November 21, 2011 – 10am Class

1. November 21, 2011 – 10am Class No class on Wednesday: Happy Thanksgiving. Make up assignment for Section 2 Star party: will be posted Work on Citizen Astronomy, go do Telescope Lab Grading: Will catch up over Thanksgiving, promise! Today: Dark Matter The Expansion of the Universe and the Big Bang Dark Energy

2. Dark Matter:Most of the Mass of the Milky Way is NOT normal atomsMost of the Mass in Galaxies is NOTstars, gas or any normal atomic material

3. Out to the edge of its visible disk, the Milky Way Galaxy contains: 200 billion solar masses, but only 20 billion solar luminosities. Conclusion: There must be dark matter in the outer regions of the Galaxy.

4. What the Milky Way REALLY is like: bright stars dark “halo”: nearly spherical distribution of dark matter

5. Dark matter could also be called “invisible matter”. The properties of invisible objects are rather difficult to determine. We know dark matter exists because of its gravitational pull on luminous matter (stars, Gas clouds); otherwise, we know very little About it.

6. Some of the dark matter in galaxy “halos” consists of Massive Compact Halo Objects (MACHOs, for short). MACHOs can be “failed stars” i.e. brown dwarfs (balls of gas smaller than a star but bigger than Jupiter) MACHOs can be “ex-stars” i.e. white dwarfs, neutron stars, black holes

7. Only 20% of the dark matter is MACHOs: Some of the dark matter in galaxy “halos” consists of exotic matter. Suppose there existed a type of massive elementary particle that didn’t absorb, emit, or scatter photons. We’d detect such a particle only by its gravitational pull on luminous matter. Neutrinos?

8. Cosmic Gall by John Updike Neutrinos, they are very small. They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a pane of glass.

9. Although we don’t know the mass of neutrinos exactly, we know it’s tiny… neutrinos electron Neutrinos provide < 10% of the dark matter.

10. Most of the dark matter must be particles other than neutrinos. ANOTHER exotic particle which is a leading candidate for the Dark Matter THE WIMP WIMP = Weakly Interacting Massive Particle

11. According to particle physics theory, WIMPs should be much like neutrinos only more massive. Neutrinos have already been detected Particle physicists are still trying to detect WIMPs.

12. To Sum up, • 90% of the Mass in the Milky Way and all other spiral and elliptical galaxies is "DARK MATTER" • Not stars, black holes, basketballs, protons, neutrons, electrons • MACHOS? (Massive Compact Halo Objects, planets, • bricks, brown dwarfs) Nope • Neutrinos? probably not, < 10% of the Dark Matter • WIMPS? (Weakly Interacting Massive Particles – • particles which interact through the weak force) • OK, but WIMPS are theoretical Particles, and have • not yet been detected in the lab We really have no idea.

13. 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.

14. 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.

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

16. Galaxy Redshifts

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

18. 1strung of the distance ladder: distances within the Solar System. Distances from Earth to nearby planets are found by radar.

19. 2ndrung of the distance ladder:distances to stars in the Milky Way. Distances to nearby stars are found by parallax

20. 3rdrung of the distance ladder:distances to other galaxiesusing STANDARD CANDLES

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

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

23. First, calibrate the properties of nearby Cepheids:

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

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

26. Type 1A Supernovae: White-dwarf supernovae can also be used as standard candles.

27. The apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years).

28. Hubble Law: Measure distance to each galaxy using Cepheid variable stars or some other standard candle Measure radial velocity from spectrum Distance Radial Velocity Mpc = Mega Parsec = million parsecs

29. 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

30. 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

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

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

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

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

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

37. 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.

38. 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.

39. 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 occurred 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.

41. 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.

42. 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.

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

44. 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".