24_cosmology - The Variety of Galaxy Morphologies Cosmology...

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1 Cosmology The Study of the Universe as a Whole Including the beginning of the Universe What is the largest kind of structure in the universe? The ~100-Mpc flaments, shells and voids? On larger scales, things look more uniform. 600 Mpc The Variety of Galaxy Morphologies Spectra of galaxies in clusters of increasing distance prominent pair of absorption lines In 1920's, Hubble used Cepheids to find distances to some of these receding galaxies. Showed that redshift or recessional velocity is proportional to distance : V = H 0 x D (Hubble's Law) velocity (km / sec) Distance (Mpc) Hubble's Constant (km / sec / Mpc) Or graphically. . . Current estimate: H 0 = 73 +/- 2 km/sec/Mpc If H 0 = 75 km/sec/Mpc, a galaxy at 1 Mpc moves away from us at 75 km/sec, etc. The Cosmological Principle On the largest scales, the universe is roughly homogeneous (same at all places) and isotropic (same in all directions). Given no evidence of further structure, assume: Hubble's Law might suggest that everything is expanding away from us, putting us at center of expansion. Is this necessarily true? (assumes H 0 = 65 km/sec/Mpc) If we were at center of expansion, universe would be isotropic (but only from our location) but not homogeneous: Us Galaxies expanding away from us into empty space But if we were not at center, universe would be neither isotropic nor homogeneous: Us
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2 So if the CP is correct, there is no center, and no edge to the Universe! Best evidence for CP comes from Cosmic Microwave Background Radiation (later). The Big Bang All galaxies moving away from each other. If twice as far away from us, moving twice as fast (Hubble's Law). So, reversing the Hubble expansion, everything must have been together once. How long ago? Hubble’s Law: v = H 0 d where H 0 gives rate of expansion. Assume H 0 = 75 km / sec / Mpc. So galaxy at 1 Mpc from us moves away at 75 km/sec. How long did it take to move 1 Mpc from us? time = = = = 13 billion years distance velocity 1 Mpc 75 km/sec The faster the expansion (the greater H 0 ), the shorter the time to get to the present separation. 1 H 0 Big Bang : we assume that at time zero, all separations were infinitely small. Universe then expanded in all directions. Galaxies formed as expansion continued. But this is not galaxies expanding through a pre-existing, static space. That would be an explosion with a center and an expanding edge. If CP is correct, space itself is expanding, and galaxies are taken along for the ride. There is no center or edge, but the distance between any two points is increasing. A raisin bread analogy provides some insight: But the cake has a center and edge. Easier to imagine having no center or edge by analogy of universe as a 2-d expanding balloon surface: To understand what it would be like in a 2-d universe, read Flatland by Edwin Abbott DEMO - Expansion in 1-D Now take this analogy "up one dimension". The Big Bang occurred everywhere at once, but "everywhere" was a small place. If all distances increase, so do wavelengths of photons as they travel and
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This note was uploaded on 12/13/2011 for the course PHYSICS 102 taught by Professor Gregtaylor during the Fall '08 term at New Mexico.

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24_cosmology - The Variety of Galaxy Morphologies Cosmology...

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