November 28, 2011

1. November 28, 2011

2. 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.

3. 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.

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

5. 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".

6. History of the Universe Hubble's Law establishes that the Universe is expanding at a certain rate. Now you might ask: Will the Universe keep expanding? Will the expansion rate speed up or slow down? Has the Universe always been expanding? First: The classical answer to these questions, which we taught in this class as recently as 10 years ago. Then: New Results -- Nature is far stranger than we had imagined.

7. Until recently, astronomers thought that the evolution of the Universe would be determined by knowing two things: (1) the rate the galaxies are expanding today (the Hubble Law) (2) the density of mass in the Universe

8. If the density is high enough, the expansion will eventually slow, and then the Universe will recollapse in a "Big Crunch". This model was called "the closed Universe".

9. If on the other hand, the density is low, the expansion won't be slowed down, and the Universe will keep expanding forever. This model was called "the Open Universe".

10. One can imagine that density and expansion could be just balanced and the Universe would "Coast". This is called the "Flat Universe".

11. Others suggested that even if the Universe is CLOSED, the Big Crunch might not be the end of the story, but the Universe might oscillate.

12. Possible Histories of the Universe: • (1) Open Universe (expands forever) • Closed Universe (expands, stops, contracts) • Flat Universe (expands until coasting) • Oscillating Universe (Expand-stop-contract- repeat forever) Three data sets in the last few years have determined the fate of the Universe very accurately, and it turns out to be "NONE OF THE ABOVE".

13. * Better measurement of distances using Cepheids observed with HST (The HST Key Project). This really nailed the slope of the Hubble Law, or the Hubble Constant. Several other techniques were developed to measure distances, serving as a useful cross-check of the different techniques. * Type Ia Supernova were used to measure distances to really distant Galaxies. This gave the surprising result that the Hubble Law is not a straight line at early times, in the sense that the expansion is accelerating! * The Type Ia Supernova results can be combined with observations of the Cosmic Microwave Background (most recently the WMAP satellite data) to measure the geometry of space in the Universe (MORE LATER).

14. Supernovae Type Ia Standard candle MUCH brighter than Cepheid star, so can be used to much more distant galaxies Adam Riess et al. 2004

15. Science magazine’s “Breakthrough of the Year” for 1999! 2011 Nobel Prize in Physics to Reiss, Perlmutter and Schmidt (former UA undergrad)

16. Now let’s consider the possible models for the Universe in terms of the CURVATURE of space

17. Will the universe continue expanding forever?

18. The fate of the universe depends on the amount of dark matter.

19. Since the amount of dark matter is ~25% of the critical density, we expect the expansion of the universe to overcome its gravitational pull.

20. In fact, the expansion appears to be speeding up! Dark energy?

21. Estimated age depends on the amount of both dark matter and dark energy.

22. Is the expansion of the universe accelerating?

24. 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

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

26. 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.

27. Coma Cluster of Galaxies

28. The Hercules Cluster

30. 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:

31. 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

32. 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

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

34. 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

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

36. 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

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

38. 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

39. What is bigger than superclusters? How far can we keep going? Since the Universe has a finite age, there is a cosmological horizon. We can't see farther than 13.7 billion light years, since light from a galaxy farther than that has not had enough time to reach us. However, we assume that the Universe is infinite, and that what we see is a fair sample of the Universe.

40. Cosmological Horizon

41. The Visible Universe (schematic)

42. Some Numbers Number of superclusters in the visible universe = 10 million Number of galaxy groups in the visible universe = 25 billion Number of large galaxies in the visible universe = 350 billion Number of dwarf galaxies in the visible universe = 7 trillion Number of stars in the visible universe = 30 billion trillion (3x10²²)