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Chapter 20

Chapter 20 Summary Cosmic Perspective

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Astronomy 1021
Chris Racknor

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CHAPTER TWENTY Galaxies and the Foundation of Modern Cosmology The universe if filled with galaxies that come in a wide variety of shapes and sizes in order to learn the histories of these galaxies we must at the same time consider how the universe itself has evolved through time Much of our current understanding of the structure and evolution of the universe is based on measurements of distances to far away galaxies these measurements rely on a carefully constructed chain of techniques in which each link in the chain builds on the links that come before it It has been less than a century since Hubble first proved that the Milky Way is only one of billions of galaxies in the universe this and the subsequent discovery universal expansion provided the foundation upon which modern cosmology has been built measurements of the rate of expansion tell us that is began about 14 billion years ago As the universe expands it carries the galaxies within it along we therefore observe red shifts in the spectra of distant galaxies and these allow us to determine how long it has been since the light from these galaxies left on its way to reach us Lives of Galaxies Connected to the History of the Universe Because nearby galaxies tend to be similar in age to the Milky Way we know that most galaxies formed when the universe was much younger and have aged a
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