Title Bar - Cosmology

Historical Background

After a discussion of Red Shift used to determine the distance to an astronomical object, students may begin to wonder about the motions of these objects, because measuring the distances to these objects using their Red Shift requires that the objects have a motion (velocity with respect to the Earth). The objects, therefore, do not have a stationary position to measure. What is even more puzzling is that the farther an object is from the Earth, the faster it moves away. These ideas may still confuse the student and therefore they deserve more discussion.

When we discussed the Hubble relation, we used it as a way to measure distance to objects, as it is most commonly used today. However, when Hubble found this relation, it was useful as much more than a simple distance measure. It showed that most galaxies were expanding at very high rates away from the Earth. Until this point, in the 1920s, most astronomers still believed in a Newtonian universe which was more or less static, meaning that it was not getting bigger or smaller, but staying the same size as it was in the distant past or future. Einstein even changed his theory of relativity to make it agree with this static model of the universe. Later, when the theory of an expanding universe became generally accepted, Einstein called this change "the greatest blunder of [his] life" footnote25. Even though most students won't know about the theory of relativity, you may want to share this example with them to show that even great scientists like Einstein make mistakes and are unsure of their work.

Actually, even before 1914 when V. M. Slipher took spectra of "spiral nebulae", some scientists and philosophers suspected that the universe was larger than the Milky Way, which until this point was considered the whole of the universe with other spiral and elliptical "clouds" in it. They thought that these clouds might be systems of "island universes" made of stars that were independent of the Milky Way, or as we know them today, other galaxies. Slipher found that 11 out of 15 of these spiral clouds showed red shift and were therefore moving away from the Earth. They were entered as evidence in the famous Shapley-Curtis Debate in which the two noted astronomers discussed the ultimate size of the galaxy and which astronomical objects were to be considered part of the galaxy and which were extragalactic. Harlow Shapley had devised a model of the Milky Way that was very large and therefore could contain these objects. Heber Curtis disagreed with Shapley's assertions andd a debate was arranged between the two scientists in 1920. The three main points discussed concerning the "island universes" were:

  1. What are the distances to the spirals?
  2. Are the spirals composed of stars or gas?
  3. Why do spirals avoid the plane of the Milky Way? footnote26

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