Ch 15 (Cosmology in the 21st Century)

Ch 15 (Cosmology in the 21st Century) - Chapter 15:...

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Cosmology in the 21 st Century Chapter 15:
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In this chapter we study cosmology, the study of the universe as a whole. Sometimes this is difficult because our “common sense” expectations about how the universe should be are incorrect. Remember, what counts are observations!
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Questions we will answer this chapter Does the universe have an edge in space or time? What evidence exists that the universe began with a big bang? How can the universe expand if it has no edge? Why is the universe the way it is?
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15-1 Introduction to the Universe We assume that the universe has no edge – that it is unbounded (it just doesn’t make sense to think of an edge to the universe) If the universe has no edge then it has no center Of course, if the universe is infinite, then it has no edge and no center – but we don’t know this (the universe could be finite and still have no edge or center The Edge-Center Problem
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Olbers’s Paradox Why is the sky dark at night? If the universe is infinite, then every line of sight should end on the surface of a star at some point. The night sky should be as bright as the surface of stars!
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Copernicus & other astronomers of his time saw no paradox because they believed in a finite universe. After Galileo and Newton, western astronomers began to think of an infinite universe and that led to the paradox.
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Solution to Olbers’s Paradox Olbers’s paradox based on an incorrect assumption Universe is not infinitely old The farther away you look the greater the “look-back” time Eventually, the look-back time reaches the age of the universe If the universe had a beginning, then we can only see light from galaxies that has had time to travel to us since the beginning of the universe.
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The universe may be infinite, but the observable universe is finite The night sky is not bright because the universe is not infinitely old
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The difference between the universe and the observable universe is important The universe is everything that exists, and it could be infinite The observable universe is that part of the universe that you can see Today we think the universe is about 14 billion years old Implies that the observable universe has a radius of 14 billion years
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Cosmic Expansion The spectra of other galaxies are redshifted Other galaxies are receding from our galaxy and from each other The size of the redshift is proportional to the distance to the galaxy (Edwin Hubble, 1929) Distant galaxies are receding from us faster than galaxies that are close to us The universe is expanding
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On large scales, galaxies are moving apart, with a velocity proportional to distance. It’s not galaxies moving through space. Space is expanding, carrying the galaxies along! The galaxies themselves are not expanding!
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This note was uploaded on 11/08/2011 for the course AS 101 taught by Professor Biegel during the Fall '11 term at Montgomery College.

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Ch 15 (Cosmology in the 21st Century) - Chapter 15:...

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