Chapter 16 Summary - M16_BENN0885_05_SE_C16.qxd 12:18 PM...

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466 Part V Galaxies and Beyond summary of key concepts 16.1 Unseen Influences in the Cosmos • What do we mean by dark matter and dark energy? Dark matter and dark energy have never been directly observed, but each has been proposed to exist because it seems the simplest way to explain a set of observed motions in the universe. Dark matter is the name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds. Dark energy is the name given to whatever may be causing the expansion of the universe to accelerate. 16.2 Evidence for Dark Matter •What is the evidence for dark matter in galaxies? The orbital velocities of stars and gas clouds in galaxies do not change much with distance from the center of the gal- axy. Applying Newton’s laws of gravitation and motion to these orbits leads to the conclusion that the total mass of a galaxy is far larger than the mass of its stars. Because no detectable visi- ble light is coming from this matter, we call it dark matter . • What is the evidence for dark matter in clusters of galaxies? We have three different ways of mea- suring the amount of dark matter in clusters of galaxies: from galaxy orbits, from the temperature of the hot gas in clusters, and from the gravitational lensing predicted by Einstein. All of these methods agree, indicating that the total mass of a cluster is about 50 times the mass of its stars, implying huge amounts of dark matter. • Does dark matter really exist? We infer that dark matter exists from its gravitational influence on the matter we can see, leaving two possibilities: Either dark matter exists, or there is something wrong with our under- standing of gravity. We cannot rule out the latter possibility, but we have good reason to be confident that our current understanding of gravity is correct and dark matter is real.
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