and axions to primordial black holes and modified gravity, highlighting their roles in astrophysics.

1. Weakly Interacting Massive Particles (WIMPs)

   • WIMPs are a leading candidate for dark matter, predicted to have mass in the range of 10 GeV to several TeV.
   • They interact via the weak nuclear force, making them difficult to detect but potentially observable through direct detection experiments.
   • WIMPs are expected to be produced in the early universe and could explain the observed structure of galaxies.

2. Axions

   • Axions are hypothetical particles proposed to solve the strong CP problem in quantum chromodynamics (QCD).
   • They are extremely light and weakly interacting, making them a viable dark matter candidate.
   • Axions could form a condensate that behaves like a classical field, potentially detectable through their effects on electromagnetic fields.

3. Sterile Neutrinos

   • Sterile neutrinos are a type of neutrino that does not interact via the standard weak interactions, making them "sterile."
   • They could account for the observed neutrino masses and may also serve as a dark matter candidate.
   • Their production in the early universe could lead to a warm dark matter scenario, affecting structure formation.

4. Primordial Black Holes

   • Primordial black holes are black holes formed in the early universe due to density fluctuations.
   • They could range in mass from very small to several solar masses and may account for a fraction of dark matter.
   • Their existence could be inferred from gravitational wave detections and their effects on cosmic structure.

5. Modified Gravity Theories

   • These theories propose alterations to general relativity to explain the effects attributed to dark matter.
   • Examples include MOND (Modified Newtonian Dynamics) and TeVeS (Tensor-Vector-Scalar gravity).
   • They aim to account for galactic rotation curves and large-scale structure without invoking dark matter.

6. Self-Interacting Dark Matter (SIDM)

   • SIDM posits that dark matter particles can interact with each other, leading to different dynamics than traditional cold dark matter.
   • This interaction could help explain the core-cusp problem in galaxy formation.
   • SIDM models can lead to observable effects in galaxy clusters and structure formation.

7. Fuzzy Dark Matter

   • Fuzzy dark matter consists of ultra-light bosons that behave like waves, leading to a "fuzzy" structure on galactic scales.
   • This model can explain the formation of galaxies and their rotation curves without requiring traditional dark matter.
   • Fuzzy dark matter may be detectable through its effects on gravitational lensing and structure formation.

8. Massive Compact Halo Objects (MACHOs)

   • MACHOs are massive objects, such as black holes or neutron stars, that could exist in galactic halos and contribute to dark matter.
   • They are detectable through gravitational microlensing events, where their mass bends light from background stars.
   • While they may account for some dark matter, they are unlikely to explain the entirety of dark matter in the universe.

9. Kaluza-Klein Particles

   • Kaluza-Klein particles arise from theories that extend the dimensions of spacetime beyond the familiar four.
   • They can be produced in high-energy collisions and may provide a link between dark matter and extra dimensions.
   • Their detection would support theories of unification and could reveal new physics beyond the Standard Model.

10. Gravitinos

    • Gravitinos are the superpartners of gravitons in supersymmetry theories and could be a candidate for dark matter.
    • They are predicted to be very light and weakly interacting, making them difficult to detect.
    • Gravitinos could play a significant role in the early universe's thermal history and structure formation.