The beginning of the Universe as we know it began about 13.7 billion years ago.

1. The beginning of the Universe as we know it began about 13.7 billion years ago.

2. What is Cosmology? Cosmology is the study of the origin, structure, and evolution of the Universe. The Universe itself is defined as all matter, energy, and spacetime.

3. Homogeneity – matter [on large scales] is uniformly spread throughout space Isotropy – universe [on large scales] looks the same in every direction (isotropic) Basic Assumptions in Cosmology • 1 and 2 give: • Cosmological Principle – any observer in any part of the universe sees the same general features • NO special place in the Universe • NO boundaries (edges or centers)

4. By “homogeneous”, we mean on large scales.

5. Olber’s Paradox If the universe is homogeneous, isotropic, infinite, and unchanging, the entire sky should be as bright as the surface of the Sun.

6. So, Why is it Dark at Night? The universe is (roughly) homogeneous and isotropic – It must not be infinite and/or unchanging.

7. The Expansion of the Universe In the 1920’s Hubble measured distances and redshifts of many galaxies. The farther away the galaxies are, the faster they are moving away. Thus, the Universe is not unchanging, rather it is expanding; this is now called the Hubble Flow.

8. Galaxy Redshifts We know that the galaxies are moving away from us because their light is redshifted.

9. Cosmological vs. Doppler Redshift • Galaxies farther away are redshifted more. • Not actually a Doppler effect (motion through space-time) • Space-time itself is expanding! (Cosmological redshift)

10. Group Question Could a galaxy be moving away from us so fast that we would never see the light from it? Yes No Can’t say Don’t know

11. The Hubble Constant The farther away the galaxies are, the faster they are moving away. Mathematically, the distance and velocity are related by the Hubble Constant (Ho). v = Ho x d The Hubble Constant is related to the age of the Universe. The older the Universe, the more time galaxies have had to move away from each other.

12. Age of the Universe Basic physics: v = d/t Hubble: v = Ho x d So, t = 1/Ho Ho is approximately 70 km/s/Mpc

13. Age of the Universe We don’t know the exact value of Ho, so we don’t know the exact age of the Universe. The Range of Ages: 10 x 109 years - 20 x 109 years Best  13.7 x 109 years

14. The Big Bang The Universe has been expanding for billions of years – maybe it was much smaller and more dense in the past? Gamow: maybe the Universe was created in a big explosion? Hoyle: disagreed, he mockingly called this the Big Bang – a name which has stuck ever since.

15. Olber’s Paradox If the universe is homogeneous, isotropic, infinite, and unchanging, the entire sky should be as bright as the surface of the Sun.

16. Olber’s Paradox Solved 1. The Universe is expanding Cosmological redshift causes energy of photons to decrease. 2. The Universe is not infinitely old Cannot see stars to an infinite distance.

17. So, where was the Big Bang? It was everywhere! No matter where in the Universe we are, we will measure the same relation between recessional velocity and distance, with the same Hubble constant.

18. The Balloon Analogy This can be demonstrated in two dimensions. Imagine a balloon with coins stuck to it. As we blow up the balloon, the coins all move farther and farther apart. There is, on the surface of the balloon, no “center” of expansion.

19. Expansion of the Universe Tutorial

20. Cosmological Redshift The same analogy can be used to explain the cosmological redshift:

21. Testing the Big Bang Theory • The Big Bang must have been incredibly hot. • The expansion of the Universe will cause this radiation to cool. • If there was a Big Bang, the Universe would be bathed in a background of cool radiation.

22. If there was a Big Bang, the Universe would be bathed in a background of cool radiation – it is! Robert Dicke and Jim Peebles at Princeton were designing an instrument to detect it. Arno Penzias and Robert Wilson of Bell Labs accidentally discovered it. We now call this the Cosmic Microwave Background Radiation (CMBR) Penzias and Wilson would later win the Nobel Prize.

23. When these photons were created, they were highly energetic. The expansion of the universe has redshifted their wavelengths so that now they are in the radio spectrum, with a blackbody curve corresponding to about 3 K.

