Chapter 20 Cosmology

1. Chapter 20Cosmology

2. Hubble Ultra Deep Field

3. Galaxies and Cosmology • A galaxy’s age, its distance, and the age of the universe are all closely related • Galaxies formed when the universe was young and have aged along with the universe

4. Parallax Measure the distances to nearby stars

5. Star clusters

6. Luminosity • Brightness = • 4π (distance)2 • Properties you can directly observe and measure: • Brightness • Change in brightness over time • Color • Rotation speed • A standard candle is an object whose luminosity we can determine without measuring its distance

7. Cepheid variable stars are very luminous standard candles

8. White-dwarf supernovae all have same peak luminosity: standard candles Can be seen up to 10 billion light years away!

9. Tully-Fisher Relation Entire galaxies can also be used as standard candles: faster rotation = greater total luminosity

10. Giant ellipticals: if you’ve seen one, you’ve seen them all… Homework assignment

12. Hubble measured the distance to nearby galaxies using Cepheid variables as standard candles (1927, Mt Wilson Obs)

13. Hubble found that the spectral features of virtually all galaxies are redshifted  They’re all moving away from us

14. velocity distance 1 time = = Hubble found that the further away a galaxy is, the faster it is receding from us! Slope = y / x Time = age of the universe! Hubble’s Law: velocity = H0 x distance

15. Distances of farthest galaxies are now measured from their redshifts!!

16. A balloon’s surface expands but has no center or edge

17. Cosmological Principle The universe looks about the same no matter where you are within it • Matter is evenly distributed on very large scales in the universe • No center & no edges • Not proved but consistent with all observations and predictions of the Big bang theory

18. Distances between faraway galaxies changes because the space between them expands! Think of lookback time rather than distance distance?

19. Redshift is NOT the Doppler shift! Expansion stretches photon wavelengths causing a cosmological redshift directly related to lookback time

20. observations show us very distant galaxies as they appeared a long time ago (Old light from young galaxies)

21. Galaxies of different ages look different from one another

22. Collisions play an important role in galaxy evolution

24. Collisions were much more common when U. was young, because galaxies were closer together

25. Many of the galaxies we see at great distances (when U. was young) look violently disturbed

26. Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies

27. Collisions may explain why giant elliptical galaxies tend to be found where galaxies are closer together

28. Quasars are the most luminous galaxies

29. The highly redshifted spectra of quasars indicate large distances • Redshift --> distance --> luminosities of some quasars are >1012LSun • Variability shows that all this energy comes from region smaller than solar system: active nucleus with supermassive black hole!!

30. Galaxies around quasars often appear disturbed by collisions

31. Dark Matter, Dark Energy, and the Fate of the Universe

32. Mass within Sun’s orbit: 1011MSun Observable stars and gas clouds: ~few 109MSun

33. Dark matter and dark energy Dark Matter: An undetected form of mass that emits little or no photons, but we know it must exist because we observe the effects of its gravity Dark Energy: An unknown form of energy that is causing the universe to expand faster over time

34. What is the Universe made of? • “Normal” Matter: ~ 4.4% • Normal Matter inside stars: ~ 0.6% • Normal Matter outside stars: ~ 3.8% • Dark Matter: ~ 25% • Dark Energy ~ 71%

36. Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

37. The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

38. measure the velocities of galaxies in a cluster from their Doppler shifts Mass is 50 x larger than the mass in stars!

39. Clusters contain large amounts hot gas: emits x rays Temperature of hot gas tells us cluster mass: 85% dark matter 13% hot gas 2% stars

40. Gravitational lensing of background galaxies also tells us the mass

44. What is dark matter made of? • Ordinary Dark Matter (MACHOS) • Massive Compact Halo Objects: dead or failed stars in halos of galaxies • Extraordinary Dark Matter (WIMPS) • Weakly Interacting Massive Particles: mysterious neutrino-like particles

45. Two Basic Options • Ordinary Dark Matter (MACHOS) • Massive Compact Halo Objects: dead or failed stars in halos of galaxies • Extraordinary Dark Matter (WIMPS) • Weakly Interacting Massive Particles: mysterious neutrino-like particles The Best Bet

46. MACHOs do not cause enough lensing events to explain all the dark matter

47. Why Believe in WIMPs? • There’s not enough ordinary matter • WIMPs could be left over from Big Bang • Models involving WIMPs explain how galaxy formation works

48.  Gravity of dark matter is what caused protogalactic clouds to contract early in time

49. WIMPs don’t contract to center because they don’t emit photons, so they can not radiate away their orbital energy

50. Maps of galaxy positions reveal extremely large structures: superclusters and voids