Summarizing Dark Matter and Closure

Dark Matter and Closure Summary

• Dark matter is non-luminous matter detected in and around galaxies and galactic clusters.
• It may be 10 times the mass of the luminous matter in the universe, and its amount may determine whether the universe is open or closed (expands forever or eventually stops).
• The determining factor is the critical density of the universe and the cosmological constant, a theoretical construct intimately related to the expansion and closure of the universe.
• The critical density ρ_c is the density needed to just halt universal expansion. It is estimated to be approximately 10^–26 kg/m³.
• An open universe is negatively curved, a closed universe is positively curved, whereas a universe with exactly the critical density is flat.
• Dark matter’s composition is a major mystery, but it may be due to the suspected mass of neutrinos or a completely unknown type of leptonic matter.
• If neutrinos have mass, they will change families, a process known as neutrino oscillations, for which there is growing evidence.

Glossary

axions

a type of WIMPs having masses about 10⁻¹⁰ of an electron mass

cosmological constant

a theoretical construct intimately related to the expansion and closure of the universe

critical density

the density of matter needed to just halt universal expansion

dark matter

indirectly observed non-luminous matter

flat (zero curvature) universe

a universe that is infinite but not curved

microlensing

a process in which light from a distant star is focused and the star appears to brighten in a characteristic manner, when a small body (smaller than about 1/1000 the mass of the Sun) passes between us and the star

MACHOs

massive compact halo objects; microlensing objects of huge mass

neutrino oscillations

a process in which any type of neutrino could change spontaneously into any other

neutralinos

a type of WIMPs having masses several orders of magnitude greater than nucleon masses

negatively curved

an open universe that expands forever

positively curved

a universe that is closed and eventually contracts

WIMPs

weakly interacting massive particles; chargeless leptons (non-baryonic matter) interacting negligibly with normal matter