For Wednesday, May 5

1. For Wednesday, May 5 Reading: Sections 22.2-4 Assignments: Mini-Project #1, Pt. 3 (due Wed. May 7) HW #8 (due Wed. May 7) FINAL EXAM: 10:30 AM Friday, May 9 TA HELP ROOM HOURS NEXT WEEK (Room PA 215A): Monday: 1-3 pm Tuesday: 11 am-3 pm; 5-6 pm Wednesday: 12-2 pm; 5-6 pm Thursday: 5-6 pm

2. Is the Universe Isotropic? isotropic: looks the same in all directions to us All-sky surveys of millions of galaxies show there are no special directions

3. Is the Universe Homogeneous?  harder to measure: need to measure galaxy distances and survey faint, distant galaxies some “clumping” of galaxies on small scales, but there seem to be no special places

4. EVIDENCE: Expansion of the Universe (1928) BLUE RED Almost all galaxies have redshifts  appear to be moving away from us  more distant galaxies have larger redshifts… BLUE RED

5. WHEW! DID SOMETHING DIE IN HERE? COINCIDENCE? HMMM…

6. Hubble’s Law vr: recession velocity d: distance H0: Hubble constant 22 km/s / Mly (1 Mly = 1 Mega light-year = 106 light-years)

7. Hubble’s Law(fast forward)

8. Consequence #1: If we see most galaxies moving away from us in this way, observers in other galaxies will also

9. Consequence #2: Imagine running time backwards:  galaxies would meet in same place at same time t in past (BIG BANG)

10. THEORY: A “Big Bang” THEORY: a scientific idea that has survived repeated tests of its predictions • Idea: • universe had a beginning • early universe was hot, dense gas Tests: • Did “Big Bang” leave evidence?

11. Evidence for a “Big Bang” • What clues do we have that there was a BEGINNING to the Universe? • Dark Sky at Night • Expansion of the Universe (Hubble’s Law) • Cosmic Microwave Background Radiation • Abundances of Chemical Elements

12. The Observable Universe • If universe is NOT infinitely old, then… • only see galaxies whose light had time to reach us • universe may be MUCH larger, but there’s an edge (“HORIZON”) to what we can see • more distant galaxies look younger to us: “LOOKBACK TIME” UNOBSERVABLE UNIVERSE OBSERVABLE UNIVERSE

13. Thought Question: Imagine you are observing a high-mass star located in a distant galaxy 100 million light-years away. By analyzing the starlight, you are able to tell that the star appears to be 10 million years old. From its properties, you predict that the star has a total lifetime of 50 million years. • How long before we receive light from this star’s supernova? (Enter your answers in millions of yrs.) ? 100 MILLION LIGHT-YEARS “I see a blue star right now.”

14. Thought Question: Imagine you are observing a high-mass star located in a distant galaxy 100 million light-years away. By analyzing the starlight, you are able to tell that the star appears to be 10 million years old. From its properties, you predict that the star has a total lifetime of 50 million years. • When will or when did the supernova actually occur? (Enter your answers in millions of yrs. If it already happened, make your answer negative.) ? 100 MILLION LIGHT-YEARS “I see a blue star right now.”

15. Lookback 100 MILLION LIGHT-YEARS 40 MILLION LIGHT-YEARS “The star has been dead for 60 million years.” “The star just blew up!!!” “The star looks like it has 40 million years to live.”

16. Universe History • How far back in time can we possibly look? “DARK AGES”: before galaxies and stars were born Universe was once OPAQUE • gas too dense to let light through US UNOBSERVABLE UNIVERSE (early in history) “DARK AGES”

17. Friday, May 9 FINAL EXAM: 10:30 AM Friday, May 9 Assignments: Mini-Project #1, Pt. 3 (due today) HW #7 (due today) TA HELP ROOM HOURS THIS WEEK (Room PA 215A): Wednesday: 12-2 pm; 5-6 pm Thursday: 5-6 pm

18. What was it like in the past?What will happen in the future?

19. Two Ways of Getting Redshifts • Motion through Space: • redshifts from Doppler effect • Stretching of Space: • universe stretches light also • galaxies don’t have to move • galaxies aren’t stretched (their own gravity holds them together) •  CAN EXPLAIN HUBBLE’S LAW

