Big Bang, Black Holes, No Math ASTR/PHYS 109 Dr. David Toback Lectures 20

1. Big Bang, Black Holes, No MathASTR/PHYS 109Dr. David TobackLectures 20

2. Was due Today – L20 • Reading: • Unit 5 • Pre-Lecture Reading Questions: • Unit 4 Revision: Stage 1 • Unit 5: Stage 1 • End-of-Chapter Quizzes: • Chapter 13 • Papers: • Paper 2 Revision (if desired): Stage 2 • Paper 3: Stage 2 due Wednesday • Honors Papers: • Stage 1 Revisions

3. A Big Bang Occurs… Then What? Universe goes through a phase of being hot and small with lots of high energy particles in thermal equilibrium Then the Universe gets bigger and cooler As it gets bigger, it “evolves”

4. The History of the Universe 0 10 billion degrees 1 degree Universe cools down as time passes 1 second 10 billion years Universe expands as time passes

5. Various Times Explain what happens during each of a number of different periods in time • The VERY early universe • The first three Minutes • The next 300,000 years • The next billion years • ~13 billion years later (now) • The ultimate fate of the universe? • The first four will take a couple of lectures } Chap 13 } Chap 14 } Chaps 15-17 } Unit 6

6. Where are we now in the history? 3 Minutes

7. After the First Three Minutes • At about the 3 minute mark the temperature of the Universe is about 1 billion degrees • Electrons and positrons have mostly annihilated each other • No particles energetic enough to produce new ones ~1 electron for every proton

8. What else? • The temperature drops so that the photons, on average, can’t even break up deuterium • Easier to start building up the heavier nuclei

9. Forming Heavy Nuclei in the Early Universe Proton + Proton  Deuterium + Electron + Neutrino Proton Electron Nuclear Reaction Deuterium Proton Neutrino

10. Nuclei and Low Energy Photons At the 3 minute mark, low energy photons only “bump” nuclei Deuterium Low Energy Photon

11. Can Now Build Heavier Nuclei Deuterium Proton + Deuterium  3He + Photon Photon Nuclear Reaction 3He Proton

12. Creating Stable Helium 3He + 3He  4He + 2 Protons 3He Replenishes the hydrogen in the Universe Proton Nuclear Reaction 4He Proton 3He

13. Why so few atoms Heavier than Helium? 0 Nuclear Physics Can build up Hydrogen and Helium one at a time  Next possibility, 5He or 5Li, isn’t stable So What? Since 5He and 5Li decay quickly they don’t have enough time to find another proton to become 6Li and be stable Almost no elements heavier than helium are produced in the early Universe

14. The Helium Story • Able to predict the fraction of the atoms in the Universe that are Helium • 75% of atomic mass in hydrogen • 25% of observed mass in helium • Same as saying about 91% of the atoms are hydrogen • Helium is the same in every direction because it was created everywhere • Entire Universe had the same temperature everywhere • If it was created mostly in stellar cooking we’d see it coming from the directions where there are more stars • I.e., the direction of galaxies • More on this in Chapter 16

15. Time marches on… • Universe continues to expand and cool • Number of protons goes up and number of neutrons goes down • Why? Neutrons can decay into protons

16. Neutron Decay Neutron  Proton + Electron + Neutrino Electron Decay Proton Neutrino Neutron

17. The End of an Era • For the next couple hundred thousand years things don’t change much • Nuclear processes eventually stop making new heavy nuclei since the Universe gets so big • But it’s still MUCH too hot for atoms to form

18. The beginning of a new Era • From 300,000-700,000 years the universe expands and cools • All the protons, neutrons and electron combine to form atoms • Translation: Universe cools until the photons in the universe can’t knock all the electrons out of the atoms any more

19. Atoms in a Lower Temperature Universe At these lower temperatures photons can’t easily break up atoms Proton ElectroMagnetic Reaction HydrogenAtom Electron Photon

20. Where are we now in the history? 300,000-700,000 years

21. The Transition From a Hot Universe to a Cool Universe Some Time Passes

22. The Universe at ~700,000 Years Old • At a temperature of about 3000K, photons, on average, can’t knock apart atoms • We can now have stable atoms and all the electrons become part of atoms • The atoms are now free to form into stars and galaxies

23. Give this special time in history a name Call this time “Recombination” Crappy Word! Atoms are combining for the first time, not recombining!

