Dark Matter Theory

Dark Matter Types

• Cosmic Structure formation depends on dark matter physics
• recall the mass budget:


• 4 types of dark matter
1. Baryonic dark matter
• Big Bang Nucleosythesis => most baryons are dark
• D abundance => more baryons than stars
• ~ 0.05
• could be in the form of MACHOs
• baryons exist!
• structure formation theory seems to require more exotic dark matter
2. Hot Dark Matter (HDM)
• massive neutrinos with m ~ 10^-8 m_p
• neutrinos exist!
• with such a low mass, they move at ~0.1c when structure starts to form
• this is above the escape velocity for a proto-galaxy
• galaxy clusters would form first and then fragment into galaxies
• But, galaxies are observed to form very early in the Universe

• dark matter cannot be only HDM but Hot + Cold DM may be ok
• structure formation theory suggests that < 0.3
• evidence for some neutrino mass from solar and atmospheric neutrino experiments
• but masses may be << 10^-8 m_p
3. Cold Dark Matter (CDM)
• WIMPs or Axions
• no evidence that this stuff exists!
• MACHOs could be CDM
• black holes that form very early, for example
• preferred DM type for structure formation theories
• theory gives 0.15 <  < 0.95
• details of structure formation theory suggest that a mixture of CDM and HDM or  is preferred
4. Cosmological Constant, , or "false vacuum" matter
• very weird
• has negative pressure
• squeeze a piston full of gas and it gains energy and pushes back with positive pressure
• false vacuum matter will accelerate the piston to close to the speed of light
• gives gravitational repulsion
• latest SN type Ia results suggest  ~ 0.7 or so