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Chapter 16

Chapter 16 Dark Matter.docx

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Natural Science
NATS 1740
Randy Hoffman

Chapter 16 Dark Matter, Dark energy and the fate of the universe! 16.1 Unseen influences in the Cosmos What do we mean by dark matter and dark energy?  Dark matter: is the name given to mass that emits no detectable radiation  We infer its existence from its gravitational effects  Dark energy: is the name given to the unseen influence that may be causing the expansion of the universe to accelerate with time 16.2 Evidence for dark matter Dark matter in the milky way - Orbital speeds in the milky way remain high even very far from the center, indicating that a large amount of dark matter lies beyond our galaxy’s visible regions  Rotation curve diagrams: summarize results of orbital measurements  Rotation curve means orbital speed increases with distance  Falling curve means orbital speed decreases with distance  Flat curve means orbital speed remains large at large distances Dark matter in other galaxies - Orbital speeds in the outer regions of other spiral galaxies indicate that they too harbor lots of dark matter What is the evidence for dark matter in clusters of galaxies? - Observations suggest that the ratio of dark matter to luminous matter is even greater than that in individual galaxies Orbits of galaxies in clusters - Fritz Zwicky one of first astronomers to think of galaxy clusters as huge swarms of galaxies bound together by gravity - The orbits of galaxies in clusters tell us that galaxy clusters contain huge amounts of dark matter Hot gas in clusters - Temperature measurements of hot gas also tell us the amount of dark matter in clusters and give results that agree with those we infer from galaxy velocities  Gravitational equilibrium: outward pressure balances gravity’s inward pull Gravitational lensing  Gravitational lenses: bend light beams as they pass by - Gravity’s light bending effects distort the images of galaxies lying behind a cluster, enabling us to measure the cluster’s mass without relying on Newton’s laws - Cluster masses measured through gravitational lensing agree with those measured from galaxy velocities and gas temperatures Does dark matter really exist? - One of these things must be true 1.) Dark matter really exists and we are observing the effects of its gravitational attraction 2.) There is something wrong with our understanding of gravity, which is causing us to mistakenly infer the existence of dark matter - Either dark matter exists or our current understanding of gravity is incorrect What might dark matter be made of? 1.) It could be made of ordinary matter that is build from protons, neutrons and electrons but that aren’t detectable 2.) It could be made of exotic particles that we have yet to discover Ordinary matter not enough - Searches for gravitational lensing by star0like or planet-like objects in our galaxys halo show that such objects cannot account for most of the dark matter Exotic matter: the best bet - Neutrinos interact with the forces of gra
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