class32a - Friday, Nov. 14 Syllabus, class notes, and...

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Friday, Nov. 14 Syllabus, class notes, and homeworks are at: www.as.utexas.edu courses AST 301, Lacy Reading for next week: chapter 16 We’ll go back to the old help session time and place next week: Wednesday at 5:00 in GRG 424
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Topics for this week How does the big bang theory explain Hubble’s law? How does the big bang theory explain the microwave background radiation? Describe some of the events that occurred in the first few minutes after the big bang. Describe how supernovae are used to measure the rate of expansion of the Universe in the past. Describe how matter and energy cause the expansion of the Universe to accelerate or decelerate. How can we measure the amount of normal matter, dark matter, and dark energy in the Universe? What do we find?
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The Background Radiation The light that is reaching us now has had its wavelengths Doppler shifted by the expansion of the Universe. For this purpose it is easiest to look at the Doppler shift as being a stretching of wavelengths of light by the same factor as distances in the Universe have been stretched. For 3000 K, λ max = 1 μ m. Stretched by a factor of 1000, this light appears now with λ max = 1 mm. It has the spectrum expected for an object at a temperature of 3 K, which would have its peak emission at 1 mm.
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If the Universe has been cooling as it expanded, and it was at a temperature of 3000K 400,000 yr after the big bang, it should have been even hotter earlier. We can ask what the temperature was at different times
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class32a - Friday, Nov. 14 Syllabus, class notes, and...

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