1. Homework for today was WORKBOOK EXERCISE: “Expansion of the Universe” (pg. 87-88 in workbook)

2. Mapping the Universe Chapter 16

3. The Local Group –group of galaxies –Milky Way is member –at least 30 galaxies 3 large spirals –Milky Way, Andromeda, M33 remainder mostly dwarf ellipticals

4. Galaxy Clusters Most galaxies found in “galaxy clusters” –Examples: Virgo cluster, Coma cluster Superclusters –extremely rich galaxy clusters

5. Structure in the Universe Galaxy surveys show structure –voids –filaments/sheets Galaxy clusters –found within the filaments/sheets Superclusters –found at intersections of several filaments/sheets

6. Evolution of Galaxies Galaxy formation –Universe ~14 billion years old –star formation more active in past Galaxies change, as a result of: –star formation –stellar evolution –galaxy collisions & cannibalism

7. Dark Matter Mass of Galaxy Clusters –determined by galaxy orbital motion –M/L ~ 300 –Universe is 99% dark matter

8. The Big Bang Chapter 17

9. Cosmological Principle Cosmological principle –on large scales, Universe is: isotropic –looks the same in all directions homogeneous –any region of space is about the same as any other

10. Age of the Universe Hubble: Universe is expanding “Run movie backward” –at beginning all matter within small region (single point) very, very hot Subsequent expansion –called Big Bang –not an explosion –galaxies not moving (flying apart) –space is expanding Age of the Universe To = 1/H (Hubble time) ~ 14 billion years

11. Big Bang Model Early universe very hot, very dense (mostly radiation) As it expanded, it cooled Eventually, atomic nuclei form –when universe about 3 minutes old –only simple atoms (hydrogen [deuterium], helium, lithium) Later, matter becomes transparent –light escapes –produces continuous spectrum (blackbody) cosmic microwave background radiation (CMB) Much later, stars and galaxies form

12. Evidence for the Big Bang 1.Expanding universe 2.Cosmic Background Radiation –discovered by Penzias & Wilson in 1960’s (won Nobel Prize 1978) –light released when matter became transparent –Observed background matches predicted temperature (3 K) 3.Nucleosynthesis –Model predicts Universe is 75% hydrogen, 25% helium. –Matches observed abundances –Details provide strong evidence

13. Fate of the Universe Big Bang: 3 possible fates –depends on mass (=gravity) of Universe –open universe (the Big Chill) not enough mass to stop the expansion universe expands forever –flat universe exactly enough mass to stop the expansion, but in infinite time –closed universe (the Big Crunch) enough mass to stop expansion followed by contraction back to single point Latest results (Supernova Cosmology) –Indicate that the Universe is ACCELERATING!

15. Current Status Careful tests via CMB observations –Universe is flat –4% normal matter 0.3% is luminous –23% dark, exotic matter what is it? –73% dark energy produces anti-gravity expansion rate is increasing what is it?