1. 18 The Big Bang Where do we come from, where are we going?

2. 18 Goals Where did the Universe come from? Where is it going? How can we see the past? How can we learn about the future from seeing the past?

3. 18 Hubble’s Law Recall: All galaxies are moving away from us. The farther away the faster they go. V = H o x D

4. 18 Expanding Universe If galaxies are all moving away, then at some point they were all much closer. Hubble’s Law implies the Universe is expanding.

5. 18 The Big Bang Big Bang: the event from which the Universe began expanding. Into what did the Universe expand? Where was the Big Bang? Where is the center of the Universe?

6. 18 Cosmological Principle Isotropy – The view from here is the same in all directions. (observation) Homogeneity – We live in an average place the same as any other. (statistics) This is the cosmological principle. Implies: –Universe has no edge! –Universe has no center!

7. 18 Cosmological Redshift Another way to think of it: Galaxies aren’t zooming through a fixed Universe. Expanding Universe is carrying galaxies along. Galaxy redshifts: – Aren’t Doppler shifts. –Are cosmological shifts.

8. 18 Age of the Universe Since all galaxies are moving away from us, how long has it been since all galaxies were together? time = distance / velocity velocity = H o x distance time = distance / (H o x distance) time = 1/H o “An expanding universe does not preclude a creator, but it does place limits on when he might have carried out his job.” -Steven Hawking, A Brief History of Time

9. 18 Age Disagreements Until recently, much disagreement on the value of H o and therefore, the age of the Universe. Need to know the distance to some galaxies in order to know the slope of velocity versus distance. Different methods yielded different distances. Some values of H o yielded an Universe younger than some of its stars. HST helped solve the problem: Cepheids.

10. 18 Hubble Space Telescope Use HST to find Cepheids in other galaxies.

11. 18 Hubble’s Constant In recent decades, H o = 50 – 100 km/s/Mpc. Difference in distances by factor of 2. Difference in age of Universe by factor of 2. Recent HST results: H o = 65 km/s/Mpc Recall: T = 1/ H o T = 15 billion years Is this older than the oldest stars? Recent HST result says the oldest white dwarfs are 13 billion years old!

12. 18 The End of the Universe Will the universe expand forever? Depends on the density of the Universe. Too big: Big Crunch –Closed Universe –Bound Universe Too small: Big Freeze –Open Universe –Unbound Universe

13. 18 Critical Density Dividing line is the critical density.  o is the ratio of measured density to the critical density. If  o > 1 then closed,  o < 1 then open.

14. 18 Density of the Universe Add up all the mass we see and  o = 0.01 But we know there is some dark matter in galaxies and clusters. How much? Think ~10 x more dark matter than “light” matter. Cosmologists think  o < 0.3 Result: Open Universe  Big Freeze!

15. 18 The Future from the Past Is the Universe: –Slowing down? –Speeding up? –Staying the same velocity? In the past, was the Universe: –Going faster? –Going slower? –Going the same velocity? The Universe is a time machine.

16. 18 Lookback Time We see everything as it once was. Young Old

17. 18 60s 50s Baby Boomer Universe 40s Farther away we look, further back in time we see! 90s 80s 70s

18. 18 What We See

19. 18 Nearby Galaxies

20. 18 Hubble Deep Field

21. 18

22. 18 V Distance (Lookback time) PresentPast Are We Slowing Down? In our experience, things slow down over time. Is the Universe slowing down at all? Plot distance versus velocity. Use supernovae as “standard candles.” –Distant supernovae (large lookback time). Accelerating Slowing V=H o D

23. 18 Are We Slowing Down? Unseen mass making stars move fast: Dark Matter Unseen energy accelerating galaxies: Dark Energy Slowing Accelerating

24. 18 Homework #18 For Friday read Bennett Ch23: Do Ch23: –Problem 4, 8