24. Atoms in the Universe A couple hundred Thousand Years Later Early Universe These days

25. Before and After the Creation of Atoms Protons and electrons combine to form atoms Universe becomes transparent for photons Lots of electrons and protons in the Universe Photons scatter from charged particles

26. Transparent Universe The Universe now LOOKS different! • The light (photons) have no free electrons to interact with • Since all the electrons are in atoms, because of Quantum Mechanics, they largely ignore the photons • Photons just travel in a straight line  Like traveling through glass

27. Atoms in the Universe Early Universe Photons bust apart atoms A couple hundred Thousand Years Later Photons can only excite atoms These days Universe is transparent to photons Hydrogen High energy photon

28. A Cartoon… Before Recombination: Photons interact with charged particles Scatter and change directions After Recombination: Atoms ignore the photons Photons travel in straight lines forever + + + + + + + +

29. Cosmic Background Radiation • The photons we see in the cosmic background radiation are essentially the photons AFTER they interacted with their last charged particle • Well… they’ve been traveling for awhile so they’ve changed a little

30. Photons in the Universe 0 After recombination, photons can travel freely through space Their wavelength is only stretched (red shifted) by cosmic expansion Observe most photons as as the cosmic background radiation!  Also can observe photons emitted from Galaxies a billion years ago (just reaching us now) as being red-shifted

31. Looking Back at the Early Universe The more distant the objects we observe, the further back into the past of the universe we are looking

32. The next few billion years After less than ~0.5 billion years, the first stars form  Stars produce heavy elements (“Stellar Cooking”)

33. A Neutral Universe • After the electrons and protons combine to form atoms the Universe is almost completely neutral i.e., neutrally charged • Why is this so important? All the particles in the Universe stop attracting each other due to their electric charge • Not completely true, just true outside the atoms • Basically can ignore electric charge from here on out

34. The Transition From a “Charged” Universe to a “Neutral” Universe Some Time Passes

35. Aside on Dark Matter • Dark matter is a BIG part of this story because there is 5 times more mass in the Universe in dark matter than in atoms • Will come back to this in Chapter 15 • However, the story doesn’t change much with Dark Matter since we think it is just another neutral particle

36. A Gravity Dominated Universe • The only thing left is gravity! • The gravitational attraction between massive things ONLY makes them move towards each other • Any single atom would be most attracted to the closest place with lots of mass

37. Wait a Half Billion Years • As the years go by, mass clumps together • By a half a billion years after the bang, most of the mass is in one of a large number of “clumps” • Billions of these clumps, each becomes a galaxy!

38. Fast Forward… • Fast forward 13 billion years • Temperature of space is now ~3 degrees • Cosmic Background Radiation • We get people… • More on Stellar and Galaxy Formation next

39. Looking Forward • Finished Unit 4 • Evolution of the Universe • Starting Unit 5 • Big Objects and Black Holes

40. For Next Time – L19 • Reading: • Unit 5 • Pre-Lecture Reading Questions: • Unit 4 Revision (if Desired): Stage 1 Due Monday before class • Unit 5: Stage 1 due Monday before class • End-of-Chapter Quizzes: • Chapter 14 (if we finished Chapter 14, else just 13) • Papers: • Don’t forget that ALL papers must be submitted to turnitin.com • Paper 2 Revision (if desired): Stage 2 due Monday before class • Paper 3: Stage 2 due Wednesday before class • Honors Papers: • Stage 1 Revision

41. For Next Time – L20 • Reading: • (Unit 5) • Pre-Lecture Reading Questions: • Unit 4 Revision (if Desired): Stage 2 • Unit 5: Stage 2 • End-of-Chapter Quizzes: • Chapter 14 (if we finished Chapter 14, else just 13) • Papers: • Don’t forget that ALL papers must be submitted to turnitin.com • Paper 3: Stage 2 due Wednesday before class • Honors Papers: • Stage 1 Revision

42. Full set of Readings So Far • Required: • BBBHNM: Chaps. 1-15 • Recommended: • TFTM: Chaps. 1-5 • BHOT: Chaps. 1-7, 8 (68-85), 9 and 11 (117-122) • SHU: Chaps. 1-3, 4(77-93), 5(95-114), 6, 7 (up-to-page 159) • TOE: Chaps. 1-3

43. End of Lecture

44. Clicker Question Why can we only see so far back into space? • Because before recombination the light in the universe was scattering off most of the particles • Because there are too many things in the way which block our ability to see

45. Clicker Question Which of the following requires the highest energy photon? • Breaking apart a Proton • Breaking apart a helium nuclei • Breaking an electron out of a hydrogen atom

46. Clicker Question Order the following in the order of highest required energy to lowest required energy • Breaking apart a Proton • Breaking apart a helium nuclei • Breaking an electron out of a hydrogen atom • A>B>C • A>C>B • B>A>C • B>C>A • C>A>B • C>B>A

47. Clicker Question What was the last thing a typical photon in the universe (part of the Cosmic Background Radiation) did before we detected it? • Broke apart a Proton • Broke apart a helium nuclei • Broke the electron out of a hydrogen atom

48. Clicker Question Why do the photons in the universe appear to be in thermal equilibrium? • Because they all have exactly the same energy • They are in thermal equilibrium • The universe was in thermal equilibrium, and the photons are just one of the particles in the Universe

49. Clicker Question Why is hydrogen the most abundant element in the universe? • It is the lightest element • Any heavy elements that might have been present would have been broken apart by high energy photons in the early universe • Both of these contribute

50. Clicker Question Q: Which of the following statements is MOST correct? • The photons in the cosmic background radiation are there because the Universe has been expanding for ~13 billion years • The photons in the cosmic background radiation are there because the universe was in thermal equilibrium • The photons in the cosmic background radiation are there because photons can exist forever