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Lec29 - Class News F o r F r i M a y 8 Read Ch 22 Do Online...

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Class News For Fri. May 8: – Read Ch. 22 + Do Online Quiz 11 For Mon. May 11: – Read Ch. 23 + Do Online Quiz 12 – Extra Credit Project Due
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Summary: Doppler shift & Hubble’s law • Doppler shift: V = c Δλ / λ o – V= velocity – c = speed of light Δλ = λ - λ o =Doppler shift from rest wavelength λ o • Hubble’s law: V = H o d –d= distance; H o = Hubble’s constant
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Pop quiz Suppose the Hubble constant is H o =60 (km/s)/Mpc, and you observe a galaxy for which spectral lines normally at a wavelength λ o =600 nm are found to be at λ =660 nm. 1. Is the galaxy moving toward or away from us? 2. About what speed is it moving? (Recall c=300,000 km/s). 3. According to Hubble’s law, what is its distance, in Mpc? 4. About how long ago did the light we see leave this galaxy?
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Pop quiz Suppose the Hubble constant is H o =60 (km/s)/Mpc, and you observe a galaxy for which spectral lines normally at a wavelength λ o =600 nm are found to be at λ =660 nm. 1. Is the galaxy moving toward or away from us? 2. About what speed is it moving? (Recall c=300,000 km/s). 3. According to Hubble’s law, what is its distance, in Mpc? 4. About how long ago did the light we see leave this galaxy? λ > λ o => redshift => away c Δ λ / λ o = 0.1c = 30,000 km/s d= 30,000/60 = 500 Mpc d=~ 500*3 = 1500 Mly => t= 1.5 billion years ago
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The Age of the Universe In the context of an expanding Universe… H o tells us the rate at which galaxies are moving apart from one another so if we run he clock backward to when the galaxies were all at one point then 1/H o tells us how long it took the Universe to expand to its current size if the expansion rate is constant; so 1/H o only estimates age of Universe
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The Age of the Universe Our best measurement of the Hubble Constant… comes from the Wilkinson Microwave Anisotropy Probe announced by NASA in February 2003 • H o = 71 km/s per Mpc This gives age of the Universe, 1/H o = 13.7 billion years
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Lookback Time
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