chapt16notes - Chapter 16: Why do we call dark matter...

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Chapter 16: Why do we call dark matter “dark”? It emits no radiation that we have been able to detect. Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it? There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales. ** Evidence to date argues against this idea, since our theory of gravity accounts well for other cosmic phenomena. But we know our theory of gravity is incomplete at the smallest scales ---its may also be incomplete at the largest scales. So unless and until we determine just what the dark matter is, we cannot rule out this alternative possibility . Spiral galaxy rotation curves are generally fair flat out to large distances. Suppose that spiral galaxies did NOT contain dark matter. How would their rotation curves be different? The orbital speeds would fall off sharply with increasing distance from the galactic center. The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter, is located within the galaxy? Yes, they tell us that the dark matter is spread throughout the galaxy, with most
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chapt16notes - Chapter 16: Why do we call dark matter...

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