PhysicsChapter 9: Cosmology
Dark Matter
Dark matter
Definition
Dark matter is the name given to non-luminous matter inferred mainly from its gravitational effects on visible matter, radiation and cosmic structure.
Galaxy Rotation
Evidence for dark matter includes observations such as: galaxy rotation; motions of galaxies in clusters; gravitational lensing; large-scale structure; * cosmological measurements. Stars and gas far from galaxy centers often move faster than expected if only visible matter supplied the gravity. This suggests additional unseen mass.
Gravitational Lensing
Mass bends the paths of light. The amount of lensing can reveal total gravitational mass, including matter that does not shine.
Formula / Key Relation
Simplified circular motion: **v²r = GM(r)/r² so: M(r)=v²rG** Observed orbital speeds can therefore provide estimates of enclosed gravitational mass.
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DARK MATTER
- Invisible directly
- Gravity detected
- Galaxy rotation
- Lensing
- Clusters
- Structure formation
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- Dark matter is inferred because observed gravity exceeds what visible matter alone can explain.
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