Thinking about the Big Bang

1. Pat Hall Department of Physics and Astronomy York University, Toronto, Ontario What are the basic concepts of the Big Bang? What is the past and future evolution of the Universe? Thinking about the Big Bang

2. Outline • Hubble’s Law • 3-D universe: like a raisin cake • 2-D universe: like the surface of a balloon • Much hotter in past: should see redshifted heat • WMAP shows universe was uniform temperature • Inflation and our Observable Universe • Our Past: Structure Formation • Our Future: Dark Energy • Olber’s Paradox

3. Steps to the Big Bang • Here in the Milky Way, we see almost all galaxies moving away from us (Edwin Hubble)

4. By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way

5. Steps to the Big Bang • Here in the Milky Way, we see almost all galaxies moving away from us (Edwin Hubble) • Either we’re at the center of the Universe…

6. Either we’re at the center of the universe ...or galaxies are all moving away from each other, and the Milky Way is just another one of those galaxies.

7. Steps to the Big Bang • Here in the Milky Way, we see almost all galaxies moving away from us (Edwin Hubble) • Either we’re at the center of the Universe… • …or galaxies are all moving away from each other, and the Milky Way is just another one of those galaxies.

8. Every galaxy is moving away from every other galaxy. (Except for galaxies so close to each other that their mutual gravity can resist the expansion of the universe.)

11. Surface of a balloon expands but has no center or edge

12. Steps to the Big Bang • Here in the Milky Way, we see almost all galaxies moving away from us (Edwin Hubble) • Either we’re at the center of the Universe… • …or galaxies are all moving away from each other, and the Milky Way is just another one of those galaxies. • Exceptions: galaxies very near to each other can orbit each other or crash into each other due to gravity. • Galaxies all moving away from each other, so…

13. …the universe must have been much denser (and hotter) early in its history

14. What is matter made of? • Water (H2O) is a molecule, like many everyday substances. • A water molecule is made up of components called atoms: 2 hydrogen atoms (H) and 1 oxygen atom (O). • Every atom consists of two parts: • One or more electrons surrounding ... • the nucleus: ≈ 100,000 times smaller than the electron cloud, but 10,000 to 100,000 times more massive. If you increase the density and temperature and density enough, you can break apart molecules into atoms, and eventually atoms into electrons and nuclei. Hydrogen & helium atoms are transparent, but become opaque when split into nuclei and electrons.

15. Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

16. Expansion stretches photon wavelengths causing a cosmological redshift directly related to lookback time

17. Background has a nearly perfect thermal spectrum at a temperature 2.73 degrees above absolute zero (-270 degrees Celsius) Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves

18. The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965

22. Background has a nearly perfect thermal spectrum at a temperature 2.73 degrees above absolute zero (-270 degrees Celsius) Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves

24. Wilkinson Microwave Anisotropy Probe (WMAP) • The WMAP satellite has shown that when our universe was about 380,000 years old, it was uniform in temperature to within 1 part in 5,000 • How flat would the Earth be if it was smooth to within 1 part in 5,000? • The highest mountain would be 640 meters high, and the deepest ocean 640 meters deep.

25. Remember that every point on this map of the microwave sky represents light that traveled 13.7 billion light years to reach us…

26. How can microwave temperature be nearly identical on opposite sides of the sky?

27. Inflation: regions now on opposite side of the sky were close together before inflation pushed them far apart

28. Inflation can make all the structure in the microwave background by stretching tiny quantum ripples to enormous size These ripples in density then become the seeds for all structures in today’s universe

29. Inflation of universe flattens overall geometry like the inflation of a balloon – however the universe started out, it will end up almost exactly flat. (Could there be island universes in a long, thin balloon?)

32. WMAP gives us detailed baby pictures of structure in the universe

33. Observed patterns of structure in universe – from WMAP etc. - agree (so far) with what inflation should produce

34. Time in billions of years 2.2 5.9 8.6 13.7 0.5 35 70 93 140 13 Size of expanding box in millions of lt-yrs Models show that gravity of dark matter pulls mass into denser regions – universe grows lumpier with time

36. Will the universe continue expanding forever?

37. Does the universe have enough kinetic energy to escape its own gravitational pull?

38. Not enough dark matter Lots of dark matter Fate of universe depends on amount of dark matter

39. Not enough dark matter Lots of dark matter Critical density of matter Fate of universe depends on amount of dark matter

40. Amount of dark matter is ~25% of the critical density suggesting fate is eternal expansion Not enough dark matter

41. But expansion appears to be speeding up! Dark Energy? Not enough dark matter

42. old older oldest Estimated age depends on dark matter and dark energy

43. The night sky tells us that the observable universe isn’t infinitely old and unchanging

44. Olbers’ Paradox If universe were 1) infinitely old, and infinite in all directions in space 2) unchanging 3) everywhere the same Then stars would cover the night sky

45. Olbers’ Paradox If universe were 1) infinite 2) unchanging 3) everywhere the same Then, stars would cover the night sky

46. Night sky is dark because the universe changes with time, and because the observable universe is not infinitely old

47. Night sky is dark because the universe changes with time, and because the observable universe is not infinitely old

48. Cosmological Horizon Universe’s age of 13.7 billion years limits how far we can see, and 13.7 billion years ago there weren’t any stars around yet.

49. The Big Bang and our Universe • The Big Bang was a point in time, not in space • The physical universe has no center or edge

50. The Big Bang and our Universe • The Big Bang was a point in time, not in space • The physical universe has no center or edge • The observable universe is centered on us: we observe light from everything that was within 13.7 billion light years of our current location when the universe became transparent. Next year, we’ll see 1 light-year farther away.