24. The Cosmic Microwave Background Radiation: Proof of the Big Bang The CMBR has a temperature of 2.73 degrees Kelvin. The CMBR has a nearly perfect blackbody spectrum. The existence of the CMBR is strong evidence in favor of the Big Bang (and led to another Nobel Prize in 2006).

25. Next Time Cosmology: The Fate of the Universe Reading: Chapter 17, Sections 1-8 (24 pages)

26. Refining the Big Bang Model A few problems remain that must be explained. 1. The Horizon Problem (or Isotropy Problem) The CMBR radiation is very smooth (too smooth). See about the same temp everywhere you look in the Universe. There are places in the Universe separated by more than 13.7 billion light years; they should not have exactly the same temperature.

27. The Horizon Problem But, the Universe is bigger than 13.7 billion light years (and looks the same in all directions). We can see back ~13.7 billion years.

28. Inflation to the Rescue One way to solve this problem is if the Universe has not always expanded at the same rate. At some time in the past, it may have expanded very quickly. This process is called inflation.

29. Inflation If the Universe underwent inflation, then all areas of the Universe were once in the same place and had the same temperature.

30. Refining the Big Bang Model II: The Flatness Problem Worksheet Here

31. The Fate of the Universe The fate of the Universe depends on how much mass there is in the Universe. If the Universe has enough mass, it will eventually contract (the Big Crunch). In this case the Universe is closed. If the Universe doesn’t have enough mass, it will continue to expand forever (the Big Chill). In this case the Universe is open.

32. The Critical Density Astronomers talk about the density of the Universe in terms of the ratio of the real density to the critical density (). The amount of mass needed to just barely make the Universe closed is called the critical density (c). c is about 14 hydrogen atoms per square meter.

33. Density and the Fate of the Universe If  > 1, the Universe will eventually collapse. If  < 1, the Universe will expand forever. If  = 1, the Universe just barely manages to expand forever.

34. Density If the density is low, the universe will expand forever. If it is high, the universe will ultimately collapse.

35. Comparison to Earth’s Gravity

36. If space is homogenous, there are three possibilities for its overall geometry: • Closed – this is the geometry that leads to ultimate collapse • Flat – this corresponds to the critical density • Open – expands forever

37. The Geometry of Space These three possibilities are illustrated here. The closed geometry is like the surface of a sphere; the flat one is flat; and the open geometry is like a saddle.

38. Summary of the Possible Geometries

39. Refining the Big Bang Model II: The Flatness Problem We don’t yet know the geometry of the Universe, but it appears to be extremely flat. However, theory says that unless is  exactly 1 after the Big Bang, it should be either much smaller or much larger today. It is unlikely that  would have been exactly 1 after the Big Bang. So, how come the Universe looks so flat today?

40. Inflation can also solve the flatness problem. A heavily curved region of space can be made to look flat if the radius increases.

41. So, which is it?

42. Fate of the Cosmos The answer to this question lies in the actual density of the Universe. Measurements of luminous matter suggest that the actual density is only a few percent of the critical density. But – we know there must be large amounts of dark matter.

43. However, the best estimates for the amount of dark matter needed to bind galaxies in clusters, still only bring the observed density up to about 0.3 times the critical density, and it seems very unlikely that there could be enough dark matter to make the density critical. We can test this by measuring the distances and redshifts of objects.

44. An Accelerating Universe? Type I supernovae can be used to measure the behavior of distant galaxies. In a decelerating Universe, we expect to see more distant galaxies receeding relatively faster than nearby galaxies.

45. Where we expected the data to be Where it really is. However, when we look at the data, we see that it corresponds not to a decelerating universe, but to an accelerating one.

46. Possible explanation for the acceleration: vacuum pressure (cosmological constant), more generically called dark energy.

47. Dark Energy and The Cosmological Constant Curiously, Einstein had introduced this idea decades before in order to balance gravity and make the Universe “static”. He later called it the biggest blunder of his career. Turns out he was right.

48. Where Did the Galaxies Come From? Cosmologists realized that galaxies could not have formed just from instabilities in normal matter. The hot radiation from the Big Bang would have kept normal matter from clumping. But, Dark Matter, being unaffected by radiation, could have started clumping long before normal matter.

49. Galaxies could then form around the dark-matter clumps, resulting in the Universe we see.

50. A simulation of structure formation in the Universe