Announcements Common Exam 3 is a week from Monday. Today’s material WILL BE on the exam.
Announcements Observing next week will count for the fourth exam. The final exam will be cumulative....
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Transcript of Announcements Observing next week will count for the fourth exam. The final exam will be cumulative....
![Page 1: Announcements Observing next week will count for the fourth exam. The final exam will be cumulative. The final will be 40 questions, 10-15 will be on cosmology,](https://reader035.fdocuments.us/reader035/viewer/2022081519/56649e3b5503460f94b2d114/html5/thumbnails/1.jpg)
Announcements
• Observing next week will count for the fourth exam.
• The final exam will be cumulative.
• The final will be 40 questions, 10-15 will be on cosmology, the remainder on material covered earlier in the semester.
![Page 2: Announcements Observing next week will count for the fourth exam. The final exam will be cumulative. The final will be 40 questions, 10-15 will be on cosmology,](https://reader035.fdocuments.us/reader035/viewer/2022081519/56649e3b5503460f94b2d114/html5/thumbnails/2.jpg)
Cosmology
• Why is the night sky dark?
• Problems with the Big Bang
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Review Questions
• Give three pieces of evidence for the Big Bang model.
• What will happen to the Universe if the density is less than the critical density?
• A few seconds after the big bang did hydrogen atoms exists? Why or why not?
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Three pieces of evidence for the Big Bang model
• Hubble expansion: galaxies are moving away from us with speed proportional to distance.
• The ratio of Helium to Hydrogen in gas clouds unaffected by stars.
• The cosmic microwave background: a 2.7 K glow seen in all directions.
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What will happen to the Universe if the density is less than the critical density?
C0
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A few seconds after the big
bang did hydrogen
atoms exists?
![Page 7: Announcements Observing next week will count for the fourth exam. The final exam will be cumulative. The final will be 40 questions, 10-15 will be on cosmology,](https://reader035.fdocuments.us/reader035/viewer/2022081519/56649e3b5503460f94b2d114/html5/thumbnails/7.jpg)
Olber’s Paradox
• Why is the night sky dark?
Assume:
• Universe is infinite
• Universe has always existed
• Universe has a flat geometry
• Universe is homogeneous
![Page 8: Announcements Observing next week will count for the fourth exam. The final exam will be cumulative. The final will be 40 questions, 10-15 will be on cosmology,](https://reader035.fdocuments.us/reader035/viewer/2022081519/56649e3b5503460f94b2d114/html5/thumbnails/8.jpg)
Olber’s Paradox
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Which is the solution to Olber’s Paradox
• Stars have finite size and hide behind each other.
• Dust blocks light from stars
• The Universe has a finite age
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Whether the Universe will continue to expand forever or eventually collapse is determined by a competition between
1. Density and the rate of Hubble expansion
2. Gravity and redshift
3. Density and redshift
4. Iowa and Michigan
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Problems with the Big Bang
• The horizon problem
• The flatness problem
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Cosmic Microwave Background
The Universe glows at 2.7 K in every direction.
The temperature is the same to < 0.1%.
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Observable Universe
We can only see the parts of the Universe from which light has had time to travel to us.
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Horizon Problem
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Flatness problem
Any tiny deviation from the critical density is amplified over time.
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Inflation solves flatness problem
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Inflation solves horizon problemSize[cm]
Time [seconds]
Whole observable universe came from a tiny region.
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Problems with Big Bang
• Problems point to an incomplete model, not an incorrect one.
• Problems can be solved by a new theoretical idea, inflation, but this new idea still needs to be tested.
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Review questions
• Why is it surprising that the microwave background has almost exactly the same temperature in all directions on the sky?
• Why is the night sky dark?