20. Stretching and Redshifts Redshift tells us about total amount of stretching universe has done while the light has been traveling EXPANDING UNIVERSE: DISTANT PAST MEDIUM PAST PRESENT • LARGER REDSHIFT FOR MORE DISTANT GALAXY

21. Universe Scale We can’t measure size of universe (especially if infinite), so compare distances at different times in history: Distances between non-moving galaxies stretch in same way light wavelengths do: PAST: density of the universe was larger in the past: PRESENT:

22. EVIDENCE: Cosmic Microwave Background Radiation (1965) Microwave and radio from all directions in sky with nearly equal brightness: MAP OF ENTIRE SKY: T = 2.728 K thermal radiation with very low temperature

23. EVIDENCE: Cosmic Microwave Background Radiation (1965) If Big Bang was an explosion at one place in space, we should see a glow only from that place… BIG BANG seems to have happened EVERYWHERE

24. Cosmic Microwave Background Radiation  thermal radiation with temperature T = 2.728 K Expansion of universe stretches light, made temperature lower today:

25. Thought Question: The most distant object that has been seen has a redshift of about z = 8. Roughly how far was it from us when it released the light we see today? • 9 times its present distance • 8 times its present distance • 7 times its present distance • Its present distance • 1/7th its present distance • 1/8th its present distance • 1/9th its present distance

26. Hot, Hot, Hot… universe starts out very hot, very dense, and opaque …but today is… very cold, mostly empty, and transparent

27. The universe was about as hot as a cool star at 380,000 yrs old:3000 K and z1100

28. Building the Universe stars make heavy atoms (like iron, carbon, oxygen) universe becomes transparent; cosmic background radiation launches 1 billion years old z  10 - 20 FIRST GALAXIES FORM 100 million years old FIRST STARS FORM 380,000 years old z  1100 T = 3000 K ATOMS FORM AS TIME PASSES, TEMPERATURE DECREASES

29. The Fate of the Universe? • “BIG CRUNCH” (or “gnaB giB”): • gravity strong enough to stop expansion and cause Universe to contract • Universe ends in a hot, violent “crunch” • “BIG CHILL”: • gravity unable to stop expansion • Universe lasts forever, stars die out… so cold, so dark….

30. What is the Universe Made of? • Count Up Mass (including Dark Matter): • Galaxies in Clusters • use orbits to get mass • see how much light gets bent ORBIT SPEEDS IMAGE OF GALAXY GRAVITATIONAL LENSING ACTUAL GALAXY US IMAGE OF GALAXY

31. Gravitational Lensing • light bent by the mass of many galaxies

32. The Cosmic Mass Pie: “Dark matter” seems to be about 5x more abundant than light-releasing matter!

33. How Fast is the Universe Decelerating? • REDSHIFTtells us how much Universe has stretched since supernova went off • DISTANCE (from its apparent brightness) tells us how far back in time a supernova happened • fainter supernovas are farther away: SUPERNOVA

34. Thought Question: Using a radar gun, you are measuring the speeds of cars and their distances (by how long radar takes to travel to them and back). If the cars close to you are moving at the speed limit, how are more distant cars moving if they are decelerating? • Slower than the speed limit • At the speed limit • Faster than the speed limit

35. Decelerating Car: car moving more quickly in past (slope = speed) DISTANCE FROM US NOW TIME BEFORE REACHING US

36. Decelerating Universe: OTHER GALAXIES WITH SUPERNOVAS: US universe CRUNCHES in future universe stretched more quickly in past AVERAGE GALAXY SEPARATION (COMPARED TO TODAY) NOW PAST FUTURE BEGINNING OF UNIVERSE TIME

37. Universe is Accelerating! OTHER GALAXIES WITH SUPERNOVAS: US universe stretched MUCH slower in past AVERAGE GALAXY SEPARATION (COMPARED TO TODAY) stretching getting faster!! NOW PAST FUTURE WHAT IS DOING THIS!?!?!? TIME

38. The Cosmic Energy Pie: about 5% about 27% about 68% “Dark energy” appears to be several times stronger than normal matter and dark matter combined

39. THE GRAND FINALE