1. Cosmology

2. The Expansion of the Universe:(The Hubble Law) Key Concepts: 1. All galaxies beyond the Local Group are moving AWAY from us. Their spectra are all REDSHIFTED • More distant galaxies are speeding away from us faster than nearby galaxies (THE HUBBLE LAW). 3. Facts #1 and #2 imply that the Universe is expanding.

3. Key Concepts (continued): • There is NO CENTER to the expansion: • the Universe is infinite, • AND is expanding everywhere. • These facts imply that the expansion • started a finite time ago • -- roughly 14 billion years ago, • in what has been dubbed THE BIG BANG.

4. All distant galaxies have redshifts. (They are moving away from us.)

5. Galaxy Redshifts

6. How do we measure distances to galaxies? Climb the “cosmic distance ladder”.

7. 1strung of the distance ladder: distances within the Solar System. Distances from Earth to nearby planets are found by radar.

8. 2ndrung of the distance ladder:distances to stars in the Milky Way. Distances to nearby stars are found by parallax

9. 3rdrung of the distance ladder:distances to other galaxiesusing STANDARD CANDLES

10. A “standard candle” is a light source of known luminosity. Know luminosity (L) measure flux or apparent brightness (f) compute distance (d).

11. Climbing the distanceladder 1) Measure flux of two standard candles: one near, one far. 2) Find distance to near standard candle from its parallax.

12. 3) Compute luminosity of near standard candle:L = 4 π d2 f. 4) Assume far standard candle has same luminosity as the near. 5) Compute the distance to the far standard candle:

13. A good standard candle: Cepheid variable star Cepheid stars vary in brightness with a period that depends on their average luminosity.

14. First, calibrate the properties of nearby Cepheids:

15. Each dot is a Cepheid variable star for which we have a parallax measurement

16. For distant Galaxies: Observe Cepheid. Measure period. Look up luminosity. Measure flux. Compute distance

17. Hubble Law: Measure distance to each galaxy using Cepheid variable stars Measure radial velocity from spectrum Distance Radial Velocity Mpc = Mega Parsec = million parsecs

18. Edwin Hubble(1889-1953) • He discovered Cepheid variables in the Andromeda galaxy. • He calculated the distance to the Andromeda galaxy. • 2 million light years • it was not in the Milky Way • He developed a classification scheme for galaxies. • He has a space telescope named after him

19. Milton Humason(1891-1972) • A former mule-team driver who dropped out of 8th grade. • He took spectra and measured the redshifts of many galaxies. • He worked with Hubble, who measured the distances to those same galaxies. What they found was that redshift and distance were correlated

20. The HUBBLE LAW states that the Doppler Shift of a galaxy is proportional to its distance.

21. Hubble’slawinmathematicalform v = radial velocity of galaxy d = distance to galaxy H0 = the “Hubble constant” (same for all galaxies in all directions)

23. 3. Hubble's Law means that the Universe is Expanding.

24. Redshift of light is a measure of how much space has expanded since the light was emitted.

25. TheUniverseisexpanding Space is huge, and it’s getting huger. As space expands,wavelength of light(distance between wave crests) increases.

26. I like to visualize the expansion of the Universe with M. C. Escher's drawing: The galaxies are at the cubes, and the rods between them are expanding.

27. There are two ways to think about a galaxy’s redshift. 1) The redshift is the result of a Dopplershift 2) The redshift is the result of expansion of space, stretching the wavelength.

28. If you play the movie backwards, space was infinitely small at a point in the past. The expansion started in an event called THE BIG BANG. The Big Bang occured 13.7 billion years ago. What came before the Big Bang? That's like asking what's south of the South Pole. At the time of the Big Bang, time was not the nice, linear quantity we know it to be today.

30. Hubble Expansion of the Universe: Key Concepts redux: The distant galaxies are all moving away from us. However, we are NOT at the center of expansion. Every galaxy sees all other galaxies moving away. The expansion of the Universe has no center. The galaxies are not really moving, the Doppler Shifts are the result of the expansion of space. Sometimes the spectral shifts are referred to as "cosmological redshifts" since they result from space expanding.

31. We assume that the Universe is infinite, it has no edge. However, it is expanding. YOU are not expanding, however, since you are held together by electromagnetic forces. The Milky Way, Local Group, Clusters of Galaxies – are not expanding, they are bound by gravity.

32. Having trouble believing the expansion? You're not alone! Hubble Einstein

33. Einstein's Theory of General Relativity predicts that the Universe should expand. Einstein recognized this, long before Hubble found the Hubble law. Expansion of the Universe seemed so implausible to Einstein that he added an arbitrary constant, called the cosmological constant, to his equations so that the Universe would not expand. After Hubble found that the Universe is expanding, Einstein called the cosmological constant, "the biggest blunder of my life".

34. Einstein at Mt. Wilson, visiting the telescope used to find the Hubble Law.

35. The Hubble Law: The Universe is expanding v = radial velocity of galaxy d = distance to galaxy H0 = the “Hubble constant” (same for all galaxies in all directions)

36. Given the Hubble Law, we can derive the distance to any galaxy, if we have a spectrum of it. (1) Measure redshift of galaxy: z=(λ-λ0)/λ0 (2) Compute radial velocity: v = cz (3) Compute distance: d = v / H0 Cheap, fast way to find distance

37. A “redshift map” of a slice through the universe. Each tiny dot represents a galaxy.

38. Groups and Clusters of Galaxies Most galaxies are found in “groups” or “clusters.” Group: a few big galaxies and some dwarfs, maybe 20 galaxies in all (e.g. Local Group) Clusters of Galaxies: Some galaxies live in big CLUSTERS OF GALAXIES, which contain several thousand galaxies which are gravitationally bound to each other.

39. Coma Cluster of Galaxies

40. The Hercules Cluster

42. Although the big clusters of galaxies are beautiful, only a small fraction of all galaxies live in big clusters. Most galaxies live in much smaller groups, like The Local Group. Here’s another nearby group of galaxies, the M81 group:

43. Number of large galaxies within 500 000 light years = 1 Number of dwarf galaxies within 500 000 light years = 12 Number of stars within 500 000 light years = 225 billion The Milky Way Source: http://www.atlasoftheuniverse.com

44. Number of large galaxies within 5 million light years = 3 Number of dwarf galaxies within 5 million light years = 46 Number of stars within 5 million light years = 700 billion The Local Group

45. Clusters and Voids Within 200 Million Light years Of the Local Group

46. Clusters and Voids Within 200 Million Light years Of the Local Group: 3D galaxy groups within 100 million light years = 200 galaxies within 100 million light years = 2500 dwarf galaxies within 100 million light years = 50 000 stars within 100 million light years = 200 trillion

47. The Universe within 1 billion Light Years: The Neighboring Superclusters Horologium Supercluster

48. The Universe within 1 billion Light Years: The Neighboring Superclusters Number of superclusters within 1 billion light years = 100 Number of galaxy groups within 1 billion light years = 240 000 Number of large galaxies within 1 billion light years = 3 million Number of dwarf galaxies within 1 billion light years = 60 million Number of stars within 1 billion light years = 250 000 trillion

49. One of the biggest superclusters is the “HorologiumSupercluster”

50. Properties of the HorologiumSupercluster (estimates) Length of the supercluster 550 million light years Mass of the supercluster 1017 solar masses Number of galaxy groups in the supercluster 5 000 Number of large galaxies in the supercluster 30 000 Number of dwarf galaxies in the supercluster 300 000 Number of stars in the supercluster 1 million billion